7.4. Effects of changes in climate and UV radiation levels on
structure and function of arctic ecosystems in the short
and long term . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .292
7.4.1. Ecosystem structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .292
7.4.1.1. Local and latitudinal variation . . . . . . . . . . . . . . . . . . . . .292
7.4.1.2. Response to experimental manipulations . . . . . . . . . . . .295
7.4.1.3. Recent decadal changes within permanent plots . . . . . .298
7.4.1.4.Trophic interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . .298
7.4.1.5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .303
7.4.2. Ecosystem function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .305
7.4.2.1. Biogeochemical cycling: dynamics of carbon and
nutrients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .305
7.4.2.2. Soil processes and controls over trace-gas exchanges . . .311
7.4.2.3.Water and energy balance . . . . . . . . . . . . . . . . . . . . . . . .314
7.4.2.4. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .314
7.5. Effects of climate change on landscape and regional
processes and feedbacks to the climate system . . . . . . . . . .315
7.5.1. Impacts of recent and current climate on carbon flux . . . . . . . .315
7.5.1.1. Recent changes in carbon dioxide fluxes . . . . . . . . . . . .316
7.5.1.2. Current circumpolar methane fluxes . . . . . . . . . . . . . . . .317
7.5.1.3. Relative contributions of methane and carbon dioxide
to the carbon budget . . . . . . . . . . . . . . . . . . . . . . . . . . . .317
7.5.2. Current circumpolar water and energy balances . . . . . . . . . . . .318
7.5.3. Large-scale processes affecting future balances of carbon,
water, and energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .319
7.5.3.1. Permafrost degradation . . . . . . . . . . . . . . . . . . . . . . . . . .319
7.5.3.2. Changes in circumpolar vegetation zones . . . . . . . . . . . .319
7.5.4. Projections of future balances of carbon, water, and energy
exchange . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .321
7.5.4.1. Carbon balance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .322
7.5.4.2. Energy and water exchange . . . . . . . . . . . . . . . . . . . . . . .324
7.5.5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .324
7.6. Synthesis: Scenarios of projected changes in the four ACIA
regions for 2020, 2050, and 2080 . . . . . . . . . . . . . . . . . . . . . .327
7.6.1. Environmental characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . .327
7.6.2.Vegetation zones and carbon balance . . . . . . . . . . . . . . . . . . . . .327
7.6.3. Biodiversity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .328
7.7. Uncertainties and recommendations . . . . . . . . . . . . . . . . . . .329
7.7.1. Uncertainties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .329
7.7.1.1. Uncertainties due to methodologies and conceptual
frameworks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .329
7.7.1.2. Uncertainties due to surprises . . . . . . . . . . . . . . . . . . . .331
7.7.1.3. Model-related uncertainties . . . . . . . . . . . . . . . . . . . . . .331
7.7.2. Recommendations to reduce uncertainties . . . . . . . . . . . . . . . . .332
7.7.2.1.Thematic recommendations and justification . . . . . . . . .332
7.7.2.2. Recommendations for future approaches to research
and monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .333
7.7.2.3. Funding requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . .334
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .335
Personal communications and unpublished data . . . . . . . . . . . . .335
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .335
Chapter 7
ArcticTundra and Polar Desert Ecosystems
Lead Author
TerryV. Callaghan
Contributing Authors
Lars Olof Björn, F. Stuart Chapin III,Yuri Chernov,Torben R. Christensen, Brian Huntley, Rolf Ims, Margareta Johansson,
Dyanna Jolly Riedlinger, Sven Jonasson, Nadya Matveyeva,Walter Oechel, Nicolai Panikov, Gus Shaver
Consulting Authors
Josef Elster, Heikki Henttonen, Ingibjörg S. Jónsdóttir, Kari Laine, Sibyll Schaphoff, Stephen Sitch, ErjaTaulavuori, KariTaulavuori, Christoph Zöckler
Contents
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .244
7.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .244
7.1.1. Characteristics of arctic tundra and polar desert ecosystems . .244
7.1.2. Raison d’être for the chapter . . . . . . . . . . . . . . . . . . . . . . . . . . .247
7.1.3. Rationale for the structure of the chapter . . . . . . . . . . . . . . . . .248
7.1.4.Approaches used for the assessment: strengths, limitations,
and uncertainties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .248
7.2. Late-Quaternary changes in arctic terrestrial ecosystems,
climate, and ultraviolet radiation levels . . . . . . . . . . . . . . . . .249
7.2.1. Environmental history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .249
7.2.2. History of arctic biota . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .250
7.2.3. Ecological history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .252
7.2.4. Human history related to ecosystems . . . . . . . . . . . . . . . . . . . .252
7.2.5. Future change in the context of late-Quaternary changes . . . . .253
7.2.6. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .254
7.3. Species responses to changes in climate and ultraviolet-B
radiation in the Arctic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .254
7.3.1. Implications of current species distributions for future biotic
change . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .255
7.3.1.1. Plants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .255
7.3.1.2.Animals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .259
7.3.1.3. Microorganisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .260
7.3.1.4. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .262
7.3.2. General characteristics of arctic species and their adaptations
in the context of changes in climate and ultraviolet-B radiation
levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .263
7.3.2.1. Plants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .263
7.3.2.2.Animals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .264
7.3.2.3. Microorganisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .269
7.3.3. Phenotypic responses of arctic species to changes in climate
and ultraviolet-B radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .271
7.3.3.1. Plants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .271
7.3.3.2.Animals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .279
7.3.3.3. Microorganisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .282
7.3.4. Genetic responses of species to changes in climate and
ultraviolet-B radiation levels . . . . . . . . . . . . . . . . . . . . . . . . . . . .284
7.3.4.1. Plants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .284
7.3.4.2.Animals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .285
7.3.4.3. Microorganisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .286
7.3.4.4. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .286
7.3.5. Recent and projected changes in species distributions and
potential ranges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .287
7.3.5.1. Recent changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .287
7.3.5.2. Projected future changes in species distributions . . . . . .287
7.3.5.3. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .291
244 Arctic Climate Impact Assessment
Summary
The dominant response of current arctic species to climate
change, as in the past, is very likely to be relocation
rather than adaptation. Relocation possibilities vary
according to region and geographic barriers. Some
changes are occurring now.
Some groups such as mosses, lichens, and some herbivores
and their predators are at risk in some areas, but productivity
and number of species is very likely to increase.
Biodiversity is more at risk in some ACIA regions than in
others: Beringia (Region 3) has a higher number of threatened
plant and animal species than any other ACIA region.
Changes in populations are triggered by trends and
extreme events, particularly winter processes.
Forest is very likely to replace a significant proportion of
the tundra and this will have a great effect on the composition
of species. However, there are environmental and
sociological processes that are very likely to prevent forest
from advancing in some locations.
Displacement of tundra by forest will lead to a decrease in
albedo, which will increase the positive feedback to the
climate system.This positive feedback is likely to dominate
over the negative feedback of increased carbon sequestration.
Forest development is very likely to also ameliorate
local climate, for example, by increasing temperature.
Warming and drying of tundra soils in parts of Alaska have
already changed the carbon status of this area from sink to
source.Although other areas still maintain their sink status,
the number of source areas currently exceeds the
number of sink areas. However, geographic representation
of research sites is currently small. Future warming of
tundra soils is likely to lead to a pulse of trace gases into
the atmosphere, particularly from disturbed areas and
areas that are drying. It is not known if the circumpolar
tundra will be a carbon source or sink in the long term,
but current models suggest that the tundra is likely to
become a weak sink for carbon because of the northward
movement of vegetation zones that are more productive
than those they displace. Uncertainties are high.
Rapid climate change that exceeds the ability of species
to relocate is very likely to lead to increased incidence of
fires, disease, and pest outbreaks.
Enhanced carbon dioxide concentrations and ultraviolet-B
radiation levels affect plant tissue chemistry and thereby
have subtle but long-term impacts on ecosystem processes
that reduce nutrient cycling and have the potential to
decrease productivity and increase or decrease herbivory.
7.1. Introduction
The Arctic is generally recognized as a treeless wilderness
with cold winters and cool summers. However,
definitions of the southern boundary vary according to
environmental, geographic, or political biases.This
chapter focuses on biota (plants, animals, and microorganisms)
and processes in the region north of the
northern limit of the closed forest (the taiga), but also
includes processes occurring south of this boundary that
affect arctic ecosystems. Examples include animals that
migrate south for the winter and the regulation of the
latitudinal treeline.The geographic area defined in this
chapter as the present-day Arctic is the area used for
developing scenarios of future impacts: the geographic
area of interest will not decrease under a scenario of
replacement of current arctic tundra by boreal forests.
7.1.1. Characteristics of arctic tundra and
polar desert ecosystems
The southern boundary of the circumpolar Arctic as
defined in this chapter is the northern extent of the closed
boreal forests (section 14.2.3).This is not a clear boundary
but a transition from south to north consisting of the
sequence: closed forest, forest with patches of tundra,
tundra with patches of forest, and tundra.The transition
zone is relatively narrow (30 to 150 km) when compared
to the width of the forest and tundra zones in many, but
not all areas. Superimposed on the latitudinal zonation of
forest and tundra is an altitudinal zonation from forest to
treeless areas to barren ground in some mountainous
regions of the northern taiga.The transition zone from
taiga to tundra stretches for more than 13400 km around
the lands of the Northern Hemisphere and is one of the
most important environmental transition zones on Earth
(Callaghan et al., 2002a,b) as it represents a strong temperature
threshold close to an area of low temperatures.
The transition zone has been called forest tundra, subarctic,
and the tundra–taiga boundary or ecotone.
The vegetation of the transition zone is characterized by
an open landscape with patches of trees that have a low
stature and dense thickets of shrubs that, together with
the trees, totally cover the ground surface.
The environmental definition of the Arctic does not
correspond with the geographic zone delimited by the
Arctic Circle (66.5º N), nor with political definitions.
Cold waters in ocean currents flowing southward from
the Arctic depress the temperatures in Greenland and the
eastern Canadian Arctic whereas the northward-flowing
Gulf Stream warms the northern landmasses of Europe
(section 2.3).Thus, at the extremes, polar bears and
tundra are found at 51º N in eastern Canada whereas
agriculture is practiced north of 69º N in Norway.
Arctic lands span some 20º of latitude, reaching 84º N in
Greenland and locally, in eastern Canada, an extreme
southern limit of 51º N.
The climate of the Arctic is largely determined by the
relatively low solar angles with respect to the earth.
Differences in photoperiod between summer and winter
become more extreme toward the north. Beyond the
Arctic Circle, the sun remains above the horizon at midnight
on midsummer’s day and remains below the horizon
at midday on midwinter’s day.
Chapter 7 • ArcticTundra and Polar Desert Ecosystems 245
Climatically, the Arctic is often defined as the area where
the average temperature for the warmest month is lower
than 10 ºC (Köppen, 1931), but mean annual air temperatures
vary greatly according to location, even at the
same latitude (see Chapter 2).They vary from -12.2 ºC at
Point Barrow,Alaska (71.3º N) to -28.1 ºC at the summit
of the Greenland Ice Sheet (about 71º N) (Weller, 2000)
and from 1.5 ºC at 52º N in subarctic Canada to 8.9 ºC
at 52º N in temperate Europe.The summer period, or
period of most biological activity, progressively decreases
from about 3.5 to 1.5 months from the southern boundary
of the Arctic to the north, and mean July temperature
decreases from 10–12 ºC to 1.5 ºC. In general, annual
precipitation in the Arctic is low, decreasing from about
250 mm in southern areas to as low as 45 mm in the
northern polar deserts (Jonasson et al., 2000), with
extreme precipitation amounts in subarctic maritime
areas (e.g., 1100 mm at 68º N in Norway). However,
owing to low rates of evaporation the Arctic cannot be
considered arid: even in the polar deserts, air humidity
is high and the soils are moist during the short growth
period (Bovis and Barry, 1974). In the Arctic context,
“desert” refers to extreme poverty of life.
The Arctic is characterized by the presence of continuous
permafrost (section 6.6.1), although there are exceptions
such as the Kola Peninsula. Continuous and deep
(>200 m) permafrost also exists south of the treeline in
large areas of Siberia extending south to Mongolia.
The depth of the active (seasonally frozen) layer of the
soil during the growing season depends on summer temperatures
and varies from about 80 cm near the treeline
to about 40 cm in polar deserts. However, active-layer
depth varies according to local conditions within landscapes
according to topography: it can reach 120 cm on
south-facing slopes and be as little as 30 cm in bogs even
in the southern part of the tundra zone. In many areas of
the Arctic, continuous permafrost occurs at greater
depths beneath the soil surface and degrades into discontinuous
permafrost in the southern part of the zone.
Active-layer depth, the extent of discontinuous permafrost,
and coastal permafrost are very likely to be particularly
sensitive to climatic warming (section 6.6).
Permafrost and active-layer dynamics lead to topographic
patterns such as polygons in the landscape.Topography
plays an important role in defining habitats in terms of
moisture and temperature as well as active-layer dynamics
(Brown et al., 1980;Webber et al., 1980), such that
arctic landscapes are a mosaic of microenvironments.
Topographic differences of even a few tens of centimeters
(e.g., polygon rims and centers) are important for
determining habitats, whereas larger-scale topographic
differences (meters to tens of meters) determine wind
exposure and snow accumulation that in turn affect plant
communities and animal distribution.Topographic differences
become more important as latitude increases.
Disturbances of ecosystems are characteristic of the
Arctic. Mechanical disturbances include thermokarst
induced by permafrost thaw (section 6.6.1); freeze–thaw
processes; wind, sand, and ice blasts; seasonal ice oscillations;
slope processes; snow load; flooding during thaw;
changes in river volume; and coastal erosion and flooding.
Biological disturbances include insect pest outbreaks,
peaks of grazing animals that have cyclic populations, and
fire.These disturbances operate at various spatial and
temporal scales (Fig. 7.1) and affect the colonization and
survival of organisms and thus ecosystem development.
Arctic lands are extensive beyond the northern limit of
the tundra–taiga ecotone, encompassing an area of
approximately 7567000 km2, including about
Fig. 7.1. Timescale of ecological processes in relation to disturbances (shown as breaks in horizontal lines) in the Arctic.The schematic
does not show responses projected as a result of anthropogenic climate change (based on Oechel and Billings, 1992; Shaver et al.,
2000;Walker D. and Walker, 1991).
246 Arctic Climate Impact Assessment
2560000 km2 in the former Soviet Union and
Scandinavia, 2480000 km2 in Canada, 2167000 km2 in
Greenland and Iceland, and 360000 km2 in Alaska (Bliss
and Matveyeva, 1992). Figure 7.2 shows the distribution
of arctic and other vegetation types based on a classification
byWalker D. (2000) and mapped by Kaplan et al.
(2003).The distribution of arctic landmasses is often
fragmented: seas separate large arctic islands (e.g.,
Svalbard, Novaya Zemlya, Severnaya Zemlya, New
Siberian Islands, andWrangel Island) and the landmasses
of the Canadian Archipelago and Greenland. Similarly,
the Bering Strait separates the arctic lands of Eurasia and
North America. Large mountains such as the east–west
running Brooks Range in Alaska and the Putorana Plateau
in Siberia separate tundra and taiga. Such areas of relief
contain outposts of boreal species on their southern
major slopes that are likely to expand northward and
higher-elevation areas that are likely to act as refuges for
arctic-alpine species.TheTaymir Peninsula is the only
continuous landmass that stretches 900 km from the
northern tundra limit to taiga without geographic barriers
to the dispersal of animals and plants (Matveyeva and
Chernov, 2000).The width of the tundra zone varies
greatly in different parts of its circumpolar distribution.
1 Cushion forb, lichen, and moss tundra
2 Graminoid and forb tundra
3 Prostrate dwarf-shrub tundra
4 Erect dwarf-shrub tundra
5 Low- and high-shrub tundra
6 Cold evergreen needleleaf forest
7 Cold deciduous forest
8 Cool evergreen needleleaf forest
9 Cool mixed forest
10 Cool–temperate evergreen needleleaf and mixed forest
11 Temperate evergreen needleleaf forest
12 Temperate deciduous broadleaf forest
13 Temperate grassland and xerophytic shrubland
14 Barren
15 Ice
Fig. 7.2. Present-day natural vegetation of the Arctic and
neighboring regions from floristic surveys.Vegetation types
1 to 5 are classified as arctic, whereas types 6 to 8 are classified
as boreal forest (Kaplan et al., 2003).
On average, it does not exceed 300 km, and in some
regions (e.g., the lower reaches of the Kolyma River),
the tundra zone extends only 60 km from the treeline to
the coast. In such areas, the tundra zone is very likely to
be highly vulnerable to climate warming.
The vegetation of the Arctic varies from forest tundra in
the south, where plant communities have all the plant life
forms known in the Arctic and have continuous canopies
in several layers extending to more than 3 m high, to
polar deserts in the north, where vegetation colonizes
5% or less of the ground surface, is less than 10 cm high,
and is dominated by herbs, lichens (symbionts of algae
and fungi), and mosses (Fig. 7.3). Species richness in the
Arctic is low and decreases toward the north: there are
about 1800 species of vascular plants, 4000 species of
cryptogams, 75 species of terrestrial mammals,
240 species of terrestrial birds, 3000 species of fungi,
3300 species of insects (Chernov, 2002; Matveyeva and
Chernov, 2000), and thousands of prokaryotic species
(bacteria and Archaea) whose diversity in the tundra has
only recently started to be estimated. However, the
Arctic is an important global pool of some groups such as
mosses, lichens, springtails (and insect parasitoids:
Hawkins, 1990; Kouki et al., 1994, Price et al., 1995)
because their abundance in the Arctic is higher than in
other biomes. Net primary production (NPP), net
ecosystem production (NEP), and decomposition rates
are low. Food chains are often short and typically there
are few representatives at each level of the chain.Arctic
soils are generally shallow and underdeveloped with low
productivity and immature moor-type humus (Brown et
al., 1980). Substantial heterogeneity of the soil cover,
owing to numerous spatial gradients, has an important
influence on the microtopographical distribution of the
soil biota (invertebrates, fungi, and bacteria) that will
possibly amplify any negative effects of climate change.
The Arctic has a long history of human settlement and
exploitation, based initially on its rich aquatic biological
resources and more recently on its minerals and fossil
hydrocarbons.At the end of the last glacial stage, humans
migrated from Eurasia to North America across the icefree
Bering land bridge and along the southern coast of
Beringia (ca. 14000–13500 years BP; Dixon, 2001).
As early as about 12200 years BP, areas north of the
Fennoscandian Ice Sheet in northernmost Finnmark
(Norway) had been settled (Thommessen, 1996). Even
earlier Paleolithic settlements (ca. 40000 years BP) have
been recorded in the eastern European Arctic (Pavlov et
al., 2001).The impacts of these peoples on terrestrial
ecosystems are difficult to assess but were probably small
given their small populations and “hunter-gatherer” way
of life.The prey species hunted by these peoples included
the megafauna, such as the woolly mammoth, which
became extinct.The extent to which hunting may have
been principally responsible for these extinctions is a
matter of continuing debate (Stuart et al., 2002) but this
possibility cannot be excluded (Alroy, 2001). It is also
uncertain to what extent the extinction of the megafauna
may have contributed to, or been at least partly a result
Chapter 7 • ArcticTundra and Polar Desert Ecosystems 247
of, the accelerated northward movement of trees and
shrubs and consequent changes in vegetation structure
(section 7.2).Although estimates of the population density
of megafaunal species have large uncertainties, it seems
unlikely that megafaunal populations were sufficient to
constrain the spread of woody taxa in response to favorable
environmental change.
During the last 1000 years, resources from terrestrial
ecosystems have been central to the mixed economies of
the Arctic: many inland indigenous communities still
derive most of their protein from subsistence activities
such as caribou/reindeer hunting (Berkes and Fast,
1996). During this period, increasing trade between
peoples of temperate latitudes and arctic indigenous
peoples is likely to have affected a few target animal
species, such as the reindeer that was domesticated in
Fennoscandia and Russia, ermine hunted for fur, and
birds of prey used for hunting as far away as the eastern
Mediterranean. However, the most dramatic impacts
occurred afterWorldWar II as a result of the exploitation
of minerals and oil and fragmentation of the arctic
landscape by infrastructure (Nellemann et al., 2001).
Vlassova (2002) suggested that industrial activities and
forestry have displaced the Russian forest tundra southward
by deforesting 470000 to 500000 km2 of land that
now superficially resembles tundra.Although this estimate
has been challenged as greatly exaggerated
(because northern taiga areas have been included in that
estimate’s definition of forest tundra), such effects have
occurred locally in theYamal Peninsula and the estimate
highlights a need for reappraisal. Knowledge of possible
past interactions between humans and the environment
that may have shaped present-day arctic ecosystems is
limited, but shows that any future increases in population
density and human activity are likely to modify the
projected responses of arctic ecosystems to changes in
climate and ultraviolet (UV) radiation levels.
7.1.2. Raison d’être for the chapter
The Arctic is experiencing dramatic environmental
changes that are likely to have profound impacts on arctic
ecosystems.The Arctic is outstanding among global
biomes in that climate change dominates the major
factors affecting biodiversity (Sala and Chapin, 2000).
Present-day arctic biota are also relatively restricted in
range and population size compared with their Quaternary
situation. For example, when the treeline advanced
northward during the early Holocene warming, a lowered
sea level allowed a belt of tundra to persist around
the Arctic Basin, whereas any future northward migration
of the treeline is very likely to further restrict tundra
areas because sea level is projected to rise.Arctic ecosystems
are known to be vulnerable to disturbances
(Crawford, 1997b; Forbes et al., 2001;Walker D. and
Walker, 1991) and to have long recovery times: subarctic
birch forest defoliated by insects can take 70 years to
recover (Tenow and Bylund, 2000). Current and
projected environmental changes are likely to create
additional stresses and decrease the potential for ecosystem
recovery from natural disturbances, while providing
thresholds for shifts to new states (e.g., disturbance opening
gaps for invasion of species new to the Arctic).
Changes in arctic ecosystems and their biota are important
to arctic residents in terms of food, fuel, and culture
(Chapter 12) and are likely to have global impacts
because of the many linkages between the Arctic and
more southerly regions. Several hundreds of millions of
birds migrate to the Arctic each year and their success in
the Arctic determines their success and impacts at lower
latitudes (section 7.3.1.2). Physical and biogeochemical
processes in the Arctic affect atmospheric circulation and
the climate of regions outside of the Arctic (section 7.5).
It is known that ecosystems have responded to past environmental
changes (section 7.2) and that environmental
Fig. 7.3. Growth forms of arctic plants (modified from Webber et al., 1980 and T. Polozova, pers. comm., 2005).
248 Arctic Climate Impact Assessment
changes are presently occurring in the Arctic (Chapman
andWalsh, 1993 as quoted inWeller, 2000; Dye, 2002;
Fioletov et al., 1997; Chapters 2, 5, and 6).This understanding
indicates that there are very likely to be responses
of arctic ecosystems to projected future and ongoing
climate change. It is also known that current levels of
ultraviolet-B (UV-B) radiation, as well as higher levels,
can affect subarctic plants (Gwynn-Jones et al., 1997;
Johanson et al., 1995; Phoenix et al., 2000).Arctic plants
may be particularly sensitive to increases in UV-B irradiance
because UV-B radiation damage is not dependent on
temperature whereas enzyme-mediated repair of DNA
damage could be constrained by low temperatures
(Björn, 2002; Li et al., 2002a,b; Paulsson, 2003).
For all of these reasons, understanding the relationships
between ecosystems and the arctic environment is
important.Although many aspects of its environment
are changing concurrently (e.g., climate, pollution,
atmospheric nitrogen deposition, atmospheric concentrations
of carbon dioxide (CO2), UV-B radiation
levels, and land use), the specific mission of this chapter
is to focus on the impacts of changes in climate and
UV-B radiation levels on arctic terrestrial ecosystems
and their species and processes.
7.1.3. Rationale for the structure of the
chapter
The effects of climate are specific to species, the age and
developmental stages of individuals, and processes from
metabolism to evolution (Fig. 7.1).Although there are
many ways in which to organize an assessment of climate
and UV-B radiation impacts, this chapter follows a logical
hierarchy of increasing organizational biological complexity
to assess impacts on species, the structure of
ecosystems, the function of ecosystems, and landscape
and regional processes.A basic understanding of biological
processes related to climate and UV-B radiation is
required before the impacts of changes in these factors
on terrestrial ecosystems can be assessed (Smaglik,
2002). Consequently, this chapter progresses from a
review of climate and UV radiation controls on biological
processes to an assessment of the potential impacts of
changes in climate and UV-B radiation levels on processes
at the species and regional levels. Some effects of
climate change on ecosystems may be beneficial to
humans, while others may be harmful.
The changes in climate and UV-B radiation levels that
are used in this chapter to assess biological impacts are
of two types: those already documented (section 2.6)
and those projected by scenarios of future change in
UV-B radiation levels (section 5.7) and climate (section
4.4) derived from models. Mean annual and seasonal
temperatures have varied considerably in the Arctic since
1965 (Chapman andWalsh, 1993 as quoted inWeller,
2000; section 2.6.2.1). Mean annual temperatures in
western parts of North America and central Siberia have
increased by about 1 ºC (up to 2 ºC in winter) per
decade between 1966 and 1995 while temperatures in
West Greenland and the eastern Canadian Arctic have
decreased by 0.25 to 1 ºC per decade (Chapman and
Walsh, 1993, quoted inWeller, 2000). Over a longer
period, from 1954 to 2003, the annual increase and
decrease in temperatures have been slightly less: about
2 to 3 ºC for the whole period (Chapter 1, Fig. 1.3).
Temperature increases in Fennoscandia over the past
century have been small, ranging from about 1 ºC in the
west to near 0 ºC in the east (Lee et al., 2000).
Precipitation has also changed.The duration of the
snow-free period at high northern latitudes increased by
5 to 6 days per decade and the week of the last observed
snow cover in spring advanced by 3 to 5 days per decade
between 1972 and 2000 (Dye, 2002). Stratospheric
ozone has been depleted over recent decades (e.g., by a
maximum of 45% below normal over the high Arctic in
spring; Fioletov et al., 1997).This has probably led to an
increase in surface UV-B radiation levels in the Arctic,
although the measurement period is short (section 5.5).
Scenarios of future change project that mean annual
temperatures in the Arctic will increase by nearly 4 ºC
by 2080 (section 4.4.2) and that spring (April) UV-B
radiation levels will increase by 20 to 90% in much of
the Arctic by 2010–2020 (Taalas et al., 2000).
The assessment of impacts on terrestrial ecosystems
presented in this chapter is based on existing literature
rather than new research or ACIA modeling activities.
Existing long-term experimental manipulations of temperature
and/or UV-B radiation relied on earlier scenarios
of climate and UV-B radiation change (IPCC, 1990).
However, the most recent scenarios (Chapters 4, 5, and
6) are used to provide a context for the assessment in
this chapter, and to modify projections of ecosystem
responses based on earlier scenarios where appropriate.
The ACIA climate scenarios (section 4.4) are also used
directly to illustrate the responses of some species to
projected climate changes.
7.1.4. Approaches used for the assessment:
strengths, limitations, and uncertainties
This chapter assesses information on interactions
between climate, UV-B radiation levels, and ecosystems
from a wide range of sources including experimental
manipulations of ecosystems and environments in the
field; laboratory experiments; monitoring and observation
of biological processes in the field; conceptual modeling
using past relationships between climate and biota
(paleo-analogues) and current relationships between climate
and biota in different geographic areas (geographic
analogues) to infer future relationships; and processbased
mathematical modeling.Where possible, indigenous
knowledge (limited to published sources) is included as
an additional source of observational evidence. Relevant
information from indigenous peoples on arctic tundra
and polar desert ecosystems is given in Chapter 3.
Each method has uncertainties and strengths and these
are discussed in section 7.7. By considering and compar-
Chapter 7 • ArcticTundra and Polar Desert Ecosystems 249
ing different types of information, it is hoped that a
more robust assessment has been achieved. However, the
only certainties in this assessment are that there are various
levels of uncertainty in the projections and that even
if an attempt is made to estimate the magnitude of these
uncertainties, surprise responses of ecosystems and their
species to changes in climate and UV-B radiation levels
are certain to occur.
7.2. Late-Quaternary changes in arctic
terrestrial ecosystems, climate, and
ultraviolet radiation levels
In order to understand the present biota and ecosystems
of the Arctic, and to project the nature of their responses
to potentially rapid future climate change, it is necessary
to examine at least the last 21000 years of their history.This
period, which is part of the late Quaternary
Period, extends from the present back to the last glacial
maximum (LGM), encompassing the Holocene, or postglacial
period, that spans approximately the last 11400
years.A review of this period of the history of the biota
and ecosystems found in the Arctic today also must
examine a spatial domain that is not restricted to the
present arctic regions.At the LGM, many of these
regions were submerged beneath vast ice sheets, whereas
many of the biota comprising present arctic ecosystems
were found at lower latitudes.
7.2.1. Environmental history
At the LGM, vast ice sheets accumulated not only on
many high-latitude continental areas but also across
some relatively shallow marine basins.The beds of relatively
shallow seas such as the North Sea and Bering Sea
were exposed as a result of a global sea-level fall of
approximately 120 m, resulting in a broad land connection
between eastern Siberia and Alaska and closure of
the connection between the Pacific and Arctic Oceans.
The reduction in sea level also exposed a broad strip of
land extending northward from the present coast of
Siberia. Most, if not all, of the Arctic Ocean basin may
have been covered by permanent sea ice.
Although details of the extent of some of the ice sheets
continue to be a controversial matter (see e.g.,Astakhov,
1998; Grosswald, 1988, 1998; Lambeck, 1995; Siegert et
al., 1999), it is certain that the majority of land areas
north of 60º N were ice-covered.The principal exceptions
were in eastern Siberia, Beringia, and Alaska,
although there is some geological evidence to suggest that
smaller ice-free areas also persisted in the high Arctic, for
example in the northernmost parts of the Canadian
Archipelago (Andrews, 1987) and perhaps even in northern
and northeastern Greenland (Funder et al., 1998).
This evidence is supported by recent molecular genetic
studies of arctic species; for example, a study of the
dwarf shrub Dryas integrifolia indicates glacial occurrences
in the high Arctic (Tremblay and Schoen, 1999) as well as
in Beringia, and a study of the collared lemming
Dicrostonyx groenlandicus indicates separate glacial populations
east and west of the Mackenzie River (Ehrich et al.,
2000; Fedorov and Goropashnaya, 1999), the latter most
probably in the Canadian Archipelago.The latter conclusion
is supported by the phylogeography (relationship
between genetic identity and geographic distribution) of
the Paranoplocephala arctica species complex, a cestode
parasite of Dicrostonyx spp., indicating that two subclades
probably survived the LGM with their host in the
Canadian High Arctic (Wickström et al., 2003). More
controversial are suggestions that elements of the arctic
flora and fauna may have survived the LGM on nunataks
(hills or mountains extending above the surface of a
glacier) in glaciated areas of high relief such as parts of
Greenland, Svalbard, and Iceland (Rundgren and
Ingolfsson, 1999).Although a recent molecular genetic
study of the alpine cushion plant Eritrichium nanum
(Stehlik et al., 2001) provides strong evidence for survival
of that species on nunataks within the heart of the
European Alps, similar studies of arctic species have so far
not supported the hypothesis of survival on nunataks in
areas such as Svalbard (Abbott et al., 2000) that experienced
extreme climatic severity as ice sheets extended to
margins beyond the current coast during the LGM.
Direct evidence of the severity of the full glacial climate
in the Arctic comes from studies of ice cores from the
Greenland Ice Sheet and other arctic ice sheets (section
2.7) that indicate full glacial conditions with mean annual
temperatures 10 to 13 ºC colder than during the
Holocene (Grootes et al., 1993). Paleotemperature
reconstructions based upon dinoflagellate cyst assemblages
indicate strong seasonal temperature fluctuations,
with markedly cold winter temperatures (deVernal and
Hillaire-Marcel, 2000; deVernal et al., 2000).
The LGM was, however, relatively short-lived; within a
few millennia of reaching their maximum extent many of
the ice sheets were decaying rapidly and seasonal temperatures
had increased in many parts of the Arctic. Deglaciation
was not, however, a simple unidirectional change;
instead a series of climatic fluctuations occurred during
the period between about 18000 and 11400 years BP
that varied in intensity, duration, and perhaps also in
geographic extent.The most marked and persistent of
these fluctuations, theYounger Dryas event (Alley, 2000;
Peteet, 1993, 1995), was at least hemispheric in its
extent, and was marked by the reglaciation of some
regions and readvances of ice-sheet margins in others.
Mean annual temperatures during this event fell substantially;
although not as low as during the glacial maximum,
they were nonetheless 4 to 6 ºC cooler than at
present over most of Europe (Walker M.J., 1995),
and as much as 10 to 12 ºC colder than at present in the
northern North Atlantic and the Norwegian Sea (Koç et
al., 1996), as well as in much of northern Eurasia
(Velichko, 1995).The end of theYounger Dryas was
marked by a very rapid rise in temperatures.At some
locations, mean annual temperature rose by more than
5 ºC in less than 100 years (Dansgaard et al., 1989).
The most rapid changes probably were spatially and temporally
transgressive, with the global mean change thus
250 Arctic Climate Impact Assessment
occurring much less rapidly. Nonetheless, in many areas
summer temperatures during the early Holocene rose to
values higher than those at present.Winter conditions
remained more severe than today in many higher-latitude
areas, however, because the influence of the decaying ice
sheets persisted into the early millennia of the Holocene.
Despite higher summer temperatures in the early to
mid-Holocene in most of the Arctic, Holocene climate
has not differed qualitatively from that at present.
Following the general thermal maximum there has been
a modest overall cooling trend throughout the second
half of the Holocene. However, a series of millennial
and centennial fluctuations in climate have been superimposed
upon these general longer-term patterns
(Huntley et al., 2002).The most marked of these
occurred about 8200 years BP and appears to have been
triggered by the catastrophic discharge of freshwater
into the northern North Atlantic from proglacial lakes
in North America (Barber et al., 1999; Renssen et al.,
2001).A reduction in strength, if not a partial shutdown,
of the thermohaline circulation in the northern
North Atlantic and Norwegian Sea was also associated
with this event, as well as with the series of less severe
climatic fluctuations that continued throughout the
Holocene (Bianchi and McCave, 1999).
The most recent of these climatic fluctuations was that
of the “Little Ice Age” (LIA), a generally cool interval
spanning approximately the late 13th to early 19th centuries
(section 2.7.5).At its most extreme, mean annual
temperatures in some arctic areas fell by several
degrees. Sea ice extended around Greenland and in
some years filled the Denmark Strait between Greenland
and Iceland (Lamb H.H., 1982; Ogilvie, 1984;
Ogilvie and Jonsdottir, 2000; Ogilvie and Jonsson,
2001), the Norse settlement of Greenland died out
(Barlow et al., 1997; Buckland et al., 1996), and the
population of Iceland was greatly reduced (Ogilvie,
1991; Sveinbjarnardóttir, 1992).Although there was
great temporal climate variability (on decadal to centennial
timescales) within the LIA, and spatial variability
in the magnitude of the impacts, it was apparently a
period of generally more severe conditions in arctic and
boreal latitudes; the marked impacts upon farming and
fisheries (Lamb H.H., 1982) imply similar impacts on
other components of the arctic ecosystem. Since the
early 19th century, however, there has been an overall
warming trend (Overpeck et al., 1997), although with
clear evidence of both spatial variability and shorterterm
temporal variability (Maxwell, 1997).The magnitude
of this recent warming is comparable to that of the
warmest part of the Holocene, at least in those parts of
the Arctic that have experienced the most rapid warming
during the last 30 years or so.
The solar variability thought to be responsible for the
LIA, and for other similar centennial to millennial climatic
fluctuations, probably also affected the ozone
layer and UV-B radiation levels. Ultraviolet-B irradiance
at ground level absorbed by DNA could have been
between 9 and 27% higher during periods of low solar
output (cool periods) than during periods of high solar
output (Rozema et al., 2002; see also section 5.4.1).
7.2.2. History of arctic biota
During the LGM, when most land areas in the Arctic
were ice-covered, biomes able to support the elements
of the arctic biota, including some species that are now
extinct, were extensive south of the Fennoscandian Ice
Sheet in Europe (Huntley et al., 2003). Similar biomes
apparently were extensive south of the Eurasian ice
sheets of northern Russia, eastward across Siberia and
the exposed seabed to the north, and via Beringia into
Alaska and the northernYukon (Ritchie, 1987), although
they were much more restricted south of the Laurentide
Ice Sheet in central and eastern North America (Lister
and Bahn, 1995).The most extensive and important of
these glacial biomes, the steppe–tundra, has been interpreted
and referred to by various authors as “tundra–
steppe” or “Mammoth steppe” (Guthrie, 2001;Walker D.
et al., 2001;Yurtsev, 2001).The vegetation of this biome
comprised a no-analogue combination of lightdemanding
herbaceous and dwarf-shrub taxa that are
found today either in arctic tundra regions or in the
steppe regions that characterize central parts of both
North America and Eurasia (Yurtsev, 2001). Evidence of
an abundance of grazing herbivores of large body mass,
some extant (e.g., reindeer/caribou – Rangifer tarandus;
muskox – Ovibos moschatus) and others extinct (e.g.,
giant deer or “Irish elk” – Megaloceros giganteus; woolly
mammoth – Mammuthus primigenius; woolly rhinoceros –
Coelodonta antiquitatis), associated with this biome suggests
that it was much more productive than is the contemporary
tundra biome.This productive biome, dominated
by non-tree taxa, corresponded to a no-analogue
environment that was relatively cold throughout the
year, with a growing season short enough to exclude
even cold-tolerant boreal trees from at least the majority
of the landscape.The “light climate”, however, was that
of the relatively lower latitudes (as low as 45º N in
Europe) at which this biome occurred, rather than that
of the present arctic latitudes; the greater solar angle and
consequent higher insolation intensities during the summer
months probably made an important contribution
to the productivity of the biome.
The productive steppe–tundra and related biomes were
much more spatially extensive during the last glacial
stage than is the tundra biome today (Fig. 7.4).The last
glacial stage was thus a time when many elements of the
present arctic biota thrived, almost certainly in greater
numbers than today. Fossil remains of both arctic plants
(see e.g.,West, 2000) and mammals (see e.g., FAUNMAPWorking
Group, 1996; Lundelius et al., 1983;
Stuart, 1982) found at numerous locations attest to their
widespread distribution and abundance. Similar conclusions
have been reached on the basis of phylogeographic
studies of arctic-breeding waders (Kraaijeveld and
Nieboer, 2000). Species such as red knot (Calidris
canutus) and ruddy turnstone (Arenaria interpres) are
Chapter 7 • ArcticTundra and Polar Desert Ecosystems 251
inferred to have had much larger populations and more
extensive breeding areas during glacial stages, although
others, such as dunlin (C.alpina), exhibit evidence of
range fragmentation during glacial stages leading to the
evolution of distinct geographically restricted infraspecific
taxa. Phylogeographic studies of other arctic taxa
show individualistic responses (seeWeider and Hobaek,
2000 for a recent review). Some species, such as Arctic
char (Salvelinus alpinus; Brunner et al., 2001), and genera,
such as whitefish (Coregonus spp.; Bernatchez et al.,
1999), exhibit evidence of sub-taxa whose origins are
apparently related to recurrent isolation of populations
throughout the alternating glacial and interglacial stages
of the Pleistocene. Collared lemmings (Dicrostonyx spp.),
however, apparently parallel C.alpina in exhibiting
genetic differentiation principally as a consequence of
the relatively recent geographic isolation of populations
during the last glacial stage (Fedorov and Goropashnaya,
1999; Fedorov et al., 1999b). Other species, such as the
polar bear (Ursus maritimus; Paetkau et al., 1999), exhibit
little or no evidence of genetic differentiation that might
indicate past population fragmentation, and Fedorov et
al. (1999a) inferred that Eurasian true lemmings (Lemmus
spp.) experienced no effective reduction in population
size during recent glacial–interglacial cycles.
In the context of their late-Quaternary history, the arctic
biota at present are relatively restricted in range and
population size.Although tundra areas were of even
smaller extent during the early part of the Holocene
than at present, as a result of greater northward extension
of the treeline (Huntley, 1997; Huntley and
Bradshaw, 1999; MacDonald et al., 2000), that reduction
in extent was small in magnitude compared to that
experienced at the end of the last glacial stage, during
which they were much more extensive than at any time
since. Similarly, while extant arctic taxa at the lower taxonomic
levels often exhibit considerable diversity that
can be related to their late-Quaternary history, the biota
as a whole has suffered a recent reduction in overall
Cushion forb, lichen, and moss tundra
Graminoid and forb tundra
Prostrate dwarf-shrub tundra
Erect dwarf-shrub tundra
Low- and high-shrub tundra
Cold evergreen needleleaf forest
Cold deciduous forest
Cool evergreen needleleaf forest
Cool mixed forest
Cool–temperate evergreen needleleaf and mixed forest
Temperate evergreen needleleaf forest
Temperate deciduous broadleaf forest
Temperate grassland and xerophytic shrubland
Barren
Ice
(a) (b)
(c)
Fig. 7.4. Northern vegetation during the mid-Holocene simulated
by forcing the BIOME4 vegetation model with output
from (a) the Institut Pierre-Simon Laplace Coupled Model 1
atmosphere–ocean general circulation model (AOGCM) and
(b) the HadCM2 AOGCM; and (c) reconstructed from pollen
data (Bigelow et al., 2003; Kaplan et al., 2003).
252 Arctic Climate Impact Assessment
diversity owing to the extinctions of many species, and
some genera, that did not survive into the Holocene.
Of at least 12 large herbivores and six large carnivores
present in steppe–tundra areas at the LGM (Lister and
Bahn, 1995; Stuart, 1982), only four and three, respectively,
survive today. Of the surviving species, only two
herbivores (reindeer/caribou and muskox) and two carnivores
(brown bear – Ursus arctos and wolf – Canis lupus)
occur today in the arctic tundra biome. Present arctic
geography also imposes extreme migratory distances
upon many tundra-breeding birds owing to the wide
separation between their breeding and wintering areas
(Davidson N. et al., 1986;Wennerberg, 2001), rendering
many of them, in common with much of the arctic
biota, extremely vulnerable to any further climatic
warming (Evans, 1997).
7.2.3. Ecological history
Although relatively few in overall number, paleoecological
studies of the late Quaternary Period have
been conducted in many parts of the Arctic (see e.g.,
Anderson and Brubaker, 1993, 1994; Lamb H.F. and
Edwards, 1988; MacDonald et al., 2000; Ritchie, 1987).
In areas that were by then ice free, the transition to the
Holocene was marked by evidence of rapid ecological
response. Elsewhere, in proximity to the decaying ice
sheets, there was a lag between the global changes and
the ecological changes because of the regional influence
of the ice sheets.Although the precise nature of the ecological
changes depended upon location, the overall picture
was one of widespread rapid replacement of the
open, discontinuously vegetated tundra and polar desert
that characterized most ice-free areas during the lateglacial
period by closed tundra.This was in turn replaced
by shrub tundra and subsequently by arctic woodlands or
northern boreal forest in southern areas of the Arctic.
In areas that were unglaciated at the LGM (e.g.,Alaska),
the ecological transition began earlier, coinciding with
the first rapid climatic warming recorded in Greenland
about 14700 years BP (Björck et al., 1998; Stuiver et al.,
1995). In Alaska, tundra was replaced by shrub tundra
during the late-glacial stage, and the first forest stands
(of balsam poplar – Populus balsamifera) were already present
before the transition to the Holocene (Anderson and
Brubaker, 1994). South of the Arctic, the extensive areas
of steppe–tundra that were present at the LGM were
rapidly replaced by expanding forests. Only in parts of
northernmost Siberia may fragments of the steppe–
tundra biome have persisted into the Holocene, supporting
the last population of woolly mammoths that persisted
as recently as 4000 years BP (Vartanyan et al., 1993).
The early Holocene was characterized by higher summer
insolation intensities at northern latitudes than at present.The
warmer summer months enabled trees to
extend their ranges further northward than at present;
positive feedback resulting from the contrasting albedo
of forest compared to tundra (sections 7.4.2.4 and
7.5.4.2) probably enhanced this extension of the forest
(Foley et al., 1994). Boreal forest trees expanded their
ranges at rates of between 0.2 and 2 km/yr (Huntley and
Birks, 1983; Ritchie and McDonald, 1986).They exhibited
individualistic responses with respect to their distributions
and abundance patterns in response to climatic
patterns that differed from those of today. Milder winters
and more winter precipitation in western Siberia
during the early Holocene, for example, allowed
Norway spruce (Picea abies) to dominate in areas where
Siberian fir (Abies sibirica) and Siberian stone pine (Pinus
sibirica) have become important forest components during
the later Holocene (Huntley, 1988, 1997; Huntley
and Birks, 1983).Throughout northern Russia, the arctic
treeline had advanced more or less to the position of the
present arctic coastline by about 10200 years BP,
although the lower sea level at that time meant that a
narrow strip of tundra, up to 150 km wide at most,
persisted north of the treeline (MacDonald et al., 2000).
Subsequently, as sea level continued to rise during the
early Holocene, tundra extent reached a minimum that
persisted for several millennia. For tundra species,
including tundra-breeding birds, the early Holocene thus
seems likely to have been a time of particular stress.
This stress may, however, have been in part relieved by
enhanced productivity in these areas, compared to modern
tundra ecosystems, as a consequence of the warmer
summers and higher insolation intensity.
In glaciated areas of the Arctic, such as northern
Fennoscandia and much of arctic Canada, peatlands
became extensive only after the mid-Holocene (see e.g.,
Lamb H.F., 1980;Vardy et al., 1997) in response to the
general pattern of climatic change toward cooler and
regionally moister summer conditions.The same cooling
trend led to the southward retreat of the arctic treeline,
which reached more or less its present location in most
regions by about 4500 years BP (MacDonald et al.,
2000).The consequent increase in tundra extent probably
relieved the stress experienced by tundra organisms
during the early Holocene, although the cooler, less productive
conditions, and the increasing extent of seasonally
waterlogged tundra peatlands, may have offset this at
least in part.While the early Holocene was a time of
permafrost decay and thermokarst development, at least
in some regions (Burn, 1997), the extent of permafrost
has increased in many areas during the later Holocene
(see e.g., Kienel et al., 1999;Vardy et al., 1997).
7.2.4. Human history related to ecosystems
Recently discovered evidence (Pavlov et al., 2001) shows
that Paleolithic “hunter-gatherers” were present about
40000 years BP (long before the LGM) as far north as
66º34' N in Russia, east of the Fennoscandian Ice Sheet.
Although it seems likely that humans did not range as far
north during the glacial maximum, it is clear that they
expanded rapidly into the Arctic during the deglaciation.
Humans entered North America via the Bering “land
bridge” and along the southern coast of Beringia about
14000 to 13500 years BP (Dixon, 2001).These so-called
Clovis hunters were hunter-gatherers who had developed
Chapter 7 • ArcticTundra and Polar Desert Ecosystems 253
sophisticated ways of working stone to produce very fine
spear- and arrowheads. Over the next few millennia, they
expanded their range and population rapidly, occupying
most of the North American continent.Their prey apparently
included many of the large vertebrate species that
soon became extinct.The extent to which human hunting
may have been principally responsible for these extinctions
is a matter of continuing debate, but recent simulations
for North America indicate that this possibility cannot
be excluded (Alroy, 2001). However, these extinctions
also coincide with an environmental change that
caused the area of the biome with which the large arctic
vertebrates were associated to be reduced to an extent
that was apparently unprecedented during previous
glacial–interglacial cycles (Sher, 1997). It thus is more
probable that the hunting pressure exerted by humans
was at most an additional contributory factor leading to
the extinctions, rather than their primary cause.
In Eurasia, Paleolithic hunter-gatherers shifted their
range northward into the Arctic at the end of the last
glacial stage, as did their large vertebrate prey.To the
south, they were replaced by Mesolithic peoples who
occupied the expanding forests. By the early Holocene
these Mesolithic peoples had expanded well into the
Arctic (Thommessen, 1996), where they probably gave
rise to the indigenous peoples that in many cases continued
to practice a nomadic hunter-gatherer way of life
until the recent past or even up to the present day in
some regions.The arrival of later immigrants has had
major impacts upon indigenous peoples and their way of
life (Chapters 3, 11, and 12). In turn, land use and natural
resource exploitation by the immigrants, as well as
the changes that they have brought about in the way of
life of indigenous peoples, have had negative impacts on
many arctic ecosystems.These impacts in some cases
have possibly increased the vulnerability of these ecosystems
to the pressures that they now face from climate
change and increased exposure to UV-B radiation.
7.2.5. Future change in the context of lateQuaternary
changes
The potential changes for the next century can be put
into context by comparing their rates and magnitudes to
those estimated for the changes documented by paleoecological
and other evidence from the late Quaternary
Period (Table 7.1).
It is apparent fromTable 7.1 that projected future
changes have several characteristics that pose a particular
threat to the biota and ecosystems of the Arctic. First,
climatic changes over the next century are likely to be
comparable in magnitude to the changes that occurred
between full glacial conditions and present conditions,
and greater than the maximum changes that occurred
during the Holocene. Second, the global increase in mean
annual temperature is projected to occur at rates that are
Late Quaternary Projected Future
Sea level ca. 120 m lower at LGM; increased at a maximum
rate of ca. 24 mm/yr (Fairbanks, 1989)
0.09–0.88 m higher by 2100; 3–10 m higher in 1000
years increasing at a rate of 1–9 mm/yr (IPCC, 2001)
Climate
Mean annual
temperature
full glacial: global mean ca. 5 ºC lower; regionally
in the Arctic 10–13 ºC lower
2100: global mean 1.5–5.8 ºC higher; regionally in
the Arctic 2.1–8.1 ºC higher (IPCC, 2001)
Holocene: global mean <1 ºC higher at maximum;
regionally in the Arctic similar to present
Winter temperature full glacial: >15 ºC cooler regionally in the
Arctic
2100: 4–10 ºC higher regionally in the Arctic (IPCC,
2001)
Holocene: ca. 2–4 ºC warmer regionally in the
Arctic at maximum
Rate of increase in mean
annual temperature
global: ≤1ºC per millennium; regionally in the
Arctic: >5 ºC in a century
global: 1.5–5.8 ºC per century; regionally in the
Arctic: 2.1–8.1 ºC in a century (IPCC, 2001)
Ecosystem responses
Treeline displacement full glacial: >1000 km southward; Holocene:
50–200 km northward at maximum (Kaplan,
2001)
2100: >500 km northward. It is possible that anthropogenic
disturbance might result in an opposite
response (see section 7.5.3.2)
Range margin
displacement rates
early Holocene: rates of 0.2–2 km/yr estimated
for trees from pollen data (Huntley, 1988)
21st century: potential rates of 5–10 km/yr estimated
from species–climate response models (Huntley
et al., 1995)
Area of tundra full glacial: 197% (ranging from 168 to 237%)
of present; Holocene: 81% (ranging from 76 to
84%) of present at minimum
2100: 51% of present (J. Kaplan, pers. comm., 2002;
see Kaplan et al., 2003)
UV-B radiation levels No long-term trend known. Due to solar variability,
levels of DNA-active UV-B wavelengths
may have varied by up to 27% within a period
of ca. 150 years (Rozema et al., 2002)
In addition to natural solar cycles, it is very likely
that anthropogenic cooling of the stratosphere will
delay recovery of the ozone layer
Table 7.1. Comparison of key aspects of projected future environmental changes with late-Quaternary changes.
254 Arctic Climate Impact Assessment
higher than the rate of global temperature increase during
the last deglaciation; in parts of the Arctic the rate of
warming is likely to match the most rapid regional
warming of the late Quaternary Period.Third, as a consequence
of this temperature increase, and the accompanying
rise in sea level, tundra extent is likely to be less
than at any time during the late Quaternary Period.
Fourth, global mean temperatures and mean annual temperatures
in the Arctic are very likely to reach levels
unprecedented in the late Quaternary Period; this is very
likely to result in a rapid reduction in the extent of
permafrost, with associated thermokarst development in
areas of permafrost decay leading to potentially severe
erosion and degradation of many arctic peatlands (section
7.5.3.1).The combination of projected future climate
change with other anthropogenic effects (including
enhanced levels of UV-B radiation, deposition of nitrogen
compounds from the atmosphere, heavy metal and acidic
pollution, radioactive contamination, and increased habitat
fragmentation) suggests that the future is very likely
to be without a past analogue and will pose unprecedented
challenges to arctic ecosystems and biota that evolved
in response to global cooling throughout the last five million
years or so (the lateTertiary and Quaternary
Periods), during which our own species also evolved.
7.2.6. Summary
At the LGM, vast ice sheets covered many continental
areas.The beds of some shallow seas were exposed, connecting
previously separated landmasses.Although some
areas were ice-free and supported a flora and fauna,
mean annual temperatures were 10 to 13 ºC colder than
during the Holocene.Within a few millennia of the glacial
maximum, deglaciation started but was not a simple
unidirectional change: a series of climatic fluctuations
occurred between about 18000 and 11400 years BP.
During theYounger Dryas event, mean annual temperatures
fell substantially in some areas and reglaciation
occurred.At the end of the event, mean annual temperatures
rose by more than 5 ºC in less than 100 years in at
least some parts of the Arctic. Following the general
thermal maximum in the Holocene, there has been a
modest overall cooling trend. However, superimposed
upon the general longer-term patterns have been a series
of millennial and centennial fluctuations in climate, the
most marked of which occurred about 8200 years BP.
The most recent of these climatic fluctuations was that
of the LIA, a generally cool interval spanning approximately
the late 13th to early 19th centuries.At its most
extreme, mean annual temperatures in some arctic areas
fell by several degrees, with impacts on human settlements
in the north.
In the context of at least the last 150000 years, arctic
ecosystems and biota have been close to their minimum
extent within the last 10000 years.They suffered loss
of diversity as a result of extinctions during the rapid,
large-magnitude global warming at the end of the last
glacial stage. Consequently, arctic ecosystems and biota
are already stressed; some are extremely vulnerable to
current and projected future climate change. For example,
migratory arctic-breeding birds today face maximal
migration distances between their wintering and
breeding areas.
Evidence from the past indicates that arctic species, especially
larger vertebrates, are very likely to be vulnerable
to extinction if climate warms.The treeline is very likely
to advance, perhaps rapidly, into tundra areas of northern
Eurasia, Canada, and Alaska, as it did during the early
Holocene, reducing the extent of tundra and contributing
to the pressure upon species that makes their extinction
possible. Species that today have more southerly distributions
are very likely to extend their ranges north, displacing
arctic species. Permafrost is very likely to decay and
thermokarst develop, leading to erosion and degradation
of arctic peatlands. Unlike the early Holocene, when
lower relative sea level allowed a belt of tundra to persist
around at least some parts of the Arctic Basin when treelines
advanced to the present coast, sea level is very likely
to rise in the future, further restricting the area of tundra
and other treeless arctic ecosystems.
The negative response of arctic ecosystems in the face
of a shift to global climatic conditions that are apparently
without precedent during the Pleistocene is likely to
be considerable, particularly as their exposure to cooccurring
environmental changes (i.e., enhanced levels
of UV-B radiation, deposition of nitrogen compounds
from the atmosphere, heavy metal and acidic pollution,
radioactive contamination, increased habitat fragmentation)
is also without precedent.
7.3. Species responses to changes in
climate and ultraviolet-B radiation in
the Arctic
The individual of a species is the basic unit of ecosystems
that responds to changes in climate and UV-B radiation
levels. Individuals respond to environmental changes
over a wide range of timescales: from biochemical,
physiological, and behavioral processes occurring in less
than a minute to the integrative responses of reproduction
and death (Fig. 7.1). Reproduction and death drive
the dynamics of populations while mutation and environmental
selection of particular traits in individuals
within the population lead to changes in the genetic
composition of the population and adaptation.
Current arctic species have characteristics that have
enabled them to pass various environmental filters associated
with the arctic environment (Körner, 1995;
Walker M.D., 1995), whereas species of more southerly
latitudes either cannot pass these filters or have not yet
arrived in the Arctic. Changes in arctic landscape
processes and ecosystems in a future climatic and UV-B
radiation regime will depend upon the ability of arctic
species to withstand or adapt to new environments and
upon their interactions with immigrant species that can
pass through less severe environmental filters.This section
focuses on the attributes of current arctic species
Chapter 7 • ArcticTundra and Polar Desert Ecosystems 255
that constrain or facilitate their responses to a changing
climate and UV-B radiation regime.
Soil characteristics will determine to some extent the
responses of vegetation to climate change.Arctic soils
(and particularly moisture content) vary from the forest
tundra to the polar deserts and within each of these
vegetation zones.
In the high-arctic polar deserts, skeletal soils and stony
ground predominate (Aleksandrova, 1988). Materials
range from boulders to gravel and the dominant erosion
process is physical weathering (e.g., freeze–thaw cracking)
rather than chemical and biochemical weathering,
which are strongly suppressed by lack of heat. Freeze–
thaw cycles lead to a sorting of stones by size and formation
of patterned ground consisting, for example, of
stone nets.An organic layer is missing from the soil profile
and organic material is restricted to small pockets
under sparse plant cover or in cracks.The soils are neutral
or only weakly acidic and the soil complexes are
almost completely saturated with moisture although this
differs between the polar deserts of Canada, Greenland,
and the Russian Arctic. Gleys are almost absent and the
active-layer depth is about 30 cm.
In the tundra biome, soil profiles are characterized by an
organic layer that is often less than 10 cm deep on dry
ridges, is deeper in moist and mesic habitats, and extends
to deep deposits of peat in wet areas (Nadelhoffer et al.,
1992). Below the organic layer is a mineral layer.The
active layer is deepest in the dry areas (~1 m) owing to
the lack of summer insulation, and is shallowest in wet
areas (~20 cm) due to efficient insulation by continuous
vegetation cover and organic soil.The pH of tundra soils
is generally acidic. Chemical and biochemical processes
are important but sorting of materials and patterned
ground are still evident, leading to landscapes with larger
polygons than those found in the polar deserts. In both
polar-desert and tundra soils, the permafrost is generally
continuous. In contrast, in soils of the forest tundra, the
permafrost generally becomes discontinuous and the
depth of the organic layer decreases except for waterlogged
depressions where peat bogs are found.
Soil formation processes in the Arctic are slow and the
type of soil is very likely to constrain potential rates of
colonization by southern species.
7.3.1. Implications of current species
distributions for future biotic change
7.3.1.1. Plants
Species diversity
About 3% (~5900 species) of the global flora occurs in
the Arctic as defined in this chapter (0.7% of the angiosperms
(flowering plants), 1.6% of the gymnosperms
(cone-bearing plants), 6.6% of the bryophytes, and 11%
of the lichens) (Table 7.2).There are more species of
primitive taxa (cryptogams), that is, mosses, liverworts,
lichens, and algae, in the Arctic than of vascular plants
(Matveyeva and Chernov, 2000). Less than half (about
1800) of arctic plant species are vascular plants.There are
about 1500 species of vascular plants common to both
Eurasia (Matveyeva and Chernov, 2000; Sekretareva,
1999) and North America (Murray, 1995). Similar numbers
of non-vascular plants probably occur in the Arctic
on both continents, although their diversity has been less
Animals Plants Fungi
Group Number of
species
% of world
biota
Group Number of
species
% of world
biota
Group Number of
species
% of world
biota
Mammals 75 1.7 Angiosperms 1735 0.7 Fungi 2500 2.3
Birds 240 2.9 monocotyledons 399 0.6
Insects 3300 0.4 dicotyledons 1336 0.7
Diptera 1600 0.9 Gymnosperms 12 1.6
Beetles 450 0.1 Pteridophytes 62 0.6
Butterflies 400 0.3 Mosses 600 4.1
Hymenoptera 450 0.2 Liverworts 250 2.5
Others 400 Lichens 2000 11.0
Springtails 400 6.0 Algae 1200 3.3
Spiders 300 1.7
Mites 700 1.9
Other groupsa
600 --
Total estimate 6000 -- 5859 3.0
aAmphibians and reptiles (7 species), centipedes (10 species), terrestrial mollusks (3 species), oligochaetes (earthworms and enchytraeids) (70 species), and nematodes
(~500 species).
Table 7.2. Biodiversity estimates in terms of species richness (number of species) for the Arctic north of the latitudinal treeline and
percentage of world biota (Chernov, 2002; Matveyeva and Chernov, 2000).
256 Arctic Climate Impact Assessment
thoroughly documented. In the Russian Arctic, for example,
735 bryophyte species (530 mosses and 205 liverworts)
and 1078 lichen species have been recorded
(Afonina and Czernyadjeva, 1995;Andreev et al., 1996;
Konstantinova and Potemkin, 1996). In general, the
North American and Eurasian Arctic are similar to one
another in their numbers of vascular and non-vascular
plant species, of which a large proportion (about 80%) of
vascular plant species occurs on at least two continents.
An even larger proportion (90%) of bryophyte species
occurs in both the North American and Eurasian Arctic.
About 40% of arctic vascular plants (and a much higher
percentage of mosses and lichens) are basically boreal
species that now barely penetrate the Arctic (Table 7.3).
They currently occur close to the treeline or along large
rivers that connect the subarctic with the Arctic.These
boreal species within the Arctic will probably be the primary
boreal colonizers of the Arctic in the event of continued
warming. Polyzonal (distributed in several zones),
arctoboreal (in taiga and tundra zones), and hypoarctic
(in the northern taiga and southern part of the tundra
zone) species have even greater potential to widen their
distribution and increase their abundance in a changing
climate.The majority of cryptogams have wide distributions
throughout the Arctic. Such species are likely to
survive a changing climate, although their abundance is
likely to be reduced (sections 7.3.3.1 and 7.4.1.2).
In contrast to the low diversity of the arctic flora at the
continental and regional scales, individual communities
(100 m2 plots) within the Arctic have a diversity similar
to or higher than those of boreal and temperate zones.
These diversities are highest in continental parts of the
Arctic such as theTaymir Peninsula of Russia, where
there are about 150 species of plants (vascular plants,
lichens, and mosses) per 100 m2 plot, 40 to 50 species
per square meter, and up to 25 species per square
decimeter (Matveyeva, 1998).
Latitudinal gradients of species diversity
Latitudinal gradients suggest that arctic plant diversity is
sensitive to climate.The number of vascular plant
species declines five-fold from south to north on the
Taymir Peninsula (Matveyeva, 1998). Summer temperature
is the environmental variable that best predicts
plant diversity in the Arctic (Young, 1971). Other factors
are also important, however: regions at different
latitudes that have a similar maximum monthly temperature
often differ in diversity.Taymir biodiversity values
are intermediate between the higher values in Chukotka
and Alaska, which have a more complicated relief, geology,
and floristic history, and the lower values in the eastern
Canadian Arctic with its impoverished flora resulting
from relatively recent glaciation.All diversity values on
theYamal Peninsula are even lower than in Canada
because of a wide distribution of sandy soils and perhaps
its young age. Similar patterns are observed with butterflies
(Fig. 7.5c) and spiders (Chernov, 1989, 1995).
Therefore, latitudinal gradients of species diversity are
best described as several parallel gradients, each of
which depends on summer heat, but which may differ
from one geographic region to another.This must be
taken into consideration when projecting future changes
in biodiversity. Figure 7.5b illustrates how current bioclimatic
distributions are related to climate change scenarios
by plotting the likely changes in the number of
ground beetles for three time slices of mean July temFig.
7.5. Relationship between mean July temperature and
(a) the number of nesting bird species in western and middle
Siberia; (b) the number of ground beetle species in local faunas
of the Taymir Peninsula; and (c) the number of day butterfly
species in the middle Siberian and Beringian areas of the Arctic
(Chernov, 1989, 1995; Matveyeva and Chernov, 2000).
Chapter 7 • ArcticTundra and Polar Desert Ecosystems 257
perature derived from the mean of the scenarios generated
by the five ACIA-designated models.
At the level of the local flora (the number of species
present in a landscape of about 100 km2), there is either
a linear or an “S”-shaped relationship between summer
temperature and the number of species (Fig. 7.6).The
number of species is least sensitive to temperature near
the southern margin of the tundra and most sensitive to
temperatures between 3 and 8 ºC.This suggests that the
primary changes in species composition are very likely
to occur in the northern part of the tundra zone and in
the polar desert, where species are now most restricted
in their distribution by summer warmth and growingseason
length. July temperature, for example, accounts
for 95% of the variance in number of vascular plant
species in the Canadian Arctic (Rannie, 1986), although
extreme winter temperatures are also important (section
7.3.3.1). Summer warmth, growing-season length,
and winter temperatures all affect the growth, reproduction,
and survival of arctic plants.The relative
importance of each varies from species to species, site
to site, and year to year.
The steep temperature gradient that has such a strong
influence on species diversity occurs over much shorter
distances in the Arctic than in other biomes. North of
the treeline in Siberia, mean July temperature decreases
from 12 to 2 ºC over 900 km, whereas mean July temperature
decreases by 10 ºC over 2000 km in the boreal
zone, and decreases by less than 10 ºC from the southern
boreal zone to the equator (Chernov, 1995).
Comparing the temperature decrease of 10 ºC with the
projected 2.5 ºC increase in mean July temperature in
the Arctic by 2080 (mean of the two extremes – 1.1
and 4.2 ºC – projected by the five ACIA-designated
models) suggests that much of the Arctic is very likely
to remain within the arctic summer climate envelope
(although the increase in winter temperature is projected
to be higher).
Because of the steep latitudinal temperature gradients
in the Arctic, the distance that plants must migrate in
response to a change in temperature is much less in the
Arctic than in other biomes, particularly where topographic
variations in microclimate enable plants to
grow far north of their climatic optima.The low solar
angle and presence of permafrost make topographic
variations in microclimate and associated plant community
composition particularly pronounced in the Arctic.
Thus, both the sensitivity of arctic species diversity to
temperature and the short distance over which this
temperature gradient occurs suggest that arctic diversity
is very likely to respond strongly and rapidly to
high-latitude temperature change.
Latitudinal patterns of diversity differ strikingly among
different groups of plants (Table 7.3). Many polyzonal,
boreal, and hypoarctic species have ranges that extend
into the Arctic. Some of these (e.g., the moss Hylocomium
splendens and the sedges Eriophorum angustifolium and
E.vaginatum) are important dominants within the Arctic.
Tussocks of E.vaginatum structure the microtopography
of broad areas of tussock tundra (Bliss and Matveyeva,
1992), and H.splendens exerts a control over nutrient
cycling (Hobbie, 1996).Tall willow (Salix spp.) and alder
(Alnus fruticosa) shrubs as well as dwarf birch (Betula exilis
and B.nana) form dense thickets in the southern part of
the tundra zone and often have outlier populations that
extend far to the north in favorable habitats (Matveyeva
and Chernov, 2000).
Species that are important community dominants are
likely to have a particularly rapid and strong effect on
ecosystem processes where regional warming occurs.
Hemiarctic species are those that occur throughout the
Arctic. Many of these species are common community
dominants, including Carex bigelowii/arctisibirica,
C. stans, Dryas octopetala/punctata, Cassiope tetragona, and
the moss Tomentypnum nitens. Due to their current widespread
distribution, their initial responses to climatic
warming are likely to be increased productivity and
abundance, probably followed by northward extension
of their ranges.The species most vulnerable to climate
change are likely to be euarctic (polar willow – Salix
polaris) and hyperarctic species that now have the greatest
abundance and widest ecological amplitude in the
northernmost part of the tundra zone and in polar
deserts, respectively.These groups of species are best
adapted to the climate conditions of the high Arctic
where they are distributed in a wide range of habitats
where more competitive southern species are absent.
In the more southerly regions of the tundra zone, they
Fig. 7.6. Relationship between mean July temperature and the
number of vascular plant species in local floras of the Taymir
Peninsula (Matveyeva and Chernov, 2000) and the number of
vascular species in the local flora of the Canadian Archipelago
(Rannie, 1986).
258 Arctic Climate Impact Assessment
Table 7.3. Current diversity changes with latitude in the Arctic, excluding limnic and marine animals (compiled and modified from
Matveyeva and Chernov, 2000). General information about how species within the various categories are likely to respond to changes
in climate and UV radiation levels is presented in the text, but insufficient information is available for most of the species in the table.
Category Optimum of
distribution
Examples
Plants Birds Mammals and
invertebrates
Polyzonal Different zones in
the Holarctic; within
the tundra zone
usually in local
habitats and wet
depressions
Soil algae; the mosses Hylocomium splendens
sensu lato, Aulacomnium turgidum, and
Racomitrium lanuginosum; the liverwort
Ptilidium ciliare; the lichens Cetraria islandica,
Psora decipiens, and Cladina rangiferina; the
vascular species Cardamine pratensis,
Chrysosplenium alternifolium, and Eriophorum
angustifolium; the sedge Carex duriusculaa;
the herb Helictotrichon kryloviia; and the moss
Tortula ruralisa
Common raven (Corvus
corax), peregrine falcon
(Falco peregrinus), white
wagtail (Motacilla alba),
northern wheatear
(Oenanthe oenanthe)
Wolf (Canis lupus),
ermine (Mustela
erminea), weasel
(M. nivalis), voles
(Microtus gregalis
and M. oeconomus),
and the mite
Chiloxanthus
pilosusb
Zonal
boreal
Not abundant and
constrained to the
southern Arctic in
benign habitats such
as river valleys,
south-facing slopes,
and wet areas
Tree species of Larix; the orchid Corallorrhiza;
the shrub Salix myrtilloides; the sedge Carex
chordorrhiza; the herbs Allium schoenoprasum,
Cortusa matthioli, Galium densiflorum, and
Sanguisorba officianalis; and the forest mosses
Climacium dendroides, Pleurozium shreberi, and
Rhytidiadelphus triquetrus
The forest birds Turdus
iliacus and T. pilaris (thrushes);
the leaf warblers Arctic
warbler (Phylloscopus
borealis) and yellowbrowed
warbler (P. inornatus);
and the “river” ducks
Anas acuta,A. penelope, and
A. crecca
Reindeer/caribou
(Rangifer tarandus),
wolverine (Gulo
gulo), and brown
bear (Ursus arctos)
Zonal Arctic
Hypoarctic Optima in the
southern tundra
subzone
The shrubs Betula nana and B. exilis and the
sedge Eriophorum vaginatumc
Ptarmigan (Lagopus
lagopus), spotted redshank
(Tringa erythropus), little
bunting (Emberiza pusilla),
bar-tailed godwit (Limosa
lapponica)
The vole Microtus
middendorffi, the
ground beetle
Carabus truncaticollis,
the bumblebee
Bombus cingulatus,
and the spider
Alopecosa hirtipes
Hemiarctic Throughout the
tundra zone but
most frequent in
the middle
Most of the dominant species, including the
grasses Arctophila fulva and Dupontia fisheri; the
sedges Carex bigelowii/arctisibirica and C. stans;
the shrub willow Salix reptans; the dwarf
shrubs Dryas punctata/octopetala and Cassiope
tetragona; the mosses Tomentypnum nitens,
Drepanocladus intermedius, and Cinclidium
arcticum; the herbs Lagotis minor and Pedicularis
hirsuta; and the moss Polytrichum juniperinum
Lapland longspur (Calcarius
lapponicus), lesser golden
plover (Pluvialis dominica),
Pacific golden plover
(P. fulva), and the dunlins
Calidris alpina and C. minuta
The lemming
Lemmus sibiricus, the
bumblebee Bombus
balteatus, the ground
beetles Curtonotus
alpinus and Pterostichus
costatus, and
the flower-fly parasyrphus
tarsatus
Euarctic Northern part of
the tundra zone, rare
in the southern part
The dwarf shrubs Salix polaris and S. arcticad
Black-bellied plover
(Pluvialis squatarola), curlew
sandpiper (Calidris ferruginea),
snowy owl (Nyctea
scandiaca), snow bunting
(Plectrophenax nivalis) and
several more
The lemming
Dicrostonyx torquatus,
the bumblebees
Bombus hyperboreus
and B. polaris, and
the crane fly Tipula
carinifrons
Hyperarctic Polar desert and the
northernmost part
of the tundra zone
Almost no plants are restricted to these
zones; the following have their highest
frequencies there: the grasses Phippsia algida
and Poa abbreviata; the herbs Cerastium regelii,
Draba oblongata, D. subcapitata, Saxifraga
hyperborea, and S. oppositifolia; the mosses
Dicranoweisia crispula, Bryum cyclophyllum,
Orthothecium chryseon, and Seligeria polaris; and
the lichens Cetrariella delisei, Arctocetraria
nigricascens, Dactylina ramulosa, D. madreporiformis,
and Thamnolia subuliformis
The wader species Calidris
alba and C. canutus
No terrestrial
mammal species are
restricted to this
zone.The collembolan
Vertagopus
brevicaudus
a“steppe” species; bbizonal steppe and tundra species; cthis group characterizes the southern tundra subzone; dthis group is relatively small, but is valuable in subdividing the
tundra zone into subzones.
Chapter 7 • ArcticTundra and Polar Desert Ecosystems 259
are able to grow only (or mainly) in snow beds (depressions
in the landscape where snow accumulates and
plant growth and diversity is limited by particularly
short snow-free periods). It is likely that their ecological
amplitude will narrow and their abundance decrease
during climate warming.
Thus, responses to climate change will be different in
various groups of plants. Some currently rare boreal
species are likely to move further north and the relative
abundance and range of habitats occupied by more common
species are likely to increase.When southern
species with currently narrow niches penetrate into the
harsher ecosystems at high latitudes, therefore, there is
likely to be a broadening of their ecological niches there.
In contrast, some true arctic species (endemics) that are
widespread at high latitudes are likely to become more
restricted in their local distribution within and among
ecosystems.They could possibly disappear in the lower
latitudes where the tundra biome is particularly narrow.
Only few high-arctic plants in Greenland are expected
to become extinct, for example, Ranunculus sabinei,
which is limited to the narrow outer coastal zone of
North Greenland (Heide-Jørgensen and Johnsen, 1998).
However, temperature is not the only factor that currently
prevents some species from being distributed in
the Arctic. Latitude is important, as life cycles depend
not only on temperature but on the light regime as well.
It is very likely that arctic species will tolerate warmer
summers whereas long day lengths will initially restrict
the distribution of some boreal species (section 7.3.3.1).
New communities with a peculiar species composition
and structure are therefore very likely to arise, and will
be different from those that exist at present.
7.3.1.2. Animals
Species diversity
The diversity of arctic terrestrial animals beyond the
latitudinal treeline (6000 species) is nearly twice as
great as that of vascular plants and bryophytes
(Chernov, 1995, 2002;Table 7.2).As with plants, the
arctic fauna accounts for about 2% of the global total,
and, in general, primitive groups (e.g., springtails, 6%
of the global total) are better represented in the Arctic
than are advanced groups such as beetles (0.1% of the
global total; Chernov, 1995; Matveyeva and Chernov,
2000).There are about 322 species of vertebrates in the
Arctic, including approximately 75 species of mammals,
240 species of birds, 2 species of reptiles, and 5 species
of amphibians. Insects are the most diverse group of
arctic animals; of approximately 3300 arctic species,
about 50% are Diptera, 10% are beetles (Coleoptera),
10% are butterflies (Lepidoptera), and 10% are Hymenoptera.The
Arctic has about 300 species of spiders
(Arachnida), 700 species of mites (Acarina), 400 species
of springtails (Collembola), 500 species of nematodes,
70 species of oligochaetes (of which most are Enchytraeidae),
only a few mollusks, and an unknown number
of protozoan species.
In the Arctic as defined by CAFF (Conservation of Arctic
Flora and Fauna) (which includes forested areas), some
450 species of birds have been recorded breeding. Some
species extend their more southern breeding areas only
marginally into the Arctic. Others are not migratory and
stay in the Arctic throughout the year.About 280 species
have their main breeding distribution in the Arctic and
migrate regularly (Scott, 1998).An estimate of the total
number of individuals involved is not possible, as too little
is known about the population size of most species or
their arctic proportion. However, a rough first approximation
suggests that there are at least several hundred
million birds. Population sizes of water birds are better
known and the Arctic is of particular importance for
most water birds, such as divers, geese, and waders.
Twelve goose species breed in the Arctic, eleven almost
entirely and eight exclusively.These comprise about 8.3
million birds.The total number of arctic-breeding sandpipers
(24 species) exceeds 17.5 million birds (Zöckler,
1998).The total number of water birds, including other
wader species, divers, swans, ducks, and gulls is estimated
to be between 85 and 100 million birds.
Latitudinal gradients of species diversity
Latitudinal patterns of diversity in arctic animals are
similar to those described for arctic plants. Species
diversity declines in parallel with decreasing temperature
in most animal groups (Fig. 7.5), including birds,
ground beetles, and butterflies (Chernov, 1995).
However, in some groups (e.g., peatland birds and
sawflies at local sites in the European Arctic), concentrations
of both species diversity and density per unit area
can increase compared with more southern territories,
perhaps because the habitat types appropriate to these
groups are more diverse in the tundra than in the boreal
forest. In general, the latitudinal decline in the number
of animal species is more pronounced (frequently
greater than 2.5-fold) than in vascular plants.As with
plants, at a given temperature there are more animal
species in Beringia, with its complicated relief, geology,
and biogeographic history than in theTaymir Peninsula.
Many animal species are restricted to the boreal zone
because they depend on the crown, wood, roots, or litter
of trees, which are absent in the tundra zone.
These groups include wood-boring insects and wooddecaying
fungi and their predators (Chernov and
Matveyeva, 1997), as well as mammals and birds that
specialize on tree seeds and leaves. Other important animals,
including the raven (Corvus corax), wolf, red fox
(Vulpes vulpes), and ermine (Mustela erminea), are primarily
boreal in distribution but remain an important component
of many arctic ecosystems.There are a few terrestrial
animals restricted to the high Arctic such as the
sanderling (Calidris alba) and a common Collembola,
Vertagopus brevicaudus. Other arctic species have their
centers of distribution in the northern, mid- or southern
Arctic (Table 7.3).The more diverse patterns of animal
distribution compared to plants make it more difficult to
project how animals will respond to climatic warming.
Some herbivores have distributions that are more limited
260 Arctic Climate Impact Assessment
than those of their host plants (Strathdee and Bale,
1998), so it is possible that warming will allow these
species to extend northward relatively rapidly.
As with plants, latitudinal patterns of diversity differ
strikingly among different groups of animals (Table
7.3).The common species tend to be more broadly distributed
in the far north. In northernTaymir, there are
only 12 species of springtails but 80% of these occur in
all microsites and topographic locations investigated
(Chernov and Matveyeva, 1997). Some boreal birds,
such as the American robin (Turdus migratorius), penetrate
only into the southern part of the tundra while
others can occur far from the area of their continuous
distribution (climatic optimum): in the vicinity of
Dixon (Taymir), the forest thrushes T. pilaris and
T. iliacus form populations in the northernmost part of
the tundra zone, 400 km from the last outposts of the
forests.At the southern limits of the tundra, there is
greater specialization among microhabitats. Many more
species occur in intrazonal habitats, occupying relatively
small and isolated sites, than in zonal habitats that contain
only a small proportion of the regional fauna.
Warming is therefore likely to lead to more pronounced
habitat and niche specialization.
An important consequence of the decline in numbers of
species with increasing latitude is an increase in dominance.
For example, one species of collembolan,
Folsomia regularis, may constitute 60% of the total
collembolan density in the polar desert (Babenko and
Bulavintsev, 1997).These “super-dominant” species are
generally highly plastic, occupy a wide range of habitats,
and generally have large effects on ecosystem processes.
Lemmings (Lemmus spp. and Dicrostonyx spp.) are superdominant
species during peak years of their population
cycles (Stenseth and Ims, 1993) and have large effects
on ecosystem processes (Batzli et al., 1980; Laine and
Henttonen, 1983; Stenseth and Ims, 1993).
7.3.1.3. Microorganisms
Species diversity
Microbial organisms are critically important for the
functioning of ecosystems, but are difficult to study and
are poorly understood compared with other species.
However, the International Biological Programme
(IBP; 1964–1974) significantly advanced understanding
of arctic microorganisms, compared with those of other
biomes, when an inventory of microbial communities
was undertaken in the tundra (Heal et al., 1981).At the
beginning of the 21st century, the knowledge of microbial
diversity in the tundra remains little better than
30 to 40 years ago, and recent outstanding progress in
molecular microbial ecology has rarely been applied to
arctic terrestrial studies.
Presently there are 5000 to 6000 named bacterial
species globally and about the same number of fungi
(Holt et al., 1994), compared with more than one million
named plant and animal species (Mayr, 1998;Wilson
E., 1992). Some scientists have interpreted this difference
to mean that the bacteria are not particularly
diverse (Mayr, 1998). However, there are several reasons
(section 7.7.1.1) to believe that the apparent limited
diversity of microbes is an artifact.
Recent progress in molecular biology and genetics has
revolutionized bacterial classification and the understanding
of microbial phylogeny (“family trees”) and biodiversity
in general.The DNA sequencing technique has
reorganized bacterial classification and brought order to
microbial taxonomy (Wayne et al., 1987). Moreover, the
microbial inventory can now be done without isolation
and cultivation of the dominant microorganisms, because
it is enough to extract the total community DNA from
the soil and amplify, clone, and sequence the individual
genes.This culture-independent approach has been
applied occasionally for analysis of microbial communities
in subarctic and arctic soils, most often to study relatively
simple communities in hot springs, subsoils, and
contaminated aquifers (section 7.3.5.2).Analysis of
Siberian subsurface permafrost samples (Gilichinsky,
2002;Tsapin et al., 1999) resulted in the formation of a
clone library of 150 clones which has been separated
into three main groups of Eubacteria. From 150 clones
so far analyzed, the authors have identified several
known species (Arthrobacter,Clostridium, and Pseudomonas),
while the most abundant phylotypes were represented
by completely unknown species closely affiliated
with iron-oxidizing bacteria.
Another area of intensive application of molecular tools
was northern wetlands (cold, oligotrophic (nutrient
poor), and usually acidic habitats) related to the
methane cycle (sections 7.4 and 7.5).The most challenging
tasks were to determine what particular microbial
organisms are responsible for the generation and
uptake of methane (so-called methanogens and methanotrophs)
in these northern ecosystems and what their
reaction might be to warming of arctic soils. It was
found that most of the boreal and subarctic wetlands
contain a wide diversity of methanogens (Hales et al.,
1996; Galand et al., 2002) and methanotrophs
(McDonald et al., 1999; Radajewsky et al., 2000), most
of them distantly related to known species. Only
recently, some of these obscure microbes were obtained
in pure culture or stable consortia (Dedysh et al., 1998;
Sizova et al., 2003).The novel microbes of methane
cycles are extreme oligotrophic species that evolved to
function in media with very low concentrations of mineral
nutrients.Taxonomically, these methanogens form
new species, genera, and even families within the
Archaea domain (Sizova et al., 2003).The acidophilic
methanotrophs form two new genera: Methylocapsa and
Methylocella (Dedysh et al., 1998, 2000), the latter affiliating
with heterotrophic Beijerinckia indica.
A DNA-based technique allows determination of the
upper limit for variation of microbial diversity in the
Arctic as compared with other natural ecosystems, that
Chapter 7 • ArcticTundra and Polar Desert Ecosystems 261
is, how many species (both cultured and unculturable)
soils contain.This technique is called DNA reassociation
(how quickly the hybrid double helix is formed from
denatured single-stranded DNA).
Arctic polar desert and tundra contain considerable
microbial diversity, comparable with boreal forest soils
and much higher than arable soils.Although extreme
environmental conditions restrain the metabolic activity
of arctic microbes, they preserve huge potential that is
likely to display the same activity as boreal analogues
during climate warming.
There is a much higher degree of genomic diversity in
prokaryotic communities (prokaryotes such as cyanobacteria
have a simple arrangement of genetic material
whereas eukaryotes such as microalgae have genetic
material arranged in a more advanced way, in that the
DNA is linear and forms a number of distinct chromosomes)
of heterogeneous habitats (virgin soils, pristine
sediments) as compared with more homogeneous samples:
the DNA diversity found in 30 to 100 cc of heterogeneous
samples corresponds to about 104 different
genomes, while in pond water and arable soils the number
of genomes decreases to 100 to 102. Based on
extrapolation and taking into account that listings of
species can significantly overlap for microbial communities
of different soils, a rough estimate is that there could
be from 104 to 109 prokaryotic species globally (Staley
and Gosink, 1999;Torsvik et al., 2002).
The conventional inventory approach based on cultivation
suggests that at present in the Arctic, it is possible to
identify in any particular soil no more than 100 prokaryotic
species from the potential of 1000 to 3000 “genome
equivalents” (Table 7.4) and no more than 2000 species
of eukaryotes. In the broadly defined Russian Arctic,
1750 named fungi species (not including yeast and soil
fungi) have been identified (Karatygin et al., 1999).
About 350 of these are macromycetes. However, the
number of fungi species in the Arctic proper is 20 to
30% less, but these data are far from complete.The
Arctic has fewer species of bacteria, fungi, and algae than
other major biomes; actinomycetes are rare or absent in
most tundra sites (Bunnell et al., 1980).While most
major phyla of microflora are represented in tundra
ecosystems, many species and genera that are common
elsewhere, even in subarctic ecosystems, are rare or
absent in tundra. Gram-positive bacteria, including
gram-positive spore forms, are absent or rare in most
tundra sites. Arthrobacter and Bacillus can rarely be isolated
and then only from drier areas. Azotobacter, the freeliving
nitrogen-fixing bacterium, is extremely rare in
tundra, and the moderate rate of nitrogen fixation
observed in situ is mainly due to the activity of cyanobacteria.
Sulfur-oxidizing bacteria are also reported to be
rare or absent. Even using enrichment techniques,
Bunnell et al. (1980) rarely found chemoautotrophic
sulfur-oxidizing bacteria. Photosynthetic sulfur bacteria
were not found in any IBP tundra biome sites and were
reported in only one subarctic site (Bunnell et al., 1980,
Dunican and Rosswall, 1974), although they are common
in coastal areas of the west and south coasts of Hudson
Bay. Sulfur-reducing bacteria, while not abundant in tundra
sites, have been reported in sites in the Arctic and
Antarctic. Iron-oxidizing bacteria are very rare in tundra
sites. Despite ample iron substrate in tundra ponds and
soils, chemoautotrophic ferrous iron oxidizers were not
found in IBP tundra sites (Dunican and Rosswall, 1974).
In contrast, methanotrophic and methanogenic bacteria
appear to be widespread in tundra areas.
As with bacteria, many generally common fungi are conspicuous
by their rare occurrence or absence in tundra
areas. Aspergillus, Alternaria, Botrytis, Fusarium, and Rhizopus
simply do not occur and even Penicillium are rare (Flanagan
and Scarborough, 1974).Yeasts can be isolated readily but
there is very low species diversity in culture media.
Only three different species were reported for Point
Barrow tundra (Bunnell et al., 1980).Aquatic fungi show
high diversity, especially Chytridiales and Saprolegniales.
However they may not be endemic, and reflect the annual
migration into the Arctic of many avian species, especially
waterfowl.The so-called higher fungi, Basidiomycetes and
Ascomycetes, also have low diversity.They are reduced in
the Arctic to 17 families, 30 genera, and about 100
species. In comparison, subarctic and temperate regions
contain at least 50 families, not less than 300 genera, and
anywhere up to 1200 species (Miller and Farr, 1975).
Mycorrhizal symbionts on tundra plants are common.
Arbuscular, ecto-, ericoid, arbutoid, and orchid mycorrhizal
fungi are associated with plants in arctic ecosystems
Table 7.4. Microbial genome size in the Arctic as compared with other habitats (after Torsvik et al., 2002).
DNA source Number of cells per cm3
Community genome
complexity (base pairs)a
Genome equivalentsb
Arctic desert (Svalbard) 7.5 x 109 5–10 x 109
1200–2500
Tundra soil (Norway) 37 x 109 5 x 109
1200
Boreal forest soil 4.8 x 109 25 x 109
6000
Forest soil, cultivated prokaryotes 0.014 x 109 0.14 x 109
35
Pasture soil 18 x 109 15–35 x 109
3500–8800
Arable soil 21 x 109 0.57–1.4 x 109
140–350
Salt-crystallizing pond, 22% salinity 0.06 x 109 0.029 x 109
7
athe number of nucleotides in each strand in the DNA molecule; bmeasure of diversity specified at a molecular level.
262 Arctic Climate Impact Assessment
(Michelsen et al., 1998).The ectomycorrhizal symbionts
are important as they form mycorrhizal associations with
Betula, Larix, Pinus, Salix,Dryas, Cassiope, Polygonum, and
Kobresia. Based on fungal fruitbodies, Borgen et al.
(in press) estimate that there are 238 ectomycorrhizal
fungal species in Greenland, which may increase to
around 250 out of a total of 855 basidiomyceteous fungi
in Greenland when some large fungal genera such as
Cortinarius and Inocybe have been revised.With the exception
of Eriophorum spp., Flanagan (unpub. data, 2004)
found endotrophic Arbuscula-like mycorrhizae on all ten
graminoid plants examined.The number of fungal species
involved in other mycorrhizal symbioses is not clear.
Tundra algae exhibit the same degree of reduction in
species diversity seen among the fungi and bacteria
(Bunnell et al., 1980; Elster, 2002; Fogg, 1998), which
documents a diversity much reduced from that of the
microflora of temperate regions. Cyanobacteria and
microalgae are among the oldest (in evolutionary terms)
and simplest forms of life on the planet that can photosynthesize.
Unicellular and filamentous photosynthetic
cyanobacteria and microalgae are among the main primary
colonizers adapted to conditions of the arctic terrestrial
environment.They are widespread in all terrestrial
and shallow wetland habitats and frequently produce
visible biomass.Terrestrial photosynthetic microorganisms
colonize mainly the surface and subsurface of
the soil and create the crust (Elster et al., 1999).Algal
communities in shallow flowing or static wetlands produce
mats or mucilaginous clusters that float in the water
but are attached to rocks underneath (Elster et al.,
1997).Terrestrial and wetland habitats represent a unique
mosaic of cyanobacteria and algae communities that
occur up to the highest and lowest possible latitudes and
altitudes as long as water (liquid or vapor) is available for
some time during the year (Elster, 2002).The arctic soil
and wetland microflora is composed mainly of species
from Cyanobacteria, Chrysophyceae, Xanthophyceae,
Bacillariophyceae, Chlorophyceae, Charophyceae,
Ulvophyceae, and Zygnemaphyceae.A wide range of
species diversity (between 53 and 150 to 160 species) has
been reported from various arctic sites (Elster, 2002).
Latitudinal gradients of species diversity
Arctic soils contain large reserves (standing crops) of
microbial (mainly fungal) biomass, although the rate of
microbial growth is generally lower than in the boreal
zone. Surprisingly, under severe arctic conditions, soil
microbes fail to produce spores and other dormant
structures that are adaptations to harsh environments
(Fig. 7.7).The species diversity of all groups of soil
microorganisms is lower in the Arctic than further
south, decreasing from about 90 species in Irish grassland,
through about 50 species in Alaskan birch forest, to
about 30 species in Alaskan tundra (Flanagan and
Scarborough, 1974).As with plants and animals, there
are large reductions in numbers of microbial species
with increasing latitude, although these patterns are less
well documented.A correlate of the decreasing number
of species with increasing latitude is increasing dominance
of the species that occur, as with plants and animals.
One yeast (Cryptococcus laurentii), for example,
constitutes a large proportion of yeast biomass across a
range of community types in the northernTaymir
Peninsula (Chernov, 1985).
The hyphal length of fungi in the Arctic shows a latitudinal
trend in which the abundance of fungi, as measured
by hyphal length, decreases toward the north.Although
it is not known if this trend also applies to the species
diversity of fungal mycelia (the belowground network of
fungal filaments or hyphae), it is clear that the amount of
fungal hyphae is low in the Arctic (Robinson et al.,
1996). In the high Arctic, fungal hyphal length was
23±1 m/g in a polar semi desert on Svalbard (78º56' N),
39 m/g on a beach ridge, and 2228 m/g in a mesic
meadow on Devon Island (75º33' N). At Barrow,
Alaska, hyphal length was 200 m/g. In a subarctic mire in
Swedish Lapland, hyphal length was 3033 m/g.These
values can be compared with 6050 to 9000 m/g in
temperate uplands in the United Kingdom and 1900 to
4432 m/g in temperate woodland soils.
7.3.1.4. Summary
Species diversity appears to be low in the Arctic, and
decreases from the boreal forests to the polar deserts of
the extreme north. Only about 3% (~5900 species) of
Fig. 7.7. Latitudinal distribution of (a) soil fungi and (b) bacilli;
curves are hand-fitted (recalculated from data in Mirchink, 1988).
Chapter 7 • ArcticTundra and Polar Desert Ecosystems 263
the global flora (excluding algae) occur in the Arctic
north of the treeline. However, primitive plant species
(mosses and lichens) are particularly abundant.
Although the number of plant species in the Arctic is low
in general, individual communities of small arctic plants
have a diversity similar to or higher than those of boreal
and temperate zones: there can be up to 25 species per
square decimeter. Latitudinal gradients suggest that arctic
plant diversity is sensitive to climate, and species number
is least sensitive to temperature near the southern margin
of the tundra.The temperature gradient that has such a
strong influence on species diversity occurs over much
shorter distances in the Arctic than in other biomes.
The diversity of arctic animals beyond the latitudinal
treeline (~6000 species) is nearly twice as great as that
of vascular plants and bryophytes.As with plants, the
arctic fauna accounts for about 3% of the global total,
and in general, primitive groups (e.g., springtails) are
better represented in the Arctic than are advanced
groups such as beetles. In general, the decline in animal
species with increasing latitude is more pronounced than
that of plants (frequently greater than 2.5-fold).An
important consequence of the decline in numbers of
species with increasing latitude is an increase in dominance.
“Super-dominant” plant and animal species occupy
a wide range of habitats, and generally have large
effects on ecosystem processes.
Microbial organisms are more difficult to enumerate.
Arctic soils contain large reserves of microbial biomass,
although diversity of all groups of soil microorganisms is
lower in the Arctic than further south. Many common
bacteria and fungi are rare or absent in tundra areas.
As with plants and animals, there are large reductions in
numbers of microbial species with increasing latitude,
and increasing dominance of the species that occur.
The latitudinal temperature gradient within tundra is
steeper than for any other biome, and outlier populations
of more southerly species frequently exist in favorable
microenvironments far to the north of their centers
of distribution. Consequently, migration of southerly
taxa is very likely to occur more rapidly in the Arctic
than in other biomes.Temperature-induced biotic
change is likely to be most pronounced at the northern
extreme of tundra, where species distributions are most
temperature-sensitive.
The initial response of diversity to warming is likely to
be an increase in the diversity of plants, animals, and
microbes, and reduced dominance of species that are
currently widespread.Taxa most likely to expand into
tundra are boreal taxa that currently exist in river valleys
and could spread into the uplands, or animal groups
such as wood-boring beetles that are presently excluded
due to a lack of food resources.Although current
extreme environmental conditions restrain the metabolic
activity of arctic microbes, they preserve huge potential
that is ready to display the same activity as boreal analogues
during climate warming.Warming could possibly
cause the extinction of a few arctic plants that currently
occur in narrow latitudinal strips of tundra adjacent to
the sea. Some animals are arctic specialists and could
possibly face extinction.Those plant and animal species
that have their centers of distribution in the high or middle
Arctic are most likely to show reduced abundance in
their current locations should projected warming occur.
7.3.2. General characteristics of arctic
species and their adaptations in the context
of changes in climate and ultraviolet-B
radiation levels
7.3.2.1. Plants
For the past 60 years, arctic plant ecologists have been
concerned with the adaptations and traits of arctic plants
that enable them to survive in harsh climates (e.g.,
Billings and Mooney, 1968; Bliss, 1971; Porsild, 1951;
Russell, 1940; Savile, 1971; Sørensen, 1941). It is now
important to consider how plants that are adapted to
harsh environments will respond to climatic warming,
and particularly how former adaptations may constrain
their survival in competition with more aggressive
species that are projected to immigrate from the south.
Only in the past 20 years have ecologists considered
arctic plant adaptations to UV-B radiation (e.g., Björn,
2002; Robberecht et al., 1980).
Plant adaptations to the arctic climate are relatively few
compared with adaptations of plants to more southerly
environments (Porsild, 1951; Savile, 1971) for the following
reasons (Jonasson et al., 2000):
• arctic plants have inhabited the Arctic (except for
ice-free refugia) for a relatively short period of
time, particularly in Canada and theYamal
Peninsula;
• life spans and generation times are long, with
clonal reproduction predominating;
• flowering and seed set are relatively low and
insecure from year to year; and
• the complexity of the plant canopy is relatively
small and the canopy is low, so that climbing plants
with tendrils, thorns, etc. are not present.
Annuals and ephemeral species are very few (e.g., cold
eyebright – Euphrasia frigida and Iceland purslane –
Koenigia islandica). Many arctic plants are pre-adapted to
arctic conditions (Crawford et al., 1994) and have
migrated to the Arctic along mountain chains (Billings,
1992) or along upland mires and bogs.Although specific
adaptations to arctic climate and UV-B radiation levels
are absent or rare, the climate and UV-B radiation
regimes of the Arctic have selected for a range of plant
characteristics (Table 7.5).
The first filter for plants that can grow in the Arctic is
freezing tolerance, which excludes approximately 75%
of the world’s vascular plants (Körner, 1995). However,
many temperature effects on plants, particularly those
264 Arctic Climate Impact Assessment
Table 7.5. Summary of major characteristics of current arctic plants related to climate and UV-B radiation.
Climatic factor General effects on plants Adaptations/characteristics of arctic plants References
Aboveground environment
Freezing
temperatures
Plant death Evergreen conifers tolerate temperatures between
-40 and -90 ºC; arctic herbaceous plants
tolerate temperatures between -30 and -196 ºC
Larcher, 1995
Ice encapsulation Death through lack of oxygen Increased anoxia tolerance Crawford et al., 1994
Low summer
temperatures
Reduced growth Increased root growth, nutrient uptake, and
respiration
Shaver and Billings,1975;Chapin
F.,1974;Mooney and Billings,1961
Minimized coupling between the vegetation surface
and the atmosphere: cushion plants can have
temperature differentials of 25 ºC
Mølgaard, 1982
Occupation of sheltered microhabitats and southfacing
slopes
Bliss, 1971;Walker M.D. et al.,
1991
Short, late
growing seasons
Constraint on available
photosynthetically active
radiation and time for
developmental processes
Long life cycles Callaghan and Emanuelsson,1985
Slow growth and productivity Wielgolaski et al., 1981
Dependence on stored resources Jonasson and Chapin, 1985
Long flowering cycles with early flowering in some species Sørensen, 1941
Increased importance of vegetative reproduction Bell and Bliss, 1980
Clonal growth; clones surviving for thousands of years Jónsdóttir et al., 2000
Long-lived leaves maximizing investment of carbon Bell and Bliss, 1977
Interannual
variability
Sporadic seed set and
seedling recruitment
Dependence on stored resources Jonasson and Chapin, 1985
Long development processes buffer effects of any
one year
Sørensen, 1941
Clonal growth Callaghan and Emanuelsson,
1985; see also Molau and
Shaver, 1997; Brooker et al.,
2001; Molau and Larsson, 2000
Snow depth and
duration
Negative: constrains length
and timing of growing season
Where snow accumulates, snow beds form in which
specialized plant communities occur
Gjærevoll, 1956
Where snow is blown off exposed ridges (fellfields),
plants are exposed to summer drought, winter
herbivory, and extreme temperatures
Billings and Bliss, 1959; Savile,
1971
Exerts mechanical pressure
on plants
Responses and adaptations not measured Positive:
Insulation in winter
(it is seldom colder than
-5 ºC under a 0.5 m layer)
Low plant stature Crawford, 1989
Reduction of plant
temperature extremes
and freeze–thaw cycles
Low stature to remain below winter snow cover
reduces the risk of premature dehardening
Ögren, 1996, 1997
Protection from wind damage,
abrasion by ice crystals,
and some herbivory
Low stature to remain below winter snow cover;
growth in sheltered locations
Sveinbjörnsson et al., 2002
Protection from winter desiccation
when water loss exceeds
water supply from frozen ground
Low stature to remain below winter snow cover;
deciduous growth
Barnes et al., 1996; Havas, 1985;
Ögren, 1996;Taulavuori E. et al.,
1997;Taulavuori K. et al., 1997b
Protection from chlorophyll
bleaching due to light damage
in sunny habitats
Low stature to remain below winter snow cover;
deciduous growth
Curl et al., 1972
Source of water and nutrients
late into the growing season
Zonation of plant species related to snow depth
and duration
Fahnestock et al., 2000;
Gjærevoll, 1956
Increased UV-B
radiation levels
Damage to DNA that can be
lethal or mutagenic
Reflective/absorptive barriers such as thick cell walls
and cuticles,waxes,and hairs on leaves;physiological
responses such as the induction or presence of UV-B
radiation absorbing pigments (e.g.,flavonoids) and an
ability to repair some UV-B radiation damage to DNA
Robberecht et al., 1980;
Semerdjieva et al., 2003
Repair is mediated through the enzyme photolyase
that is induced by UV-A radiation.There is so far no
indication of adaptations to UV-B radiation that are
specific to plants of the Arctic
Li et al., 2002a,b
Chapter 7 • ArcticTundra and Polar Desert Ecosystems 265
Climatic factor General effects on plants Adaptations/characteristics of arctic plants References
Aboveground environment
Variable
atmospheric CO2
concentrations
Increased atmospheric
CO2 concentrations
usually stimulate photosynthesis
and growth if
other factors are nonlimiting;
an increased ratio
of carbon to nitrogen in
plant tissues
Photosynthesis inAlaskan graminoids acclimated to high CO2
concentrations within six weeks,with no long-term gain
Tissue and Oechel, 1987
The dwarf willow (Salix herbacea) has been able to alter its
carbon metabolism and morphology in relation to changing
CO2 concentrations throughout the last 9000 years
Beerling and Rundgren,
2000
Species such as the moss Hylocomium splendens are already
adapted to high CO2 concentrations; frequently experiencing
400–450 ppm, and sometimes over 1100 ppm, to compensate
for low light intensities under mountain birch woodland.These
high CO2 concentrations are caused by both
soil and plant respiration close to the forest understory surface
in still conditions and when light intensities are low
Sonesson et al., 1992
Soil environment
Low rates of
nutrient
availability,
particularly
nitrogen
Reduced growth and
reproduction
Conservation of nutrients in nutrient-poor tissues Wielgolaski et al., 1975
Long nitrogen retention time resulting from considerable
longevity of plant organs and resorption of nutrients
from senescing tissues and retention of dead leaves
within plant tufts and cushions
Berendse and Jonasson,
1992
Substantial rates of nutrient uptake at low temperatures Chapin F. and Bloom, 1976
Increased surface area for nutrient uptake by increased
biomass of roots relative to shoots (up to 95% of plant
biomass can be below ground)
Chapin F., 1974; Shaver and
Cutler, 1979
Associations with mycorrhizal fungi Michelsen et al., 1998
Nitrogen uptake by rhizomes Brooker et al., 1999
Some arctic plants can take up nutrients in organic
forms, bypassing some of the slow decomposition and
mineralization processes
Chapin F. et al., 1993
Dependence on atmospheric nutrient deposition in
mosses and lichens
Jónsdóttir et al. 1995
Soil movement at
various spatial
scales resulting
from freeze–thaw
cycles, permafrost
dynamics, and
slope processes
Freeze–thaw cycles heave
ill-adapted plants from the
soil and cause seedling
death
Areas of active movement select for species with elastic
and shallow roots or cryptogams without roots
Perfect et al., 1988;Wager,
1938; Jonasson and
Callaghan, 1992; Chapter 5
Shallow active
layer
Limits zone of soil biological
activity and rooting depth;
shallow rooting plates of
trees can lead to falling
Shallow-rooting species, rhizome networks Shaver and Cutler, 1979
Biotic environment
Herbivory Removal of plant tissue
sometimes leading to widespread
defoliation and
death
Arctic plants do not have some morphological defenses
(e.g., thorns) found elsewhere
Porsild, 1951
Many plants have secondary metabolites that deter
herbivores; some substances are induced by vertebrate
and invertebrate herbivores
Haukioja and Neuvonen,
1987; Seldal et al. 1994
Protected growing points; continuous leaf growth in summer;
rapid modular growth in some graminoids; regeneration
from torn fragments of grass leaves, mosses, and lichens
Savile, 1971; Sørensen, 1941
Competition Suppression of some species
and increased dominance of
others leading to changes in
community structure
Secondary metabolites in some arctic species inhibit
the germination and growth of neighboring species
Michelsen et al., 1995;
Zackrisson and Nilsson,
1992
Facilitation Mutual benefits to plant
species that grow together
Positive plant interactions are more important than
plant competition in severe physical environments
Brooker and Callaghan,
1998; Callaway et al., 2002
Nitrogen-fixing species in expanding glacial forefields facilitate
the colonization and growth of immigrant plant species
Chapin F. et al., 1994
Plant aggregation can confer advantages of shelter from wind Carlsson and Callaghan,1991
Hemiparasites can stimulate nutrient cycling of potential
benefit to the whole plant community
Quested et al., 2002
266 Arctic Climate Impact Assessment
with roots and in the long term, are indirect (Chapin F.,
1983). Plant nutrients in arctic soils, particularly nitrogen,
are available to higher plants (with roots) at low
rates (Russell R., 1940) because of slow microbial
decomposition and mineralization rates of organic matter
constrained by low temperatures (Heal et al., 1981).
Arctic plants use different strategies for nutrient uptake
(Callaghan et al., 1991), and different sources of nitrogen,
which reduces competition among plants and facilitates
greater plant diversity (McKane et al., 2002).
Many of the adaptations of arctic species to their current
environments, such as slow and low growth, are likely to
limit their responses to climate warming and other environmental
changes. If changes in climate and UV radiation
levels adversely affect species such as mosses that
play an important role in facilitation, normal community
development and recovery after disturbance are likely to
be constrained. Many arctic plant characteristics are likely
to enable plants to cope with abiotic selective pressures
(e.g., climate) more than biotic pressures (e.g.,
interspecific competition).This is likely to render arctic
organisms more susceptible to biological invasions at
their southern distributional limits, while populations at
their northern range limit (e.g., boreal species in the tundra)
are likely to respond more than species at their
southern limit to warming per se.Thus, as during past
environmental changes, arctic species are very likely to
change their distributions rather than evolve significantly.
Summary
Plant adaptations to the arctic climate are absent or rare:
many species are pre-adapted.The first filter for arctic
plants is freezing tolerance, which excludes approximately
75% of the world’s vascular plants. Short growing seasons
and low solar angles select for long life cycles in
which slow growth often uses stored resources while
development cycles are often extended over multiple
growing seasons. Some plant species occupy microhabitats,
or exhibit behavior or growth forms that maximize
plant temperatures compared with ambient temperatures.
Low soil temperatures reduce microbial activity
and the rates and magnitude of nutrient availability to the
roots of higher plants. Mechanisms to compensate for
low nutrient availability include the conservation of
nutrients in nutrient-poor tissues, resorption of nutrients
from senescing tissues, enhanced rates of nutrient uptake
at low temperatures, increased biomass of roots relative
to shoots, associations with mycorrhizal fungi, uptake of
nutrients in organic forms, and uptake of nitrogen by rhizomes.Temperature
fluctuations around 0 ºC cause frostheave
phenomena that can uproot ill-adapted plants.
Snow distribution determines the period over which
plants can intercept solar radiation and grow. Snow
cover insulates plants against low air temperatures in
winter and extremes of temperature in spring, protects
plants from physical damage from abrasion by ice crystals,
and provides a source of water often late into the
growing season.Where snow cover is thin (e.g., on
exposed ridges), growing seasons are usually long but
water can become limiting; where snow accumulates in
sheltered depressions, snow beds form in which specialized
plant communities occur.
Many arctic plants are pre-adapted to relatively high levels
of UV-B radiation.They exhibit various mechanisms
to protect DNA and sensitive tissues from UV-B radiation
and an ability to repair some UV-B radiation damage
to DNA.Thick cell walls and cuticles, waxes, and
hairs on leaves, and the presence or induction of UV-B
radiation absorbing chemical compounds in leaves, protect
sensitive tissues.There appear to be no specific
adaptations of arctic plant species to high atmospheric
CO2 concentrations.
Arctic plant species do not show the often-complex
interactions with other organisms prevalent in southern
latitudes.Arctic plants are adapted to grazing and
browsing mainly through chemical defenses rather than
the possession of spines and thorns. Facilitation increases
in importance relative to competition at high latitudes
and altitudes.
Thus, many of the adaptations of arctic species to their
current environments are likely to limit their responses
to climate warming and other environmental changes.
Many characteristics are likely to enable plants to cope
with abiotic selective pressures (e.g., climate) more than
biotic pressures (e.g., inter-specific competition).This is
likely to render arctic organisms more susceptible to
biological invasions and they are very likely to change
their distributions rather than evolve significantly in
response to warming.
7.3.2.2. Animals
Classical arctic zoology typically focused on morphological
and physiological adaptations to life under conditions
of extremely low winter temperatures (SchmidtNielsen,
1979; Scholander et al., 1950). Physiological
studies contribute to a mechanistic understanding of
how arctic animals cope with extreme environmental
conditions (especially low temperatures), and what
makes them different from their temperate counterparts.
Ecological and evolutionary studies focus on how
life-history strategies of arctic animals have evolved to
tolerate environmental variation in the Arctic, how flexible
life histories (in terms of both phenotypic plasticity
and genetic variation) are adapted to environmental
variation, and how adjustments in life-history parameters
such as survival and reproduction translate into
population dynamics patterns.
Adaptations to low temperatures
Arctic animals have evolved a set of adaptations that
enable them to conserve energy in low winter temperatures.Warm-blooded
animals that persist throughout the
arctic winter have thick coats of fur or feathers that often
turn white (Scholander et al., 1950).The body shapes of
Chapter 7 • ArcticTundra and Polar Desert Ecosystems 267
high-arctic mammals such as reindeer/caribou, collared
lemmings,Arctic hares (Lepus arcticus), and Arctic foxes
(Alopex lagopus) are rounder and their extremities shorter
than their temperate counterparts (Allen’s rule). Body
size within some vertebrate taxa increases toward the
north (Bergman’s rule), but there are several notable
arctic exceptions to this (e.g., reindeer/caribou; Klein,
1999; muskox; Smith P. et al., 2002).There are few
physiological adaptations in homeotherms (warmblooded
animals) that are unique to arctic animals.
However, several adaptations may be considered to be
typical of the Arctic, including fat storage (e.g., reindeer/
caribou and Arctic fox; Prestrud and Nilssen, 1992) and
reduced body-core temperature and basal metabolism in
the winter (e.g.,Arctic fox; Fuglei and Øritsland, 1999).
While hibernation during the winter is found in a few
arctic mammals such as the Arctic ground squirrel
(Spermophilus parryii), most homeothermic animals are
active throughout the year. Small mammals such as
shrews (Sorex spp.), voles (Microtus,Clethrionomys spp.),
and lemmings with relatively large heat losses due to a
high surface-to-volume ratio stay in the subnivean space
(a cavity below the snow) where they are protected from
low temperatures during the winter. Even medium-sized
birds and mammals such as ptarmigan (Lagopus mutus)
and hares seek thermal refuges in snow caves when resting.
In the high Arctic, the normal diurnal activity patterns
observed at more southerly latitudes are replaced
by activity patterns that are independent of the time of
the day (e.g., Svalbard ptarmigan – Lagopus mutus hyperboreus;
Reierth and Stokkan, 1998).
In heterothermic (cold-blooded) invertebrates, hairiness
and melanism (dark pigmentation) enable them to warm
up in the summer season. Invertebrates survive low winter
temperatures in dormancy mainly due to two coldhardiness
strategies: freeze tolerance and freeze avoidance
(Strathdee and Bale, 1998).Typically, supercooling points
are lower in arctic than in temperate invertebrates.
Freeze tolerance, which appears to be an energetically
less costly strategy than extended supercooling, is a common
strategy in very cold regions.Wingless morphs
occur frequently among arctic insects, probably because
limited energy during the short growth season is allocated
to development and reproduction, rather than in an
energetically costly flight apparatus.A short growth season
also constrains insect body size and number of generations
per year. Life cycles are often extended in time
and/or simplified because invertebrates may need several
seasons to complete their life cycles. Small body size in
arctic insects seems to be a strategy to shorten generation
time (Strathdee and Bale, 1998). Moreover, individuals
from arctic populations are able to grow faster at a given
temperature than southern conspecifics (e.g., Birkemoe
and Leinaas, 2000).Thus, arctic invertebrates may be
particularly efficient in utilizing relatively short warm
periods to complete life-cycle stages.
A short breeding season also underlies several lifehistory
adaptations in birds and mammals, such as synchronized
breeding, shortened breeding season, specific
molting patterns, and mating systems (Mehlum,
1999). Although adjustments to low temperatures and
short growth seasons are widespread in arctic animals,
successful species cannot be generalized with respect
to particular life-history traits (Convey, 2000). Both
flexible and programmed life cycles are common in
polar arthropods (Danks, 1999).
While there are many examples that show that winter
temperatures lower than species-specific tolerance limits
set the northern borders of the geographic distribution of
animals, there are hardly any examples that demonstrate
that high temperatures alone determine how far south
terrestrial arctic animals are found. Southern range borders
are typically set by a combination of abiotic factors
(e.g., temperature and moisture in soil invertebrates) or,
probably most often, by biotic factors such as food
resources, competitors, and natural enemies.
Migration and habitat selection
Many vertebrates escape unfavorable conditions through
movement (either long-distance migration or more
short-range seasonal movement) between different habitats
in the same landscape. Seasonal migration to southern
overwintering areas is almost the rule in arctic birds.
Climate may interfere in several ways with migrating
birds, such as mismatched migration timing, habitat loss
at stopover sites, and weather en route (Lindström and
Agrell, 1999), and a mismatch in the timing of migration
and the development of invertebrate food in arctic
ponds (section 8.5.6). Many boreal forest insects invade
the low-arctic tundra in quite large quantities every
summer (Chernov and Matveyeva, 1997), but few of
these are likely to return in the autumn. Birds residing
in the tundra throughout the year are very few and
include species such as Arctic redpoll (Carduelis hornemanni),
willow grouse (Lagopus lagopus), ptarmigan,
raven, gyrfalcon (Falco rusticolus), and snowy owl (Nyctea
scandiaca). Like several other arctic predators that specialize
in feeding on lemmings and Arctic voles, the
snowy owl emigrates when cyclic lemming populations
crash to seek high-density prey populations elsewhere in
the Arctic and subarctic.A similar nomadic lifestyle is
found in small passerine seed-eating birds such as redpolls
and crossbills (Loxia spp.) in the forest tundra.
These birds move between areas with asynchronous mast
years (years with exceptionally abundant seed production)
in birch and conifers.A substantial fraction of the
Arctic fox population migrates after lemming peaks and
sometimes these migrations may extend far into the
taiga zone (Hersteinsson and Macdonald, 1992). Most
reindeer and caribou populations perform seasonal
migrations from coastal tundra in summer to continental
areas of forest tundra and taiga in the winter. Inuit ecological
knowledge explains caribou migrations as triggered
by seasonal “cues”, such as day length, temperature,
or ice thickness (Thorpe et al., 2001). Reindeer on
isolated arctic islands are more sedentary without pronounced
seasonal migrations (Tyler and Øritsland,
1989). Lemmings and ptarmigans shift habitat seasonally
268 Arctic Climate Impact Assessment
within the same landscapes (Kalela, 1961). In peak population
years, the seasonal habitat shifts of the Norway
lemming (Lemmus lemmus) may become more longdistance
mass movements (Henttonen and Kaikusalo,
1993). For small mobile animals (e.g., wingless soil
invertebrates such as Collembola and mites), habitat
selection on a very small spatial scale (microhabitat
selection) enables individuals to find spatial refuges with
temperature and moisture regimes adequate for survival
(Hodkinson et al., 1994; Ims et al., 2004).The variability
in microclimatic conditions may be extremely large in
the high Arctic (Coulson et al., 1995).
Adaptations to the biotic environment
Generalists in terms of food and habitat selection seem to
be more common among arctic animals than in communities
further south (e.g., Strathdee and Bale, 1998).
This may be either due to fewer competitors and a less
tightly packed niche space in arctic animal communities
and/or because food resource availability is less predictable
and the appropriate strategy is to opt for more
flexible diets. Notable exceptions to food resource generalism
are lemming predators (e.g., least weasels – Mustela
nivalis, and several owls – Asio, Nyctea spp., raptors –
Buteo spp., and skuas – Stercorarius spp.) and a number of
host-specific phytophagous insects (e.g., aphids –
Acyrthosiphon spp. and sawflies – Symphyta). Many water
birds, such as geese and sandpipers (Calidris spp.) with 75
and 90%, respectively, of species breeding in the Arctic,
are habitat specialists. Some species exhibit a large flexibility
in their reproductive strategy based on food
resources. Coastal populations of Arctic foxes with a relatively
predictable food supply from the marine ecosystem
(e.g., seabird colonies) have smaller litter sizes than
inland “lemming foxes” that rely on a highly variable food
supply (Tannerfeldt and Angerbjörn, 1998). Specialists on
highly fluctuating food resources such as seeds from birch
and conifers, as well as lemmings and voles, respond to
temporary superabundant food supplies by having
extraordinarily high clutch or litter sizes.
High Arctic environments contain fewer natural enemy
species (e.g., predators and parasites) and some animals
seem to be less agile (e.g., Svalbard reindeer;Tyler and
Øritsland, 1989) and are possibly less resistant to disease
(Piersma, 1997).
Ultraviolet-B radiation
Little is known about animal adaptations to UV-B radiation.
Non-migrant species, such as reindeer/caribou,
Arctic foxes, hares, and many birds, have white fur or
feathers that presumably reflect some UV-B radiation.
There is some evidence, however, that feathers can be
affected by high UV-B radiation levels (Bergman, 1982),
although this early research needs to be repeated.There
is also a possibility that fur absorbs UV-B radiation. Eyes
of arctic vertebrates experience extremes of UV-B radiation,
from dark winter conditions to high UV-B radiation
environments in spring. However, mechanisms of tolerance
are unknown. Invertebrates in general have DNA
that is robust to UV-B radiation damage (Koval, 1988)
and various adaptations to reduce the absorption of
UV-B radiation. Some subarctic caterpillars possess
pigmented cuticles that absorb in the UV-B wavelengths,
while pre-exposure to UV-B radiation can induce pigmentation
(Buck and Callaghan, 1999). Collembolans
and possibly other invertebrates have dark pigmentation
that plays a role in both thermoregulation and UV-B
radiation protection (Leinaas, 2002).
Patterns of population dynamics
In tundra habitats, population cycles in small- to medium-sized
birds and mammals are the rule, with few
exceptions.The period of the population cycle in lemmings
and voles varies geographically, and is between
three and five years. Cyclicity (e.g., spatial synchronicity
and period between peak population years) seems to
be associated with spatial climate gradients (coast to
inland and south to north) in Fennoscandia (Hansson
and Henttonen, 1988; Strann et al., 2002), although the
biotic mechanisms involved are still much debated
(Hanski et al., 2001). Some lemming populations show
geographic variation in the cycle period within arctic
Siberia; and also (for example) exhibit a long period
(5 years onWrangel Island) and a relatively short period
(3 years inTaymir) between peak population years
(Chernov and Matveyeva, 1997).Within regions (e.g.,
northern Fennoscandia), small rodent cycles may show
distinct interspecific synchrony over large spatial scales
(Myrberget, 1973). However, recent spatially extensive
surveys in northern Canada (Predavec et al., 2001) and
Siberia (Erlinge et al., 1999) have indicated that the
spatial synchrony of lemming populations is not as
large-scale as the snowshoe hare (Lepus americanus)
cycles in boreal North America (Elton and Nicholson,
1942).This is at least partly due to the geographically
variable cycle period.
Populations of small- and medium-sized bird and mammal
predators follow the dynamics of their lemming and
vole prey species (Wiklund et al., 1999).The signature of
lemming and vole population dynamics can also be found
in the reproductive success and demography of mammals
and birds, for example, waders and geese (e.g., Bety et
al., 2002), that serve as alternative prey for lemming
predators.Among northern insects, population cycles are
best known in geometrid moths, particularly the autumnal
moth (Epirrita autumnata), which exhibits massive
population outbreaks with approximately 10-year intervals
that extend into the forest tundra (Tenow, 1972,
1996). In the tundra biome, no herbivorous insects are
known to have population cycles (Chernov and
Matveyeva, 1997). However, the population dynamics of
tundra invertebrates is poorly known due to the lack of
long-term data. Soil invertebrates such as Collembola
(Birkemoe and Sømme, 1998; Hertzberg et al., 2000)
sometimes exhibit large interannual fluctuations in population
density. Large fluctuations in numbers have also
been observed in arctic ungulate populations
Chapter 7 • ArcticTundra and Polar Desert Ecosystems 269
(reindeer/caribou and muskox), and seem to be the result
of several biotic factors in combination with climatic variation
(Klein, 1996, 1999; Morneau and Payette, 2000).
Summary: Implications for animal responses to
climate change
Terrestrial arctic animals possess many adaptations that
enable them to persist in the arctic climate. Physiological
and morphological traits in warm-blooded vertebrates
(mammals and birds) include thick fur and feather
plumages, short extremities, extensive fat storage before
winter, and metabolic seasonal adjustments, while coldblooded
invertebrates have developed strategies of cold
hardiness, high body growth rates, and pigmented and
hairy bodies.Arctic animals can survive under an amazingly
wide range of temperatures, including high temperatures.The
short growing season represents a challenge
for most arctic animals and life-history strategies have
evolved to enable individuals to fulfill their life cycles
under time constraints and high environmental unpredictability.The
biotic environment (e.g., the ecosystem
context) of arctic species is relatively simple with few
enemies, competitors, and available food resources.
For those reasons, arctic animals have evolved fewer traits
related to competition for resources, predator avoidance,
and resistance to diseases and parasites than have their
southern counterparts. Life cycles that are specifically
adjusted to seasonal and multi-annual fluctuations in
resources are particularly important because such fluctuations
are very pronounced in terrestrial arctic environments.
Many arctic animals possess adaptations for escaping
unfavorable weather, resource shortages, or other
unfavorable conditions through either winter dormancy
or by selection of refuges at a wide range of spatial scales,
including microhabitat selection at any given site, seasonal
habitat shifts within landscapes, and long-distance
seasonal migrations within or across geographic regions.
Based on the above general characteristics, if climate
changes, terrestrial arctic animals are likely to be most
vulnerable to the following conditions: higher summer
temperatures that induce desiccation in invertebrates;
climatic changes that interfere with migration routes and
staging sites for long-distance migrants; climatic events
that alter winter snow conditions and freeze–thaw cycles
resulting in unfavorable temperature, oxygen, and CO2
conditions for animals below the snow and limited
resource availability (e.g., vegetation or animal prey) for
animals above the snow; climatic changes that disrupt
behavior and life-history adjustments to the timing of
reproduction and development that are currently linked
to seasonal and multi-annual peaks in food resource
availability; and the influx of new competitors, predators,
parasites, and diseases.
7.3.2.3. Microorganisms
As a group, microorganisms are highly mobile, can
tolerate most environmental conditions, and have short
generation times that can facilitate rapid adaptation to
new environments associated with changes in climate
and UV-B radiation levels.
Adaptations to cold
The ability to resist freezing (and to restore activity after
thawing) and the ability to metabolize below the freezing
point are fundamental microbial adaptations to cold climates
prevailing at high latitudes.
Cell viability depends dramatically on the freezing rate,
which defines the formation of intracellular water crystals
(Kushner, 1981; Mazur, 1980). Cold-adapted microbial
species are characterized by remarkably high resistance to
freezing due to the presence of specific intracellular compounds
(metabolic antifreeze), stable and flexible membranes,
and other adaptations. Lichens are extreme examples
(Kappen, 1993): the moist thalli of such species as
Xanthoria candelaria and Rhizoplaca melanophthalma fully
tolerated gradual or rapid freezing to -196 ºC, and even
after being stored for up to several years, almost immediately
resumed normal photosynthetic rates when warmed
and wetted. For five to seven months of cold and continuous
darkness, they remain green with intact photosynthetic
pigments. However, freeze resistance is not a unique
feature of arctic organisms.
The ability of microorganisms to grow and metabolize in
frozen soils, subsoils, or water is generally thought to be
insignificant. However, microbial growth and activity
below the freezing point has been recorded in refrigerated
food (Larkin and Stokes, 1968) as well as in arctic
and antarctic habitats such as sea ice, frozen soil, and
permafrost (Kappen et al., 1996; Schroeter et al.,
1994). Such activity has important implications for
ecosystem function. Field measurements of gas fluxes in
Alaska and northern Eurasia revealed that winter CO2
emissions can account for up to half of annual CO2 emissions
(Oechel et al., 1997; Panikov and Dedysh, 2000;
Sommerfeld et al., 1993; Zimov et al., 1993), implying
significant cold-season activity in psychrophilic (“coldloving”)
soil microbes. Soil fungi (including mycobionts,
the symbiotic fungal component of lichens) have been
considered as the most probable candidates for the
majority of the tundra soil respiration occurring at temperatures
below 0 ºC (Flanagan and Bunnell, 1980)
because their live biomass was estimated to be ten times
larger than that of cohabiting bacteria.
Winter CO2 emissions have been also explained by other
mechanisms (e.g., the physical release of summeraccumulated
gases or abiotic CO2 formation due to
cryoturbation; Coyne and Kelley, 1971). Most recent
studies (Finegold, 1996; Geiges, 1996; Mazur, 1980;
Rivkina et al., 2000; Russell N., 1990) agree that microbial
growth is limited at about -12 ºC and that occasional
reports of microbial activity below -12 ºC (e.g., continuous
photosynthesis in arctic and antarctic lichens down
to -17 ºC; Kappen et al., 1996; Schroeter et al., 1994,
and photosynthetic CO2 fixation at -24 ºC; Lange and
Metzner, 1965) were not carefully recorded and con-
270 Arctic Climate Impact Assessment
firmed. Under laboratory conditions, Rivkina et al.
(2000) quantified microbial growth in permafrost samples
at temperatures down to -20 ºC. However, the data
points below -12 ºC turned out to be close to the detection
limits of the highly sensitive technique that they
employed.The authors concluded that nutrient uptake at
-20 ºC could be measured, but only transiently, “whereas
in nature (i.e., under stable permafrost conditions)…
the level of activity, if any, is not measurable …”
(Rivkina et al., 2000).
Recently, a new precise technique was applied to frozen
soil samples collected from Barrow,Alaska, and incubated
at a wide range of subzero temperatures under laboratory
conditions (Panikov et al., 2001).The rate of CO2
production declined exponentially with temperature and
unfrozen water content when the soil was cooled below
0 ºC, but it remained surprisingly positive and measurable
(e.g., 8 ng C/d/kg) at -39 ºC.A range of experimental
results and treatments confirmed that this CO2 production
at very low temperatures was due to microbial
respiration, rather than to abiotic processes.The demonstration
that microorganisms can survive low temperatures
suggests the possibility that ancient bacteria of distinctive
genotypes trapped in permafrost will be
released and become active during permafrost thawing
(Gilichinsky, 1994). However, the period over which
ancient permafrost is likely to thaw will be significantly
longer than the next 100 years (section 6.6.1.3).
Dark pigmentation causes higher heat absorption in
lichens, which is especially favorable in the cold polar
environment (Kershaw, 1983; Lange, 1954).
Adaptations to drought
Freezing is always associated with a deficiency of available
water.Thus, true psychrophilic organisms must also
be “xerotolerant” (adapted to extremely dry environments).A
number of plants and microorganisms in polar
deserts, such as lichens, are termed “poikilohydrous”,
meaning that they tend to be in moisture equilibrium
with their surroundings (Blum, 1974).They have high
desiccation tolerance and are able to survive water loss
of more than 95% and long periods of drought. Rapid
water loss inactivates the thallus, and in the inactive state
the lichen is safe from heat-induced respiratory loss and
heat stress (Kappen, 1974; Lange, 1953). In unicellular
microorganisms, drought resistance can also be significant,
although mycelial forms of microbial life (fungi and
actinomycetes) seem to have a much higher resistance to
drought due to their more efficient cytoplasm compartmentalization
and spore formation.
Adaptations to mechanical disturbance
Wind, sand, and ice blasts, and seasonal ice oscillations,
are characteristic features of arctic environments that
affect the colonization and survival of organisms. Most
lichens are adapted to such effects by forming a mechanically
solid thallus firmly attached to the substrate.
Windswept habitats such as hillsides can be favorable if
they provide a suitably rough substrate and receive sufficient
moisture from the air. In contrast, shallow depressions
or small valleys, although more sheltered, are bare
of lichens because snow recedes from them only for very
brief periods each season or persists over several years.
This phenomenon is one reason for the so-called trimline
effect (a sharp delineation on rocks between zones
with and without lichens: Corner and Smith, 1973;
Koerner, 1980; Smith R., 1972).The abrasive forces of
the ice at the bottoms of glaciers may destroy all epilithic
(rock-attached) lichen vegetation, but lichens once
established are able to survive long periods of snow
cover, and even glacial periods (Kappen, 1993).
Adaptations to irradiance
Strong pigmentation is typical for numerous microorganisms
inhabiting tundra and polar deserts, especially
for those that are frequently or permanently exposed to
sunlight at the soil surface (lichens and epiphytic bacteria).
Pigments (melanin, melanoids, carotenoids, etc.)
are usually interpreted as a protection against strong
irradiation. Pigmentation may be constitutive for particular
species or appear as a plastic response to irradiance,
for example, originally colorless Cladonia and Cladina
lichens quickly develop dark-pigmented thalli after
exposure to higher levels of solar radiation (Ahmadjian,
1970). Buffoni Hall et al. (2002) demonstrated that in
Cladonia arbuscula ssp. mitis an increase in phenolic substances
is specifically induced by UV-B radiation, and
that this increase leads to attenuation of the UV-B radiation
penetrating into the thallus.The accumulation of the
protective pigment parietin in Xanthoria parietina is
induced specifically by UV-B radiation (Gauslaa and
Ustvedt, 2003), while in Cladonia uncialis and Cladina
rangiferina only UV-A radiation had a stimulating effect
on the accumulation of usnic acid and atranorin, respectively.
Photorepair of radiation-damaged DNA in
Cladonia requires not only light, but also high temperature
and a hydrated thallus (Buffoni Hall et al., 2003).
As in higher plants, carotenoids protect algae, fungi, and
lichens from excessive photosynthetically active radiation
(MacKenzie et al., 2002) and perhaps also have a role in
protecting them from ultraviolet radiation. In contrast to
higher plants, flavonoids do not act as screening compounds
in algae, fungi, and lichens.
Braga et al. (2001a,b) surveyed the UV radiation sensitivity
of conidia (spore-forming bodies) of 30 strains
belonging to four species of the fungus Metarhizium, an
important biological insecticide. Exposing the fungus to
UV-B radiation levels within an ecologically relevant
range revealed great differences between the strains,
with strains from low latitudes generally more tolerant
of UV-B radiation than strains from high latitudes.
Algae
Seven inter-related stress factors (temperature, water,
nutrient status, light availability and/or UV radiation,
Chapter 7 • ArcticTundra and Polar Desert Ecosystems 271
freeze–thaw events, and growing-season length and
unpredictability) are important for life in arctic terrestrial
and shallow wetlands (Convey, 2000). Cyanobacteria
and algae have developed a wide range of adaptive
strategies that allow them to avoid or at least minimize
injury.Three main strategies for coping with life in
arctic terrestrial and wetland habitats are avoidance,
protection, and the formation of partnerships with
other organisms (Elster and Benson, 2004). Poikilohydricity
(tolerance of desiccation) and shelter strategies
are frequently interconnected, and when combined
with cell mobility and the development of complex life
cycles, afford considerable potential for avoidance.
The production of intracellular protective compounds,
which control the cell solute composition and viscosity
(changes in the carbohydrate and polyols composition of
the cell), together with changes in cell wall structures
(production of multi-layered cell walls and mucilage
sheets) are very common phenomena.The association of
cyanobacteria and algae with fungi in lichens provides
the benefit of physical protection.
Summary
Arctic microorganisms are not only resistant to freezing,
but some can metabolize at temperatures down to
-39 ºC. During winter, this process could be responsible
for up to 50% of annual CO2 emissions from tundra
soils. Cold-tolerant microorganisms are usually also
drought-tolerant. Microorganisms are tolerant of
mechanical disturbance and high irradiance. Pigmentation
protects organisms such as lichens from high irradiance,
including UV radiation, and pigments can be
present in considerable concentrations. Cyanobacteria
and algae have developed a wide range of adaptive
strategies that allow them to avoid, or at least minimize,
UV injury. However, in contrast to higher plants,
flavonoids do not act as screening compounds in algae,
fungi, and lichens.
As a group, microorganisms are highly adaptive, can tolerate
most environmental conditions, and have short
generation times that can facilitate rapid adaptation to
new environments associated with changes in climate
and UV-B radiation levels.
7.3.3. Phenotypic responses of arctic
species to changes in climate and
ultraviolet-B radiation
Species responses to climate change are complex.
They respond individualistically to environmental variables
such as temperature (Chapin F. and Shaver, 1985a),
and various processes within a given species (e.g., reproductive
development, photosynthesis, respiration, leaf
phenology in plants) respond individualistically to a given
environmental change. Knowledge of species responses
to changes in temperature comes from many sources
including indigenous knowledge, current species distributions
related to climate, and experimental manipulations
of temperature in the laboratory and field.
7.3.3.1. Plants
The information presented in this section relates to individual
plant species and how they have responded to
changes in various aspects of climate and UV radiation.
The information is taken mainly from experiments in
which climate variables or UV radiation levels were
modified and the responses of the individual species
determined while they were growing in natural communities.
Indigenous knowledge is also included.
Responses to current changes in climate
Indigenous knowledge studies in Canada describe poor
vegetation growth in eastern regions associated with
warmer summers and less rain (Fox, 2002), but describe
increased plant biomass and growth in western regions,
particularly in riparian areas and of moisture-tolerant
species such as shrubs (Riedlinger, 2001;Thorpe et al.,
2001), due to lengthening of the growing season,
marked spring warming, and increased rainfall.
Inuit participating in theTuktu and Nogak Project in the
Kitikmeot region of Nunavut (Thorpe, 2000;Thorpe et
al., 2001) observed that vegetation was more lush,
plentiful, and diverse in the 1990s compared to earlier
decades.Willows and alders were described as taller,
with thicker stem diameters and producing more
branches, particularly along shorelines. Other indigenous
communities have also reported increases in vegetation,
particularly grasses and shrubs – stating that
there is grass growing in places where there used to be
only gravel. On Banks Island, in the western Canadian
Arctic, Inuvialuit point to observations that the umingmak
(muskox) are staying in one place for longer periods
of time as additional evidence that vegetation is richer
(Riedlinger, 2001). In addition, Riedlinger (2001) has
documented Inuvialuit observations of an increase in
forbs such as qungalik (Arctic sorrel – Oxyria digyna),
which is described as coming out earlier in the spring,
and noticeably “bigger, fresher, and greener”.
The Arctic Borderlands Ecological Knowledge Co-op
monitors the annual quality and quantity of salmonberries
locally called akpiks (cloudberry – Rubus chamaemorus),
and has documented recent observations of high
temperatures early in the year that “burn” berry plant
flowers, early spring melt that results in inadequate
moisture for the plants later in the year, and intense
summer sun that “cooks” the berries before they can be
picked (Kofinas et al., 2002). On Banks Island, local residents
report years where the grass remained green into
the autumn, leaving it vulnerable to freezing (Riedlinger,
2001).This corresponds to experiments that show a similar
effect on Svalbard (Robinson et al., 1998).
In northern Finland, marshy areas are said to be drying
up. Sami reindeer herders from Kaldoaivi in Utsjoki have
observed that berries such as bog whortleberry
(Vaccinium uliginosum) have almost disappeared in some
areas. Other berries such as cloudberry and lingonberry
272 Arctic Climate Impact Assessment
(V.vitis-idaea) are said to have declined in the last 30
years (Helander, 2002). Indigenous peoples’ observations
of declining cloudberry production are supported
by experiments that postulate declines in growth in
warm winters (Marks andTaylor, 1978) and provide
detailed mechanisms of fruit production (Jean and
Lapointe, 2001; Korpelainen, 1994;Wallenius, 1999).
Indigenous knowledge also records changes in species
distribution: some existing species have become more
widespread and new species have been seen. In addition
to increased shrub abundance,Thorpe et al. (2001,
2002) documented reports of new types of lichens and
flowering plants onVictoria Island in Nunavut and more
individual plants of the same species (Thorpe, 2000).
The increases in shrubs in this area correspond to aerial
photographic evidence of increases in shrub abundance
in Alaska (Sturm et al., 2001b). However, the reports of
new types of plants, and lichens in particular, contrast
with experimental evidence that shows a decrease in
lichens and some mosses when flowering plant biomass
increases (Cornelissen et al., 2001; Potter et al., 1995).
A possible reason for this is that results from warming
experiments cannot be extrapolated throughout the
Arctic because of variations in recent and projected climate
owing to both cooling and warming (sections
2.6.2.1 and 4.4.2): warming experiments in continuous
vegetation show declines of lichens, whereas lichens may
expand their distribution during warming in the high
Arctic where vascular plant competitors are sparse
(Heide-Jørgensen and Johnsen, 1998).
In contrast to observed responses of plants to recent
warming, remote sensing by satellites has shown that
the start dates of birch pollen seasons have been
delayed at high altitudes and in the northern boreal
regions of Fennoscandia (Høgda et al., 2002). In the
Faroe Islands, there has been a lowering of the alpine
zone in response to a 0.25 ºC cooling over the past 50
years (Fossa, 2003).
Projected responses to future temperature
changes
Warming per se is very likely to be favorable to the
growth, development, and reproduction of most arctic
plant species, particularly those with high phenotypic
plasticity (flexible/responsive growth and development).
However, other limiting factors such as nutrients and
moisture or competition from immigrant species are
likely to modify plant responses to warming. In some
cases, the direct and indirect effects of warming are
projected to generate negative responses:
• Increased respiration relative to photosynthesis
can result in negative carbon balances, particularly
in clonal plants that accumulate old tissues, for
example, the cushion form ecotype (Fig. 7.3) of
purple saxifrage (Saxifraga oppositifolia; Crawford,
1997a) and some species of the herb Ranunculus
(Cooper, 1996).
• It is possible that cushion forms of arctic plants,
including mosses, that have low atmospheric coupling
and experience high temperatures will experience
thermal death during warming, particularly
when combined with reduced cooling by evapotranspiration
under drought conditions.
• Exposure to high levels of solar radiation and
increases in temperature could possibly cause damage
and death to some species, particularly those
occupying shady and wet habitats, that have low
thermal tolerances (as low as 42 ºC in Arctic sorrel;
Gauslaa, 1984).
• During warming, arctic species with conservative
nutrient-use strategies, slow growth, and particularly
inflexible morphologies such as those of
cushion and mat plants, are likely to be at a competitive
disadvantage to more responsive, faster
growing, taller species immigrating from southern
latitudes.After six years of shading (simulating
competition), increasing temperatures, and fertilizing
a heath and a fellfield community in Swedish
Lapland, shading was found to have the greatest
effect on aboveground growth (Graglia et al.,
1997). In another experiment, flowering of the
dwarf heather-like shrub Cassiope tetragona stopped
when it was shaded (Havström et al., 1995a).
In contrast, a meta-analysis by Dormann and
Woodin (2002) found no significant effect of
shading on biomass.
Populations at the most environmentally extreme
boundary of their distributions (in terms of latitude,
altitude, and habitat mosaics within landscapes) tend to
be responsive to amelioration of physical environmental
factors such as temperature that limit their distributions;
these populations have the potential to expand their distribution.
In contrast, populations at the most environmentally
benign boundary of their distribution tend to
be constrained by competition with more responsive
species of more benign environments (Wijk, 1986) and
tend to be displaced by environmental amelioration.
An InternationalTundra Experiment (ITEX; Henry and
Molau, 1997) meta-analysis of arctic vascular plant
species responses to simulated summer warming (1.2 to
1.8 ºC mean daily near-surface and soil temperature
increase) using standard open-top chambers compared
key species from 13 sites over a period of one to four
years (Arft et al., 1999).The simulated temperature
increase is comparable to the projected increase in mean
arctic summer air temperature of 1.8 ºC by 2050 (mean
of the five ACIA-designated model scenarios). In ITEX
and earlier experiments, phenology (bud burst and flowering)
was advanced in warming treatments at some
sites (Wookey et al., 1993, 1995). In Swedish Lapland,
growth accelerated and the period between thawing and
anther appearance advanced by two weeks (Welker et
al., 1997). In contrast, there was little change in growth
cessation at the end of the season in response to higher
temperatures. However, nutrient addition prolonged the
growth period of polar semi-desert species on Svalbard
Chapter 7 • ArcticTundra and Polar Desert Ecosystems 273
in autumn but reduced frost hardening, leading to dramatic
loss of aboveground biomass during November
1993, which was extremely warm and wet (Robinson et
al., 1998).This corresponds with the indigenous observations
noted above.
Experiments conducted by ITEX showed that initial
increases in vegetative growth were generally, but not
always, reduced in later years, probably because temperature
increases stimulated the use of stored resources
more than the uptake of new resources. Similarly,
growth responses of subarctic dwarf shrubs to soil
warming increased initially but soon returned to former
levels.This response followed an initial increase in nitrogen
mineralization as a result of soil warming, but
increases in mineralization did not persist (Hartley et
al., 1999). In contrast, reproductive success improved in
later years in the ITEX experiments (Arft et al., 1999)
due to the extended period between flower bud initiation
and seed set in arctic flowering plants. Similarly,
over an 18-year period, flowering of the widespread
sedge Carex bigelowii was strongly correlated with July
temperature of the previous year (Brooker et al., 2001).
Eriophorum species exhibited even more dramatic interannual
variation in flowering than Carex species, but
there was no simple correlation with weather in the
flowering year or the previous year (Shaver et al., 1986).
The ITEX experiments showed that responses of growth
and reproduction to temperature increases varied among
vascular plant life forms. Herbaceous species responded
more strongly and consistently to warming than did
woody forms over a four-year period (Arft et al., 1999).
Over longer time periods, the growth form, number, and
position of meristems in some woody plants such as
Betula nana (Fig. 7.3) allowed a much greater response
that completely changed the height and structure of the
whole canopy (Bret-Harte et al., 2001; but see Graglia et
al., 2001a for a different response). In the subarctic,
Graglia et al. (2001a) showed that initial plant responses
(abundance) to temperature increases and other treatments
persisted throughout a ten-year period.
Graminoids were particularly responsive to fertilizer
additions in the subarctic and their increased growth and
litter production suppressed the growth of mosses and
lichens (Cornelissen et al., 2001; Graglia et al., 2001a;
Molau and Alatalo, 1998; Potter et al., 1995). Evergreens
were more responsive to nutrient addition and temperature
increases than deciduous species (Arft et al., 1999).
Mosses and lichens appear to be particularly vulnerable
to climate warming, at least in areas of continuous vegetation
cover.A meta-analysis of lichen responses to
warming experiments across the Arctic showed that
lichen biomass decreased as vascular plant biomass
increased following warming (Cornelissen et al., 2001).
This group of plants is particularly important because a
large proportion of global lichen diversity is found in the
Arctic, some species are important winter forage for
reindeer/caribou, and some are important nitrogen fixers
in strongly nitrogen-limited systems.A 22-year study
of the lichen flora of the Netherlands showed changes
that researchers suggest are related to an increase in
temperature, although the subtropical species might be
more sensitive to nitrogen (Palmqvist et al., 2002).
Fifty percent of the arctic–alpine/boreal–montane lichen
species were declining while subtropical species were
invading (van Herk et al., 2002).The widespread moss
Hylocomium splendens shows a complex response to
warming (Callaghan et al., 1999): in warming experiments
growth is reduced (Graglia et al., 2001a; Potter et
al., 1995), whereas growth increases in relation to
increases in mean annual temperature throughout its
arctic distribution range (section 7.3.5.2, Fig. 7.15;
Callaghan et al., 1997).This suggests some limitation in
the experimental simulation of natural/anthropogenic
warming. If, however, moss growth and abundance are
reduced by higher temperatures, soil thermal regimes,
biogeochemical cycling, and energy and heat exchange
between the biosphere and atmosphere will be significantly
affected (Hobbie, 1996).
Plant species respond differently to warming according
to previous temperature history related to latitude, altitude,
interannual temperature variations, and interactions
among species. Phenological responses to warming
are greatest at cold sites in the high Arctic (Arft et al.,
1999;Wookey et al., 1993), whereas growth responses
to warming are greatest at sites in the low Arctic.
Growth responses of Cassiope tetragona to warming were
greatest at a site in the high Arctic and a high-altitude
site in the low Arctic when compared with the warmest
low-altitude site in the low Arctic (Havström et al.,
1993). Over a period of five years, shoot elongation
responses to warming were greatest in cold summers
(Molau, 2001; Richardson, 2000). Laine (1988) showed
that the reproduction of bilberry (Vaccinium myrtillus)
depended to some extent on the climate in the previous
years (see section 14.7.3 for examples of this in trees),
whereas Shevtsova et al. (1995, 1997) showed no such
response for co-occurring lingonberry and crowberry
(Empetrum nigrum).
Most information on plant responses to climate warming
is limited to the short term and small plot – even if the
short term is two decades. Because of the great longevity
and clonal growth of arctic plants, it is difficult to
extrapolate plant responses from an individual plant to
the population. However, the impacts of climate change
(temperature, nutrients, CO2) on demographic parameters
and population growth statistics were determined
for the sedge Carex bigelowii by Carlsson and Callaghan
(1994) and Callaghan and Carlsson (1997), who showed
that climate change increased tiller size, vegetative production
of young tiller generations, survival of young
tillers, and flowering, and reduced the age of a tiller at
flowering and tiller life span.Two mathematical models
showed that the changes in demographic parameters led
to an increase in the population growth rate, with young
tillers dominating this increase.The rate of vegetative
spread more than doubled, while cyclical trends in flowering
and population growth decreased substantially.
274 Arctic Climate Impact Assessment
Responses to precipitation changes
Precipitation in the Arctic is extremely variable
between seasons and from place to place, but the
amount of snow is difficult to measure (section 2.4).
Precipitation varies from over 1000 mm in coastal areas
(e.g., Norway and Iceland) to less than 45 mm in the
polar deserts, where most of the annual precipitation
occurs as snow.The interaction between precipitation
and temperature is extremely important for plant
growth and ecosystem processes and it is difficult to
separate their effects.
Observations show that precipitation has increased by
up to 20% in northern latitudes within the last 40 years
(section 2.6.2.2), although there has been a 10%
decrease in snow-cover extent in the Northern Hemisphere
in the last 20 years (section 2.4.1).The most
recent climate scenarios for the North Atlantic region
suggest increased mean annual temperatures and precipitation
for the entire region over the next 100 years
(IPCC, 2001; section 4.4).
Effects of changes in snow depth,duration,and timing of the
snow-free period
The interaction between snow amount and temperature
determines the start and duration of the snow-free
period.The duration of the snow-free period at high
northern latitudes has increased by five to six days per
decade and the week of the last observed snow cover in
spring has advanced by three to five days per decade
between 1972 and 2000 (Dye, 2002). Even if precipitation
increases, therefore, temperature increases may
still result in shorter duration of snow and less snow
cover (sections 2.6 and 4.4.3). In contrast, the start of
the growing season has been delayed by up to one
week over the last 20 years in the high-altitude and
northern boreal regions of Fennoscandia (Høgda et al.,
2001). Hydrological models applied to theTana River
Basin of northernmost Finland project increases in
growing-season length, from 30 days in the mountains
to 70 days near the coast of the Barents Sea, by 2100
(Dankers, 2002).This change is associated with an earlier
start to the growing season of about three weeks
and a delayed end of two to three weeks.
The timing of the start of the snow-free period is of
critical importance, and more important than the timing
of autumn snowfall, because solar angles are
already high when plants start growth and each extra
snow-free day at the beginning of the growing season
will enable plants to access high levels of photosynthetically
active radiation (Fig. 7.8; see also section
7.5). In an alpine area, productivity decreased by
about 3% per day that the snow release date was
delayed (Ostler et al., 1982).The timing of snowmelt
has also been found to have considerable effects on
plant phenology (more so than temperature in some
cases: Hollister and Webber, 2000), with a contracting
of development time that is associated with a decrease
in productivity and reproductive output (Callaghan,
1974). Some plant species, such as the deciduous
shrub species Salix pulchra and Betula nana, can
respond to changes in growing-season timing (Pop et
al., 2000), but others, particularly evergreen and
early-flowering species appear to be particularly
vulnerable (Kudo, 1991, 1993).
An experiment that manipulated snow conditions by
using snow fences atToolik Lake, Alaska, showed that
drifts increased winter temperatures and CO2 flux
(Jones et al., 1998,Walker M.D. et al., 1999;Welker
et al., 2000). Under the drifts, temperatures were
more constant than in control plots. Plant growth
increased despite a shorter growing season, although
this was thought to be a transitory response and contrasts
with the reduced growth of plants associated
with late snow beds (Fig. 7.9).
Fig. 7.8. Relationship between timing of the growing season and
the seasonal pattern of irradiance together with an indication of
where transient switches from carbon (C) sink to carbon source
could occur (modified from Chapin F. and Shaver, 1985b).
Fig. 7.9. Snow bank vegetation showing increasing vegetation
development with increasing growing-season length represented
as distance from the snow patch, Disko Island,West Greenland
(photo:T.V. Callaghan).
Chapter 7 • ArcticTundra and Polar Desert Ecosystems 275
Frost resistance and avoidance
Changes in snow depth and duration are likely to negatively
affect the frost resistance and avoidance of many
plants at high latitudes. Damage to foliage and apical
meristems occurs when they are “triggered” to premature
bud burst and development by an earlier onset of the
growing season (resulting from early snowmelt) when
the annual hardening/dehardening process is at its most
sensitive phase, and when there is a risk of short periods
of cold weather. Bilberry is a species whose requirement
for cool temperatures to enable it to break dormancy
(i.e., chilling requirement) is fulfilled early (Havas,
1971).Accelerated dehardening of bilberry was found as
consequence of a minor (2 to 3 ºC) increase in temperature
(Taulavuori K. et al., 1997b), suggesting that climatic
warming is very likely to entail a real risk of early
dehardening and subsequent frost damage of shoots.
The explanation for this may be the higher but fluctuating
temperatures, which increase the cryoprotectantconsuming
freeze–thaw cycles (Ögren, 1996, 1997;
Ögren et al., 1997). In addition to frost resistance, frost
avoidance is likely to be disturbed by a thin or lacking
snow cover.The risk is likely to be highest at high latitudes,
where plants that are genetically adapted to the
presence of snow may have lost some potential for frost
resistance during their evolution. Subarctic provenances
of bilberry, for example, have shown reduced frost resistance
compared to provenances from southern Finland
(Havas, 1971).
Other global change factors might affect frost resistance,
but few, sometimes conflicting, reports have been published
of studies performed at high latitudes. Nitrogen
pollutant (or fertilizer) can impair the frost resistance of
plants. Such an effect was demonstrated for mountain
avens (Dryas octopetala) on Svalbard during a warm period
in early winter (Robinson et al., 1998; see previous subsection
on projected temperature responses). However,
recent studies with the heather Calluna vulgaris (Caporn et
al., 1994), bilberry (Taulavuori E. et al., 1997), and lingonberry
(Taulavuori K. et al., 2001) have demonstrated
improved frost resistance caused by extra nitrogen, probably
because these ericaceous species are plants adapted to
low-nutrient habitats, such as those at high latitudes.
Snow depth and duration vary greatly with topography
at the landscape level. High summer temperatures are
very likely to decrease the abundance and size of snow
beds. Changes in snow patches observed by indigenous
peoples are already causing concern in Baker Lake,
Clyde River, and Iqualuit; Fox (2002) describes aniuvak
(permanent snow patches) that are melting in the hills
around those communities. Aniuvak are good areas for
caching meat and provide a sanctuary for caribou to
evade flying insects. Indigenous peoples’ explanations for
the melting snow patches relate more to changes in precipitation
and mean relative humidity than to temperature
increases.The specialized plants characteristic of
late snow beds (Gjærevoll, 1956) are very likely to be at
particular risk as temperatures increase.
Summer precipitation
Altered timing and rate of snowmelt are very likely to
differentially alter the availability of water in different
facies of the tundra landscape mosaic, which are very
likely to in turn significantly affect the predominant vegetation
type and its growth dynamic through the active
season (Molau, 1996).Artificial increases in summer
precipitation produced few responses in arctic plants
compared with manipulations of other environmental
variables (Dormann andWoodin, 2002). However, mosses
benefited from moderate summer watering (Phoenix
et al., 2001; Potter et al., 1995; Sonesson et al., 2002)
and nitrogen fixation rates by blue-green algae associated
with the moss Hylocomium splendens increased (Solheim et
al., 2002).Addition of water to a polar semi-desert
community in summer produced surprisingly few
responses (Press et al., 1998a).Also in the high Arctic,
comparisons were made between sites with high and low
plant densities.Although there was little difference in
soil moisture and plant water relations, and water availability
did not constrain the adult vascular plants,
surface water flow in snow-flush areas (accumulations of
snow that provide water during summer) allowed
greater development of cyanobacterial soil crusts,
prolonged their nitrogen fixing activity, and resulted in
greater soil nitrogen concentrations (Gold and Bliss,
1995). Because of their importance in facilitating
vascular plant community development, Gold and Bliss
(1995) projected that the effects of climate change on
non-vascular species are very likely to be of great
consequence for high-arctic ecosystems.
Responses to increased atmospheric carbon
dioxide concentrations
There are very few arctic experiments that have manipulated
atmospheric CO2 concentrations in the field
(Gwynn-Jones et al., 1996, 1999;Tissue and Oechel,
1987), but more laboratory experiments have been conducted
on arctic vascular plants (Oberbauer et al.,
1986), mosses, and lichens (Schipperges and Gehrke,
1996; Sonesson et al., 1992, 1995, 1996).
The first experiment that manipulated CO2 in the
Arctic concluded that elevated CO2 concentrations had
no long-term effects because photosynthetic acclimation
(i.e., down-regulation, the physiological adjustment
of photosynthetic rate so that no differences are
found between plants grown at ambient and elevated
CO2 levels) in cottongrass (Eriophorum vaginatum) was
apparent within three weeks and biomass did not
increase. However, there was prolonged photosynthetic
activity in autumn and more biomass was allocated to
roots (Tissue and Oechel, 1987).The lack of response
and enhanced root biomass were attributed to nutrient
limitation (Oechel et al., 1997). Although increases in
tiller production of cottongrass were not considered to
be an important response, this can lead to long-term
increases in population growth (Carlsson and
Callaghan, 1994).
276 Arctic Climate Impact Assessment
Longer-term CO2 enrichment experiments in the subarctic
also show that growth responses are dominated
by early, transient responses (Gwynn-Jones et al.,
1997). Four dwarf shrubs were studied over the first
three years of the experiment; one, the deciduous bilberry,
showed increased annual stem growth (length) in
the first year whereas two other evergreen dwarf shrubs
(mountain crowberry – Empetrum hermaphroditum and
lingonberry) showed reduced growth. In the seventh
year, increased CO2 concentrations significantly
increased the leaf ice nucleation temperature (i.e.,
reduced frost resistance, which can be harmful during
the growing season) in three of the four species tested
(Beerling et al., 2001). Bog whortleberry (or bog bilberry
– Vaccinium uliginosum), lingonberry, and mountain
crowberry showed increases in leaf ice nucleation
temperature exceeding 2.5 ºC whereas bilberry showed
no significant effect, as in another study (Taulavuori E.
et al., 1997). Increased CO2 concentrations interacted
with high UV-B radiation levels to increase leaf ice
nucleation temperature by 5 ºC in bog whortleberry.
This effect coincides with indigenous knowledge and
other experiments that show increased frost sensitivity
of some arctic plants to changes in climate and UV-B
radiation levels (see responses to cloudiness and photoperiod
in this section).
An expected (and subsequently observed) response to
increased atmospheric CO2 concentrations was a
change in leaf chemistry (e.g., an increase in the carbon
to nitrogen ratio) that was expected to affect
herbivory (Fajer et al., 1989) and decomposition
(Robinson et al., 1997). Surprisingly, herbivory was
not affected. However, increased CO2 concentrations
were found to play a role in nutrient cycling by altering
the composition of microbial communities after
five years (Johnson et al., 2002; section 7.3.3.3), suggesting
that chemical changes are occurring in plants
exposed to high CO2 concentrations but the changes
have not yet been identified.
In laboratory studies, the moss Hylocomium splendens that
naturally experiences high CO2 levels in the birch woodlands
of the Swedish subarctic was shown to have photosynthetic
rates that were limited by light, temperature,
and water for most of the growing season (Sonesson et
al., 1992). Enhanced CO2 concentrations for five
months decreased photosynthetic efficiency, light compensation
point, maximum net photosynthesis, and surprisingly,
growth (Sonesson et al., 1996). Similar experiments
with three lichen species, Cladonia arbuscula,
Cetraria islandica, and Stereocaulon paschale, failed to show
any response of fluorescence yield to enhanced CO2
concentration (1000 ppm), although there was an interaction
between CO2 and UV-B radiation levels
(Sonesson et al., 1995). Perhaps the lack of response in
moss and lichens reflects their adaptation to the currently
high levels of CO2 that they experience close to the
ground surface (Sonesson et al., 1992) via the process of
down-regulation.
In contrast to some views that responses of plants
(mainly growth) to increased CO2 concentrations are
relatively small and by inference insignificant (Dormann
andWoodin, 2002), recent results show that in the long
term, increased CO2 concentrations can have the wideranging
and important effects discussed previously
(Beerling et al., 2001; Johnson et al., 2002).
Responses to increased ultraviolet-B radiation
levels
One common method for simulating the effects of ozone
depletion has been to irradiate organisms and ecosystems
with artificial UV-B radiation. Results are often reported
in relation to the equivalent percentage of ozone depletion.
It should be noted, however, that the radiation spectrum
from the lamps used in experiments differs from
the spectrum of the radiation increase that would ensue
from real ozone depletion.Therefore, the degree of simulated
ozone depletion depends on the “weighting func-
Bilberry
(Vaccinium myrtillus)
Bog whortleberry
(Vaccinium uliginosum)
Lingonberry
(Vaccinium vitis-idaea)
Mountain crowberry
(Empetrum hermaphroditum)
Ambient to
enhanced
UV-B
Zero to
ambient
UV
Ambient to
enhanced
UV-B
Zero to
ambient
UV
Ambient to
enhanced
UV-B
Zero to
ambient
UV
Ambient to
enhanced
UV-B
Stem length - 0 0 0 0 0 0
Branching 0 0 0 0 0 0 0
Leaf thickness + 0 0 0 0
Flowering + + 0 0
Berry production + 0 0 0 0 0
Phenology 0 0 0 0 0 0 0
Total UV-B radiationabsorbing
compounds
- - - - 0 0 0
Leaf ice nucleation temperature 0 + + 0 +
+ indicates an increase;- indicates a decrease;0 indicates no effect compared to control;blank indicates no information
Table 7.6. Summary of UV-B radiation effects on subarctic dwarf shrubs (based on Phoenix, 2000 and other sources referred to in
the text).
Chapter 7 • ArcticTundra and Polar Desert Ecosystems 277
tion” applied in the calculations, and the knowledge of
the appropriate weighting function is very incomplete.
Weighting functions are also species-specific: a certain
amount of applied artificial radiation does not correspond
to the same degree of ozone depletion for a plant and a
tadpole, for example.The information in the following
sections should be read with this in mind.
Relatively little is known about plant responses to
changes in UV-B radiation levels. Field experiments on
subarctic (Table 7.6) and high-arctic ecosystems show
species-specific responses to ambient UV-B radiation
levels and to enhanced UV-B radiation levels equivalent
to a 15% decrease in stratospheric ozone from 1990 levels.
(The 15% decrease is equivalent to losses of ozone
projected to occur throughout much of the Arctic by
2015. However, the values do not apply to Beringia for
April and October 2015;Taalas et al., 2000.) On the
whole, the effects of increased UV-B radiation levels are
relatively few compared with effects of increased temperature
and nitrogen (Dormann andWoodin, 2002).
A global meta-analysis of plant responses to increased
UV-B radiation levels showed that there was a small but
significant reduction in biomass and plant height
(Searles et al., 2001). In the subarctic, measurements of
stem length, branching, leaf thickness, flowering, berry
production, phenology, and total UV-B radiation
absorbing compounds were affected significantly by
ambient UV-B radiation levels in only two of three
dwarf shrubs (i.e., bog whortleberry and lingonberry;
Phoenix, 2000). Mountain crowberry and lingonberry
showed no responses to enhanced UV-B radiation levels
after seven years of exposure whereas bog whortleberry
and bilberry showed few responses (Table 7.6).
Enhanced UV-B radiation levels have been shown to
reduce the height growth, but not biomass, of the mosses
Sphagnum fuscum and Hylocomium splendens in the subarctic
(Gehrke et al., 1995).
The UV-B radiation studies (Table 7.6) showed that arctic
species were more tolerant of enhanced UV-B radiation
levels than previously thought, and that the production
of UV-B radiation absorbing compounds did not
show the simple relationship with UV-B radiation dose
expected from laboratory studies.Another surprise
effect was the responsiveness of frost hardiness in some
arctic dwarf shrubs to increased UV-B radiation levels.
Dunning et al. (1994) pioneered investigation of the
relationship between UV-B radiation levels and frost
resistance in a Rhododendron species and concluded that
increased exposure to UV-B radiation increases
(although only marginally) cold resistance. In contrast,
K.Taulavuori et al., (unpubl. data, 2004) found
decreased frost resistance in bilberry in response to elevated
UV-B radiation levels and Beerling et al. (2001)
showed decreased frost resistance in bog whortleberry,
lingonberry, and mountain crowberry.A combination of
elevated CO2 and UV-B radiation levels increased lateseason
frost sensitivity of leaves of bog whortleberry
from -11.5 to -6 ºC. Increased frost sensitivity at the
beginning and/or end of the short arctic growing season
is likely to curtail the season even further.As some models
of vegetation redistribution related to temperature
change use the critical freezing temperatures for leaf
damage in temperate trees and shrubs (Prentice et al.,
1992), modeled past and future northward migration of
temperate vegetation should be reconsidered in relation
to changing CO2 and UV-B radiation levels.
The resilience of the subarctic dwarf shrubs to enhanced
UV-B radiation levels probably reflects pre-adaptation to
higher levels than are currently experienced in the Arctic
(Phoenix, 2000).These species currently extend southward
to about 40º N and they probably existed even further
south in a higher UV-B radiation regime during the
early Holocene.The increased UV-B radiation levels currently
applied in experiments are equivalent to the difference
in ambient UV-B radiation levels between the
site of the experiment (68º N) and Helsinki (59º N)
(Fig. 7.10). In addition, many arctic plants have thick
leaves that might attenuate UV-B radiation entering leaf
tissues. However, one particular climate–UV radiation
interaction that could possibly increase the damage
experienced by plants is the combination of possible
earlier snow-free periods (Dankers, 2002) with higher
spring UV-B radiation levels at the surface of the earth
(Taalas et al., 2000). Such a combination of effects
would expose young, potentially sensitive plant shoots
and flower buds to particularly high UV irradiation
(Zepp et al., 2003).
Responses to changes in cloudiness and
photoperiod
An important characteristic of the arctic environment is
the daily and seasonal patterns of the light period or
photoperiod.At midwinter, intermediate latitudes (40º
Fig. 7.10. Dwarf shrub distributions in relation to latitude and
solar UV-B radiation incident at the surface of the earth
(Gwynn-Jones et al., 1999; Hultén, 1964; Phoenix, 2000).
278 Arctic Climate Impact Assessment
to 50º N) have about an eight-hour day length, whereas
a polar night without sunrise prevails north of the
Arctic Circle (66.5º N). Consequently, day-length
change during spring and autumn occurs much faster at
high latitudes.
Frost-resistance patterns change seasonally and are
environmentally controlled, mainly by temperature and
day length; which of these factors predominate
depends on the seasonal growth cycle (Fuchigami et
al., 1982).The development of frost resistance by
almost all woody plants at high latitudes is characterized
by strong dependency on the photoperiod for
growth cessation and cold hardening. Scots pine (Pinus
sylvestris) seedlings from the northern boreal forest
develop a high degree of frost resistance during the late
summer as a consequence of the shortening days
(Taulavuori K. et al., 1997a).The frost-hardening
process is initiated even at high temperatures (20 ºC)
in experimental conditions that mimic the ambient
photoperiod (Taulavuori K. et al., 2000). Given the
marked photoperiodic control of the frost-hardening
process in woody species at high latitudes, it is understandable
that they harden more extensively compared
to populations at lower latitudes under similar temperatures.
For example, the lowest survival temperature of
bilberry in the central Alps (~50º N) at midwinter is
around -35 ºC (Sakai and Larcher, 1987 and references
therein), while the same level of frost resistance is
already achieved at the end of September in northern
Finland (65º N) (Taulavuori E. et al., 1997).
In a changing climate, photoperiod will not change, but
species that are migrating will experience changes in
photoperiod. It is unlikely however, that this will constrain
species initially. Many northern boreal species, for
example, experienced arctic photoperiods earlier in the
Holocene before they were displaced southward by climate
cooling (section 7.2). If and when species with a
more southerly distribution migrate into the Arctic,
photoperiod constraints could possibly affect growth
and flowering but this is largely unknown. However,
experiments with transplanting herbs between the
Austrian Alps,Abisko, and Svalbard showed that allocation
of biomass in some species such as glacier buttercup
(Ranunculus glacialis) was affected by photoperiod
and this constrained any potential increases of vigor that
might have occurred due to climate warming (Prock
and Körner, 1996). In contrast, herbs such as Geum
(Prock and Körner, 1996) and some grasses (Heide et
al., 1995) that are not sensitive to photoperiod could
possibly benefit from climate warming.
It has been suggested that increased UV-B radiation
effects might be small in the future because of increased
cloudiness (section 4.4.4) that is likely to counteract to
some extent the effect of decreasing ozone levels
(Dormann andWoodin, 2002). However, projections of
increased cloudiness, and particularly future cloud types,
are uncertain. It is more likely that UV-B radiation
effects will be reduced by decreases in albedo as snow
and ice distribution and seasonal duration decline, and as
the boreal forest displaces part of the current tundra.
Arctic plants differ in the degree to which they gain or
lose carbon in photosynthesis at “night” (22:00 to 04:00
hours when light intensity is low but it is not necessarily
dark). Under conditions of cloudy nights, those species
that gain carbon at night, for example,Arctic dryad
(Dryas integrifolia), alpine foxtail (Alopecurus alpinus),
glaucous willow (Salix glauca), and arctic willow (Salix
arctica) (25–30% of diurnal carbon gain; Semikhatova et
al., 1992), are likely to have reduced competitive ability
compared with species that do not. In contrast, increased
cloudiness during the day probably favors those species
that gain carbon at night.Those species that lose carbon
at night (e.g., Eriophorum angustifolium; Semikhatova et
al., 1992) are likely to be disadvantaged by warming.
Responses to potential changes in pollinator
abundance and activity
The rapid phenological changes that have been observed
in response to simulated climate change have the potential
to disrupt the relationships that plants have with
animal, fungal, and bacterial species that act as pollinators,
seed dispersers, herbivores, seed predators, and
pathogens (Dunne et al., 2003).These disruptions are
likely to have the strongest impact if the interacting
species are influenced by different abiotic factors or if
their relative responses to the same factors (e.g., elevated
temperatures) are different. However, wind and selfpollination
are more widespread among arctic flowering
plants, so any mismatch between pollinator activity and
flowering phenology is likely to be of greater significance
to any plants immigrating to the Arctic as temperatures
increase. Little appears to be known about
these processes.
Summary
Species responses to changes in temperature and other
environmental variables are complex. Species respond
individualistically to each environmental variable.
Plant species also respond differently to warming
according to previous temperature history related to
latitude, altitude, interannual temperature variations,
and interactions among species. Some species are
already responding to recent environmental changes.
Indigenous knowledge, aerial photographs, and satellite
images show that some arctic vegetation is becoming
more shrubby and productive.
Summer warming experiments showed that initial
increases in the growth of vascular species were generally
reduced with time, whereas reproductive success
improved in later years. Over short periods (four years),
herbaceous plants responded more than woody plants,
but over longer periods, woody plant responses were
dominant and could change the canopy height and structure.
Mosses and lichens were generally disadvantaged by
higher-plant responses to warming.
Chapter 7 • ArcticTundra and Polar Desert Ecosystems 279
Responses to warming are critically controlled by moisture
availability and snow cover.Already, indigenous
observations from North America and Lapland show a
drying trend with reduced growth of economically important
berries. However, experimental increases in summer
precipitation produced few responses in arctic plants,
except for mosses, which showed increased growth.
An experiment that manipulated snow conditions showed
that drifts increased winter temperatures and CO2 flux
and, surprisingly, that plant growth increased despite a
shorter growing season. In general, however, any earlier
onset of the snow-free period is likely to stimulate
increased plant growth because of high solar angles,
whereas an increase in the snow-free period in autumn,
when solar angles are low, will probably have little impact.
Carbon dioxide enrichment experiments show that plant
growth responses are dominated by early, transient
responses. Surprisingly, enhanced CO2 did not affect levels
of herbivory, but significantly increased the leaf icenucleation
temperature (i.e., increased frost sensitivity) of
three of four dwarf shrub species, and altered the composition
of microbial communities after five years.A general
lack of responses of mosses and lichens reflects their adaptation
to the currently high levels of CO2 that they experience
close to the ground surface.
Ambient and supplemental UV-B radiation levels produced
complex, individualistic, and somewhat small
responses in species. Overall, arctic species were far more
tolerant of enhanced UV-B radiation levels than previously
thought, and the production of UV-B absorbing compounds
did not show the simple relationship with UV-B
radiation dose expected from laboratory studies. Some
arctic dwarf shrubs exhibited increased frost sensitivity
under increased UV-B radiation levels.The arctic photoperiod
is unlikely to be a general constraint to species
migrations from the south, as trees and southern species
previously occurred further north than at present.
7.3.3.2.Animals
In contrast to plants, there are relatively few experiments
that have addressed how animal populations respond to
simulated climate change and UV-B radiation levels in the
Arctic.The few experiments have focused on invertebrates
(e.g., insects and soil animals) for which the microclimate
can be manipulated on small experimental plots.
Experiments on free-ranging vertebrate populations may
not be feasible for logistical reasons. On the other hand,
more time series of population data are available for conspicuous
vertebrates such as reindeer/caribou and lemmings
than, for example, soil invertebrates.Time series
can be analyzed with respect to the influence of current
climate variability (including recent changes).
Responses to current changes in climate and
ultraviolet radiation levels
Ice-crust formation on the tundra as a result of freeze–
thaw events during the winter affects most terrestrial arctic
animals. Dense snow and ice severely limit forage availability
for large ungulates such as reindeer/caribou and
muskox (Klein, 1999). Dramatic reindeer population
crashes resulting from periodic ice crusting have been
reported from the western coastal part of the Russian
Arctic, Svalbard, and Fennoscandia (Aanes et al., 2000;
Putkonen and Roe, 2003; Reimers, 1982; Syroechovski
and Kuprionov, 1995). Similar events have been reported
for muskox in the southern parts of their range in Greenland
(Forchhammer and Boertmann, 1993). Inuit in
Nunavut report that caribou numbers decrease in years
when there are many freeze–thaw cycles (Thorpe et al.,
2001) and the probability of such freeze–thaw events is
said to have increased as a result of more short-term fluctuations
in temperature. In central Siberia, where winter
climate is colder and more stable, reindeer population
dynamics are less climate-driven (Syroechovski and
Kuprionov, 1995). Swedish Saami note that over the last
decade, autumn snow lies on unfrozen ground rather than
on frozen ground in the summer grazing areas and this
results in poor-quality spring vegetation that has rotted
(Nutti, pers. comm., 2004); certain microfungi seem to
be responsible for this (Kumpula et al., 2000).
Long and accurate time series data on Svalbard reindeer
populations (Aanes et al., 2000; Solberg et al., 2001)
show that the amount of precipitation during the winter,
which is highly variable and is well described by the
Arctic Oscillation index (Aanes et al., 2002), provides
the most important check on the reindeer population
growth rate in concert with population density.Winters
with freezing rain were associated with severe population
crashes both in one reindeer population (although the
natural dynamics of an introduced herd may have contributed
to this) and in an introduced population of sibling
voles (Microtus rossiaemeridionalis; Fig. 7.11).
Episodes of mild weather and wet snow lead to a collapse
of the subnivean space and subsequent frost encapsulates
food plants in ice, making them unavailable to
small mammal herbivores, and even killing plants in
some cases (Callaghan et al., 1999; Robinson et al.,
1998).Accordingly, the survival rate of tundra voles
(Microtus oeconomus) decreases dramatically in winters
with many freeze–thaw cycles (Aars and Ims, 2002;
Fig. 7.12). For example, the lemming increases observed
at Kilpisjärvi (northwest Finnish Lapland) in 1997 and
2001 were probably curtailed by warm spells and rain in
January that resulted in freezing of the ground layer
(Henttonen, unpubl. data, 2004). Inuit residents of the
western Canadian Arctic are also concerned with the
impacts of thaw slumping on lemming populations and
their predators (owls).Thaw slumps at lake edges have
been occurring more extensively and at a faster rate in
recent years, linked to warmer temperatures and an
increase in wind activity and rain; thawing of ice-bound
soil destroys lemming burrows (IISD and the community
of Sachs Harbour, 2000).
There has been speculation about whether the recent
dampened amplitude of population cycles and more
280 Arctic Climate Impact Assessment
spatially asynchronous dynamics of voles and lemmings
in northern Fennoscandia may be the result of occasionally
unfavorable winters disrupting the normal population
dynamics (Yoccoz and Ims, 1999). Figure 7.13
illustrates changes since the beginning of the 1990s in
populations of the formerly cyclic and numerically
dominant grey-sided vole (Clethrionomys rufocanus) and
other vole species. In long qualitative time series (up to
100 years), periods with loss of cyclicity and synchrony
are evident (Angerbjörn et al., 2001; Steen et al., 1990;
Stenseth and Ims, 1993), but it is unclear whether this
is related to fluctuations in climate.There is a correlation
between sunspot activity and snowshoe hare cycles
in North America (Sinclair et al., 1993), but no such
relationship has been found for the mountain hare
(Lepus timidus) in northern Finland (Ranta et al., 1997).
There are no relationships between sunspot activity and
outbreak years in the autumnal moth in Fennoscandia
(Ruohomäki et al., 2000), although there are few studies
of the role of climatic variability in arctic insect and
soil arthropod populations because of the lack of long
quantitative time series.
Arctic indigenous peoples are rich sources of information
about recent changes in animal health and behavior,
in particular reindeer/caribou. Increases in vegetation
(longer grass, riparian areas with denser vegetation) are
linked to increased forage availability and more mosquitoes
and flies, resulting in increased insect harassment of
reindeer/caribou (Riedlinger, 2001). Changes in “the
warmth of the sun”, day length, and the timing of the
growing season may trigger reindeer/caribou to cross a
frozen lake or river when the ice is no longer thick
enough to support their weight (Thorpe et al., 2001).
However, some of the environmental changes may be
beneficial. Stronger and more frequent winds are said to
provide reindeer/caribou with relief from insect harassment,
meaning they can spend more time inland and not
in coastal areas (Riedlinger, 2001). Qitirmiut in Nunavut
know that caribou adapt to the heat by staying near
coastal areas and shorelines, lying on patches of snow,
drinking water, standing in the water, eating moist
plants, and sucking mushrooms (Thorpe et al., 2001).
However, increases in the number of extremely hot days,
combined with changing water levels and vegetation patterns,
are likely to affect the ability of reindeer/caribou
to respond in these ways.
Climatic cooling has to some extent caused habitat
degradation in some coastal areas as a result of grubbing
by snow geese on their staging ground.The lesser snow
goose (Anser caerulescens caerulescens) breeds in coastal
areas of the Hudson Bay region, which has experienced
climatic cooling since the mid-1970s.This has delayed
migration of the breeding populations (Hansell et al.,
1998). Huge aggregations of staging and local geese in
the coastal marshes have led to intense grubbing and
degradation of salt-marsh sward (Srivastava and
Jefferies, 1996). Long-term observations and modeling
have shown that goose reproductive variables are directly
and indirectly dependent on selected climatic variables,
particularly those relating to spring (SkinnerW.
et al., 1998). Nest initiation date, hatching date, and
clutch size were associated with the date of the last
snow on the ground and mean daily temperature
between 6 and 20 May. Early snowmelt allows geese to
forage and females to build up nutrient stores before
nest initiation. Goslings that hatch earlier in the spring
have a higher probability of survival than those hatching
later. Inclement weather, such as cumulative snowfall,
freezing rain, and northerly and easterly winds can
result in nest abandonment by females and even adult
starvation while incubating eggs.
Fig. 7.12. Yearly winter survival rate (with 95% confidence intervals)
of experimental tundra vole populations plotted against the
number of days with temperatures above 0 ºC during December
through February. Mean winter temperature and the year are
denoted above the survival rate estimates (Aars and Ims, 2002).
Fig. 7.11. (a) Population dynamics of Svalbard reindeer at
Brøggerhalvøya and sibling voles at Fuglefjella on Svalbard (Aanes
et al., 2000;Yoccoz and Ims, 1999); (b) observed and projected
changes in vegetation (Robinson et al., 1998; Callaghan et al., 1999).
Chapter 7 • ArcticTundra and Polar Desert Ecosystems 281
Responses to projected changes in climate
Despite adaptations to low temperatures, warming
experiments have shown that temperatures higher than
normal do not present any physiological problem for
arctic arthropods provided that water is available
(Hodkinson et al., 1998).Arctic aphids were more successful
in terms of the number of completed generations
through the summer when temperature was experimentally
elevated (Strathdee et al., 1993).The effects of
experimental warming were more pronounced in the
high Arctic at Svalbard than in subarctic Abisko
(Strathdee et al., 1995).The combination of high temperatures
and drought seem to be very problematic for
terrestrial invertebrates (Strathdee and Bale, 1998), but
the hydrological aspect of climate change in tundra habitats
is an important issue that has rarely been addressed
in studies of arctic animals (Hodkinson et al., 1999).
Some of the most important effects of higher summer
temperatures on arctic terrestrial animals are likely to be
mediated through intensified interspecific interactions
(parasitism, predation, and competition). Higher temperatures
in the Arctic are very likely to lead to invasions
of species with more southerly distributions. Such range
expansions are projected to be particularly rapid in those
species for which food resources (e.g., host plants) are
already present (Hodkinson and Bird, 1998). For example,
the mountain birch Betula pubescens ssp. czerepanovii,
the main food plant of the autumnal moth, occurs in the
continental parts of the Fennoscandian forest tundra
where winter temperatures are occasionally lower than
the tolerance limit for over-wintering eggs (Tenow,
1972); however, warmer winters are likely to lead to the
exploitation of this existing food source. Many insects
belonging to the boreal forest already invade the lowarctic
tundra in quite large quantities every summer
(Chernov and Matveyeva, 1997) and the Arctic is subject
to a “steady rain” of wind-dispersed small invertebrates
(Elton, 1925) that are likely to rapidly become established
when environmental conditions are adequate.
Due to the lack of long-term monitoring programs,
there are presently no arctic equivalents of the detailed
and quantitative documentation of the northward spread
of insects in Europe (e.g., Parmesan et al., 1999).
Several generalist predators not yet present in the Arctic
are likely to spread northward with increased ecosystem
productivity due to warming.The red fox has already
expanded into the Arctic, probably at the expense of the
Arctic fox (Hersteinsson and MacDonald, 1992).
Winter warming will alter snow cover, texture, and
thickness.A deeper snow cover is likely to restrict reindeer/caribou
access to winter pastures, their ability to
flee from predators, and energy expenditure traveling
across snow. Changes in snow depth and texture are very
likely to also determine whether warm-blooded small
vertebrates will find thermal refuges for resting in snow
dens (ptarmigan and hares) or for being active in the
subnivean space (Pruitt, 1957). Ice-crust formation
reduces the insulating properties of the snowpack
(Aitchinson, 2001) and makes vegetation inaccessible to
herbivores.There is ample observational evidence that
the current incidence and degree of winter ice crusting
clearly affects the population dynamics patterns of both
large and small mammal herbivore species (see previous
subsection). Moreover, there is experimental evidence
that population densities of numerically dominant tundra
Collembola (springtail) species such as Folsomia quadrioculata
and Hypogastrura tullbergi can be halved following an
episode of freezing rain on Spitzbergen (Coulson et al.,
2000).The projected winter temperature increase of
6.3 ºC by 2080 (mean of the five ACIA-designated
model scenarios) is very likely to result in an increase of
alternating periods of melting and freezing (section
6.4.4). Putkonen and Roe (2003) found that episodes
with rain-on-snow in the winter presently occur over an
area of 8.4 x 106 km2 in the Arctic and they projected
that this area would increase 40% by 2080–2089.The
projected increase in the frequency of winter warming is
very likely to severely suppress population densities; distort
the cyclic dynamics and degree of geographic synchrony
in lemmings, voles, and geometrid moths; and in
some cases even lead to population extinctions.
Responses to projected increases in ultravioletB
radiation levels
The extent to which animals are adapted to incident UV-B
radiation levels must be inferred in most cases. Hairs and
feathers necessary for insulation against low temperatures
also presumably protect the skins of mammals and birds
from UV-B radiation, while white winter hair and feathers
reflect UV-B radiation to some extent.The eyes of nonmigratory
animals must be extremely well adapted to
UV-B radiation in order to be effective in the dark arctic
winter yet also cope with high UV-B radiation levels in the
bright, snowy spring. Invertebrates have coloring that may
serve many functions. Melanic forms of invertebrates
might have advantages in thermoregulation and UV-B
radiation protection (Leinaas, 2002). If white coloration,
insulation, and melanistic thermal regulation decrease due
to reduced snow cover and higher temperatures, sensitivity
to increased UV-B radiation levels is likely to increase.
Fig. 7.13. Population dynamics of the grey-sided vole and other
vole species (combined) at Kilpisjärvi, northern Finland between
1950 and 2000 (Henttonen and Wallgren, 2001).
282 Arctic Climate Impact Assessment
Four species of Collembola on Svalbard were investigated
by Leinaas (2002) with respect to UV-B radiation tolerance:
Hypogastrura viatica, Folsomia sexoculata, Onychiurus
groenlandicus, and O.arcticus.The first three species coexist
in wet shore habitats, with the very heavily pigmented
H.viatica on the surface and F.sexoculata, which as an
adult is also very heavily pigmented, lower down.
O.groenlandicus is a soil-dwelling, unpigmented species.
Although O.arcticus is most commonly found under
small stones and in rock crevices, and is thus rather
unexposed, it has some pigmentation. In an experiment
with enhanced UV-B radiation levels (0.5W/m2 in the
300 to 320 nm band for 12 to 14 hours per day, approximately
equivalent to clear sky summer conditions in
southern Norway) the unpigmented O.groenlandicus
experienced 100% mortality within one week, while the
heavily pigmented H.viatica was not affected.
Caterpillars of subarctic moths have skins that absorb UV-B
radiation to varying extents and the degree of absorption
may depend on previous exposure to high UV-B radiation
levels (Buck and Callaghan, 1999). However, UV-B radiation
levels affect animals indirectly via the quality and
quantity of food that is available to them as a result of
UV-B radiation impacts on plant growth and secondary
metabolite production (section 7.4.1.4).
It is possible to infer some responses of animals to future
increases in UV-B radiation levels by comparing them to
the effects of natural UV-B radiation levels on animals
along latitudinal gradients.Along a south-to-north gradient
starting at 55.7º N, ambient UV-B radiation levels
reduced hatchling size in frogs at sites up to 66º N, with
no latitudinal gradient in UV-B radiation tolerance
(Pahkala et al., 2002). Surprisingly, for a given time of
the year, although UV-B radiation levels decrease with
increasing latitudes, the frogs were exposed to higher
UV-B radiation levels during the sensitive stages of their
life cycles (egg and tadpole) at high latitudes than at low
latitudes (Merilä et al., 2000).These studies suggest that
an increase in UV-B radiation levels due to anthropogenic
ozone depletion is likely to reduce the populations
of those amphibians that have distribution ranges
extending into the Arctic.
Enhanced UV-B radiation levels are thought to improve
the immune system of the autumnal moth in the subarctic
and to destroy the polyhedrosis virus.As this virus
and the parasitoid wasp Cotesia jucunda are both important
controllers of the survival of moth caterpillars,
increased UV-B radiation levels could possibly lead to
increased moth populations and birch forest defoliation.
However, no direct effects of enhanced UV-B radiation
levels on moth fecundity or survival have been detected
(Buck and Callaghan, 1999).
Summary
Evidence for animal responses to climate change is
scarcer than for plants because field experiments are less
feasible for mobile animals, especially vertebrates. In
many cases inferences are made based on time-series
analyses of population abundance data for a few conspicuous
species such as ungulates and lemmings.
Winter climate impacts, especially those events that
affect properties of snow and ice, are particularly important.
Freeze–thaw cycles leading to ice-crust formation
have been shown to severely reduce the winter survival
rate of a variety of species, ranging from soil-dwelling
springtails (Collembola) to small mammals (lemmings
and voles) to ungulates (in particular reindeer/caribou).
Such icing induces conditions of anoxia that affect invertebrates,
creates unfavorable thermal conditions for animals
under the snow, and renders vegetation unavailable
for herbivores.A deeper snow cover is likely to restrict
reindeer/caribou access to winter pastures and their ability
to flee from predators.The projected increase in the
frequency of freeze–thaw cycles is very likely to disrupt
the population dynamics of many terrestrial animals, and
indications that this is already happening to some extent
are apparent in the recent loss of the typical three-tofour
year population cycles of voles and lemmings in
subarctic Europe.
Experimental elevation of summer temperature has
shown that many invertebrates respond positively to
higher temperatures in terms of population growth, as
long as desiccation is not induced. Many invertebrates,
such as insects, are very likely to rapidly expand their
ranges northward into the Arctic if climate warming
occurs, because they have vast capacities to become passively
or actively dispersed and host species (both plants
and animals) are already present north of their present
range borders.
Little is known about the responses in arctic animals to
expected increases in UV-B radiation levels. However,
there are some indications that arctic animals are likely
to be more exposed and susceptible to such changes than
their southern counterparts.The effects of increased
UV-B radiation levels on animals are likely to be subtle
and indirect, such as reduced food quality for herbivores
and increased disease resistance in insect pest species.
7.3.3.3. Microorganisms
Recent experiments that manipulate the environment
(e.g., soil heating, changing the water table, atmospheric
CO2 enrichment, and UV-B radiation supplementation
and attenuation) have added new information about the
effects of environmental change on the soil microbial
community at the species level. In general, climate change
is likely to alter microbial community composition and
substrate utilization (Lipson et al., 1999).Tundra soil
heating, atmospheric CO2 enrichment, and amendment
with mineral nutrients generally accelerate microbial
activity (leading to a higher growth rate). Higher CO2
concentrations tend to intensify root exudation, which is
the main source of available carbon for soil and rhizosphere
bacteria. Much less is known about the transient
changes in the species composition of soil microorganisms
Chapter 7 • ArcticTundra and Polar Desert Ecosystems 283
induced by manipulation of UV radiation levels, although
supplementation of UV-B radiation in the field resulted in
changes in the composition of microbial communities
(Johnson et al., 2002). Laboratory incubation of tundra
soils from Barrow,Alaska, at different temperatures had
strong effects on community composition assessed from a
molecular biology approach, but only after a temperature
shift of more than 10 ºC (Panikov, 1999).
A mathematical simulation of the changes in tundra
microbial community structure (Panikov, 1994, 1997)
showed, surprisingly, that the effects of temperature on
the soil microbial community were less significant compared
with effects on the plant community. Probable
reasons for this include strong metabolic interactions
between individual populations within the microbial
community (in which the product of one organism is
used as a nutrient substrate by other organisms) that stabilize
community structure in a wide range of environmental
conditions; the wide temperature tolerance of
microbial species; and the lower resolution power of
microbial taxonomy as compared with plant taxonomy.
The model (Panikov, 1994, 1997) generated realistic
patterns of mass and energy flow (primary productivity,
decomposition rates, and soil respiration) under presentday
conditions and in response to warming, pollution,
fertilization, drying/rewetting of soil, etc. (Fig. 7.14).
Figure 7.14a shows that L-selected species (Bacillus) display
only sporadic occurrence under normal cold tundra
conditions, in agreement with observations, and attain
high population density after soil warming. Simulated
soil warming accelerated both primary productivity and
organic matter decomposition, but the latter was more
affected. Soil warming also led to a negative carbon balance
in the soil, as respiration exceeded photosynthesis
leading to a decline in accumulated organic carbon (Fig.
7.14b; sections 7.4.2.1, 7.4.2.2, 7.5.1.1, and 7.5.4).
Conidia (spores) of the fungus Metarhizium are sensitive
to UV-B radiation.There are great differences between
strains, but strains from high latitudes are less tolerant
than those from lower latitudes (Braga et al., 2001a,b).
In one species (M.anisopliae) it was shown that UV-A
also had a negative effect and, when comparing strains,
the sensitivity to UV-A radiation did not correlate with
the sensitivity to UV-B radiation (Braga et al., 2001c).
Several groups have studied the effects of UV-B radiation
on phylloplane (leaf surface-dwelling) fungi and litterdecomposing
fungi. Moody et al. (1999) found that five
of the investigated species were sensitive and seven relatively
insensitive to UV-B radiation.The spore production
in the litter decomposers was generally inhibited by
UV-B radiation (except for one species), while that in
phylloplane species was unaffected. However, the sensitivity
of spores is not equivalent to the sensitivity of the
metabolic machinery of the vegetative body of a fungus
(i.e., the thallus or mycelium) that produces the spores.
In the subarctic (Abisko), a study of the decomposition
rates of a standard litter type showed that there was a
change in the composition of fungal species resulting
from elevated UV-B radiation levels (Moody et al.,
2001).These results to some extent resemble those from
an earlier experiment studying the decomposition of
dwarf-shrub litter at the same site (Gehrke et al., 1995).
Fig. 7.14. Simulation of changes in a tundra microbial community (Barrow,Alaska) induced by climate warming: (a) population
dynamics of dominant soil bacteria (right axis applies to Bacillus); (b) carbon budget including gross primary production (GPP), soil
respiration, and litter dynamics.The simulation assumed that average air temperature instantly increased by 10 ºC in year two of the
simulation, indicated by the vertical arrows in each panel (Panikov, 1994).
284 Arctic Climate Impact Assessment
The arctic periglacial environment represents a unique
mosaic of unstable habitats (gradation between terrestrial
and shallow wetland environments) where large variations
in cyanobacterial and algal diversity, productivity,
and life strategy exist (Elster and Svoboda, 1995, 1996;
Elster et al., 1997, 2002; Kubeckova et al., 2001).
Prokaryotic cyanobacteria and eukaryotic algae have different
life strategies with respect to their susceptibility
to severe and unstable conditions (Elster, 2002).
Cyanobacteria are well adapted to changeable conditions
involving low and high radiation levels (including UV-B
radiation), and cycles of desiccation and rehydration,
increasing and decreasing salinity, and freezing and thawing.This
gives them a great ecological advantage and
allows them to be perennial. In contrast, eukaryotic
algae have higher rates of photosynthesis and lower
resistance to changes in irradiation, desiccation, rehydration,
and freeze–thaw cycles.These features predetermine
their annual character. If the arctic terrestrial environment
becomes colder, the cyanobacteria are very
likely to become the dominant community. In contrast,
if temperatures become warmer, the eukaryotic algae
are very likely to start to predominate. In addition, the
ongoing temperature increase in the Arctic is very likely
to influence cyanobacteria and algal production, as well
as the balance between cyanobacteria and algae and
invertebrate herbivore activity. Invertebrate grazing
pressure is likely to increase and much of the visible
cyanobacteria and algae biomass could possibly disappear
from arctic locales (Elster et al., 2001).
Summary
Tundra soil heating, CO2 enrichment, and amendment
with mineral nutrients generally accelerate microbial
activity. Higher CO2 concentrations tend to intensify
root exudation, which is the main source of available
carbon for soil and rhizosphere bacteria. Supplementation
of UV-B radiation in the field resulted in changes in
the composition of microbial communities. Laboratory
incubation of tundra soils had strong effects on community
composition after a temperature shift of more than
10 ºC. Surprisingly, the effects of many factors on the
soil microbial community were less significant compared
with effects on the plant community. However, a mathematical
simulation of the changes in microbial community
structure in the tundra showed that soil warming
resulted in stimulation of bacterial growth.
The effects of increased UV-B radiation levels on microorganisms
include damage to high-latitude strains of
fungal spores, and damage to some species of leafdwelling
fungi and soil-dwelling decomposer fungi that
resulted in a change in the composition of the fungal
communities.
Cyanobacteria are better adapted to changeable and
harsh conditions than algae, and in milder climates are
likely to be dominated by algae. However, herbivory of
both cyanobacteria and algal biomass is likely to
increase in a warmer climate.
7.3.4. Genetic responses of species to changes
in climate and ultraviolet-B radiation levels
Many widely distributed arctic species show large ecological
amplitude (broad niches) and are taxonomically
complex, often with many subspecies, while species with
a narrower distribution range often show a more
restricted ecological amplitude. It is necessary to know
the extent of genetic variation in arctic species and the
underlying causes of differentiation or homogenization
(biogeography, historical bottlenecks, reproductive biology,
and demography) in order to assess responses of
species to climate change.
7.3.4.1. Plants
In spite of rapid development in recent years of different
molecular techniques suited for population genetic
studies, there are still few studies of arctic plants, most
of which have focused on biogeographical and phylogeographical
questions related to vascular plant species.
Such studies may reveal the migratory potential of the
species in response to climate change. During the
Pleistocene glaciations, arctic plants were restricted to
refugia within or south of the present-day Arctic, from
which they could re-colonize areas as conditions
improved during interglacial periods (Abbott et al.,
2000;Tremblay and Schoen, 1999).The rate of colonization
by different species during the Holocene probably
depended on the location of their closest refugia,
their dispersal biology, and their genetic makeup.
Genetic phylogeographical studies provide evidence for
relatively fast migration rates in most vascular species
(Abbott and Brochmann, 2003; Bennike, 1999;
Brochmann et al., 2003) and possibly bryophytes as
well (Derda andWyatt, 1999). However, in the modern
context of rapid climate change, migration rates need to
be considered on somewhat shorter timescales than
thousands of years.
The level of genetic variation within and between populations
indicates the potential for local adaptation to
environmental change and hence population resilience
to environmental change. Based on the relatively young
age of populations and low recruitment of sexually
reproduced offspring, it was long believed that genetic
variation in arctic plants would be low. However, the
number of genetic studies is limited and no such general
pattern of genetic variation has been identified.
Arctic plants show the same range of genetic variation
as temperate plants, ranging from comparatively high
levels (Bauert, 1996; Gabrielsen and Brochmann, 1998;
Jefferies and Gottlieb, 1983; Jonsson et al., 1996;
Philipp, 1997; Stenström et al., 2001) to very low
levels (Bayer, 1991; Max et al., 1999; Odasz and
Savolainen, 1996; Philipp, 1998; Stenström et al.,
2001) of variation. However, genetic variation among
arctic plants may be of greater value in terms of biodiversity
than in other biomes due to much lower species
diversity. Furthermore, high levels of polyploidy in
many arctic vascular plant species may promote the
Chapter 7 • ArcticTundra and Polar Desert Ecosystems 285
proportion of the genetic variation partitioned within
individuals, which may be important when passing
through evolutionary bottlenecks (Brochmann and
Steen, 1999).
By comparing 19 different populations of three rhizomatous
Carex taxa, distributed among 16 sites within arctic
Eurasia, ranging from northern Scandinavia in the west
toWrangel Island in the east, Stenström et al. (2001)
showed that the levels of genetic variation were not
related to climate, but were to a large extent explained
by differences in glaciation history at the sampling sites.
Populations in areas deglaciated about 10000 years BP
had significantly lower genetic variation than populations
in areas deglaciated 60000 years BP or those in areas not
glaciated at all during theWeichselian. Relatively young
population age may also be responsible for a low genetic
variation in some other populations (e.g., Bayer, 1991;
Max et al., 1999), while in yet others, breeding systems
apparently play a large role (e.g., Odasz and Savolainen,
1996; Philipp, 1998). In general, populations of insector
self-pollinated plant species have lower genetic variation
than populations of wind-pollinated species
(Hamrick and Godt, 1990), and this seems to apply to
arctic plants as well.
Those plant species representing populations with relatively
high levels of genetic variation usually have a large
geographic distribution, for example purple saxifrage
(Abbott et al., 1995), nodding saxifrage (Saxifraga
cernua; Gabrielsen and Brochmann, 1998), moss
campion (Silene acaulis;Abbott et al., 1995; Philipp,
1997), Carex bigelowii sensu lato (Jonsson et al., 1996;
Stenström et al., 2001), and C. stans (Stenström et al.,
2001). In these species, the genetic variation among
populations (GST) is a relatively small proportion of the
total genetic variation (i.e., they show low degrees of
population differentiation). Large variation within populations,
however, increases possibilities for ecotypic
differentiation. In the Arctic, extremely steep environmental
gradients are frequent on a microtopographical
scale and ecotypic differentiation has been demonstrated
over such short distances for alpine timothy (Phleum
alpinum; Callaghan, 1974), Carex aquatilis (Shaver et al.,
1979), mountain avens (McGraw and Antonovics,
1983), and purple saxifrage (Crawford and Smith,
1997), all widely distributed plant species in the Arctic.
Ecotypic differentiation in response to this small-scale
heterogeneity may preserve genetic variation and in that
way contribute to resilience to change at the species
rather than the population level.Thus, an initial
response to climate change in such species is likely to
be a change in the distribution and abundance of ecotypes
within a species distribution (Crawford and
Smith, 1997). In addition, many arctic plants show large
phenotypic plasticity, which is likely to further increase
their resilience (Stenström et al., 2002;Table 7.5).
If the degree of genetic variation can be used as an indication
of resilience of populations to change, it is likely
that this resilience will be greatest among plants in old
populations of widely distributed, wind-pollinated
vascular species (e.g., rhizomatous Carex populations in
eastern Siberia). However, generation time and seedling
recruitment may affect the adaptation rate. Many of the
dominant arctic plants such as the rhizomatous Carex
species are clonal, that is, they do not rely on seed production
through sexual reproduction for short-term
population maintenance. Genetic individuals of these
plant species may live to be thousands of years old
(Jónsdóttir et al., 2000), which may decrease the adaptation
rate. However, experiments with plants from
outside of the Arctic have shown that UV-B radiation
may increase the rate of genetic change. Exposure to
high UV-B radiation levels can activate mutator transposons
that amplify the mutation effect beyond the
immediate UV-B radiation damage (Walbot, 1999), and
increased levels of UV-B radiation may lead to an
increased tendency to mutations in future generations
(Ries et al., 2000).
For plants with long-lived seed, further genetic variation
is preserved in the seed banks. Dormant seed populations
may be genetically different from the aboveground
populations (McGraw, 1995) and potentially able to better
exploit a new climate.
Genetic variation has been studied in fewer moss and
lichen species than in vascular plants. Boreal and antarctic
bryophytes usually show high levels of variation
(Cronberg et al., 1997; Derda andWyatt, 1999;
Skotnicki et al., 1998, 1999), but the partitioning of
genetic variation among and within populations depends
on species. Scandinavian populations of the widely distributed
moss Hylocomium splendens, including two subarctic
alpine populations, showed high genetic variation
within populations and low GST, a pattern similar to
widely distributed, wind-pollinated vascular species
(Cronberg et al., 1997). In contrast, North American
and European populations of Polytrichum commune have
low within-population variation and high GST (Derda and
Wyatt, 1999).
7.3.4.2. Animals
The genetics of arctic terrestrial animals have been
thoroughly studied mainly for a few well-known mammal
species such as reindeer/caribou (Flagstad and
Røed, 2003), lemmings (Ehrich et al., 2000; Fedorov et
al., 1999a,b), and Arctic fox (Dalén et al., 2005).
These studies have focused on phylogeographical patterns
and the relative roles of present gene flow and
historic processes (especially glacial–interglacial cycles;
see section 7.2) based on neutral genetic markers (especially
mitochondrial DNA).The present genetic differentiation
reflects to a large extent historic processes
and the presence of current migration barriers. For
mammals with relatively restricted mobility such as
lemmings, even small-scale barriers (e.g., large rivers)
can form the borders between subspecies (Fedorov et
al., 1999a,b), while a very mobile animal such as the
Arctic fox, which readily moves between continents and
286 Arctic Climate Impact Assessment
islands on sea ice, appears to be relatively panmictic
(i.e., shows little genetic structuring) at the circumpolar
scale (Dalén et al., 2005).
Current gene flow (an indication of mobility) and population
history (origin and differentiation) indicate the
ability of a species to track the location of its habitats
through time (i.e., a species is able to relocate its distributional
range according to any changes in the distribution
of its habitat through the process of dispersal).
A mobile species will have better prospects for survival
than a relatively sedentary species. Moreover, a species
with high genetic/racial diversity has proved an ability
to adapt to different environmental conditions in the
past and is likely to do the same in the future. It should
be noted, however, that markers of genetic variation/
differentiation currently used (e.g., mitochondrial
DNA) may have little bearing on the genetic variation
in morphology and life-history traits (see Flagstad and
Røed, 2003). It is these latter traits that decide whether
a species or a morph will be able to adapt to future
changes. Currently, there are few studies of arctic animals
using a quantitative genetics approach (Roff, 1997)
that address the potential for rapid adaptations to climatic
change. Elsewhere, using a quantitative genetic
research protocol, Réale et al. (2003) showed that
northern boreal red squirrels (Tamiasciurus hudsonicus)
were able to respond genetically within a decade to
increased spring temperatures.
7.3.4.3. Microorganisms
Assessment of genetic responses of microorganisms to
climate change is based on laboratory models, as observations
made within arctic terrestrial ecosystems are
absent. Short generation times and the impressive genetic
plasticity of bacteria make them a favorite topic in
theoretical studies of general population genetics.
Because most mutations are deleterious, mutation rates
are generally thought to be low and, consequently, mutator
alleles should be selected against. However, up to
1% of natural bacterial isolates have been found to be
mutators.A mutator can be viewed as behaving altruistically
because, although it reduces individual fitness, it
increases the probability of an adaptive mutation appearing.These
results may help to explain observations that
associate high mutation rates with emerging pathogens
that cause spontaneous epidemic outbreaks (Sniegowski
et al., 1997;Wilke et al., 2001).
In the arctic environment, intensive mutagenic effects
are likely to result from increased UV radiation levels
and also from aerosols and volatile chemical mutagens
transported to the cool polar atmosphere from the midand
low latitudes.The direct mutagenic effects are very
likely to be weak, especially if the protective shielding
effects of soil particles and impressive genetic plasticity
of bacteria are taken into account. However, it is possible
that mutants could lead to epidemic outbreaks that
could have profound and unexpected consequences for
the whole ecosystem.
7.3.4.4. Summary
Arctic plants show the same range of genetic variation as
temperate plants, ranging from comparatively high levels
to very low levels. In widespread Carex taxa, levels of
genetic variation were not related to climate, but were
to a large extent explained by differences in glaciation
history at the sampling sites: populations in areas
deglaciated approximately 10000 years BP had significantly
lower genetic variation than populations in areas
deglaciated 60000 years BP.
Plant species representing populations with relatively
high levels of genetic variation usually have a large geographic
distribution. On a microtopographical scale,
extremely steep environmental gradients are frequent
and ecotypic differentiation has been demonstrated over
short distances for several widespread species.This heterogeneity,
together with large phenotypic plasticity, is
likely to contribute to resilience to change at the population
and species levels. For plants with long-lived seed,
further genetic variation related to former environments
is preserved in the seed banks.Thus, there are several
mechanisms for widespread arctic plant species to
respond to environmental change.
Experiments with plants from outside the Arctic have
shown that increased levels of UV-B radiation can speed
up genetic change and may lead to an increased tendency
for mutations in future generations.
The present genetic differentiation of arctic terrestrial
animals that have been studied thoroughly (e.g., reindeer/caribou,
lemmings,Arctic fox) to a large extent
reflects historic processes and the presence of current
migration barriers. For mammals with relatively
restricted mobility such as lemmings, even small-scale
barriers (e.g., large rivers) can form the borders
between subspecies, while a very mobile animal such as
the Arctic fox shows little genetic structuring at the
circumpolar scale.A species with high genetic/racial
diversity has proved an ability to adapt to different environmental
conditions in the past and is likely to do the
same in the future.
There is a paucity of studies of arctic animals that have
addressed the potential for rapid adaptations to climatic
change. Elsewhere, it was shown that northern boreal
red squirrels were able to respond genetically within a
decade to increased spring temperatures.
Up to 1% of natural bacterial isolates have been found to
be mutators, and high mutation rates are associated with
emerging pathogens causing spontaneous epidemic outbreaks.
In the Arctic, intensive mutagenic effects are
likely to result from increased UV radiation levels and
also from aerosols and volatile chemical mutagens.
Although the effects are very likely to be weak, it is possible
that mutants could lead to epidemic outbreaks that
could have profound and unexpected consequences for
the whole ecosystem.
Chapter 7 • ArcticTundra and Polar Desert Ecosystems 287
7.3.5. Recent and projected changes in
species distributions and potential ranges
Paleoecological research (section 7.2) and observations
over many decades demonstrate that the geographic
ranges of terrestrial species in general are well correlated
with bioclimatic variables. Furthermore, the strength
of these relationships is independent of trophic level
(Huntley et al., 2003). Major climate-related species distributions
at the large scale include the limit of trees,
which is associated with the isoline for mean July air
temperatures of about 10 ºC (Brockmann-Jerosch, 1919
as discussed in Körner, 1999) and soil temperature of
7 ºC (Körner, 1998), and the limit of woody plants such
as dwarf shrubs that are one indicator of the boundary of
the polar desert biome (Edlund and Alt, 1989). Such
relationships suggest that species distributions at the
macrogeographical and landscape scale are very likely to
change as temperature changes.This section assesses the
effects of climate change on recent changes in species
distributions and those projected to occur in the future.
7.3.5.1. Recent changes
Indigenous knowledge projects have documented recent
changes in the ranges of caribou in relation to changes
in weather, based on hunters’ understanding of how
environmental conditions affect seasonal caribou distribution
patterns (Kofinas et al., 2002). Hunters’ explanations
of caribou distributions may provide indications
of potential range changes given projected climate
change. For example, in the El Niño year of 1997–
1998, several thousand Porcupine Caribou overwintered
on theYukon Coast in arctic Canada. Hunters in
Aklavik, NorthwestTerritories, explained this phenomenon
in terms of the Beaufort Sea ice pack, which was
farther from theYukon North Slope than in most years,
resulting in warmer coastal temperatures and thus more
abundant forage for caribou. In July 1997, as the caribou
moved into Canada from their Alaskan calving
grounds, several large groups remained on the coast,
taking advantage of the rich forage opportunities.
A mild autumn and the lack of icing events that push
the caribou south for the winter kept the caribou in the
area into October, as the animals could continue to
access summer forage.The herd that remained on the
coast for the winter was reported to be in better condition
than the herd wintering in the usual locations.
Indigenous knowledge has also documented recent
changes in the ranges of other animals in relation to
changes in the weather. In the Canadian Arctic, Inuit in
communities such as Baker Lake have reported insects
previously associated with areas south of the treeline
(Fox, 2002). In more western regions, there have been
more frequent sightings of “mainland ducks” such as pintail
ducks (Anas acuta) and mallard (A.platyrhynchos;
Riedlinger, 2001).
Working in the Canadian Arctic using a conventional
scientific approach, Morrison et al. (2001) summarized
the trends in population data for breeding waders, and
found that almost all arctic-breeding species were
declining.The reasons for the trends were not always
clear and were probably of multiple origins. Long-term
monitoring in Finland has shown a substantial decline in
the populations of many arctic and subarctic bird
species over the past 20 years (Väisänen et al., 1998),
but the trend in bird populations is not always negative.
Zöckler et al. (2003) found that almost half of the
arctic-breeding, long-distance migrants studied are
presently in decline. For many species there are still
insufficient data available, and only a few species (8%)
show an increasing trend. In most cases, it is not easy to
correlate trends with climate change.As the trends in
some species are different outside and inside the Arctic,
there is an indication that factors of a more global
nature are involved.An example is the drastic decline of
the ruff (Philomachus pugnax) in almost all breeding sites
outside the Arctic in contrast to their stable or even
increasing populations in some (but not all) northern
arctic areas (Zöckler, 2002).This coincides with the
recent northern expansion of other wet-grassland
waders, such as the common snipe (Gallinago gallinago)
in the Bolshemelzkaya tundra (Morozov, 1998), the
black-tailed godwit (Limosa limosa), and the northern
lapwing (Vanellus vanellus) in northern Russia concomitant
with a northward expansion of agriculture including
sown meadows (Lebedeva, 1998). Several other bird
species have recently been recorded in more northerly
locations in the Arctic (Zöckler et al., 1997), suggesting
that some species are shifting their distribution in
response to alteration of habitats by climate change.
The emerging picture is that the ruff is being forced to
retreat to its core arctic habitats owing to the effects of
global climate change in combination with increasing
nutrient enrichment on the quality of wet grassland
habitats (Zöckler, 2002).
A recent global meta-analysis of plants claims that a climate
change signal has been identified across natural
ecosystems (Parmesan andYohe, 2003). Range shifts of
plants averaging 6.1 km per decade toward the poles and
6.1 m per decade in altitude have been identified in
response to a mean advancement of spring (initiation of
greening) by 2 to 3 days per decade.Although some
northern treeline data were included in the analysis,
little information was available for arctic ecosystems.
7.3.5.2. Projected future changes in species
distributions
Models of species–climate response surfaces based upon
correlations between species ranges and bioclimatic variables
are able to simulate the recently observed range
changes of at least some species of birds (Zöckler and
Lysenko, 2000) and butterflies (Hill et al., 1999; 2003;
Virtanen and Neuvonen, 1999). Related studies have
shown that, at least in the case of butterflies, the extent
to which species have realized their projected range
changes over the last 30 to 50 years is strongly related to
their degree of habitat restriction: generalist species are
288 Arctic Climate Impact Assessment
much more able to achieve the projected range expansions
than are specialist species (Warren et al., 2001).
Such models (Hill et al., 2003; Huntley et al., 1995)
project future ranges of arctic species that are often
markedly reduced in spatial extent compared to the
species’ present ranges.The range limits of boreal and
temperate species shift poleward in response to the
same future climate scenarios. However, the large magnitude
of the shifts in projected range margins results in
potential reductions in the ranges of many boreal
species because they are limited to the north by the
Arctic Ocean.
The extent to which arctic plant species experience the
rapid range reductions simulated by such models will
depend principally upon two factors. First, such reductions
are likely to happen most rapidly in species that
experience some physiological constraint at their southern
range margin, for example, the winter thermal constraint
postulated for cloudberry (Marks, 1978; Marks
andTaylor, 1978) or the summer thermal constraints
postulated for the great skua (Catharacta skua; Furness,
1990). Species whose southern range margin is determined
by biotic interactions are likely to experience less
rapid range reductions. Second, such reductions are very
likely to happen more rapidly where the northward
migration of boreal or temperate species is not limited
either by habitat availability or propagule (dispersal stage
of a plant or animal, such as fertilized eggs, larvae, or
seeds) dispersal. “Fugitive” species of the early successional
communities that characteristically follow disturbance
of the boreal forests have the required dispersal
ability to achieve rapid poleward range expansions.
Unless other factors (e.g., herbivore pressure or a lack
of microsites for successful seedling establishment)
exclude them, these species are likely to extend into the
Arctic rapidly, forming transient ecosystems that will
persist until the arrival of the more slowly expanding
late-successional boreal species.
Loss of habitat is a particularly important possibility that
would constrain species ranges.The most dramatic
change in habitat for many water birds is the projected
loss of tundra habitat, which varies between 39 and 57%
by the end of the 21st century (Harding et al., 2002;
Haxeltine and Prentice, 1996).Vegetation models
(Neilson and Drapek, 1998) combined with maps of
water-bird distributions show a large variation in the
impact of projected vegetation changes on 25 selected
Table 7.7. Loss of breeding area habitat projected by two different general circulation models for arctic water-bird species, and their
globally threatened status (based on Zöckler and Lysenko, 2000).
Loss of habitat (%)a
Red ListdHadCM2GSa1b UKMOc
Tundra bean goose Anser fabalis rossicus/serrirostris 76 93
Red-breasted goose Branta ruficollis 67 85 VU
Spoon-billed sandpiper Eurynorhynchus pygmaeus 57 57 VU/EN
Emperor goose Anser canagicus 54 54 !
Ross’s gull Rhodostethia rosea 51 73
Red-necked stint Calidris ruficollis 48 68
Sharp-tailed sandpiper Calidris acuminata 46 74
Little stint Calidris minuta 45 65
Curlew sandpiper Calidris ferruginea 41 70
Pectoral sandpiper Calidris melanotos 38 60
Dunlin Calidris alpina 36 58
White-fronted goose Anser albifrons 36 57
Long-billed dowitcher Limnodromus scolopaceus 31 54
Great knot Calidris tenuirostris 31 42
Lesser white-fronted goose Anser erythropus 28 29 VU
Barnacle goose Branta leucopsis 21 27
Western sandpiper Calidris mauri 19 21
Brent goose Branta bernicla 16 44
Red knot Calidris canutus 16 33
Greater snow goose Anser caerulescens 14 46
Canada goose Branta canadensis 13 22
Pink-footed goose Anser brachyrhynchus 10 10
Sanderling Calidris alba 5 25
aValue could be substantially higher as unclassified areas in these analyses may contain different tundra types; bmoderate warming; cextreme warming; dVU=vulnerable as a
globally threatened species (BirdLife International, 2001), EN=suggested for upgrading to endangered as a globally threatened species, !=suggested for inclusion in the Red List.
Chapter 7 • ArcticTundra and Polar Desert Ecosystems 289
species (Zöckler and Lysenko, 2000).Vegetation scenarios
derived from the HadCM2GSa1 model project that
76% of tundra bean goose (Anser fabalis rossicus/
serrirostris) habitat will be affected by the alteration of
tundra vegetation, while only 5% of sanderling habitat
will be affected (Neilson and Drapek, 1998). However,
the sanderling, similar to many other high-arctic breeders,
might be affected even more strongly, as southern
tundra habitat types are projected to replace their specific
high-arctic habitats.Whereas the more southerly
breeding species can shift northwards, it is likely to be
increasingly difficult for high-arctic breeders to compete.
For two of the three water-bird species that are
considered globally threatened, namely the red-breasted
goose (Branta ruficollis) and the spoon-billed sandpiper
(Eurynorhynchus pygmaeus), 67 and 57% of their current
breeding range is projected to change from tundra to
forest, respectively (Table 7.7).This additional loss of
habitat is likely to place these two species at a higher risk
of extinction.The emperor goose (Anser canagicus),
already in decline and with 54% of its small range projected
to be affected, is highlighted as needing further
conservation attention.
Geographic ranges of plants
Strong relationships between growth and temperature in
the circumpolar ericaceous dwarf shrub Cassiope tetragona
and the feather moss Hylocomium splendens can be used to
model range changes.The growth of C.tetragona is strongly
related to mean July temperature (Havström et al.,
1995b) and that of H.splendens is related to mean annual
temperature (Callaghan et al., 1997) throughout their
northern ranges (Fig. 7.15a). Mean July and mean annual
temperatures are to some extent representative of latitude,
as they decrease toward the north.The natural climatic
warming from the beginning of the Little Ice Age to
the present is the equivalent of only a minor shift in latitude
for C.tetragona. On the other hand, projected future
warming is likely to produce a greater latitudinal displacement
that, at the northern limit of the current ranges of
the two species, is very likely to result in a northern range
extension (Fig. 7.15a). In contrast, at the southern edge of
the ranges, future warming is very unlikely to increase
growth beyond the genetic capabilities of the species, and
the dynamics of the species in this part of their ranges are
very likely to be determined by the responses of competiFig.
7.15. (a) Number of leaves produced per year in shoots of Cassiope tetragona as a function of mean July temperature and
(b) correlation between a growth parameter in Hylocomium splendens and mean annual temperature at seven arctic and subarctic sites.
The boxes on the regression lines contain scenarios of growth in ACIA time slices resulting from temperature increases projected by
the five ACIA-designated models.The hatched line in (a) depicts projected growth that is probably outside the capability of the species.
The expanded boxes to the right of each panel depict uncertainty ranges associated with each of the projections (doubled-ended
arrows). Open circles represent sequences of present-day relationships between climate and growth, while filled circles represent
present-day and projected growth at three sites (based on Callaghan et al., 1997 and Havström et al., 1995b).
290 Arctic Climate Impact Assessment
tors to warming.A similar analysis for H.splendens shows
how a current alpine population is very likely to resemble
a population from a lowland forested area under projected
future climate conditions (Fig. 7.15b).
At the landscape scale, plants are distributed in mosaics
associated with microhabitats, and the larger-scale latitudinal
range changes are very likely to be associated with
initial changes in landscape mosaics. Cushion plants and
other species characteristic of wind-exposed patches are
likely to become restricted in distribution by increased
snow cover. In contrast, plants of snow beds might
become more restricted if snow duration decreases.
Wetland species will become restricted by drying and
species in drier areas will become more restricted by
increased soil moisture. Plants currently restricted to
south-facing slopes and warm springs (to some extent
analogues of future warmer habitats and hot spots of biodiversity)
north of their main distribution areas are likely
to provide an “inoculum” for rapid colonization of surrounding
habitats when climate becomes warmer,
although they themselves are likely to be displaced from
their current niches by less diverse shrub–thicket communities.
Examples include orchids, ferns, and herbs in
warm springs onWest Greenland (although orchids and
ferns are unlikely to become widely distributed), ericaceous
dwarf shrubs in some inner fjords of Svalbard, and
the large shrubs/small trees of the North Slope of Alaska.
Geographic ranges of animals
Often, causes of observed trends in migrant bird population
numbers cannot easily be attributed to local, siterelated
factors within and outside the Arctic, such as
drainage, land use change, hunting and persecution by
humans, and predation. Even among global factors, climate
change is one of an array of drivers, such as
eutrophication, often working in synergy with climate
change and reinforcing the effect. In addition, migratory
birds are also strongly affected by climate change outside
of their arctic breeding grounds. Desertification,
droughts, and wetland loss; eutrophication of staging and
wintering wetlands; changes in land use; and application
of chemicals and nutrients on wintering grounds lead to
changes in vegetation and biomass on coastal staging and
wintering grounds.The effects of sea-level rise on the
extent of coastal staging and wintering grounds are very
likely to be particularly harmful, and the hunting pressure
on wintering waders in certain areas will also
reduce bird populations.
There are few studies of the impacts of climate change on
migratory species, although recent trends in some species
(e.g., arctic geese) are well known (Madsen et al., 1999).
Very little can be concluded from the observed impacts
of current climate variability on migratory birds, as existing
monitoring programs are few (e.g., Soloviev et al.,
1998) and often began only recently.
An analysis of Hadley Centre spring and summer temperature
and precipitation data over the last 50 years, interpolated
over the currently known arctic distribution areas
of the white-fronted goose (Anser albifrons) and theTaymir
population of the red knot (Calidris canutus canutus),
demonstrates a significant correlation between the mean
June temperature and the percentage of juveniles in the
population as a measure of breeding success.The Nearctic
population of the red knot (C.c.islandica) and the curlew
sandpiper (Calidris ferruginea) breeding on theTaymir
Peninsula do not show such a correlation (Zöckler and
Lysenko, 2000).The HadCM2GSa1 model, forced with a
1% per year increase in atmospheric CO2 concentrations,
projects a moderate increase in mean June temperature
(by the time CO2 concentrations double) in theTaymir
breeding area of the white-fronted goose, which is likely
to favor the goose population.The conditions for the
Taymir population are projected to be particularly favorable
in the period around 2020. However, a considerable
initial cooling and lack of warming over present-day
values by 2080 in the breeding grounds of theWest
Greenland goose population is likely to lead to a drop in
the size of the fragile Greenland population.Although the
ACIA-designated model projections differ from those
used in Zöckler and Lysenko (2000), possible decreases
in temperature in ACIA Region 1 (section 18.3.1) are
within the range of the projections (section 7.6,Table
7.14).The Zöckler and Lysenko (2000) study must be
interpreted in relation to other factors, such as other
weather parameters and natural predation that often
fluctuates in three- to four-year cycles according to the
abundance of the main prey (i.e., the lemming; see section
7.4.1.4). Furthermore, hunting by humans, mainly
outside of the Arctic, and the effects of climate change
outside of the Arctic, in particular sea-level rise, need to
be taken into account.
Investigations of the breeding wader population in
northeast Greenland for over 30 years showed that
spring snow cover is the main factor governing initiation
of egg laying in high-arctic waders, such as the red knot
and other sandpipers, while June temperature does not
appear to be important (Meltofte, 1985, 2000; pers.
comm., 2004). In fact, waders breed earlier in the arid
but cool far north of Greenland than they do in the
“mild”, humid southern areas of the high Arctic, where
snow cover is much deeper and more extensive.
Projections of future climate for northeast Greenland
include cooler summers, later snowmelt, and less snowfree
space for the arriving waders to feed, which are
likely to lead to later breeding and smaller populations.
Snow cover must still be considered the prime regulating
factor for initiation of egg laying, but temperature –
so important for determining invertebrate food availability
(section 8.5.6) – is important as well when sufficient
snow-free habitat is already present.
Although global climate change in synergy with global
eutrophication is likely to lead to an increase in biomass in
the Arctic, a change in vegetation height and density, and a
general change in vegetation structure with shifts in
species distribution that are very likely to have an enormous
impact on water birds (which are highly dependent
Chapter 7 • ArcticTundra and Polar Desert Ecosystems 291
on open landscapes and lightly vegetated breeding sites),
global climate change and eutrophication are very likely to
provide opportunities for other birds with more southerly
distributions, such as owls and woodpeckers. Some birds,
including most goose species and a few waders, have
demonstrated a certain ability to adjust to new and changing
habitats (Lugert and Zöckler, 2001), but the majority
of birds breeding in the high Arctic are likely to be to be
pushed to the edge of survival with little habitat left.
Geographic ranges of microorganisms
Studies of geographic ranges of microbes related to
extremely cold environments such as the Arctic, and also
to climate change, are in their infancy. Unlike plant and
animal ecology, soil microbiology still does not have a
solution to the central biogeographical problem of
whether soil microorganisms are cosmopolitan (widely
distributed) or endemic (restricted to one location)
species. Until the ranges of species are known, the bacteria
that might be threatened by climate change cannot
be identified (Staley, 1997).
The prevailing hypothesis for bacterial biogeography is
based on the axiom of the Dutch microbiologists BaasBecking
and Beijerinck, who stated, “Everything is everywhere,
but the environment selects” (Beijerinck, 1913).
This hypothesis assumes that free-living bacteria are cosmopolitan
in their geographic distribution; they are readily
disseminated from one location on earth to another by
water and air currents or by animal vectors such as birds
that migrate between regions. Only recently has it been
possible to rigorously test the cosmopolitan distribution
of bacteria with unbiased molecular biology approaches.
Studies outside of the Arctic demonstrate that the
cyanobacterium Microcoleus chthonoplastes is a cosmopolitan
species (Garcia-Pichel et al., 1996). Using different
molecular biology techniques, Stetter et al. (1993) discovered
that hyperthermophilic (heat-loving) Archaea
isolated from Alaskan oil reservoirs showed a high degree
of DNA–DNA reassociation with selected Archaeoglobus,
Thermococcus, and Pyrococcus species, and concluded that
the species were the same as those from European thermal
marine sources. In a separate study, DNA–DNA
reassociation of a strain of Archaeoglobus fulgidus isolated
from North Sea crude oil fields showed 100% relatedness
to an Archaeoglobus fulgidus strain from Italian hydrothermal
systems (Beeder et al., 1994).These two studies
comprise some of the best evidence to date supporting
the cosmopolitan hypothesis of Baas-Becking.
However, 3-chlorobenzoate-degrading bacteria isolated
from soils in six regions on five continents (Fulthorpe et
al., 1998) were found to have restricted or unique ranges.
Also, plant species have been reported to harbor their own
unique symbiotic species of fungi associated with leaves,
bark, roots, etc (Hawksworth, 1991), so, by definition, the
existence of endemic plants should imply the existence of
respective microbial symbionts.Therefore, arctic microbial
communities may consist of a mixture of species: some
that are endemic and some that are cosmopolitan.
7.3.5.3. Summary
Monitoring of distribution ranges with a spatial representation
as good as for temperate latitudes is not available
for the terrestrial Arctic. Indigenous knowledge
projects have documented recent changes in caribou
ranges in relation to changes in weather. Hunters’ explanations
of caribou distributions may provide indications
of potential range changes under scenarios of warming
temperatures, such as overwintering of caribou in
coastal areas during warm winters. Other arctic indigenous
observations include insects previously associated
with areas south of the treeline and more frequent sightings
of “mainland ducks”. In contrast, almost all arcticbreeding
species are declining.The reasons for the trends
are not always clear and probably of multiple origins,
although there are suggestions that some species are
shifting their distribution in response to alteration of
habitats by climate change.
Quantitative monitoring of conspicuous and popular
species such as birds and butterflies has demonstrated
that many formerly southern species are rapidly
approaching arctic regions and some have already
entered.Arctic birds, especially arctic-breeding water
birds and waders that can be counted on staging and
wintering grounds, show mostly declining population
trends; some species have declined dramatically. It is
likely that these changes result from the combined action
of eutrophication and habitat loss on wintering and staging
sites as well as concurrent climate change, although
separating the relative contributions of these factors is
difficult. Based on climate models, dramatic reductions
in the populations of tundra birds are projected as a generally
warmer climate is likely to increase vegetation
height and decrease the Arctic’s landmass.
Species–climate response-surface models are able to simulate
the recently observed range changes of at least
some species of both birds and butterflies.At least in the
case of butterflies, the extent to which species have realized
their projected range changes over the last 30 to
50 years is strongly related to their degree of habitat
restriction: generalist species are much more able to
achieve the projected range expansions than are specialist
species. Simulated potential future ranges are often
markedly reduced in spatial extent compared to present
ranges.The range limits of boreal and temperate species
shift poleward but the large magnitude of the shifts in
projected range margins results in potential reductions in
the ranges of many boreal species because they are limited
to the north by the Arctic Ocean. Species that experience
some physiological constraint at their southern
range margin are likely to be affected sooner than those
that are affected by biotic relationships such as competition
from immigrant species. Loss of habitat, such as tundra
ponds for many arctic birds, is a particularly important
possibility that would constrain species ranges.
In contrast, plant populations that are outliers of more
southerly regions and restricted to particularly favorable
habitats in the Arctic, are likely to spread rapidly during
292 Arctic Climate Impact Assessment
warming. Models of moss and dwarf-shrub growth along
latitudinal gradients show considerable potential for
range expansion in the north, but considerable uncertainty,
in relation to ACIA scenarios of warming.
Most microorganisms detected in northern ecosystems,
such as free-living bacteria, are probably cosmopolitan
in their geographic distribution and readily disseminated
from one location to another, and the environment
selects those that can proliferate. However, some
species, particularly symbionts with endemic plants,
can themselves be candidates for endemic status.
7.4. Effects of changes in climate and UV
radiation levels on structure and
function of arctic ecosystems in the
short and long term
Section 7.3 assessed the responses of individual species
to changes in climate and UV-B radiation levels.The
present section assesses the responses of species aggregated
into communities and ecosystems.The two main
attributes of ecosystems that respond to environmental
change are structure and function: each is assessed separately
although the two attributes strongly interact.
In this section, ecosystem structure is defined in terms
of spatial structure (e.g., canopy structure and habitat),
trophic interactions, and community composition in
terms of biodiversity; while ecosystem function is
defined in terms of carbon and nutrient cycling including
dissolved organic carbon export, soil processes, controls
on trace gas exchange processes, primary and secondary
productivity, and water and energy balance.
Although ecosystem structure and function are closely
interconnected, this section focuses on the two aspects
separately for clarity, and limits the discussion here to
plot (single square meter) scales: processes at the landscape
and regional scales are covered in sections 7.5 and
7.6. Community responses to climate and UV radiation
change presented in this section include effects on the
diversity of plant growth forms in terms of biomass contribution,
but the details of impacts on biodiversity in
terms of organism survival and population dynamics are
included in section 7.3.
7.4.1. Ecosystem structure
7.4.1.1. Local and latitudinal variation
The Arctic is characterized by ecosystems that lack
trees.There is a broad diversity in ecosystem structure
among these northern treeless ecosystems that follows a
latitudinal gradient from the treeline to the polar
deserts.Typical communities for a particular latitude
are called “zonal”, but local variation at the landscape
level occurs and these “intrazonal communities” are frequently
associated with variations in soil moisture and
snow accumulation (Chernov and Matveyeva, 1997;
Walker M.D. et al., 1989).
According to Bliss and Matveyeva (1992), zonal communities
south of the arctic boundary near the mean July
isotherms of 10 to 12 ºC consist of taiga (i.e., the northern
edge of the boreal forest).This is characterized by a
closed-canopy forest of northern coniferous trees with
mires in poorly drained areas.To the north of this transition
zone is the forest tundra. It is characterized by white
spruce (Picea glauca) in Alaska, mountain birch (Betula
pubescens ssp. czerepanovii) in Fennoscandia (Fig. 7.16), by
birch and Norway spruce in the European Russian Arctic
(Kola Peninsula and the Pechora lowlands), by Dahurian
larch (Larix dahurica) in central and eastern Siberia, and
by evergreen coniferous trees in Canada (Hustich, 1983).
The vegetation of the forest tundra is characterized by
sparse, low-growing trees with thickets of shrubs. North
of this zone is the low Arctic, which is characterized by
tundra vegetation in the strict sense (Fig. 7.17), consisting
of communities of low, thicket-forming shrubs with
sedges, tussock-forming sedges with dwarf shrubs, and
mires in poorly drained areas.To the north of this zone is
the high Arctic, which consists of polar semi-desert communities
(Fig. 7.18) in the south, characterized by
Fig. 7.16. Forest tundra vegetation represented by the
Fennoscandian mountain birch forest,Abisko, northern Sweden
(photo:T.V. Callaghan).
Fig. 7.17. Zonal tussock tundra near Toolik Lake,Alaska, with
large shrubs/small trees of Salix in moist sheltered depressions
(photo:T.V. Callaghan).
Chapter 7 • ArcticTundra and Polar Desert Ecosystems 293
cryptogam–herb, cushion plant–cryptogam, and, to a
limited extent, mire communities.To the extreme north
is the polar desert where only about 5% of the ground
surface is covered by herb–cryptogam communities
(Fig. 7.19). In this zone, the mean July temperature is
below 2 ºC and precipitation, which falls mainly as snow,
is about 50 mm per year.
The tundra zone can be further subdivided into three
subzones: the southern tundra with shrub–sedge,
tussock–dwarf shrub, and mire communities; the typical
tundra with sedge–dwarf shrub and polygonal mire communities
(Fig. 7.20); and the northern arctic tundra that
consists of dwarf shrub–herb communities.The northern
end of the latitudinal gradient, occurring primarily on
islands and on the mainland only at Cape Chelyuskin
(Taymir Peninsula), is occupied by polar deserts where
woody plants are absent, and forbs and grasses with
mosses and lichens are the main components of plant
communities (Matveyeva and Chernov, 2000).
This vegetation classification has geographic connotations
and cannot be applied easily to reconstructions of past
vegetation throughout the circumpolar Arctic (Kaplan et
al., 2003).A recent classification of tundra vegetation at
the biome level (Walker D., 2000) has been proposed by
Kaplan et al. (2003;Table 7.8, Fig. 7.2).
Within the biomes or zonal vegetation types, there are
intrazonal habitats that are frequently associated with
variations in soil moisture and snow accumulation, and
that have a microclimate that deviates from the general
macroclimate associated with flat surfaces.The intrazonal
habitats form a mosaic of communities. Each of these
tend to have fewer species than the “plakor,” or zonal,
communities. For example, poorly drained areas are
often dominated by sedges with an understory of mosses
and liverworts, but lack fruticose lichens (Matveyeva and
Chernov, 2000).Although each intrazonal community
has relatively few species, together they are more differentiated
and diverse than zonal ones, and are responsible
for about 80% of total species diversity in the regional
flora and fauna. Disturbances, particularly freeze–thaw
cycles and thermokarst (Fig. 7.21) that form patterned
ground, also create landscape mosaics (Fig. 7.20).
Diversity “focal points/hot spots” (Walker M.D., 1995)
and “oases” (Edlund and Alt, 1989; Svoboda and
Freedman, 1994) enrich landscapes by possessing a larger
number of species, including those of more southerly
distribution. Examples include dense willow thickets
Fig. 7.18. Polar semi-desert dominated by mountain avens
(Dryas octopetala), Ny Ålesund, Svalbard (photo:T.V. Callaghan).
Fig. 7.19. Polar desert, Cornwallis Island, Northwest Territories,
Canada (photo: J. Svoboda).
Table 7.8. Circumpolar tundra biome classification (Walker D., 2000; Kaplan et al., 2003).
Biome Definition Typical taxa
Low- and high-shrub
tundra
Continuous shrubland, 50 cm to 2 m tall, deciduous or
evergreen, sometimes with tussock-forming graminoids
and true mossesa, bog mosses, and lichens
Alnus, Betula, Salix, Pinus pumila (in eastern
Siberia), Eriophorum, Sphagnum
Erect dwarf-shrub tundra Continuous shrubland 2 to 50 cm tall, deciduous or
evergreen, with graminoids, true mossesa, and lichens
Betula, Cassiope, Empetrum, Salix,Vaccinium,
Poaceae, Cyperaceae
Prostrate dwarf-shrub
tundra
Discontinuous “shrubland” of prostrate deciduous dwarfshrubs
0 to 2 cm tall, true mossesa, and lichens
Salix, Dryas, Pedicularis, Asteraceae,
Caryophyllaceae, Poaceae, true mossesa
Cushion forb, lichen, and
moss tundra
Discontinuous cover of rosette plants or cushion
forbs with lichens and true mossesa
Saxifragaceae, Caryophyllaceae, Papaver, Draba,
lichens, true mossesa
Graminoid and forb tundra Predominantly herbaceous vegetation dominated by
forbs, graminoids, true mossesa, and lichens
Artemisia, Kobresia, Brassicaceae,Asteraceae,
Caryophyllaceae, Poaceae, true mossesa
a“true” mosses exclude the genus Sphagnum
294 Arctic Climate Impact Assessment
two meters in height in sheltered valleys at 75º N in
Taymir and stands of balsam poplar north of the treeline
in the northern foothills of the Brooks Range,Alaska,
that are likely to respond rapidly to warming.There are
numerous other types of plant communities, such as the
moss-dominated tundra of Iceland (Fig. 7.22).
The vertical structure of arctic ecosystems is as important
as horizontal structure in explaining their current
and future functioning.This structure is most pronounced
in low-arctic shrub communities, where there is
a well-developed shrub canopy and an understory of
mosses, similar to the vertical structure of boreal forests.
Vertical structure is also pronounced below ground, with
mosses and lichens lacking roots, some species rooted in
the moss layer, others rooted just beneath the mosses,
and a few species rooted more deeply.
The most striking latitudinal trend in plant functional
types is the decrease in height of woody plants (from
trees to tall shrubs, to low and prostrate shrubs, to dwarf
shrubs, and eventually the loss of woody plants with
increasing latitude).These functional types often occur in
low abundance in zones north of their main areas of
dominance, suggesting that they are likely to expand rapidly
in response to warming through vegetative reproduction
(Bret-Harte et al., 2001; Chapin F. et al., 1995) and
sexual reproduction (Molau and Larsson, 2000), although
range expansion will depend on geographic barriers such
as mountains and seas (section 7.6,Table 7.14). Recent
warming in Alaska has caused a substantial increase in
shrub density and size in the moist tundra of northern
Alaska (Sturm et al., 2001b). In areas where shrubs are
absent, shrubs are likely to exhibit time lags in migrating
to new habitats (Chapin F. and Starfield, 1997). Shrubs
colonize most effectively in association with disturbances
such as flooding in riparian zones, thermokarst, and frost
boils (patterned ground formation caused by soil heave)
throughout their latitudinal range, so migration may be
strongly influenced by climate- or human-induced
changes in the disturbance regime.Woody species affect
ecosystem structure and function because of their potential
to dominate the canopy and reduce light availability
to understory species (Bret-Harte et al., 2001; Chapin F.
et al., 1996) and to reduce overall litter quality (Hobbie,
1996) and rates of nutrient cycling.
A similar latitudinal decline in abundance occurs with
sedges, which are absent from polar deserts, suggesting
that this group is also likely to expand northward with
warming (Matveyeva and Chernov, 2000). Carex stans
and C.bigelowii now mark the northernmost boundary of
the tundra zone and might be a sensitive indicator of
species responses to warming. Sedges have important
effects on many ecosystem processes, including methane
flux, because of their transport of oxygen to soils, transport
of methane to the atmosphere, and inputs of labile
carbon to the rhizosphere (Joabsson and Christensen,
2001;Torn and Chapin, 1993). Prostrate and dwarf
shrubs such as Dryas spp., arctic willow, and polar willow
are likely to decline in abundance with warming in the
Fig. 7.22. Racomitrium/Empetrum heath in Iceland showing erosion
(photo:T.V. Callaghan).
Fig. 7.20. Polygonal wet tundra near Prudhoe Bay,Alaska
(photo:T.V. Callaghan).
Fig. 7.21. Thermokarst in the Russian tundra, New Siberian
Islands (photo:T.V. Callaghan).
Chapter 7 • ArcticTundra and Polar Desert Ecosystems 295
low Arctic, due to competition with taller plants, but are
likely to increase in abundance in the current polar
deserts.These changes in distribution are very likely to
substantially reduce the extent of polar desert ecosystems
(section 7.5.3.2), which are characterized by the
absence of woody plants.
7.4.1.2. Response to experimental manipulations
Experimental manipulation of environmental factors
projected to change at high latitudes (temperature,
snow, nutrients, solar radiation, atmospheric CO2 concentrations,
and UV-B radiation levels) has substantial
effects on the structure of arctic ecosystems, but the
effects are regionally variable.The effects of these variables
on individual species are discussed in section 7.3;
while this section focuses on overall community structure
and species interactions.
Plant communities
Nutrient addition is the environmental manipulation that
has the greatest effect on the productivity, canopy
height, and community composition of arctic plant communities
(Jonasson et al., 2001; Press et al., 1998a; van
Wijk et al., 2004; Fig. 7.23). Fertilization also increases
biomass turnover rate, so eventual biomass may or may
not change in response to nutrient addition. In northern
Sweden, for example, nutrient addition to a mountain
birch (Betula pubescens ssp. czerepanovii) site (cf. a Swedish
treeline heath and fellfield) caused an initial biomass
increase.This biomass increase was not maintained over
the long term, however, because expansion of the grass
Calamagrostis lapponica negatively affected the growth of
mosses and evergreen shrubs, leading to a negligible
change in community biomass (Parsons et al., 1994;
Press et al., 1998b). Similarly, addition of nitrogen and
phosphorus at a site in northern Alaska increased productivity
and turnover within three years (Chapin F. et
al., 1995).There was, however, little change in biomass
because the rapidly growing sedges, forbs, and deciduous
shrubs responded most strongly, whereas evergreen
shrubs and mosses declined in abundance (Fig. 7.24).
After 9 and 15 years, competitive interactions altered
the relative abundance of plant functional types, with the
tallest species (the deciduous shrub Betula nana; Fig. 7.3)
responding most strongly (Bret-Harte et al., 2001;
Chapin F. et al., 1995; Shaver et al., 2001). Litter and/or
shade from this species reduced the growth of lichens,
mosses, and evergreen shrubs. In vegetation types without
any pronounced change in relative proportions of
dominant species or life forms following fertilizer addition,
as in Swedish treeline and high-altitude heaths and
in Alaskan wet-sedge tundra, the biomass of most dominant
life forms increased.This resulted in up to a doubling
of biomass after five to nine years of treatment
Fig. 7.23. Results of long-term (generally 10 years or more) experiments in a range of habitats at Toolik Lake,Alaska, and Abisko,
Sweden, showing the responsiveness of aboveground biomass ordered by treatment and degree of responsiveness (X is the mean
value of the analyzed characteristic for the experimental (Xe) and control (Xc) groups). Data are given for (a) total vascular plant biomass
and (b) lichen biomass. Codes relate to the geographical region (To=Toolik,Ab=Abisko), the site name, and the duration of the
experiment (van Wijk et al., 2004).
296 Arctic Climate Impact Assessment
(Jonasson et al., 1999b; Shaver et al., 1998). In polar
semi-deserts, nutrient addition generally had a negative
effect on vascular plants, due to enhanced winterkill, but
stimulated the growth of mosses (Robinson et al.,
1998), an effect opposite to that in low-arctic tundra.
This difference is probably due to the immigration of
nitrogen-demanding mosses from nearby bird-cliff communities
in the high Arctic compared with loss of existing
moss species in the low Arctic.
Water additions to simulate increased precipitation have
generally had only minor effects on total biomass and
production (Press et al., 1998a).
Experimental summer warming of tundra vegetation
within the range of projected temperature increases
(2 to 4 ºC over the next 100 years) has generally led to
smaller changes than fertilizer addition (Arft et al.,
1999; Jonasson et al., 2001; Shaver and Jonasson, 2000;
vanWijk et al., 2004; Fig. 7.23). For example, temperature
enhancement in the high-arctic semi-desert
increased plant cover within growing seasons but the
effect did not persist from year to year (Arft et al.,
1999; Robinson et al., 1998). In the low Arctic, community
biomass and nutrient mass changed little in response
to warming of two Alaskan tussock sites (Chapin F. et
al., 1995; Hobbie and Chapin, 1998) and two wet-sedge
tundra sites (Shaver et al., 1998), coincident with relatively
low changes in soil nutrient pools and net mineralization.Tussock
tundra showed little response to warming,
as some species increased in abundance and others
decreased (Chapin F. and Shaver, 1985a,b; Chapin F. et
al., 1995), similar to a pattern observed in subarctic
Swedish forest floor vegetation (Press et al., 1998b).
The responses to warming were much greater in
Swedish treeline heath and in fellfield (Jonasson et al.,
1999b). Biomass in the low-altitude heath increased by
about 60% after air temperatures were increased by
about 2.5 ºC, but there was little additional effect when
temperatures were further increased by about 2 ºC.
In contrast, biomass approximately doubled after the
first temperature increase (2.5 ºC) and tripled after the
higher temperature increase (an additional 2 ºC) in the
colder fellfield. Hence, the growth response increased
from the climatically relatively mild forest understory
through the treeline heath to the cold, high-altitude
fellfield where the response to warming was of the
same magnitude as the response to fertilizer addition
(Jonasson et al., 2001).A general long-term (10 years or
more) response to environmental manipulations at sites
in subarctic Sweden and in Alaska was a decrease in total
nonvascular plant biomass and particularly the biomass
of lichens (vanWijk et al., 2004: Fig. 7.23).
Animal communities
Air-warming experiments at Svalbard (79º N) had
greater effects on the fauna above ground than below
ground, probably because the soil is more buffered
against fluctuations in temperature and moisture than
the surface (Hodkinson et al., 1998). Species with rapid
life cycles (aphids and Collembola) responded demographically
more quickly than species (e.g., mites) with
slow life cycles (Coulson et al., 1996). Responses to
warming differed among sites.The abundance of
Collembola declined at barren sites where higher temperatures
also caused drought and mortality due to desiccation,
whereas the abundance of Collembola increased
at moister sites. In summer, water availability is probably
much more important to many invertebrates than is
temperature. Mites are more resistant than Collembola
to summer desiccation (Hodkinson et al., 1998) and to
anoxic conditions in winter due to ice-crust formation
following episodes of mild weather (Coulson et al.,
2000). Ice-crust formation during the winter may
increase winter mortality by 50% in Collembola
(Coulson et al., 2000). Freeze–thaw events in spring
may also cause differential mortality among species, thus
altering community composition (Coulson et al., 1995).
In experiments conducted simultaneously at several sites
and over several years, the natural spatial and temporal
variability in community structure and population density
of soil invertebrates was larger than the effects of the
Fig. 7.24. Effects of long-term fertilizer addition and experimental
warming and shading during the growing season on aboveground
net primary production (NPP) of different plant functional
types at Toolik Lake,Alaska, showing NPP by functional type
and treatment (a) in 1983, after three years of treatment, and
(b) in 1989, after nine years of treatment (Chapin F. et al., 1995).
Chapter 7 • ArcticTundra and Polar Desert Ecosystems 297
experimental manipulation within years and sites.
This demonstrates that there is a large variability in the
structure and function of high-arctic invertebrate communities
due to current variation in abiotic conditions.
It also indicates that arctic invertebrate communities can
respond rapidly to change.
Compared to the high Arctic, subarctic invertebrate
communities at Abisko responded less to experimental
temperature increases (Hodkinson et al., 1998).
However, nematode population density increased substantially,
and the dominance changed in favor of plantand
fungal-feeding species with elevated summer temperatures
and nitrogen (N), phosphorus (P), and potassium
(K) fertilization, indicating a shift in the decomposition
pathway (Ruess et al., 1999a,b).
Microbial communities
Although the biomass of microorganisms is a poor predictor
of the productivity and turnover of microorganisms
and their carbon, alternative methods focusing
on population dynamics of microbial species within
communities are extremely difficult to employ in the
field. Microbial biomass has therefore been used to
quantify microbial processes within ecosystems.The sensitivity
of microbial biomass, generally measured as biomass
carbon (C), and nutrient content to changed environmental
conditions in the Arctic has not been well
studied. Long-term addition of easily processed C generally
increases the microbial biomass; and addition of
inorganic nutrients generally, but not always, increases
microbial nutrient content without appreciable effect on
the biomass (Jonasson et al., 1996, 1999b; Michelsen et
al., 1999; Schmidt et al., 2000). In some cases, however,
a combination of C and nutrient addition has led to a
pronounced increase in both microbial biomass and
nutrient content (Schmidt et al., 2000).This suggests a
general C limitation of microbial biomass production,
and increased sink strength for soil nutrients (i.e.,
increased sequestration of nutrients) if the amounts of
both labile C and nutrients increase, but relatively weak
effects from increased nutrient availability alone. In the
widespread drier ecosystem types in the Arctic, the soil
microbial biomass is likely to be further limited by low
water supply.Water addition to a high-arctic semi-desert
led to a substantial increase in microbial biomass C and
microbial activity (Illeris et al., 2003).
Data on the effects on ecosystems of growing-season
temperature increases of 2 to 4 ºC over five (Jonasson
et al., 1999b; Ruess et al., 1999a,b) and ten (Jonasson
and Michelsen, unpub. data, 2005) years have not
shown appreciable long-term changes in microbial biomass
and nutrient stocks.This suggests that an increase
in growing-season temperature alone is unlikely to have
any strong impact on microbial C and nutrient sequestration,
and that changes in soil nutrient availability are
likely to lead to greater changes than the direct effects
of increased temperature.Temperature effects on
ecosystem processes are likely, however, to be different
from the observed relatively small effects on microbial
biomass and nutrient stocks, because temperature
changes are likely to affect rates of decomposition and
nutrient mineralization, rather than pool sizes, resulting
in altered C balance and rates of nutrient supply to the
plants (section 7.4.2.1).
Appreciable seasonality in microbial biomass and nutrient
mass have been reported, however, that seemingly
are independent of ambient temperature. In general, the
masses change little, or fluctuate, during summer
(Giblin et al., 1991; Jonasson et al., 1999b; Schmidt et
al., 1999, 2002). In contrast, pronounced increases in
both biomass and nutrient mass have been reported in
autumn (Bardgett et al., 2002; Jaeger et al., 1999;
Lipson et al., 1999), probably as a function of increased
input of labile C and nutrients from plants as they
senesce, although these data are from mountain and
alpine, rather than from arctic soils.The increase seems
to continue through winter, although at a slower rate
(Grogan and Jonasson, 2003; Schmidt et al., 1999),
despite soil temperatures below 0 ºC (Clein and
Schimel, 1995; see section 7.3.2.3). It is followed by a
sharp biomass decline in the transition between winter
and spring (Bardgett et al., 2002; Brooks et al., 1998;
Grogan and Jonasson, 2003), which may (Brooks et al.,
1998) or may not (Grogan and Jonasson, 2003; Larsen
et al., 2002) coincide with a decrease in microbial N and
an increase in mineralized N, indicating a pronounced
transformation of microbial N to soil inorganic N
(Giblin et al., 1991; Schmidt et al., 1999). Indeed, this
seasonal pattern suggests a temporal partitioning of
resource uptake with low competition between plants
and microbes for nutrients, as microbes absorb most
nutrients in autumn and plants in spring, coincident with
the nutrient release from declining microbial populations.
However, it may also be an indication that plants
compete well for nutrients during the growing season
(Schimel and Chapin, 1996), and microbes access nutrients
efficiently only when the sink strength for nutrients
in plants is low (Jonasson et al., 1999b).
Laboratory experiments have shown that the spring
decline of microbial mass is likely to be an effect of
repeated freeze–thaw cycles (Schimel and Clein, 1996).
Indeed, Larsen et al. (2002) reported a microbial decline
in soils subjected to repeated freezing and thawing but
not in the same soils kept constantly frozen before thawing.The
seasonal dynamics of microbial biomass and
microbial and inorganic soil nutrients therefore suggests
that “off growing-season” changes in climate during the
transition between winter and spring (e.g., changed frequency
of freeze–thaw events and warmer winters) are
likely to have greater impacts on nutrient transformations
between microbes, soils, and plants than changes
during the growing season.
Manipulations simulating enhanced UV-B radiation levels
(equivalent to a 15% reduction in stratospheric ozone
levels) and a doubling of atmospheric CO2 concentrations
for seven years altered the use of labile C sub-
298 Arctic Climate Impact Assessment
strates by gram-negative bacteria (Johnson et al., 2002).
Although these rhizosphere bacteria are a relatively small
component of the belowground microbial biomass, they
are likely to be particularly responsive to environmentally
induced changes in belowground plant C flow.
Ultraviolet-B radiation also affects the structure of fungal
communities. Microcosms of subarctic birch forest
floor litter exposed to enhanced UV-B radiation levels
showed a reduction in fungal colonization of leaf veins
and lamina (Gehrke et al., 1995). Fungal composition
was also altered in the UV-B radiation treatments, with
a reduction in Mucor hiemalis and a loss of Truncatella
truncata. Similar findings of fungal community change
were obtained in subarctic Abisko, in an ecosystem that
was the source of the litter used by Gehrke et al.
(1995). In this field study of the decomposition rates of
a standard litter type, there was also a change in the
composition of the fungal community associated with
litter resulting from elevated UV-B radiation levels
(Moody et al., 2001). So far, no change in plant community
structure has been found in the Arctic in
response to artificially enhanced or reduced UV-B radiation
levels and CO2 concentrations.
7.4.1.3. Recent decadal changes within
permanent plots
Satellite measurements suggest a widespread increase in
indices of vegetation greenness (e.g., the normalized difference
vegetation index – NDVI) and biomass at high
latitudes (Myneni et al., 1997, 2001; Fig. 7.25), although
changes in satellites and sensor degradation may have
contributed to this trend (Fung, 1997).Aerial photographs
show a general increase in shrubbiness in arctic
Alaska (Sturm et al., 2001b) and indigenous knowledge
also reports an increase in shrubbiness in some areas.
These observations are consistent with the satellite observations.
However, it has been difficult to corroborate
these with studies of permanent plots, because of the
paucity of long-term vegetation studies in the Arctic. In
arctic Alaska, for example, a trend toward reduced abundance
of graminoids and deciduous shrubs during the
1980s was reversed in the 1990s (Shaver et al., 2001).
In Scandinavia, decadal changes in vegetation were affected
more strongly by the cyclic abundance of lemmings
than by climatic trends (Laine and Henttonen, 1983).
7.4.1.4.Trophic interactions
Trophic-level structure is simpler in the Arctic than further
south. In all taxonomic groups, the Arctic has an
unusually high proportion of carnivorous species and a
low proportion of herbivores (Chernov, 1995).As herbivores
are strongly dependent on responses of vegetation
to climate variability, warming is very likely to substantially
alter the trophic structure and dynamics of arctic
ecosystems.The herbivore-based trophic system in most
tundra habitats is dominated by one or two lemming
species (Batzli et al., 1980; Oksanen et al., 1997;
Wiklund et al., 1999) while the abundance of phytophagous
(plant-eating) insects relative to plant biomass is
low in arctic tundra (Strathdee and Bale, 1998). Large
predators such as wolves, wolverines, and bears are less
numerous in the tundra than in the boreal forest
(Chernov and Matveyeva, 1997) and predation impacts
on tundra ungulates are usually low.Thus, the dynamics
and assemblages of vertebrate predators in arctic tundra
are almost entirely based on lemmings and other small
rodent species (Microtus spp. and Clethrionomys spp.;
Batzli, 1975;Wiklund et al., 1999), while lemmings and
small rodents consume more plant biomass than other
herbivores. Climate has direct and indirect impacts on
the interactions among trophic levels, but there is
greater uncertainty about the responses to climate
change of animals at the higher trophic levels.
Plant-herbivore interactions
Plant tissue chemistry and herbivory
Arctic and boreal plant species often contain significant
concentrations of secondary metabolites that are important
to the regulation of herbivory and herbivore abundance
(Haukioja, 1980; Jefferies et al., 1994). Secondary
compounds also retard decomposition of leaves after litter
fall (Cornelissen et al., 1999).These secondary
metabolites are highly variable in their chemical composition
and in their antiherbivore effects, both within and
among species. One hypothesis about the regulation of
these compounds that has received widespread discussion
is the carbon–nutrient balance hypothesis of Bryant
et al. (1983; Coley et al., 1985), which attempts to
explain this variation in part on the basis of C versus
nutrient limitation of plant growth.Although many
<5 25 >55
Increase in NDVI (%)
Fig. 7.25. Changes in greenness of northern vegetation
(depicted by NDVI) between 1981 and 1991 as measured by
satellite instruments (Myneni et al., 1997).
Chapter 7 • ArcticTundra and Polar Desert Ecosystems 299
other factors in addition to carbon–nutrient balance are
probably important to the regulation of plant–herbivore
interactions in the Arctic (e.g., Iason and Hester, 1993;
Jefferies et al., 1994; Jonasson et al., 1986), the abundance
of secondary chemicals is often strongly responsive
to changes in the environment including temperature,
light, and nutrient availability (e.g., Graglia et al.,
2001b; Haukioja et al., 1998; Laine and Henttonen,
1987). In a widespread arctic shrub species, Betula nana
(Fig. 7.3), Graglia et al. (2001b) found that fertilization
and shading generally led to decreased condensed and
hydrolyzable tannin concentrations in leaves, whereas
warming in small field greenhouses increased condensed
tannins and decreased hydrolyzable tannins.There was
also a large difference in both the average concentrations
and the responsiveness of the concentrations of phenolics
in plants from northern Alaska versus northern
Sweden, with the plants from Sweden having generally
higher concentrations but being less responsive to environmental
changes. Such data suggest that the effects of
climate change on plant–herbivore interactions are likely
to be highly variable, species-specific, and also dependent
on the nature of the change and on ecotypic or subspecific
differences, perhaps related to local evolution in
the presence or absence of herbivores.
Plant exposure to UV-B radiation has the ability to
change the chemistry of leaf tissues, which has the
potential to affect the odor that herbivores such as
reindeer/caribou use to detect food, and the quality of
food in terms of palatability and digestibility (GwynnJones,
1999). In general, enhanced UV-B radiation
levels can reduce soluble carbohydrates and increase
phenolic compounds and flavonoids. Such changes are
expected to reduce forage quality.
Plant exposure to increased CO2 concentrations can also
affect plant tissue quality and consequently herbivory
(Agrell et al., 1999). Enriched CO2 concentrations may
lead to the accumulation of carbohydrates and phenolic
compounds while reducing N concentrations in leaves.
However, these phytochemical responses can be significantly
modified by the availability of other resources
such as nutrients, water, and light. Unfortunately, little
information about the impacts of increased CO2 concentrations
on herbivory is available for the Arctic.
Herbivore abundance and vegetation production
Invertebrates
Insect population outbreaks seldom extend into the tundra.
However, in the forest near the treeline, insect
defoliators can have devastating impacts on the ecosystem.
Climate change is very likely to modify the population
dynamics of such insects in several ways (Bylund,
1999; Neuvonen et al., 1999). In the autumnal moth,
eggs laid on birch twigs in autumn cannot tolerate winter
temperatures lower than -36 ºC. For this reason, the
moth is destroyed in parts of the terrain (e.g., depressions)
where winter temperatures drop below this critical
minimum (Tenow and Nilssen, 1990;Virtanen and
Neuvonen, 1999).Warmer winters are very likely to
reduce winter mortality and possibly increase outbreak
intensity. Moreover, lower minimum temperatures are
likely to allow the autumnal moth and the related, less
cold-tolerant winter moth (Operophtera brumata) to
extend their geographic distributions into continental
areas with cold winters (Tenow, 1996). However, predicting
the effect of a changing climate is not straightforward
because moth responses are season-specific. For
instance, increasing spring temperatures are likely to
cause a mismatch between the phenology of birch leaves
and hatching of larvae that are currently synchronized
(Bale et al., 2002). Moreover, natural enemies such as
parasitoid wasps and ants are likely to increase their
abundances and activity rates if summer temperature
rises. Currently, there is cyclicity in the populations of
the autumnal moth and outbreak proportions occur
approximately every 10 to 11 years (Tenow, 1972,
1996).The defoliated forests require about 70 years to
attain their former leaf area, although insect outbreaks in
subarctic Finland followed by heavy reindeer browsing
of regenerating birch shoots have led to more or less
permanent tundra (Kallio and Lehtonen, 1973;
Lehtonen and Heikkinen, 1995).There are no population
outbreaks in the autumnal moth further south in
Fennoscandia, most likely due to the high abundance of
generalist parasitoids that keep moth populations below
outbreak levels (Tanhuanpää et al., 2001). However, the
border between outbreaking and non-outbreaking populations
of geometrid moths is likely to move northward
if climate changes.
Enhanced UV-B radiation levels applied to birch leaves
alters the chemistry or structure of the leaves such that
caterpillars eat three times as much leaf biomass to
maintain body development (Buck and Callaghan, 1999;
Lavola et al., 1997, 1998).There is also a tendency for
enhanced UV-B radiation levels to increase the immunocompetence
of the caterpillars, which could possibly
make them more tolerant to the wasp parasitoid (Buck,
1999).Although the effects of winter warming on eggs,
increased UV-B radiation levels on leaves, and immunocompetence
on caterpillars are likely to increase future
damage to subarctic birch forests, it is not known to
what extent other processes susceptible to spring and
summer climate variability may alleviate these effects.
Vertebrates
The herbivore-based trophic system in most tundra habitats
is dominated by one or two lemming species (Batzli
et al., 1980; Oksanen et al., 1997;Wiklund et al.,
1999). Lemming abundance is the highest in coastal tundra,
especially in moist sedge meadows that are the optimum
habitat for Lemmus. Collared lemmings (Dicrostonyx)
usually do not reach as high densities in their preferred
habitats on drier ridges where herbs and dwarf shrubs
dominate.Voles (Microtus and Clethrionomys spp.) are likely
to become more abundant than lemmings in some
low-arctic tundra habitats and forest tundra (Chernov
300 Arctic Climate Impact Assessment
and Matveyeva, 1997).At the landscape scale, lemmings
and voles are very patchily distributed according to the
abundance of their preferred food plants, as well as the
distribution of snow (Batzli, 1975; section 7.3.2.2).
Lemming peak densities exceed 200 individuals per
hectare in the most productive Lemmus habitats in both
Siberia and North America (Batzli, 1981) and the standing
crop of lemmings may approach 2.6 kg dry weight
per hectare.The population builds up during the winter
(due to winter breeding) and peak densities may be
reached in late winter/early spring when the standing
crop of food plants is minimal.The diet of Lemmus consists
mainly of mosses and graminoids, while Dicrostonyx
prefers herbs and dwarf shrubs (Batzli, 1993). Lemmings
have a high metabolic rate, and Lemmus in particular has a
low digestive efficiency (about 30%, compared to 50%
in other small rodents). Consequently, their consumption
rate and impact on the vegetation exceeds that of all
other herbivores combined (with the exception of local
effects of geese near breeding colonies). Moreover, lemmings
destroy much more vegetation than they ingest
and after population peaks typically 50% of the aboveground
biomass has been removed by the time the snow
melts (Turchin and Batzli, 2001). In unproductive snow
beds, which are favored winter habitats of the Norway
lemming (Kalela, 1961), up to 90–100% of the mosses
and graminoids present during the winter may be
removed (Koskina, 1961). If winters become so unfavorable
for lemmings that they are unable to build up cyclic
peak densities, the species-rich predator community
relying on lemmings is likely to collapse (see next subsection).
Moreover, their important, pulsed impact on
vegetation as a result of grazing and nutrient recycling is
likely to cease. Changes in snow conditions, relative
abundances of preferred food plants, and climate
impacts on primary production are all very likely to
affect lemming populations, and are likely to result in a
northward displacement of the climatically determined
geographic borders between cyclic and non-cyclic populations
of small herbivores (small rodents and moths), as
well as the species distributions per se.
Wild populations of other herbivorous mammal species
in the tundra, such as hares, squirrels, muskox, and
reindeer/caribou, never reach population densities or
biomass levels that can compare with peak lemming
populations (Chernov and Matveyeva, 1997). Moving
herds of reindeer/caribou represent only patchy and
temporary excursions in numbers, biomass, and impacts
on vegetation; averaged over space and time some of the
largest herds approach only 0.01 individuals and 0.5 kg
of dry weight per hectare (Batzli, 1981) on their summer
pastures and usually take less than 10% of the vegetation
(Jefferies et al., 1994).The only cases where reindeer/caribou
have been shown to have large impacts on
vegetation seem to be in unusual circumstances (stranding
on islands; Klein, 1968) or under human intervention
(e.g., removing top predators or introductions to
islands) where overshooting reindeer/caribou populations
have led to vegetation destruction, habitat degradation,
and subsequent population crashes.
Although the cooling since the mid-1970s in the Hudson
Bay region has affected the reproduction of snow geese
(Anser caerulescens), the mid-continental population is
currently growing by 5% per year (SkinnerW. et al.,
1998).This, in combination with the staging of snow
geese in La Pérouse Bay, Manitoba, because of bad
weather further north, leads to increasing foraging for
roots and rhizomes of the graminoids Puccinellia phryganodes
and Carex subspathacea (Jefferies et al., 1995).The
rate of removal of belowground organs in the salt marshes
combined with intense grazing of sward during summer
exceeds the rate of recovery of the vegetation. It is
estimated that geese have destroyed 50% of the salt
marsh graminoid sward of La Pérouse Bay since 1985.
This loss of vegetation cover exposes the sediments of
the salt marshes, which have become hypersaline (salinities
exceeding 3.2) as a result of increased evapotranspiration.This
further reduces plant growth and forage
availability to the geese. In turn, this is reducing goose
size, survivorship, and fecundity. Other factors that are
affected by the trophic cascades initiated by the geese
include reduced N mineralization rates and declines in
the populations of soil invertebrates, waders, and some
species of duck such as the widgeon (Anas americana).
Cyclic populations
Herbivore–plant interactions have been proposed to produce
population cycles in arctic herbivores through several
mechanisms including nutrient recycling (Schultz,
1969), production cycles inherent in food plants (Tast
and Kalela, 1971), induced chemical defense in plants
(Haukioja, 1991), and recurrent overgrazing (Oksanen et
al., 1981).The empirical evidence is mixed.There is at
least partly supporting evidence for induced chemical
defense in the Epirrita–birch system (Ruohomäki et al.,
2000) and for overgrazing in the Lemmus–plant system in
unproductive tundra habitats (Turchin et al., 2000).
There is little evidence, however, for mechanisms involving
nutrient cycling and chemical defense in the case of
lemmings and voles (Andersson and Jonasson, 1986;
Jonasson et al., 1986). Climate is somehow involved in
all the hypotheses of population cycles related to
plant–herbivore interactions. For example, allocation
strategies in plants and the amount of secondary compounds
(induced chemical defense hypothesis) depend on
temperature and growing-season length (see plant tissue
chemistry subsection). Plant production and biomass are
also controlled by temperature (overgrazing hypothesis).
Climate change is thus likely to modify the population
dynamics patterns and roles of key herbivores such as
lemmings and moths because the dynamics of herbivoreplant
interactions are likely to change.As early as 1924,
Charles Elton pointed out the potentially decisive role of
climate in determining the generation of cycles in northern
animal populations (Elton, 1924).
Mathematical modeling shows that specialist resident
predators such as small mustelids and the Arctic fox can
also impose prey population cycles due to sufficiently
strong numerical and adequate functional responses (Gilg
Chapter 7 • ArcticTundra and Polar Desert Ecosystems 301
et al., 2003;Turchin and Hanski, 1997). Moreover,
nomadic specialists such as birds of prey can dampen
lemming cycles and decrease the degree of regional
asynchrony if their predation rates are sufficiently high
(Ims and Andreassen, 2000; Ims and Steen, 1990).The
impacts of bird predators have a strong seasonal component
since most migrate south for the winter (Ims and
Steen, 1990). Reliable estimates of predation rates on
cyclic lemming populations are rare. Indirect estimates
based on the energy requirements of predators at Point
Barrow,Alaska, indicated that avian predators could
account for 88% of the early summer mortality, but it
was concluded that neither this nor winter predation by
weasels could stop lemming population growth under
otherwise favorable winter conditions (Batzli, 1981).
In the KarupValley, Greenland, the combined impact of
different predators both limited population growth and
caused population crashes in collared lemmings (Gilg et
al., 2003). In a declining lemming population in an
alpine area in Norway, almost 50% predation was
demonstrated by following the fates of radio-tagged individuals
(Heske et al., 1993). Using the same methodology,
Reid et al. (1995),Wilson D. et al. (1999), and Gilg
(2002) showed that predation was the predominant mortality
factor in populations of collared lemmings at various
locations in northern Canada and eastern Greenland.
Predator–prey interactions
The dynamics and assemblages of vertebrate predators in
arctic tundra are almost entirely based on lemmings and
other small rodent species (Microtus spp. and Clethrionomys
spp.; Batzli, 1975;Wiklund et al., 1999). Birds of prey
such as snowy owls, short-eared owls (Asio flammeus),
jaegers (skuas – Stercorarius spp.), and rough-legged buzzards
(Buteo lagopus) are lemming and vole specialists that
are only able to breed at peak lemming densities and
which aggregate in areas with high lemming densities.
Since lemming cycles are not synchronized over large
distances (Erlinge et al., 1999; Predavec et al., 2001), the
highly mobile avian predators can track lemming population
peaks in space. Mammal lemming and vole specialists
in the Arctic, such as the least weasel (Mustela nivalis)
and the ermine, are less mobile than birds but both have
high pregnancy rates and produce large litters in lemming
peak years (MacLean et al., 1974). In lemming low
years, weasel and ermine reproduction frequently fails
and mortality rates increase (Gilg et al., 2003; Hanski et
al., 2001). In coastal and inland tundra habitats where
bird colonies are lacking, the Arctic fox also exhibits the
population dynamics typical of a lemming specialist
(Angerbjörn et al., 1999).The lemming cycles also
impose cyclic dynamics in other animals such as geese
and waders because they serve as alternative prey for
predators in lemming crash years (Bety et al., 2002;
Sutherland, 1988). Recently observed increased predation
pressure on water birds in various arctic regions
might reflect a change of the lemming cycle in response
to climate change, with secondary effects on predators
and water birds as an alternative prey (Soloviev et al.,
1998; Summers and Underhill, 1987).Thus, a large part
of the tundra vertebrate community cycle is in a rhythm
dictated by the lemming populations (Chernov and
Matveyeva, 1997; Stenseth and Ims, 1993).
This rhythm is likely to be disrupted by projected future
variations in snow properties (e.g., snow-season length,
snow density, and snow-cover thickness;Yoccoz and Ims,
1999). For small mammals living in the subnivean space,
snow provides insulation from low temperatures as well
as protection from most predators such as foxes and raptors
(Hansson and Henttonen, 1988) and increases in
snow are likely to be beneficial.The effect on large mammal
prey species (ungulates) is likely to be the opposite,
as deeper snow makes reindeer/caribou and moose
(Alces alces) more vulnerable to predators such as wolves
(Post et al., 1999), but more extensive snow patches provide
relief from insect pests (section 7.3.3.2). If climate
change results in more frequent freeze-thaw events leading
to a more shallow and icy snowpack (section 6.4.4),
this is likely to expose small mammals to predators,
disrupt population increases, and thereby prevent cyclic
peak abundances of lemmings and voles. For nomadic
predators whose life-history tactic is based on asynchronous
lemming populations at the continental scale, an
increased frequency of large-scale climatic anomalies that
induces continent-wide synchrony (the “Moran effect”;
Moran, 1953) is very likely to have devastating effects.
Long-term monitoring (>50 years) of small rodents near
the treeline at Kilpisjärvi in subarctic Finland has shown
a pronounced shift in small rodent community structure
and dynamics since the early 1990s (Henttonen and
Wallgren, 2001; see Fig. 7.13). In particular, the previously
numerically dominant and cyclically fluctuating
grey-sided vole has become both less abundant and less
variable in abundance.The Norway lemming and
Microtus voles also have lower peak abundances, and the
small rodent community is currently dominated by the
relatively more stable red-backed vole (Clethrionomys
rutilus). Similar changes took place in the mid-1980s in
the northern taiga (Hanski and Henttonen, 1996;
Henttonen, 2000; Henttonen et al., 1987) and are still
prevailing. For predators that specialize in feeding on
small rodents, the lack of cyclic peak abundances of
small rodents, especially in the spring (Oksanen et al.,
1997), is likely to have detrimental consequences, as
they need to breed successfully at least every three to
four years to sustain viable populations.At Kilpisjärvi,
the least weasel has become rare. Moreover, the severe
decline of the Arctic fox and the snowy owl in Fennoscandia,
both of which prey on Microtus voles and lemmings
in mountain and tundra habitats, may be due to
lower peak abundances of small mammal prey species in
their habitats (Angerbjörn et al., 2001). In Alaska, a similar
decrease in lemming cyclicity occurred in the 1970s
(Batzli et al., 1980).
Large predators such as wolves, wolverines, and bears are
less numerous in the tundra than in the boreal forest
(Chernov and Matveyeva, 1997). Consequently, predation
impacts on tundra ungulates are usually low.While 79% of
302 Arctic Climate Impact Assessment
the production in small herbivores (voles, lemmings,
ptarmigan, and Arctic hares) was consumed by predators
averaged across a number of sites in arctic Canada, the
corresponding number was only 9% for large herbivores
(caribou and muskox; Krebs et al., 2003).
Insect pests, parasites, and pathogens
Plants
Disease in plants is likely to increase in those parts of
species distribution ranges where a mismatch between
the rate of relocation of the species and the northward/
upward shift of climatic zones results in populations
remaining in supra-optimal temperature conditions.
Under these conditions, species can experience thermal
injury (particularly plants of wet and shady habitats;
Gauslaa, 1984), drought, and other stresses that make
plants more susceptible to disease.
Very little is known about the incidence and impacts of
plant diseases in arctic ecosystems. However, recent
work has shown that a fungal pathogen (Exobasidium) of
Cassiope tetragona and Andromeda polifolia reduces host
plant growth, reproductive investment, and survival
(Skinner L., 2002).As the incidence of disease increases
with an increase in temperature downward along an altitudinal
gradient, climate warming is likely to increase
the incidence of at least this naturally occurring disease
in the Arctic.The incidence of new diseases from
increasing mobility of pathogens with a southern distribution
is a possibility.
Animals
Ultraviolet-B radiation can reduce the impact of viral
and fungal pathogens on insects.The nuclear polyhedrosis
virus is a major cause of death of the defoliating
autumnal moth. However, this virus is killed by UV-B
radiation (Killick andWarden, 1991). Species and strains
of the fungus Metarhizium are important agents of insect
disease, but some, particularly high-latitude strains, are
sensitive to UV-B radiation (Braga et al., 2001a,b).
Parasitism is perhaps the most successful form of life,
but until recently has been underestimated, especially in
the Arctic (Henttonen and Burek, 2001; Hoberg et al.,
2003). Parasitism in the Arctic has been poorly studied
with respect to both taxonomy and biodiversity as well
as the ecological impact parasites may have on animal
species and communities.
Recent research on the evolution and phylogeography of
typical arctic animals like lemmings has revealed how
greatly the alternating glacial and interglacial periods
have influenced their distribution and genetic diversity
(Fedorov et al., 1999a,b).The impact seems to be at
least as profound on the helminth parasites of arctic
rodents (Haukisalmi et al., 2004; Hoberg et al., 2003).
Such impacts of past climatic fluctuations can be used to
project some possible consequences of the present
warming. If the arctic host populations become fragmented
due to the northward expansion of southern
biogeographic elements, extinction of parasites in small
host populations and/or cryptic speciation (isolation
events seen in parasites, often only by using molecular
methods, that are not evident in host populations) in
refugia are likely to follow. Phylogeographic structure
(often cryptic speciation) can be seen in rodent cestodes
in the Arctic even if there is no such structure in the
host.This is true also for ruminant parasites.
Phylogenetic studies have shown that host switches have
occurred in many clades of rodent cestodes (Wickström
et al., in press). It seems plausible that host switches
have been promoted by climatic events that force host
assemblages, earlier separated by geography or habitat,
to overlap in their distribution.
Macroparasites, such as intestinal worms, often have
complicated life cycles. In the main host, in which the
parasite reproduces, parasites are controlled by host
immunity. On the other hand, the free-living intermediate
stages (eggs and larvae), and those in intermediate
hosts, are subject to extrinsic environmental conditions
like temperature and humidity.Temperature strongly
affects the development rate of parasite larvae. For
example, a small increase in temperature has a clear
effect on the development of the muskox lungworm
Umingmakstrongylus pallikuukensis in its gastropod intermediate
hosts (Kutz et al., 2002).Therefore, a slight
increase in temperature and in growing-season length is
very likely to profoundly affect the abundance and geographic
distribution of potentially harmful parasites such
as lungworms. Lungworm infections have become conspicuous
in recent years as summer temperatures in the
Arctic have increased.
The free-living stages of parasites are prone to desiccation.
In addition to temperature effects on their development,
the survival and abundance of free-living intermediate
stages depend greatly on humidity. In addition, the
same factors affect drastically the abundance, survival,
and distribution of the intermediate hosts of parasites,
like insects, gastropods, and soil mites. Haukisalmi and
Henttonen (1990) found that precipitation in early summer
was the most important factor affecting the prevalence
of common nematodes and cestodes in Clethrionomys
voles in Finnish Lapland.Temperature and humidity
also affect the primary production and development
of the free-living stages of abomasal nematodes of reindeer/caribou
(Irvine et al., 2000). Recently,Albon et al.
(2002) showed that abomasal nematodes affect the
dynamics of Svalbard reindeer through fecundity.
Consequently, even slight climatic changes are likely to
have surprising effects on the large ungulates, and possibly
on humans exploiting them, through enhanced parasite
development (Chapter 15).
The complicated life cycles of parasites cause intrinsic
lags in their capacity to track the changes in the population
density of their hosts, and these lags are further
Chapter 7 • ArcticTundra and Polar Desert Ecosystems 303
retarded by unfavorable arctic conditions.Any climatic
factor promoting the development of a parasite, so that
it can respond in a density-dependent way to host
dynamics, is likely to alter the interaction between parasite
and host, and the dynamics of both.
There is considerable uncertainty about the possibilities
for invasion of pathogens and parasites into the Arctic as
a result of climate warming (but see section 15.4.1.2).
However, increased tourism combined with a warmer
climate could possibly increase the risk of such invasions.
Climate change is likely to affect the important interaction
between parasitic insects and reindeer/caribou.
Insect harassment is already a significant factor affecting
the condition of reindeer/caribou in the summer
(section 7.3.3.2).These insects are likely to become
more widespread, abundant, and active during warmer
summers while many refuges for reindeer/caribou on
glaciers and late snow patches are likely to disappear.
Microbe–plant and microbe–microbivore
interactions
Although data on the dynamics and processes in arctic
microbial communities and on processes in the soil–
microbial–plant interface are accumulating rapidly, it is
not yet possible to reach firm conclusions about how
the dynamics and processes will change in a changing
climate. However, the following can be stated. First,
short-term (seasonal) changes in microbial processes
may not necessarily have major influences on longerterm
(annual to multi-annual) processes. Second,
microbes and plants share common nutrient resources,
although they may not be limited by the same resource.
For example, while nutrient supply rates generally control
plant productivity, microbial productivity may be
constantly or periodically controlled by the abundance
of labile C.Third, the nutrient supply rate to the pool
available to plants may not be controlled principally by
continuous nutrient mineralization, but rather by pulses
of supply and sequestration of nutrients linked to
microbial population dynamics and abiotic change, such
as freeze–thaw cycles.
Jonasson et al. (1999b) showed that despite no appreciable
effect on the microbial biomass and nutrient
mass, warming increased plant productivity. Because
plant productivity was limited principally by a low rate
of N supply, it appears that the mineralization of litter
or soil organic matter, or microbial solubilization of
organic N, increased, and that the plants rather than the
microbes sequestered the “extra” N in inorganic or
organic form. However, microbes increased their nutrient
content in cases when the sink strength for nutrients
in the plants decreased (e.g., after shading) at the
same time as soil inorganic N also increased.This suggests
either that plants compete successfully with
microbes for nutrients, or that the microbial requirement
for nutrients was satisfied, and the microbes
absorbed a “surplus” of nutrients, which is likely if they
were limited by C rather than nutrients.This does not
fully preclude nutrient competition, however, because it
is possible that the plants accessed the nutrients from
pulse releases from microbes during periods of population
dieback. If so, seasonal changes in the frequency of
such pulses are of importance for projecting changes in
ecosystem function and need further investigation.
This is particularly obvious, considering that the microbial
N and P content typically exceeds the amounts
annually sequestered by plants several-fold, and should
constitute an important plant nutrient source (Jonasson
et al., 1999a, 2001).
The plant–microbe interaction may also be mutualistic
through the mycorrhiza by which the fungal partner supplies
nutrients to the plant in exchange for C supplied
by the plant.A large proportion of the plant species in
shrubby vegetation, common in the Arctic, associate
with ecto- or ericaceous mycorrhizal fungi.These mycorrhiza
types have enzyme systems able to break down
complex organic molecules and thereby supply the plant
partner with N (Read et al., 1989), the most common
production-limiting element for plants. Changes in plant
species composition as a consequence of climatic
changes are very likely to substantially affect microbial
community composition, including that of mycorrhizal
fungi. Unfortunately, studies of the effects of projected
climate change on mycorrhizal associations in the Arctic
are virtually nonexistent. However, a decade of warming
of a fellfield led to a substantial increase in willow (Salix)
biomass, but few changes in the community of associated
ecto-mycorrhizal fungi (Clemmensen and Michelsen,
unpub. data, 2004).
The effects of microbivores on the microbial community
are yet poorly explored and can only be listed as
potentially important for projecting effects of global
change. It appears, however, that populations of nematodes
increase strongly with warming. Because nematodes
are the main predators of fungi and bacteria, it
may be that increased biomass production of microbes
is masked in a warmer environment because of predation
by strongly responding microbivores (Ruess et al.,
1999a,b). If so, the release rate of plant-available nutrients
is likely to increase (e.g., Ingham et al., 1985),
which may explain the enhanced nutrient sequestration
by plants in warmer soils (rather than pulse sequestration
after microbial dieback).
7.4.1.5. Summary
Changes in climate and UV radiation levels are very
likely to affect three important attributes of ecosystem
structure: spatial structure (e.g., canopy structure and
habitat), trophic interactions, and community composition
in terms of biodiversity. Ecosystem structure varies
along a latitudinal gradient from the treeline to the
polar deserts of the high Arctic.Along this gradient
there is a decreasing complexity of vertical canopy
structure and ground cover ranging from the continuous
and high canopies (>2 m) of the forest tundra in the
304 Arctic Climate Impact Assessment
south to the low canopies (~5 cm) that occupy less than
5% of the ground surface in the polar deserts.Within
each arctic vegetation zone, there are often outliers of
more southerly zones. Changes in vegetation distribution
in relation to climate warming are likely to occur
by local expansion of these intrazonal communities and
northward movement of zones. Satellite measurements,
aerial photographs, and indigenous knowledge show a
recent increase in shrubbiness in parts of the Arctic.
Experimental manipulations of environmental factors
projected to change at high latitudes show that some of
these factors have strong effects on the structure of arctic
ecosystems, but the effects are regionally variable.
Nutrient addition has the greatest effect on the productivity,
canopy height, and community composition of
arctic plant communities. Nutrients also increase biomass
turnover, so biomass may or may not respond to
nutrient addition. Summer warming of tundra vegetation
within the range of projected temperature increases
(2 to 4 ºC over the next 100 years) has generally led
to smaller changes compared with fertilization and
always to greater responses compared with irrigation.
Plant growth response increased from a climatically relatively
mild forest understory through a treeline heath
to a cold, high-altitude fellfield.Total nonvascular plant
biomass and particularly the biomass of lichens
decreased in response to 10 years or more of environmental
manipulations at sites in subarctic Sweden and in
Alaska.Warming experiments in the high Arctic had a
greater effect on the fauna above ground compared with
fauna below ground and in the subarctic. Spring
freeze–thaw events are important, and will probably
cause differential mortality among species, thus altering
community composition. In general, arctic invertebrate
communities are very likely to respond rapidly to
change. In contrast, long-term data on the effects of
summer warming (2–4 ºC) of ecosystems have not
shown appreciable changes in microbial biomass and
nutrient stocks.This suggests that a temperature
increase alone is unlikely to have any strong impact on
microbial carbon and nutrient sequestration. Manipulations
simulating enhanced UV-B radiation levels and a
doubling of atmospheric CO2 concentration for seven
years altered the use of labile carbon substrates by
gram-negative bacteria, suggesting a change in community
composition. UV-B radiation also affects the structure
of fungal communities. So far, no change in plant
community structure has been found in the Arctic in
response to manipulations of UV-B radiation levels and
CO2 concentration.
Trophic interactions of tundra and subarctic forest
plant-based food webs are centered on a few dominant
animal species, which often have cyclic population fluctuations
that lead to extremely high peak abundances in
some years. Small herbivorous rodents of the tundra
(mainly lemmings) are the main trophic link between
plants and carnivores. Small-rodent population cycles
with peak densities every three to five years induce
strong pulses of disturbance, energy, and nutrient flows,
and a host of indirect interactions throughout the food
web. Lemming population cycles are crucial for nutrient
cycling, structure and diversity of vegetation, and
for the viability of a number of predators and parasites
that are specialists on rodent prey/hosts.Trophic interactions
are likely to be affected by climate change.
Ice crusting in winter is likely to render vegetation
inaccessible for lemmings, deep snow is likely to render
rodent prey less accessible to predators, and increased
plant productivity due to warmer summers is likely to
dominate food-web dynamics. Long-term monitoring of
small rodents at the border of arctic Fennoscandia provides
evidence of pronounced shifts in small rodent
community structure and dynamics that have resulted in
a decline in predators (including Arctic fox, snowy
owls, buzzards, and skuas) that specialize in feeding on
small rodents.
In subarctic forests, a few insect defoliators such as the
autumnal moth that exhibit cyclic peak densities at
approximately 10-year intervals are dominant actors in
the forest food web.At outbreak densities, insects can
devastate large tracts of birch forest and play a crucial
role in forest structure and dynamics.Trophic interactions
with either the mountain birch host or its insect
parasitoids are the most plausible mechanisms generating
cyclic outbreaks in Epirrita. Climate is likely to alter
the role of Epirrita and other insect pests in the birch
forest system in several ways.Warmer winters are likely
to increase egg survival and expand the range of the
insects into areas outside their present outbreak ranges.
However, the distribution range and activity of natural
enemies are likely to keep the insect herbivore populations
below outbreak densities in some areas.
Climate change is likely to also affect the important
interaction between parasitic insects and reindeer/
caribou. Insect harassment is already a significant factor
affecting the condition of reindeer in the summer.
These insects are likely to become more widespread,
abundant, and active during warmer summers while
refuges for reindeer/caribou on glaciers and late snow
patches are likely to disappear.There are large uncertainties
about the outcome of the potential spread of
new trophic interactants, especially pests and
pathogens, into the Arctic.
Disease in plants is likely to increase in those parts of
species distribution ranges where a mismatch between
the rate of relocation of the species and the northward/
upward shift of climatic zones results in populations
remaining in supra-optimal temperature conditions.The
incidence of new diseases from increasing mobility of
pathogens with a southern distribution is a possibility,
but increases in UV-B radiation levels could possibly
reduce the impact of viral and fungal pathogens.
Microbe–plant interactions can be competitive for
nutrients and also mutualistic through mycorrhizal associations.Warming
will probably affect both types of
relationship, but information is scarce.
Chapter 7 • ArcticTundra and Polar Desert Ecosystems 305
7.4.2. Ecosystem function
7.4.2.1. Biogeochemical cycling: dynamics of
carbon and nutrients
Arctic ecosystems are characterized by low primary productivity,
low element inputs, and slow element cycling,
yet they tend to accumulate organic matter, C, and other
elements because decomposition and mineralization
processes are even more strongly limited than productivity
by the arctic environment, particularly the cold, wet
soil environment (Jonasson et al., 2001). Because of this
slow decomposition, the total C and element stocks of
wet and moist arctic tundra frequently equal and may
exceed the stocks of the same elements in the much
more productive systems of temperate and even tropical
latitudes (Table 7.9).
Low-arctic sites with warmer and dryer soils, and
extremely unproductive high-arctic polar deserts and
semi-deserts, have smaller accumulations of organic matter
(Table 7.10). Most of the organic matter and element
accumulation occurs in soils, while large accumulations
of biomass are limited by a lack of tall woody
plant forms such as trees; by selection for slow-growing,
low, compact plant forms; and by low productivity and
low availability of soil-available elements such as N or P.
Typically, the majority of the biomass consists of roots
and belowground stems, with aboveground plant mass
accounting for less than one-third, and sometimes only
5 to 10%, of the total.
In addition to the large C stocks within the seasonally
thawed active layer of the soil (Table 7.10), an equally
large pool of organic C may be held in the upper permafrost,
within 1 to 2 m of the surface (Michaelson et al.,
1996).While these frozen C stocks are not actively
involved in C cycling on a seasonal or yearly basis, in the
long term, they represent an important C sink, and they
are likely to be of particular importance if climate
change leads to greater soil thawing or to loss of permafrost
(section 6.6.1.3).
The largest body of information on organic matter, C,
and nutrient budgets of a wide range of arctic ecosystems
comes from the IBPTundra Biome program, which
took place during the late 1960s and early 1970s (Bliss
et al., 1981). Since then, research on arctic element
cycling has tended to focus on controls over individual
biogeochemical processes rather than on comparisons of
overall budgets and element stocks.The recent surge of
interest in climate change and feedbacks from the Arctic
to the globe has highlighted the relevance and utility of
those earlier studies, particularly as currently only a few
sites are being studied at the whole-system level.
Microbes in arctic soils contain only one or a few percent
of the ecosystem C pool. However, the proportions
of ecosystem N and P are appreciably higher due to high
concentrations of N and P in the microbial tissue compared
to the concentrations in plants and soil organic
matter (Jonasson et al., 1999a).As a proportion of the
total soil organic matter, microbial biomass and nutrient
content are similar to ecosystems outside the Arctic, but
as a proportion of the total organic matter in the ecosystem
(soil plus vegetation) microbial biomass and nutrient
content are high in comparison with other systems
due to the relatively small vegetation component in the
Arctic. Data from various arctic and subarctic sites have
shown that microbes commonly contain appreciably less
C, slightly less or comparable amounts of N, and much
higher amounts of P than the entire plant biomass
(Cheng andVirginia, 1993; Hobbie and Chapin, 1996;
Jonasson et al., 1996, 1999a; Schmidt et al., 2002).
Nutrient mineralization rates are low, however: typically
ten-fold lower than in the boreal region.The low rate is
mainly due to low soil temperatures, and it leads to low
supply rates of nutrients to the plant-available pool and
nutrient-constrained plant productivity in most arctic
ecosystems (Nadelhoffer et al., 1992).The combination
of low mineralization rates and high proportions of
nutrients in microbes compared to plants leads to possible
competition for nutrients between microbes and
plants during periods of rapid microbial growth (Kaye
and Hart, 1997). However, microbes are likely to also
release a pulse of nutrients during periods of population
decline when the cells are lysed and nutrients are
leached (Giblin et al., 1991; Jonasson et al., 2001).
To project microbial effects on nutrient-constrained
plant productivity as a result of environmental changes,
it is essential to understand not only how the microbial
processing rate of organic matter will change, but also
the controls on microbial population sizes and how
Table 7.9. Average carbon (C) pools and total C in arctic and alpine tundra, the neighboring boreal zone, and all terrestrial ecosystems.The
soil pools do not include organic C in permafrost beneath the seasonally thawed active layer (from Jonasson et al., 2001,
after data in McGuire et al., 1997).
Area
(106 km2)
Soil
(g C/m2)
Vegetation
(g C/m2)
Soil:
Vegetation
ratio
Total C (1012 kg)
Soil Vegetation Soil and
vegetation
Arctic and alpine tundra 10.5 9200 550 17 96 5.7 102
Boreal woodlandsa
6.5 11750 4150 2.8 76 27 103
Boreal forest 12.5 11000 9450 1.2 138 118 256
Global terrestrial 130.3 5900 7150 0.8 772 930 1702
acomparable to forest tundra
306 Arctic Climate Impact Assessment
changes in the populations affect nutrient cycling and
interact with plant processes (see also section 7.4.1.4).
Spatial variability
Although the productivity of the most productive tundra
may rival that of highly productive shrub and marsh
systems at lower latitudes, most arctic systems lie at the
low end of the global productivity range.What is striking
is the wide range of variation (about three orders of
magnitude) in NPP and standing stocks of organic matter
in soils and vegetation within the Arctic (Table
7.10). In general, productivity and organic matter
stocks decrease with temperature and precipitation
from south to north, but local variation in productivity
in relation to topography is dramatic (often 10- to
100-fold).Among the most important correlates of
topographic variation in productivity are the duration
and depth of winter snow cover and degree of protection
from winter wind damage, as well as variation in
soil moisture, soil thaw, and soil temperature. Local
variation in these factors can be nearly as great as that
across a wide range of latitudes (Billings, 1973;
Jonasson et al., 2001; Shaver et al., 1996). Local variation
in productivity is also associated with dramatic
shifts in the relative abundance of plant functional types
including both vascular and nonvascular plants (sections
7.3 and 7.4.1). Because of this dramatic local variability,
primary production and organic matter accumulation
are distributed in a mosaic fashion across the Arctic,
with a higher frequency of more productive sites
(usually wet or moist lowlands) at lower latitudes.
The spatial distribution of productivity and organic
matter in the Arctic is broadly predictable in relation to
temperature, soil moisture, and other soil factors such
as pH, topography, and snow cover (Walker D. et al.,
1998, 2002;Walker M.D. et al., 1989, 1994).The proximate
controls on C cycling in these ecosystems, however,
are much more closely tied to the inputs and
turnover of other elements, especially N and P (Shaver
et al., 1992). Because N and P inputs (by deposition,
fixation, or weathering) are low, even where rapid
photosynthesis is possible C cannot be stored in organic
matter any faster than the rate of N or P accumulation.
Thus, for example, in the Canadian High Arctic where a
large portion of the surface is bare ground, C fixation
and accumulation is closely tied to low, wet areas where
anaerobic soil conditions favor N fixation (Gold and
Bliss, 1995). In other arctic systems, such as Alaskan
wet and moist tundras, the total amounts of N and P in
soil organic matter may be very high but, due to slow
rates of decomposition, the availability of these elements
to plants is low, leading to low productivity
despite high element and soil organic matter stocks
(Giblin et al., 1991).
Temporal variability
Interannual variation in biogeochemical cycles has
received little attention in the Arctic, although a few
multi-year records of ecosystem C exchange, N deposition,
and C and N losses at the watershed or catchment
level do exist (Hershey et al., 1997; Kling et al., 2000;
Oechel et al., 2000a; Steiglitz et al., 2000). Clearly, the
trend over the Holocene, at least, has been one of overall
accumulation of elements in organic matter since the
loss of the glacial ice cover, but the variation in rates of
accumulation (or loss) at the scale of years to decades is
particularly poorly understood.
The net C balance of arctic ecosystems, as in any terrestrial
ecosystem, may be positive or negative depending
on the timescale over which it is measured and the
environmental conditions during the measurement period.This
dynamic balance is called net ecosystem production
(NEP3) and is defined as the difference between
two large, opposing fluxes: gross ecosystem production
Table 7.10. Soil organic matter, plant biomass, and net primary production (NPP) in the primary arctic ecosystem types (after
Jonasson et al., 2001; based on data from Bliss and Matveyeva, 1992 and Oechel and Billings, 1992).
Soil organic
matter (g/m2)
Vegetation
biomass
(g/m2)
NPP
(g/m2/yr)
Soil:
Vegetation
Ratio
Soil:
NPP Ratio
(yr)
Vegetation:
NPP Ratio
(yr)
Percentage
of total area
High Arctic
Polar desert 20 2 1 10 20 2.0 15
Semi-desert 1030 250 35 4.1 29 7.1 8
Wet sedge/mire 21000 750 140 28 150 5.4 2
Low Arctic
Semi-desert 9200 290 45 32 204 6.4 6
Low shrub 3800 770 375 4.9 10 2.1 23
Wet sedge/mire 38750 959 220 40 176 4.3 16
Tall shrub 400 2600 1000 0.2a
0.4 2.6 3
Tussock/sedge dwarf shrub 29000 3330 225 8.7 129 16 17
asource data for this value, which is surprisingly low, have not been found
3The biological definition of NEP results in positive values when carbon accumulates in the biosphere, whereas measurements of carbon fluxes and the
concept of carbon sinks and sources use a convention of negative fluxes when carbon accumulates in the biosphere.
Chapter 7 • ArcticTundra and Polar Desert Ecosystems 307
(GEP, or gross ecosystem photosynthesis) and ecosystem
respiration (RE), both measured in units of C (mass
or moles) per unit area and time. Ecosystem respiration
has two major components, RA (autotrophic or plant
respiration) and RH (heterotrophic or animal plus
microbial respiration). Each of these components of
NEP has different relationships to current temperature,
moisture, and light conditions. Because they are measured
at the whole-system level, all three components
are also a function of the current functional mass or
surface area of the organisms as well as their current
nutritional status.Thus, for example, even though longterm
NEP must be positive for the large C accumulations
in tundra ecosystems to occur, on a daily or seasonal
basis NEP swings from strongly negative (at
“night”, even under the midnight sun, or in winter) to
strongly positive (at midday in midsummer).These daily
and seasonal fluctuations have been measured at an
increasing number of arctic sites in recent years
(Nordström et al., 2001; Oechel et al., 2000a; Søgaard
et al., 2000;Vourlitis and Oechel, 1997, 1999).
Carbon balance may also vary sharply among years, and
may be either positive or negative on an annual basis.
Recent work in Alaska (Oechel et al., 2000a) indicates
that although C was accumulating in wet and moist tundras
in the 1960s and 1970s (i.e., “negative” C balance),
during much of the 1980s and 1990s there was a net loss
of C from these ecosystems in both winter and summer
(i.e., “positive” C balance). In the late 1990s, the summer
C balances changed again so that the ecosystems were net
sinks, but it is not yet clear whether the C balances for
the full year have returned to net C accumulation (Fig.
7.26; see also section 7.5.1.1). It is also unclear whether
the shifts in NEP (from C sink to C source) that have
occurred over the past 40 years are related in any direct
way to weather, because the entire period has been one
of general warming in northern Alaska (section 7.5).
Modeling studies (e.g., Clein et al., 2000; McGuire et
al., 2000; McKane et al., 1997) suggest that in the short
term (within one or a few years) the response of RE
(both RA and RH) to temperature is more rapid than the
response of GEP, leading to a short-term loss of C with
warming. In the long term, however, the interaction
between temperature and soil nutrient availability might
increase GEP sufficiently to cause an eventual return to
net C accumulation.There is also evidence from manipulation
experiments (Shaver and Chapin, 1991; Shaver et
al., 1998) and latitudinal gradients (Callaghan and
Jonasson, 1995) that increases in air temperature can
result in soil cooling after long periods, as higher air temperatures
lead to increased leaf area indices that intercept
a greater proportion of incoming radiation before it
reaches the soil surface, thereby leading to soil cooling.
Species and functional type composition of the vegetation
are keys to long-term change in productivity,
because of differences in nutrient use and allocation,
canopy structure, phenology, and relative growth rates
among plants (Chapin F. et al., 1993; Hobbie, 1995).
Large differences exist, for example, in the rate at which
tundra plants can respond to changes in weather and
climate, due to differences in allocation to stems versus
leaves or to secondary chemistry versus new growth
(Shaver et al., 2001), in the ability to add new meristems
(Bret-Harte et al., 2001), and in the constraints on
the amount of growth that can be achieved by a single
meristem within a single year (i.e., determinate versus
indeterminate growth). Species and functional types also
differ in their growth phenology and thus in their ability
to take advantage of a change in the timing and duration
of the growing season. For example, moss-dominated
ecosystems in Iceland have limited ability to respond to
climate change without a complete change to a vascular
plant-dominated community (Jónsdóttir et al., 1995),
whereas shrubs and small trees already present in sheltered,
moist depressions on the North Slope of Alaska
seem to be already expanding their distribution (Sturm
et al., 2001b) and therefore productivity.
The chemical composition of primary productivity
(leaves versus wood, secondary chemistry, species composition)
is important as a long-term feedback to producFig.
7.26. Long-term trends in (a) summer net CO2 flux,
(b) average summer temperature, (c) total summer precipitation,
and (d) summer precipitation (P) less potential evapotranspiration
(PET) for Alaskan coastal wet-sedge tundra (Oechel et al.,
1993, 2000b).
308 Arctic Climate Impact Assessment
tivity and its responsiveness to climate change. It is also
important in terms of both animal community composition
and secondary (herbivore) productivity. Carbon to
nitrogen ratios; lignin and protein content; and tannin,
resin, and phenolic content are all important in determining
forage quality (section 7.4.1.4) and the susceptibility
of plant litter to decomposition, and thus the remineralization
of essential limiting nutrients like N and P
(Hobbie, 1996).
Despite the critical importance of NPP together with
NEP and the considerable research already conducted on
these parameters, additional field measurements and
focused process studies are needed to resolve issues
relating to the different methodologies used for measuring
NPP and NEP (Hobbie et al., 2000;Williams et al.,
2000). Results from different methodologies also need
to be reconciled (section 7.7.1.1).
Budgets of N and P were developed for several arctic
sites during the IBP studies 30 years ago (Chapin F. et
al., 1980) but complete documentation of inputs and
outputs of any element other than C has not been
attempted since then for any arctic site. Part of the
problem is that individual N and P inputs and outputs in
arctic ecosystems, such as N fixation, N deposition, denitrification,
rock weathering, or losses in streamflow
(see next subsection), are even smaller than the amounts
annually recycled by mineralization of organic matter
(Fig. 7.27; Nadelhoffer et al., 1991; Peterson et al.,
1992; Shaver et al., 1992), except perhaps in the high
Arctic (Gold and Bliss, 1995).Thus, very long-term
records are needed to evaluate the significance of interannual
variation in N and P budgets, while most studies
of the component processes last only one to three years.
Inputs/outputs, primary production, and net
ecosystem production
The dominant C input to arctic ecosystems is from
photosynthesis in vascular and nonvascular plants, which
in total comprises GEP.The relative (apparent) importance
of various controls on primary production differs
depending on the level (leaf, canopy, or whole vegetation)
and timescale (daily, seasonal, or decadal) of examination
(Williams et al., 2001). Carbon inputs at the leaf level are
clearly limited in the short term by generally low irradiance
and consequent low temperatures during usually
short, and late, growing seasons (Fig. 7.8), despite a wide
range of specific photosynthesis-related adaptations to the
arctic environment (section 7.3). Photosynthesis in arctic
plants is also often sensitive to changes in CO2 concentrations
(in the short term), moisture conditions, and snow
(UV radiation effects are variable and comparatively
small).Although arctic plants in general are well adapted
to the arctic climate, there is considerable variation in the
responses of photosynthesis to microclimate among plant
functional types. In the longer term and at the level of
whole vegetation canopies, however, C inputs are limited
by generally low canopy leaf areas, leaf phenology and
duration, and light interception (Williams et al., 2001).
Canopy leaf area is low because low soil nutrient availability,
particularly N, limits the ability of the vegetation to
develop a large, photosynthetically efficient leaf area
(Williams and Rastetter, 1999), and it also limits the ability
of the vegetation to use newly fixed C for new growth,
because growth requires adequate supplies of multiple
elements in addition to C (Jonasson et al., 2001; Shaver
et al., 1992). It is also low because of the low stature of
the vegetation, which prevents development of a multilayered
canopy. Other environmental factors such as wind
and soil disturbance also limit C gain. Storage of photosynthate
and nutrients acquired in previous years plays a
key role in determining the current year’s productivity
(Chapin F. and Shaver, 1985b).
Carbon outputs from arctic ecosystems occur via a
wider array of processes and are regulated very differently
from C inputs (section 7.4.2.2).The dominant
form of C loss is as CO2, produced by both plants and
soil biota.Autotrophic or plant respiration (RA) typically
accounts for about half of GEP on an annual basis
(Williams et al., 2000, 2001) but follows a very different
seasonal and daily pattern (discussed previously).
Fig. 7.27. Nitrogen budget for wet-sedge tundra at Barrow,Alaska. Numbers in boxes are N stocks in g/m2; numbers in parentheses are N
fluxes in g/m2/yr (Shaver et al., 1992; adapted from Chapin F. et al., 1980).
Chapter 7 • ArcticTundra and Polar Desert Ecosystems 309
Heterotrophic respiration (RH), mostly by soil organisms,
accounts for most of the other half of GEP,
although in the long term the sum of RA and RH must be
slightly less than GEP if C is to accumulate in soil organic
matter. Heterotrophic respiration produces both CO2
and methane (CH4), the latter produced anaerobically in
wet soils (section 7.4.2.2). Much of the CH4 produced
in arctic soils is oxidized to CO2 before it reaches the
atmosphere; net CH4 emissions thus are normally only a
fraction of CO2 emissions from arctic soils (less than
5%), but CH4 is a much more powerful greenhouse gas
than CO2. Other aspects of C balance are important yet
difficult to quantify. Examples are plant root respiration,
the sloughing of dead material from roots, root exudation,
and the growth and respiration of microorganisms
intimately associated with plant roots.
Most of the respiratory CO2 and CH4 losses from arctic
systems move directly to the atmosphere. Significant
fractions of these gases, however, travel in dissolved
forms in soil water, eventually reaching streams and
lakes where they are released to the atmosphere (Kling
et al., 1991, 1992). In addition, soil and surface waters
contain significant amounts of dissolved organic forms of
C, much of which is eventually consumed by aquatic
microbes, producing more CO2 (Chapter 8).Together,
these losses to aquatic systems may add up to a significant
component of the net C balance of arctic systems.
Synoptic, simultaneous analysis of aquatic C losses at the
same time, place, and scale as direct atmospheric
exchanges has not been completed, but estimates of
aquatic C losses suggest that these may equal as much as
20 to 30% of GEP.
Winter CO2 losses are a second major gap in the understanding
of C losses from arctic ecosystems.Although
winter CO2 losses have long been recognized (Coyne and
Kelley, 1971), more recent research indicates that these
losses are greater than was previously thought and may be
the product of significant respiratory activity during the
winter (Fahnestock et al., 1999; Hobbie et al., 2000;
Oechel et al., 1997;Welker et al., 2000; section 7.3.2.3),
when recently fixed C is respired (Grogan et al., 2001).
There are few studies of inputs and outputs of N in the
Arctic, largely because early work suggested that they
were small relative to standing N stocks and internal recycling,
and thus were less important, at least on a shortterm
basis (Shaver et al., 1992). In the long term (several
decades or more), however, understanding of N inputs
and outputs is essential to understanding how the total
pool sizes of N change over time. Changes in standing
stocks of N are closely tied to the accumulation or loss of
organic matter and C in the Arctic (Gold and Bliss, 1995).
Nitrogen enters arctic ecosystems through atmospheric
deposition and microbially mediated N fixation (Fig.
7.27). Nitrogen deposition rates are low in the Arctic relative
to other parts of the world, mostly because the
atmosphere is cold enough that it cannot hold the high
concentrations of N species such as nitrate (NO3) that are
deposited at lower latitudes.Thus, N deposition can
account for only about 5% or less of the annual plant N
uptake requirement in Alaskan wet-sedge tundra (Chapin
F. et al., 1980), although this might increase with
increased industrial activity at lower latitudes. In regions
such as northern Scandinavia that are subject to N deposition
from lower-latitude anthropogenic sources, however,
N deposition may be greater than 0.1 g/m2/yr, which
if continued for many years is sufficient to affect plant
growth and productivity (Back et al., 1994). Nitrogen
fixation rates are usually assumed to be of similar magnitude,
although the only relatively recent studies (Chapin
D. and Bledsoe, 1992; Lennihan et al., 1994) indicate
that, at least in the high Arctic, N fixation might account
for more than 10% of plant requirements with the
remaining 90% supplied by recycling from the soils.
Nitrogen losses are also poorly understood.There have
been no recent, published studies of denitrification in
the Arctic; although anaerobic soils might be expected to
have high potential for denitrification, the generally low
rates of NO3 production in tundra soils suggest that this
is also a small component of the annual N budget.
Possible spring losses of N in the form of nitrous oxide
(N2O) have been suggested (ChristensenT. et al., 1999a)
but not yet verified in the Arctic. Nitrogen losses in
streams have been monitored at several locations, and
are of roughly the same magnitude as N deposition
(Hershey et al., 1997).
Responses to climate change
Responses of element cycles in arctic ecosystems to climate
change factors have been studied in multi-year
manipulation experiments in several contrasting ecosystem
types (Dormann andWoodin, 2002; Shaver and
Jonasson, 2000; Fig. 7.28).These experiments include
manipulations of air temperature, CO2 concentration,
light, water (both excess and deficit), nutrients, and UV-B
radiation levels. One common observation from these
experiments is that although short-term responses to single
factors like changing CO2 concentrations or temperature
increases are measurable and often significant, these
responses are often not sustained due to other limitations.
A general conclusion is that nutrient limitation dominates
the multi-year responses and is linked to changes in other
factors (e.g., temperature and water) through their indirect
effects on nutrient mineralization and availability to
plants. In wet systems, water-table depth and soil drainage
are critical variables limiting nutrient turnover in the soil;
increases in C turnover in these systems are not linked to
increases in C accumulation because increased C accumulation
requires increased N and/or P supply (Oechel et
al., 1998, 2000a; Shaver et al., 1998).A large pool of
nutrients exists in organic matter, and may drive large
changes in organic matter stocks if the nutrients can be
mineralized and not leached from the system. Similarly,
short-term increases in photosynthesis and growth in
response to high CO2 concentrations are often not sustained
due to nutrient limitation (Grulke et al., 1990;
Hartley et al., 1999; Oechel et al., 1994; section 7.3).
310 Arctic Climate Impact Assessment
The results of several manipulation experiments indicate
that nutrient mineralization stimulated by increased soil
temperatures is unlikely to be sustained in the long
term. Soil temperature increases cause an immediate
increase in soil respiration in laboratory studies, but few
arctic field studies have shown increased mineralization
rates in response to increased air temperatures (but see
Schmidt et al., 1999) and longer-term field studies show
an acclimation to increased soil temperature (Hartley et
al., 1999; Luo et al., 2001). Some studies have shown
soil cooling in response to increasing air temperatures in
experiments (Shaver and Chapin, 1991) and along latitudinal
gradients (Callaghan and Jonasson, 1995). Higher
air temperatures stimulate leaf area development
(Myneni et al., 1997) and a greater leaf area index
would be expected to intercept thermal radiation before
it reaches the soil, leading to soil cooling. In addition,
organic matter in deeper soil profiles is less responsive
to temperature increases than that in surface layers
(ChristensenT. et al., 1999b; Grogan et al., 2001), again
suggesting that any temperature-induced mineralization
is likely to be transitory.
Overall, however, multi-year experiments suggest that
the arctic ecosystems most responsive to climate change
are very likely to be those in which the environmental
change is linked to a large change in nutrient inputs or
soil nutrient turnover, and/or large changes in leaching
or erosional losses of soil-available nutrients (Fig. 7.27).
The effects of UV radiation on overall organic matter
cycling are generally unknown, but not unimportant.
Recent work by Niemi et al. (2002) showed that
increased UV-B radiation decreased CH4 emissions from
a peatland in northern Finland, while three studies show
UV-B radiation effects on Sphagnum growth (Gehrke et
al., 1995; Searles et al., 1999; Sonesson et al., 2002)
with potential implications for C sequestration. Longterm
responses of biogeochemical cycling to increased
CO2 concentrations and UV-B radiation levels are small
in magnitude but are likely to lead to longer-term
changes in biogeochemical cycling and ecosystem structure
(sections 7.3.3.3 and 7.4.1.2). However, most of
the current understanding of UV radiation responses is
based on species- and tissue-level research.
Biodiversity and species effects on
biogeochemistry
It is important to determine if species or growth form
composition of the vegetation have any impact on biogeochemistry
of arctic ecosystems, or if biogeochemistry
is largely regulated by climate and resource availability
irrespective of species composition.Although only partial
answers to this question are currently available, there
are at least five main mechanisms by which species composition
are likely to have important consequences for
biogeochemistry.These are:
1. Species composition is likely to affect the rate of
change in ecosystems in response to environmental
change, through differences in species growth,
reproduction, and dispersal rate potential (e.g.,
Bret-Harte et al., 2001).
2. Species are likely to affect nutrient availability and
C cycling through differences in the turnover of
elements in their living tissues and in the decomposability
of their dead parts (Hobbie and Chapin,
1998; Quested et al., 2003).
3. Species are likely to affect element accumulations in
living plants through differences in their biomass
allocation patterns and in their biomass element
concentrations and element ratios (Bret-Harte et
al., 2002; Quested et al., 2003; Shaver et al., 2000).
4. Species are likely to differ in their effects on snow
accumulation and snowmelt, surface energy balance,
and soil temperature regimes, with important
feedbacks to element cycles (McFadden et al.,
2001; Sturm et al., 2001a).
5. Physiological mechanisms, for example, wetland
species that act as conduits for CH4 transport from
the soil to the atmosphere (Fig. 7.29; Joabsson and
Christensen, 2001; Niemi et al., 2002; Öquist and
Svensson, 2002).
All five of these species effects have been documented in
arctic systems, although it is often uncertain how to
scale up from small experimental communities to larger
units of the landscape.
Species richness or diversity itself is likely to also affect
the biogeochemistry of arctic ecosystems, although the
magnitude of the effect is hard to judge.There is a weak
positive correlation between productivity and vascular
species richness in arctic vegetation, but, like most vegetation,
richness declines when productivity is increased artificially
by fertilizer addition or other disturbance (Gough
et al., 2000). Recent evidence suggests that arctic plants
obtain their N from diverse sources in the soil (Michelsen
et al., 1996, 1998; Nadelhoffer et al., 1996) and that the
Fig. 7.28. Mean response of plants to environmental perturbations,
shown as a percentage of untreated controls (vertical
lines depict ±1 standard deviation). Black and grey symbols
indicate that means differ significantly from 100% (P<0.05 and
0.1>P>0.05, respectively; two-tailed t-test). Numbers along the
horizontal axis refer to corrected sample size (df+1)
(Dormann and Woodin, 2002).
Chapter 7 • ArcticTundra and Polar Desert Ecosystems 311
relative abundance of different species reflects different
abilities to acquire the different forms of N (McKane et
al., 2002).These latter studies suggest that diversity will
probably increase productivity in arctic vegetation by
increasing total uptake of different forms of a strongly
limiting element, N. Partial support for this conclusion
comes from experiments involving removal of individual
species from arctic vegetation, in which the remaining
species failed to increase in abundance (Fetcher, 1985;
Jonasson, 1992; Shevtsova et al., 1995, 1997).
Vascular plants directly affect the substrate availability
for methanogens and have the capability to transport
gases between the anaerobic parts of soils and the atmosphere
(Bubier and Moore, 1994; Joabsson and
Christensen, 2001; Joabsson et al., 1999; Niemi et al.,
2002). Different vascular plant species have different
effects, however, and the vascular plant species composition
in wet tundra ecosystems may be a key determinant
of the scale of CH4 emissions; for example, Fig. 7.29
shows that the minor constituents of the total vascular
plant biomass (Carex and Eriophorum) seem to be “driving”
net CH4 emissions from the site, suggesting that
shifts in vascular plant species composition alone could
lead to significant effects on trace-gas exchange
(Joabsson and Christensen, 2001; Ström et al., 2003).
Changes in species composition per se caused by climate
warming and increased UV-B radiation levels are likely
to cause a change in CH4 emissions, adding to the direct
effect of a changing soil climate on these emissions
(Niemi et al., 2002; see section 7.4.2.2).
Role of disturbance
Disturbances are expected to increase with climatic
warming, mainly through thermokarst (section 7.5.2) and
possibly also through increased fire in some northern
ecosystems and insect pest outbreaks in subarctic forests
(section 7.4.1.4). In general, physical disturbance of arctic
ecosystems results in greater soil warming and permafrost
thawing, which tend to increase soil organic matter
and nutrient turnover.Typically, vegetation productivity
increases dramatically although soil respiration also
increases. It is not yet clear whether the increased plant
growth is sufficient to compensate for losses of soil organic
matter. In the long term, however, arctic landscapes
should gain organic matter in both soils and vegetation on
disturbed sites.The timing and trajectory of these changes
are key unknowns requiring further research.
7.4.2.2. Soil processes and controls over tracegas
exchanges
During the last decade, trace-gas production, emissions,
and assimilation have attracted considerable attention
from the scientific community, for the following reasons:
• most of these gases are “radiatively active”
(i.e., they affect the heat balance and contribute to
the “greenhouse effect” responsible for climate
instability and change);
• the atmospheric concentration (mixing ratios) of
these gases underwent remarkably rapid changes
after the industrial revolution (e.g., the concentration
of atmospheric CH4 has been growing at an
annual rate of 0.8 to 1%, which can have a significant
impact on the biosphere apart from the
greenhouse effect); and
• most trace gases are intermediate or end products/
substrates of key biogeochemical processes, such
that monitoring these gas species can be used for
early detection of any anomaly in ecosystem
functioning.
Table 7.11 lists the major trace gases and their potential
impacts on ecosystems, although not all of the listed
gases are of primary importance for arctic terrestrial
ecosystems.This review is restricted to CO2 (section
7.4.2.1), CH4, and N2O.
Soil and ecosystem processes responsible for
trace-gas emissions
Trace-gas exchange with the atmosphere occurs through
a set of coupled soil/ecosystem processes, including:
• production of substrate(s) for processing by trace
gas-producing organisms;
• conversion of substrate(s) to respective gaseous
species in parallel with gas consumption (Table
7.12); and
• mass transfer of produced gas to the free
atmosphere, which includes three main mechanisms:
molecular diffusion, vascular gas transfer
(i.e., through plant “conduits”), and ebullition
(i.e., bubble formation).
Substrates are formed by one of three processes: decomposition
(hydrolytic breakdown of plant litter, oxidation,
fermentation); N mineralization, and photosynthesis and
photorespiration. However, the starting point for almost
all substrates is the primary production of organic matter
by plant photosynthesis or (occasionally) bacterial
chemosynthesis.There are two main flows of C subFig.
7.29. Mean seasonal methane emissions from a high-arctic
fen in Zackenberg, northeast Greenland, as a function of leaf
biomass of the grasses Eriophorum scheuchzeri, Dupontia psilosantha,
and Carex subspathacea, and total leaf biomass of the three
species (Joabsson and Christensen, 2001).
312 Arctic Climate Impact Assessment
strates from plants: plant litter formation with lignocellulose
as a main component resistant to microbial breakdown;
and the continuous supply of readily available C
monomers (root and foliage exudation).The chain of
events leading to the formation of immediate precursors
of trace gases can be long and intricate (Panikov, 1999).
It is worthwhile to note that the most successful model
simulations of trace gas emissions include vegetation or
primary productivity modules.
Trace-gas transport
There are three main transport mechanisms: molecular
diffusion, vascular transport of gas through plant roots,
and ebullition.Vascular transport can be described as a
diffusion process through plant root aerochyma
(parenchyma containing large air spaces typical of
emergent and marginal wetland species), which is a
continuous network of gas-filled channels.Vascular
transport is two or three orders of magnitude more
rapid than diffusion in water. Ebullition is probably the
most difficult process to simulate and describe mathematically
due to its stochastic nature. In northern soils,
ebullition and vascular transport were shown to be the
major transport mechanisms, accounting for up to 98%
of total CH4 emissions (ChristensenT. et al., 2003a).
Environmental controls on methane fluxes
Methane is produced from anaerobic decomposition of
organic material in waterlogged, anaerobic parts of the
soil.Wet and moist tundra environments are known to
be significant contributors to atmospheric CH4 (Bartlett
and Harriss, 1993; Fung et al., 1991). Methane is
formed through the microbial process of methanogenesis,
which is controlled by a range of factors: most
notably temperature, the persistence of anaerobic conditions,
gas transport by vascular plants, and the supply
of labile organic substrates (Joabsson and Christensen,
2001; Schimel, 1995; Ström et al., 2003;Whalen and
Reeburgh, 1992). Figure 7.30 shows the variety of controls
on CH4 formation rates at different spatial and
Table 7.11. Trace gases produced in tundra soils and their potential impacts on terrestrial ecosystems.
Gas species Main soil sources Main soil sinks Environmental control
on“source–sink” balance
Impacta
Reference related to
the Arctic
Carbon dioxide
(CO2)
Organic matter
decomposition
Photosynthetic
uptake; formation of
carbonates
Temperature;
moisture; available
nutrients
GHG; productivity of
plant community
(indirectly)
Nordström et al.,2001;
Oechel et al, 2000b;
Søgaard et al., 2000;
Methane (CH4) Methanogenesis
(anaerobic decomposition
of organic matter)
Uptake by methanotrophic
bacteria
Temperature;
moisture; nutrients;
plant community
structure
GHG; highly
combustible
Christensen T. et al.,
2000; Friborg et al.,
2000;Whalen and
Reeburgh 1990
Nitrous oxide
(N2O)
Denitrification; nitrifi-
cation
Uptake by aerobic and
anaerobic soil bacteria
Temperature; moisture;
N fertilizers
GHG Christensen T. et al.,
1999a
Carbon monoxide
(CO)
Anaerobic decomposition
of organic matter;
airborne pollution
Uptake by
carboxydobacteria
Temperature;
moisture
Atmospheric
photochemical
reactions; toxicity
Conrad, 1996;Whalen
and Reeburgh, 2001
Molecular
hydrogen (H2)
Fermentation,
Pollution
Uptake by methanogens,
acetogens, sulfite
reducers, and H2oxidizing
bacteria
Temperature;
moisture
Atmospheric photochemical
reactions;
toxicity
Conrad, 1996; no
specific arctic works
non-methyl
hydrocarbons
(terpene and isoprene
derivatives)
Plants and microorganisms
(bacteria,
fungi)
Photooxidation;
uptake by aerobic soil
bacteria
Temperature;
moisture; plant
phenology and
physiological state
GHG; ozone and
aerosol formation
within plant canopy
Guenther et al., 1993;
Isidorov et al., 1983;
Isidorov and Jdanova,
2002
Methylated halogens
(methyl
bromide, methyl
iodide, etc.)
Soil affected by
oceanic water
Bacterial decomposi-
tion
Soil hydrology;
location within
landscape relative to
oceanic shoreline
Ozone depletion;
phytotoxicity
Dimmer et al., 2001
(data for Irish peatland
ecosystems)
Dimethyl sulfide
(DMS)
Hydrolysis; uptake as
a sulfur source by
plants
Soil hydrology and
location within
landscape relative to
oceanic shoreline
Phytotoxicity Legrand, 1995
Sulfur oxides
(SO2; SO)
Fuel combustion:
airborne contamination
of soil
Sulfate and sulfite
formation; ion
exchange; plant and
microbial uptake
Temperature;
moisture
Acid rain;
phytotoxicity
Conrad, 1996
Ammonia
(NH3)
Airborne soil contam-
ination
Conversion to
ammonium (NH4
+);
ion exchange; plant
and microbial uptake
Temperature;
moisture
Plant productivity Conrad, 1996
aGHG=greenhouse gas
Chapter 7 • ArcticTundra and Polar Desert Ecosystems 313
temporal scales. Methane is not only produced but also
consumed in the aerobic parts of the soil through the
microbial process of methanotrophy, which can even
take place in dry soils with the bacteria utilizing atmospheric
CH4 (ChristensenT. et al., 1999a; Panikov,
1999; Sjögersten andWookey, 2002;Whalen and
Reeburgh, 1992). Methanotrophy is responsible for the
oxidation of approximately 50% of the CH4 produced
at depth in the soil and therefore is as important to net
CH4 emissions as methanogenesis.The anaerobic
process of methanogenesis is much more responsive to
temperature than CH4 uptake, so soil warming in the
absence of any other changes is very likely to accelerate
emissions (the difference between production and consumption),
in spite of the simultaneous stimulation of
the two opposing processes.
Apart from temperature, water regime, and plant cover,
methanogenic bacteria are strongly affected by biological
interactions within the soil community. Competition
with acetogenic and sulfidogenic bacteria for molecular
hydrogen (the outcome of which depends on the affinity
to hydrogen, the temperature, and density of the various
populations) determines the pattern of gas formation
not only quantitatively, but also in qualitative terms.
For example, ecosystems can be a source of CH4 (if
methanogenic bacteria prevail), or hydrogen sulfide and
other sulfides (if sulfate-reducing bacteria dominate), or
acetic acid (if a large population of acetogens is present).
Early empirical models of northern wetland/tundra
CH4 exchanges suggested sensitivity to climate change
(Harriss et al., 1993; Roulet et al., 1992).A simple
mechanistic model of tundra CH4 emissions that included
the combined effects of temperature, moisture, and
active-layer depth also suggested significant changes in
CH4 emissions as a result of climate change (Christensen
T. and Cox, 1995). More complex wetland CH4 emission
models suggest that winter processes have a strong
influence on net annual CH4 emissions (Panikov and
Dedysh, 2000).Variations in CH4 emissions at the
regional and global scale are driven largely by temperature
(Crill et al., 1992; Harriss et al., 1993) with the
important modulating effects of vascular plant species
composition superimposed (ChristensenT. et al., 2003b;
Ström et al., 2003).An initial warming is, hence, projected
to lead to increased CH4 emissions, the scale of
which will depend on associated changes in soil moisture
conditions and the secondary effects of changes in vegetation
composition (section 7.5.1.2).
Table 7.12. Examples of soil processes where trace gases are formed or consumed.
Gas species Source reaction Sink reaction
CO2 Oxidative decomposition of dead organic matter
(OM): OM+O2 →CO2+H2O+“ash”
Oxidation of root exudates
Fermentation: (CH2O)n
a → CO2+VOCb+H2
sulfurc (S) oxidizing bacteria
Photosynthesis (plant and green bacteria): nCO2+nH2O →
nCH2Oa +nO2
Soil chemosynthesis (nitrifying, hydrogen-oxidizing, iron (Fe2+)oxidizing,
and sulfurc (S) bacteria)
Carbonate formation and leaching
CO Fermentation
O2-limited oxidation reactions
Activity of carboxydobacteria:
CO + 1/2 O2 → CO2
Spontaneous chemical oxidation
CH4 Methanogenesis:
CO2 + H2 → CH4 or
CH3COO-(acetate)→ CH4
Methanotrophy:
CH4 → CO2 + H2O
Photochemical reactions with hydroxyl and chlorine radicals
N2O Nitrification:
NH4
+→≈ 99% NO3
- + ≈ 1% N2O
Denitrification
NO3
- → N2O → N2 (molecular nitrogen)
The N2O is finally converted to stable end products such as NO3
(nitrification,
aerobic conditions) or N2 and NH3 (denitrification,
anaerobic conditions ) upon completion of the respective processes
Nitric oxide
(NO)
Nitrate reduction
NO3
- → NO
NO oxidation
Isoprene
(C5H8)
Secondary metabolic reactions in plants:
pyruvate → …→ isoprene
Fermentation
Microbial oxidation
Formation of phytogenic aerosols and sedimentation
acarbohydrate; bvolatile organic carbon (acetic, propionic, or butyric acid; aldehydes; alcohols; ketones; ethers; etc.); cthese bacteria can use several S-containing reduced compounds
(S, S2-, SO3
2-, etc.) as an energy source.
Fig. 7.30. Controls on methanogenesis (redrawn from Davidson
E. and Schimel, 1995).
314 Arctic Climate Impact Assessment
Controls on nitrous oxide fluxes
The simulation of N2O emissions requires consideration
of the combined N and C cycles, because the substrates
of denitrification include electron acceptors (NO3) as
well as oxidizable C substrates.The ecosystem–
atmosphere fluxes of N2O are associated with fundamental
transformations of N in the soil, namely the processes
of nitrification and denitrification.Very few field studies
of N2O fluxes are available from the Arctic (Christensen
T. et al., 1999a), but very small releases of N2O are
expected from arctic soils due to their general nutrient
limitation. However, one issue of potentially great
importance in the Arctic is early spring fluxes of N2O.
Denitrification has been found to take place even below
the freezing point (Dorland and Beauchamp, 1991;
Malhi et al., 1990). During freezing and thawing, C is
liberated and this may increase denitrification activity in
the soil (Christensen S. and Christensen, 1991). During
the spring thaw of the soil, a significant percentage of
annual N2O emissions can take place (Papen and
Butterbach-Bahl, 1999).The early spring fluxes may also
explain the significant N2O production measured in fertilized
plots on a subarctic heath (ChristensenT. et al.,
1999a) on soils that showed no emissions from control
plots. Hence, as with CH4, the winter and “shoulder”
season processes are generally very important but also
the least well understood.
7.4.2.3.Water and energy balance
Arctic ecosystems exhibit the greatest seasonal changes
in energy exchange of any terrestrial ecosystem because
of the large changes in albedo from late winter, when
snow reflects most incoming radiation, to summer when
the ecosystem absorbs most incoming radiation.About
90% of the energy absorbed during summer is transferred
to the atmosphere, with the rest transferred to
the soil in summer and released to the atmosphere in
winter (Eugster et al., 2000). Consequently, arctic
ecosystems have a strong warming effect on the atmosphere
during the snow-free season.
Vegetation profoundly influences the water and energy
exchange of arctic ecosystems. In general, ecosystems
with high soil moisture have greater evapotranspiration
than dry ecosystems, as in any climatic zone.Arctic
ecosystems differ from those at lower latitudes, however,
in that there is no consistent relationship between CO2
flux and water vapor flux, because vascular plants
account for most CO2 flux, whereas mosses account for
most water vapor flux (McFadden et al., 2003).This
contrasts with other major biomes on Earth, where
these two fluxes are strongly correlated (Kelliher et al.,
1995; Schulze et al., 1994).
Within tundra, vegetation strongly influences the winter
energy budget through its effects on snow depth and
density. Shrubs increase snow depth by reducing the
velocity of blowing snow and reducing sublimation rates;
models suggest that in northern Alaska this shrubinduced
reduction in sublimation can increase ecosystem-scale
winter snow accumulation by 20% (Sturm et
al., 2001a). By acting as a “snow fence”, shrubs also
cause snow to accumulate within shrub patches and to
be depleted from shrub-free zones, increasing the spatial
heterogeneity of snow depth. Snow within shrub
canopies is deeper and less dense, which reduces heat
transfer through the snowpack and increases winter soil
temperatures by 2 ºC relative to adjacent shrub-free tundra.Warmer
soil temperatures beneath shrubs may
increase winter decomposition and enhance nutrient
availability, creating a positive feedback that promotes
shrub growth (Sturm et al., 2001a).
Midsummer vegetation feedbacks to regional climate are
determined largely by midsummer patterns of water and
energy exchange (Chapin F. et al., 2000a). Midsummer
albedo is greatest in sedge communities, whose standing
dead leaves reflect much of the incoming radiation
(Chapin F. et al., 2000a; McFadden et al., 1998).
Evergreen forests and forest tundra, in contrast, have a
particularly low albedo because of the dark absorptive
nature of evergreen leaves and the effectiveness of complex
forest canopies in capturing light (Chapin F. et al.,
2000b; Eugster et al., 2000).Tundra and forest tundra
canopies dominated by deciduous plants are intermediate
in albedo and therefore in the quantity of energy that
they absorb and transfer to the atmosphere (Baldocchi et
al., 2000; Chapin F. et al., 2000b).A larger proportion
of the energy transfer to the atmosphere occurs as sensible
heat flux in forests, forest tundra, and shrub tundra
than in wet tundra (Boudreau and Rouse, 1995; Chapin
F. et al., 2000a; Eugster et al., 2000; Lafleur et al.,
1992; McFadden et al., 1998).
All arctic ecosystems exhibit greater ground heat flux
during summer (5 to 15% of net radiation) than do temperate
ecosystems (generally close to zero), due to the
strong thermal gradient between the ground surface and
permafrost and the long hours of solar radiation (Chapin
F. et al., 2000a). Ground heat fluxes are reduced in tundra
ecosystems with a large leaf area, which shades the
ground surface (McFadden et al., 1998), or where the
ground cover is highly insulative, as with Sphagnum
mosses (Beringer et al., 2001a). Grazing and other
processes causing surface disturbance increase ground
heat flux and thaw depth (Walker D. et al., 1998).
Future changes in vegetation driven by climate change
are very likely to profoundly alter regional climate.
7.4.2.4. Summary
Arctic ecosystems tend to accumulate organic matter
and elements despite low inputs because organic matter
decomposition is very slow.As a result, soil-available
elements like N and P place key limits on increases in C
fixation and further biomass and organic matter accumulation.
Key issues for projecting whole-system responses
to climate change include the importance of carbon–
nutrient interactions; the interactions of C and nutrient
cycles with temperature, water, and snow cover; the
Chapter 7 • ArcticTundra and Polar Desert Ecosystems 315
magnitude of dissolved organic and inorganic C losses in
soil water; and the magnitude and role of wintertime
processes. Most disturbances are expected to increase C
and element turnover, particularly in soils.This is likely
to lead to initial losses of elements but eventual, slow
recovery. Individual species and species diversity have
clear impacts on element inputs and retention in arctic
ecosystems, but their magnitude relative to climate and
resource supply is still uncertain. Similarly, the current
information about the long-term effects of increasing
CO2 concentrations and UV-B radiation levels on whole
ecosystems indicates that direct effects of these variables
will probably be small relative to changes in soil
resources and element turnover. Indirect effects of
increasing CO2 concentrations and UV-B radiation levels
are likely to be more important at the ecosystem level
(e.g., through changes in species composition).
The most important trace gases in arctic ecosystems are
CO2 and CH4.Trace-gas exchange with the atmosphere
occurs through a set of coupled soil ecosystem processes.Wet
and moist tundra environments are known to be
significant contributors to atmospheric CH4. However,
CH4 is also consumed in aerobic parts of the soil.
Methane emissions from the ecosystems are a balance
between production and consumption, with production
more responsive to warming than consumption. Soil
warming in the absence of any other changes is very
likely to accelerate emissions.Winter processes and vegetation
type also affect CH4 emissions. Nitrous oxide
emissions are also sensitive to winter conditions and
potential winter warming.
Arctic ecosystems exhibit the largest seasonal changes in
energy exchange of any terrestrial ecosystem because of
the large changes in albedo from late winter, when snow
reflects most incoming radiation, to summer when the
ecosystem absorbs most incoming radiation.Vegetation
profoundly influences the water and energy exchange of
arctic ecosystems.Vascular plants account for most CO2
flux, whereas mosses account for most water vapor flux.
Albedo during the period of snow cover declines from
tundra to forest tundra to deciduous forest to evergreen
forest. Shrubs and trees increase snow depth, which in
turn increases ground heat fluxes; ecosystems with a
large leaf area and insulating moss carpets reduce
ground heat fluxes and conserve permafrost. Future
changes in vegetation driven by climate change are very
likely to profoundly alter regional climate.
7.5. Effects of climate change on
landscape and regional processes and
feedbacks to the climate system
Biological and physical processes and phenomena in the
arctic system operate at various temporal and spatial
scales to affect large-scale feedbacks and interactions
with the earth system. Understanding these processes at
multiple scales is critical because the effects of the complex
interactions between physical, biological, and
human dimensions on system performance cannot be
projected by simply applying a different scale to existing
results.Therefore, a multidisciplinary and quantitative
approach is necessary to understand and project the
response of the arctic system to variability in temperature
and moisture.The large scale, inter-related processes
described in this section include:
• ecosystem processes extrapolated to the landscape
or regional scale (e.g., trace-gas exchange, water
and energy exchange, and disturbance);
• changes in ecosystem distribution and abundance
in the landscape;
• changes in vegetation zonation (e.g., treeline
movement);
• interactions between terrestrial and freshwater
ecosystems; and
• regional feedbacks.
Paleoclimate studies and studies of the contemporary
Arctic together have identified four potential feedback
mechanisms between the impacts of climate change on
the Arctic and the global climate system; these are:
• albedo (reflectivity);
• greenhouse gas emissions and/or uptake through
biological responses to warming;
• greenhouse gas emissions from methane hydrates
released from thawing permafrost; and
• freshwater fluxes that affect thermohaline
circulation.
In the past, three of the potential feedbacks have been
generally positive and only one negative.
Some of the feedbacks such as energy and water
exchange operate at local to regional scales whereas others,
particularly trace-gas fluxes, have the potential to
operate at regional to global scales.This section assesses
the impacts of changes in climate (but not UV radiation
levels, for which data are lacking) on ecosystem processes
at the larger scale.The section explores the implications
of these changes for feedbacks from terrestrial
ecosystems to the climate system, but does not calculate
changes in forcing (section 4.7.1). Nor does it consider
freshwater discharge (sections 6.8.3 and 8.4.2) or
methane hydrate feedbacks (section 6.6.2).
7.5.1. Impacts of recent and current
climate on carbon flux
There are two complementary approaches to solve the
carbon-flux inventory problem: “bottom-up” and “topdown”.The
first is based on the long-term monitoring of
gas emissions within networks of field stations or sites
that cover the main types of habitats.At its simplest,
total circumpolar emissions are estimated from the number
and area of the types of northern ecosystems differentiated
in terms of easily mapped features (e.g., vegetation,
soil properties, relief, and geomorphology) and the
characteristic annual exchange of CO2 and CH4 from
316 Arctic Climate Impact Assessment
each ecosystem.The data on CO2 and CH4 fluxes come
from three main groups of available techniques that
operate at different spatial scales: closed and open-top
chambers (0.1 to 1 m2), micrometeorological towers
based on eddy covariance and gradient methods (10 to
10000 m2), and aircraft sensing (up to tens and hundreds
of square kilometers).All three groups of techniques
have their advantages and disadvantages.
However, continuous measurements with towers seem
to be the most appropriate for providing reliable information
on the temporal variation of gas emissions at the
ecosystem and landscape spatial levels.
7.5.1.1. Recent changes in carbon dioxide fluxes
Recent variations in arctic climate have had profound
effects on some ecosystem- and regional-level C fluxes
and, in general, these fluxes reflect the recent spatial
variability in climate change.The assessment in this section
is restricted to C in the active layer of soils and in
plants, and does not consider C in permafrost and
methane hydrates (section 6.6).
The North Slope of Alaska has experienced a rise in
temperature (Fig. 7.26;Weller, 2000), an increase in
growing-season length, and a decrease in available soil
moisture (Oechel et al., 1993, 1995, 2000a; Serreze et
al., 2000) over the last three to four decades.This has
resulted in North Slope ecosystems changing from a sink
for C throughout the Holocene (Marion and Oechel,
1993) to a source of C to the atmosphere beginning in
the mid-1970s to early 1990s (Oechel et al., 1993,
1995, 2000a; Fig. 7.26). However, as there has been a
change in climate, with progressive warming, drying,
and lengthening of the growing season, there has been
physiological, community, and ecosystem level adjustment
that has reduced the rate of C loss from North
Slope ecosystems (Fig. 7.26). In addition, wetter areas of
the North Slope are not showing the same increase in
CO2 emissions (Harazono et al., 2003).The interannual
variations in the C balance measured on the coastal plain
of Alaska are very large, ranging from a net summer
CO2 uptake of about 25 g C/m2/yr to a summer loss of
over 225 g C/m2/yr. If the latter fluxes held worldwide
for wet coastal and moist tussock tundra, this would
result in a net loss of up to 0.3 Pg C/yr from these two
ecosystem types alone.
In northeast Greenland, the recent climatic history is
different than that of Alaska: there has been no significant
trend toward higher temperatures (Weller, 2000)
and integrating over all vegetation types shows that the
Zackenberg valley is a small net sink (2.3 g C/m2/yr)
with a large uncertainty range (±16.2 g C/m2/yr).
This integrated study of the valley shows that Landsatderived
C flux estimates are in good agreement with
ground-based eddy correlation flux measurements covering
all the dominant vegetation types in the valley.
The Landsat method estimated a midday uptake rate in
August 1997 of 0.77 g C/m2/d for the valley as a whole
whereas the ground-based measurements showed the
uptake rate to be 0.88 g C/m2/d (Søgaard et al., 2000).
The measured annual balance in the valley varies from
significant uptake in the intensively studied fen areas (on
the order of 18.8±6.7 g C/m2/yr) to net C losses in the
dry heath (ChristensenT. et al., 2000; Nordström et al.,
2001; Søgaard et al., 2000).
Like Alaska, northern Scandinavian areas have experienced
warming in recent years.The ecosystem C balance
for a subarctic Swedish peatland was found to be a sink
of between 15 and 25 g C/m2/yr (Friborg, pers. comm.,
2005). Similarly, in Finland, a net annual uptake of about
20 g C/m2/yr was reported for a subarctic fen at
Kaamanen (Aurela et al., 2002). Six years of continuous
measurements at this fen show marked interannual variation
in the CO2 balances (sinks ranging from 4 to 52 g
C/m2/yr), which mainly reflect the variations in spring
temperatures and the timing of the snow melt (Aurela et
al., 2004). Studies of fluxes in high-arctic barren tundra
on Svalbard show a very limited source of around 1 g
C/m2/yr (Lloyd C., 2001). Overall, the synthesis of
regional C flux information from measurements at several
sites in northern Europe and Greenland (the Land
Arctic Physical Processes project; Laurila et al., 2001)
indicates that arctic landscapes are remarkably similar in
their C fluxes during midsummer, but the length of the
growing season and the shoulder season fluxes are the
key determinants for the net annual fluxes.This causes
substantial interannual variability at the individual sites,
and general uncertainty about whether the circumpolar
Arctic is presently a source or a sink for C.
Recent work in East European tundra indicates a
substantial current source of C in the northeastern
European tundra areas (Heikkinen et al., 2004).
When combined with the areas of the northern Alaska
tundra mentioned above that are also a source of C,
source areas (East European tundra, Svalbard, and
Alaska) may exceed sink areas (northeast Greenland,
northern Scandinavia). However, data are available for
only a small part of the Arctic.
There may be a correlation between C balance and
recent climatic history in areas that have seen a significant
warming and drying: these areas experienced at
least a temporary release of CO2, while areas that have
not experienced the same extent of warming and drying,
or have possibly experienced a warming and wetting,
remain atmospheric CO2 sinks and could possibly
even become large sinks.A complete synthesis of the
available information from the circumpolar Arctic is
underway but not yet available.
New models and approaches make estimation of current
and future global C balances possible.The modeling
approach has been used to explore potential changes
in arctic terrestrial ecosystems from C sink to source
status (Clein et al., 2000; McGuire et al., 2000).
TheTerrestrial Ecosystem Model (Marine Biological
Laboratory,Woods Hole) has been used to estimate current
and future C fluxes, while the model Hybrid v4.1
Chapter 7 • ArcticTundra and Polar Desert Ecosystems 317
(Institute ofTerrestrial Ecology, United Kingdom:
Friend et al., 1997) has been used to simulate vegetation
and carbon-pool changes at high latitudes for the
period from 1860 to 2100 (White et al., 2000). For the
present day, the models simulate a mix of C sinks and
sources that reflect variations in current and past climate.
McGuire et al. (2000) estimate that average
circumpolar C fluxes presently constitute a small sink
of 17 g C/m2/yr with a standard deviation of 40 g
C/m2/yr that crosses the boundary between sink and
source status.This uncertainty range is comparable to
the Lund-Potsdam-Jena Dynamic GlobalVegetation
Model (LPJ) outputs (section 7.5.4.1; Sitch et al.,
2003) and the calculation of current sink status corresponds
to the projections byWhite et al. (2000).
Although it can be concluded that source areas currently
exceed sink areas, there is great uncertainty about the
current CO2 balance of the Arctic, owing to geographically
sparse measurements and inadequate representation
of ecosystem dynamics in current models.
7.5.1.2. Current circumpolar methane fluxes
Probably the most intensive studies and the longest
observations of CH4 fluxes were performed in North
America, mainly within the central Alaskan and North
Slope sites at Barrow,Atqasuk,Toolik Lake, and Prudhoe
Bay (ChristensenT., 1993; Morrissey and Livingston,
1992;Vourlitis et al., 1993;Whalen and Reeburgh,
1990, 1992). In northern Eurasia including Russia,
extensive measurements of gas emissions were initiated
in the late 1980s, either as short-term measurements
across geographic transects or as long time series of flux
measurements at individual sites.The first approach is
illustrated by chamber measurements of CH4 (and CO2)
fluxes across the Russian Arctic (ChristensenT. et al.,
1995, 1998).The second approach is found at a number
of field stations where gas fluxes are measured mainly
during the summer season (Panikov et al., 1995, 1997;
Wagner et al., 2003; Zimov et al., 1993).
The general characteristics of spatial and temporal flux
variations can be formulated as follows. First, there are
evident temperature-related variations: even within
northern wetlands, the highest net fluxes occur in
warmer soils, with maximum values attained in the boreal
zone.This trend is especially evident with respect to
CH4 emissions, which increase along the latitudinal
sequence Barrow–Toolik Lake–Fairbanks orTaymir–
Surgut–Tomsk. Seasonal variations also follow a temperature
curve, although winter, autumn, and spring emissions
are often measurable (sections 7.3.2.3 and
7.4.2.1). Seasonal measurements of CH4 emissions from
five different wetland sites along a transect from northeast
Greenland across Iceland and Scandinavia to Siberia
also showed a clear positive relationship to the mean seasonal
temperatures of the sites (ChristensenT. et al.,
2003b). Second, there are always enhanced emissions
from wetland patches covered by vascular plants (e.g.,
Eriophorum, Carex, and Menyanthes) as compared with pure
Sphagnum lawn (section 7.4.2.2).Third, variations in the
water table affect CH4 (and CO2) emissions in opposite
ways, with CH4 fluxes stimulated and CO2 suppressed by
an increase in the water table. However, the range of
fluxes varies so widely that uncertainty in regional and
global estimates remains too large and is very dependent
on the site-specific features of a particular study.
For example, extensive measurements using various
techniques over the Hudson Bay Lowland (Roulet et al.,
1994) led to the conclusion that northern wetlands are
modest sources of atmospheric CH4 (average July emissions
as low as 10 to 20 mg CH4/m2/d). On the other
hand,Alaskan wet meadow and shrub/tussock tundra
have average summer emissions as high as 100 to 700 mg
CH4/m2/d (ChristensenT., 1993;Whalen and Reeburgh,
1992).The uncertainty in regional and global estimates
that follows from these differences in measured fluxes is
very frustrating and calls for alternative ways to solve the
problem of scaling up fluxes.
One such alternative is the inverse modeling approach.
In this top-down approach, information about the temporal
and spatial variation of CH4 and CO2 emissions
from soils is deduced from observations of gas mixing
ratios in the atmosphere (obtained from a network of
National Oceanic and Atmospheric Administration/
Climate Monitoring and Diagnostics Laboratory field
stations scattered over the globe, mainly in oceanic
regions far from industrial effects).These data are fitted
to a three-dimensional atmospheric transport model,
which is combined with a tropospheric background
chemistry module and accounts for all essential sources
and sinks of gases.The model is validated against an
“internal standard” such as methyl chloroform. Presently
available results from inverse modeling (Hein et al.,
1997) do not deviate significantly from data obtained by
the bottom-up approach.The contribution of highlatitude
regions (>60º N) to the global CH4 source was
less than 13% (70Tg CH4/yr), and northern wetlands
were responsible for emissions of less than 30Tg
CH4/yr.This estimate appears to contradict the latitudinal
gradient of atmospheric CH4 concentration that has a
well-expressed maximum in the north, however, the
higher concentration of CH4 in the high-latitude atmosphere
can be explained by low concentrations of the
hydroxyl radical and, hence, lower rates of photochemical
reactions that break down atmospheric CH4.
7.5.1.3. Relative contributions of methane and
carbon dioxide to the carbon budget
The formation of CO2 and CH4 is a result of aerobic
and anaerobic decomposition, respectively.The ratio of
respired CO2 to CH4 is hence an indication of how
reduced the soil environment is.An increasingly
reduced soil environment (i.e., higher CH4/CO2 ratio)
also leads to slower overall decomposition rates, as
anaerobic decomposition is less efficient in absolute C
terms (i.e., less C is released compared to aerobic
decomposition).This generally leads to a buildup of
stored organic C in wet tundra soils, as net primary
318 Arctic Climate Impact Assessment
production is not normally limited by wet soil conditions
to the same extent as respiration.
The net CH4/CO2 ratio of the total respiration is also a
function of the amount of CH4 that is oxidized in the
aerobic soil layers above a given anaerobic zone of production
and even the atmospheric CH4 uptake that
takes place in some dry tundra soil environments.
The CH4/CO2 ratio or the percentage contribution of
CH4 to total respired C varies from <1% in dry
ecosystems to >20% in extreme cases in wet tundra
ecosystems.Typical annual average contributions of
CH4 to the total C flux range from 2 to 10% for wet
tundra and northern wetlands (e.g., ChristensenT. et
al., 1996; Clymo and Reddaway, 1971; Klinger et al.,
1994; Svensson, 1980; Svensson et al., 1999).
In the context of climate change, it is very important to
note that the relative contribution of CH4 to total radiative
forcing is much greater on a per molecule basis
than CO2 (IPCC, 2001).The “global warming potential”
(GWP) indicates how many times stronger a given
greenhouse gas is compared to CO2 on a per molecule
basis, and this is dependent on the time horizon of
interest. For example, over a 100-year time horizon,
the GWP of CH4 is 23 and over a 20-year horizon it is
63 (IPCC, 2001).
From the perspective of climate change, it is, hence, not
very informative to look only at the C balance of an
ecosystem if there are fluxes of CH4 or other greenhouse
gases such as N2O (ChristensenT. and Keller,
2003). Calculations have shown that ecosystems such as
the huge western Siberian lowlands, despite being strong
sinks for C, are sources of radiative forcing due to considerable
CH4 emissions (Friborg et al., 2003). Data are,
however, scarce when it comes to full annual budgets of
both CO2 and CH4 fluxes from tundra regions. Figure
7.31 shows calculations based on accumulated continuous
eddy correlation measurements of CO2 and CH4
fluxes in the ZackenbergValley, northeast Greenland
during the summer of 1997 (Friborg et al., 2000;
Søgaard et al., 2000).The figure illustrates that the
effect of a net C accumulation (“minus” in the budget)
during the season is completely offset if the CO2 equivalent
of CH4 is calculated (using a 20-year time horizon)
and added to the budget. Using the 100-year time horizon,
the ecosystem is still a small sink of CO2 equivalent
at the end of the growing season. However, if the
autumn and winter fluxes (which are entirely sources)
were included, the annual total would probably add up
to a source as well.
In general, there are significant CH4 emissions from the
most productive tundra areas due to the predominantly
wet soil conditions. It is very likely that, at the landscape,
regional, and global scales, the tundra represents a
source of radiative forcing due to emissions of CH4,
which is the most important greenhouse gas driving the
ecosystem influence on atmospheric radiative forcing.
7.5.2. Current circumpolar water and
energy balances
Arctic ecosystems exhibit the greatest seasonal changes in
energy exchange of any terrestrial ecosystem because of
the large changes in albedo from late winter, when snow
reflects most incoming radiation (albedo about 0.7), to
summer when the ecosystem absorbs most incoming
radiation (albedo about 0.15).This change in albedo
combined with greater incoming solar radiation in summer
than in winter causes much greater energy absorption
in summer than in winter.About 90% of the energy
absorbed during summer is transferred to the atmosphere,
with the rest transferred to the soil in summer
and released to the atmosphere in winter (Eugster et al.,
2000).Also, snow within shrub canopies is deeper and
less dense, which reduces heat transfer through the snowpack
and increases winter soil temperatures by 2 ºC relative
to adjacent shrub-free tundra. Consequently, arctic
ecosystems have a strong warming effect on the atmosphere
during the snow-free season, and any increase in
the duration of snow-free conditions results in a strong
positive feedback to regional climate warming (Bonan et
al., 1995; Foley et al., 1994).
Climate influences the partitioning of energy between
sensible and latent heat fluxes. Cold moist air from coastal
oceans, for example, minimizes latent heat flux (evapotranspiration),
as does extremely warm dry air, which can
induce stomatal closure (Eugster et al., 2000; Rouse,
2000); evapotranspiration is therefore greatest at intermediate
temperatures. Conversely, sensible heat flux is a larger
proportion of the energy transfer to the atmosphere
when air is cold and moist or when drought limits stomatal
conductance under dry conditions. Heat that is conducted
into the ground during summer is released to the
atmosphere in winter, with any seasonal imbalance causing
changes in permafrost temperature leading to a probability
of thermokarst (Osterkamp and Romanovsky, 1999).
Fig. 7.31. Accumulated carbon and global warming potential
(GWP, calculated as CO2 equivalent) from CO2 and CH4 fluxes
measured throughout the summer of 1997 at Zackenberg, northeast
Greenland (data from Friborg et al., 2000; Søgaard et al., 2000).
Chapter 7 • ArcticTundra and Polar Desert Ecosystems 319
There are large regional differences among arctic ecosystems
in energy exchange and partitioning.Albedo during
the period of snow cover is extremely high in tundra and
declines with increasing development of a plant canopy
above the snow, from tundra to shrub tundra to forest
tundra to deciduous forest to evergreen forest (Betts A.
and Ball, 1997).These differences in albedo are an
important feedback to climate during spring, when the
ground is snow-covered, and incoming radiation is high.
As a result of differences in albedo and sensible heat
flux, forests at the arctic treeline transfer about 5W/m2
more energy to the atmosphere than does adjacent tundra
(Beringer et al., 2001b).This difference in energy
transfer to the atmosphere depending on vegetation type
is an order of magnitude less than the heating contrast
that had been hypothesized to be required for treeline to
regulate the position of the Arctic Front (Pielke and
Vidale, 1995).Thus, the location of the Arctic Front is
more likely to govern the position of the treeline than
the other way around (Bryson, 1966).
7.5.3. Large-scale processes affecting future
balances of carbon, water, and energy
This section assesses the effects of climate change on
permafrost degradation and vegetation redistribution as
a prerequisite for assessing changes in feedbacks from
future terrestrial ecosystems to the climate system.
7.5.3.1. Permafrost degradation
Soil C storage is greatest where the drainage is slight and
the limited precipitation is held near the surface by
permafrost and modest topography.This results in ponds,
wetlands, and moist tundra with a saturated seasonal
active layer that limits microbial activity. Increases in
active-layer depth can cause subsidence at the surface, a
lowering of the soil water table (Hinzman et al., 2003),
and, potentially, thermokarst erosion (Hinzman et al.,
1997).This can drain surrounding areas, often increasing
the decomposition rate of soil organic matter, which
accelerates the loss of belowground C stores (Oberbauer
et al., 1991, 1996) and results in a change in plant communities
and their abilities to sequester atmospheric
CO2. Initially, increased soil decomposition rates can
increase mineralization rates (Rastetter et al., 1997) and
result in increased NPP (section 7.4.2.1). However, continued
thawing of permafrost and increased drainage of
surface water in areas with low precipitation are likely to
lead to drier soils, a decrease in NPP, and possibly even
desertification (section 7.5.3.2).
Full permafrost disintegration in subarctic discontinuous
permafrost regions may in some cases show a different
response. Monitoring of changes in permafrost distribution
in subarctic Sweden as part of the Circumpolar
Active Layer Monitoring Program (Brown et al., 2000)
shows that permafrost loss causes mires to shift from
ombrotrophic moss- and shrub-dominated systems to
wetland systems dominated by minerotrophic vascular
plants (ChristensenT. et al., 2004; Svensson et al.,
1999).This, in turn, leads to a significant lowering of
soil redox potentials, an increase in anaerobic decomposition,
and increased CH4 emissions.Wet minerotrophic
soils and vegetation are in general associated with the
highest CH4 emissions in subarctic and arctic tundra
environments. Discontinuous permafrost regions are
considered some of the most vulnerable to climate
warming; given projected temperature increases over
the next 100 years, effects such as the changes in vegetation
composition discussed here are likely.
Permafrost degradation and disintegration are therefore
very likely to have major effects on ecosystem C balances
and CH4 emissions.The rate of permafrost thawing,
the amount of ground surface subsidence, and the
response of the hydrological regime to permafrost
degradation will depend on numerous site characteristics.
Changes in the hydrological regime will also alter
the soil thermal regime. In areas with significant topographic
variations, flowing water can carry heat into
drainage channels, causing increased soil temperatures
and increased active-layer thickness (Hastings et al.,
1989; Kane et al., 2001). In regions with minor topographic
variations, subtle differences in elevation can
create cooler, saturated wetlands (as mentioned in the
previous paragraph) or markedly drier, warmer uplands
(Rovansek et al., 1996).
7.5.3.2. Changes in circumpolar vegetation zones
While climate-driven changes in the structure and distribution
of plant communities affect trace gas fluxes and
water and energy balances at the landscape scale (section
7.4.2), changes in the location and extent of broad vegetation
zones is a longer-term integrative process that is
likely to lead to regional and even global impacts on
feedbacks to the climate system (Betts R., 2000; Chapin
F. et al., 2000a; Harding et al., 2002; McGuire et al.,
2002). Such vegetation zone changes are also likely to
affect permafrost dynamics (section 6.6), biodiversity
(section 7.3.1), and ecosystem services (Vlassova,
2002). Past climate-driven changes in vegetation zones
such as forest and tundra (section 7.2; Payette et al.,
2002) lead to the expectation that future climate warming
is very likely to result in vegetation and ecosystem
change, but projecting future changes is complex and
relies on modeling.
Dynamics of the treeline and changes in the
areas of tundra and taiga vegetation
The latitudinal treeline or tundra–taiga boundary is an
exceptionally important transition zone in terms of
global vegetation, climate feedbacks, biodiversity, and
human settlement.
The treeline stretches for more than 13000 km around
the Northern Hemisphere and through areas that are
experiencing different types of environmental change, for
example, cooling, warming, or marginal temperature
change and increasing or decreasing land use. Climate is
320 Arctic Climate Impact Assessment
only one of a suite of environmental factors that are now
changing, and a critically important challenge is to determine
how human impacts in the ecotone will modify its
projected response to climate change (Vlassova, 2002).
The lack of standardized terminology and the wide variation
in methodology applied to locate, characterize, and
observe changes in the boundary have resulted in a poor
understanding of even the current location and characteristics
of the boundary. Particular areas of uncertainty
include the Lena Delta of Siberia (Callaghan et al.,
2002a) and forests in Iceland that have been subjected to
major environmental and land-use changes since colonization
by people about 1100 years BP (section
14.3.4.4). One of the major problems with current
studies of the latitudinal “treeline” is the concept of “line”
inappropriately applied to the transition from forest,
through an area dominated by forest in which patches of
tundra occur, to tundra in which patches of forest occur,
and then eventually to tundra without trees. Often there
are east–west gradients related to the presence of a river
valley, bogs, mires, uplands, etc. that also confound the
concept of a linear boundary.
Dynamics of the boundary
Current and projected changes in the location of the
tundra–taiga boundary should be seen in the context of
the longer-term past cooling trend during which the
treeline has been at its lowest latitudinal and altitudinal
locations for several thousands of years (section 7.2.4).
Examples of recent treeline advance include a 40 m
increase in the elevation of the subarctic treeline in
northern Sweden during the 20th century
(Emanuelsson, 1987; Kullman, 1979; Rapp, 1996;
Sonesson and Hoogesteger, 1983), an increase in shrub
growth in Alaska (Sturm et al., 2001b), and an increase
in shrubbiness and larch advance in the northeast
Russian European Arctic (Katenin, unpub. data, 2004).
In contrast, other studies show a surprising southward
displacement of the tundra–taiga boundary (Crawford
et al., 2003; Kozhevnikov, 2000;Vlassova, 2002). Part
of this is a counter-intuitive response to warming in
which an increasingly oceanic climate (e.g., in the western
Russian Arctic: see section 8.4.1, Fig. 8.9) together
with permafrost thawing have led to paludification
(waterlogging and peat formation) and the death of
treeline trees (Skre et al., 2002). Part is associated with
human activities, including mining, farming, and
forestry, that have led to ecosystem degradation in the
forest tundra zone and the movement of its northern
boundary southward in some locations (Vlassova,
2002). In the Archangelsk region and the Komi
Republic, the southern border of the forest tundra zone
now lies 40 to 100 km further south than when previously
surveyed. One report claims that anthropogenic
tundra now covers about 470000 to 500000 km2 of the
forest tundra stretching from Archangelsk to Chukotka
(Vlassova, 2002), although it is likely that this estimate
includes deforestation in some of the northern boreal
forest zone.
Although records of recent changes in the location of the
latitudinal treeline are surprisingly rare, there is good evidence
of increased growth in current northern forests.
Comparisons of the greenness index (NDVI) from satellite
images show that May to September values for the
Northern Hemisphere between 55º and 75º N increased
by around 5 to 50% from 1982 to 1990 (Myneni et al.,
1997; Fig. 7.25), with greater increases in North America
than in Eurasia.The increased greenness was associated
with an increase in growing-season length of between 3.8
and 4.3 days for the circumpolar area, mainly due to an
earlier start of the growing season.
Projecting future changes in the tundra–taiga boundary
In order to model changes in the location of the tundra–
taiga ecotone and estimate future areas of tundra to the
north and taiga to the south, it is necessary to understand
the causes of the treeline. Opinions on the mechanisms
controlling the location of the treeline vary greatly.
Some researchers see the limit of tree growth as a
universal mechanism related to a specific process such as
sink (C and nutrient) limitation (Körner, 1998, 1999) or
C limitation (Nojd and Hari, 2001). Others see a range
of possible mechanisms that operate in different places
and at different times (Sveinbjörnsson et al., 2002).
These mechanisms are in turn affected by environmental
factors such as incident radiation, temperature, wind,
moisture, and soil nutrients, which affect tree reproduction,
seedling establishment, and the growth and physiology
of mature trees. Extreme conditions such as icecrystal
abrasion and soil movement also directly damage
tree tissues (e.g., conifer needles) and displace individuals.
Diseases, pests, fires, and human activities all exert
some control on the treeline at certain places and at certain
times (Chapter 14).
Models of vegetation redistribution resulting from global
change utilize more general driving variables, related to,
for example, biogeography and biogeochemistry. Most
current global vegetation models and regional models
project that a major part of the tundra (between 11 and
50% according to location) will be displaced by a northward
shift of the boreal forest over the period in which
atmospheric CO2 concentrations double (Harding et al.,
2002; Kaplan et al., 2003; Skre et al., 2002;Table 7.13;
section 7.6,Table 7.14; Fig. 7.32).The treeline is projected
to move north in all sectors of the Arctic, even in
Greenland and Chukotka where only fragments of forest
exist today (Kaplan et al., 2003). However, to date there
have been fewer observations of this type of forest
response than projections suggest, even though temperature
has already risen dramatically in some areas.
The observations of the latitudinal treeline that show a
recent southern displacement suggest that there is very
unlikely to be a general northward displacement of the
latitudinal treeline throughout the circumpolar region as
the models project. In addition to potential paludification
(Crawford et al., 2003) and local human activities displacing
treelines southward, permafrost thawing, surface-
Chapter 7 • ArcticTundra and Polar Desert Ecosystems 321
water drainage, and soil drying in areas of low precipitation
are likely to lead to the formation of vegetation similar
to the tundra–steppe (Yurtsev, 2001; section 7.2.2).
Increased disturbances such as pest outbreaks, thermokarst,
and fire are also likely to locally affect the direction
of treeline response. In addition, some tree species show
reduced responsiveness to increases in temperature with
increasing continentality of their location and decreased
precipitation (Linderholm et al., 2003; section 14.7).
This suggests that increased temperatures with no comparable
increase in precipitation are likely to lead to
reduced tree growth and/or changes in species and lack
of treeline advance. Even in areas projected to undergo
warming with none of the moderating factors listed
above, it is uncertain if the rate of tree migration can
keep up with the rate of projected temperature increases.
Past tree migration rates were generally of the order of
0.2 to 0.4 km/yr but sometimes reached 4.0 km/yr (section
7.2; Payette et al., 2002).These rates suggest that in
those areas of the Arctic that have warmed substantially
in the last 30 years, treeline should already have advanced
by about 6 to 120 km. Such shifts have not been recorded
in the Arctic, although Parmesan andYohe (2003) claim
to have identified a poleward displacement of species
ranges of 6.1 km per decade globally.
Overall, it is likely that treeline will show many different
responses throughout the circumpolar north according
to different degrees of temperature change associated
with various changes in precipitation, permafrost
dynamics, land use, and tree species migration potential.
Projecting future changes in the areas of tundra
and polar desert
The LPJ model was used to project vegetation changes
in the northern areas of the Arctic (Sitch et al., 2003;
Table 7.13; section 7.6,Table 7.14), using climate scenarios
generated by four of the five ACIA-designated
models (CGCM2, GFDL-R30_c, HadCM3, and
ECHAM4/OPYC3; section 4.2.7) forced with the B2
emissions scenario (section 4.4.1).Although the results
and interpretations are preliminary, LPJ simulations are
consistent in projecting a decrease in the area of polar
desert that will be replaced by northward-moving tundra
(Table 7.13).The area of the Arctic covered by polar
desert is projected to decrease by 17.6% (range 14 to
23%) between 1960 and 2080. In this model, the two
vegetation zones were defined by plant functional types:
woody species for the tundra, and absence of woody
species for the polar desert. In the BIOME4 (coupled
biogeography and biogeochemistry model) simulations
by Kaplan et al. (2003), driven by output from the
HadCM2GSa1 model forced with the IS92a emissions
scenario, five tundra biomes were constructed (Table
7.8).The most significant changes appear to be a significant
northward advance of the cold evergreen needleleaf
forest that is particularly dramatic in the region of arctic
Russia between Chukotka and theTaymir Peninsula.
This greatly reduces the area of tundra. However, lowand
high-shrub tundra in the Canadian Archipelago
remains as a wide zone and displaces prostrate dwarfshrub
tundra (Figs. 7.2 and 7.32). Earlier modeling by
White et al. (2000) projected that the area of tundra
would be halved by forest expansion by 2100.
7.5.4. Projections of future balances of
carbon, water, and energy exchange
Because the Arctic contains huge stores of C in the soil
and permafrost (section 7.4.2), and because the Arctic
has the capacity for unlimited additional storage or significant
loss (Billings et al., 1982, 1984), it can be a
major positive or negative feedback to increasing tracegas
concentrations in the atmosphere and to global climate
change. Loss of CO2 from arctic ecosystems could
potentially lead to enormous positive feedbacks to global
climate change by release to the atmosphere of the
estimated 250 Pg C stored in the large arctic soil pool,
although this is unlikely to happen (Billings, 1987;
Average Range
Temperature change (°C)
2000–2100 5.0 4.7 to 5.7
Precipitation change (mm/yr)
2000–2100 42.9 9.0 to 78.0
NPP (Pg C/yr)
1960s 2.83 2.77 to 2.88
2080s 4.87 4.57 to 5.19
Change (%) 72.4 60.9 to 87.4
Change in C storage 1960–2080 (Pg C)
Vegetation 5.73 3.59 to 7.65
Soil 6.98 1.6 to 15.6
Litter 5.6 3.4 to 9.6
Total 18.3 12.2 to 31.3
Percent areal vegetation changea
Increase in taiga areab
1960–2020 4.4 3.1 to 5.3
1960–2050 7.4 6.4 to 8.4
1960–2080 11.3 9.8 to 14.4
Change in polar desert areac
1960–2020 -7.5 -13.3 to -4.2
1960–2050 -13.2 -18.5 to -10.6
1960–2080 -17.6 -23.0 to -14.2
aOnly a proxy as the change is derived from functional characteristics of the vegetation
produced by the model rather than projections of specific vegetation composition
per se. For a proper vegetation distribution estimate, it would be more
appropriate to use a coupled biogeography and biogeochemistry model such as
BIOME4; bbased on the percentage increase in woody plants simulated by LPJ;
cbased on the percentage reduction in bare ground simulated by LPJ
Table 7.13. Averages and ranges of the drivers and responses
of a leading dynamic global vegetation model (LPJ) driven with
the outputs for the terrestrial Arctic (>60º N) from four different
climate models (CGCM2, GFDL-R20_c, HadCM3, ECHAM4/
OPYC3) forced with the B2 emissions scenario (Sitch et al.,
2003; Sitch et al., submitted).
322 Arctic Climate Impact Assessment
Billings et al., 1982; Lal et al., 2000; Oechel and
Billings, 1992; Oechel et al., 1993, 1995, 2000a).
In addition, an increasing snow-free period (Engstrom et
al., 2002; Keyser et al., 2000), increasing shrub cover
(Silapaswan et al., 2001; Sturm et al., 2001a,b), and the
northward migration of the treeline (Lloyd A. et al.,
2003) are very likely to decrease arctic albedo and further
increase regional warming (Bonan et al., 1992;
Chapin F. et al., 2000a,b; Foley et al., 1994; McFadden
et al., 1998;Thomas and Rowntree, 1992).This section
assesses likely changes in the C balance and water and
energy exchange in relation to vegetation change.
7.5.4.1. Carbon balance
The vegetation distribution model BIOME3, forced
with present-day and doubled atmospheric CO2 concentrations,
projected decreases in the extent of the
Scandinavian, central northern Siberian, and Eurasian
tundra areas ranging from 10 to 35% as a result of displacement
by taiga (Harding et al., 2002).This vegetation
change was projected to significantly increase CO2
uptake and reduce CH4 emissions, with net C sequestration
in the biosphere of a magnitude (4.6 Pg C) that
would alter the radiative forcing of the earth. Using
another model, McGuire et al. (2000) estimated that
circumpolar mean C uptake would increase from a
current 12 g C/m2/yr to 22 g C/m2/yr by 2100, because
NPP increased more than respiration throughout the
period (McGuire et al., 2000). It should be noted,
however, that throughout the 200-year model run, the
standard deviation of C uptake always crosses the zero
line (McGuire et al., 2000).White et al. (2000)
obtained comparable results from their Hybrid v4.1
model, projecting that high-latitude terrestrial ecosystems
would remain a sink for C.
The LPJ model (Sitch et al., 2003; Box 7.1) was used
to produce estimates of future changes in arctic C storage
and fluxes for this assessment, based on four different
general circulation model (GCM) outputs.The
results and analyses are preliminary but indicate a consistent
net additional sink in the Arctic in 2080 compared
to 2000, with increases in accumulated arctic C
storage varying between 12 and 31 Pg C depending on
the climate scenario used. Figure 7.33 shows the C
storage anomalies projected by the LPJ model, and
Table 7.14 (section 7.6) shows further details of the
regional subdivision of these projections.
There are great uncertainties associated with these estimates
due to the complex differential response of NPP
and respiration to the climate drivers (temperature,
precipitation), which themselves are highly spatially
variable and interact. However, the general response of
the model seems to be as follows. In areas with little or
no vegetation (e.g., polar desert), increasing CO2 concentrations
and temperature (e.g., increasing growingseason
length) lead to increased vegetation growth and
northward plant migration, resulting in an increase in C
stocks.This seems to be a general pattern acting
through increased productivity throughout the Arctic,
all else being equal (see NPP projections inTable 7.13).
However, increased temperature leads also to increased
heterotrophic (soil microorganism) respiration.
Therefore, areas that at present contain large soil C
stocks are likely to release larger amounts of C from
the soil as respiration responds to a warmer climate.
Whether these areas become net sources or sinks
depends on the balance between temperature-enhanced
respiration and increased productivity (hence increased
biomass and litterfall) due to increased CO2 concentrations
and longer growing seasons.When the LPJ model
was forced with the climate projections of the
ECHAM4/OPYC3 model, which projects very large
temperature increases, projected respiration was
enhanced more than productivity. Over the entire
Arctic, C storage is projected by the LPJ model to balance,
due to northward migration of plants, etc., with
C loss in areas that experience large temperature
changes and have large stocks of soil C. On the whole,
all runs of the LPJ model project that C storage will
increase.The scenario with the highest projected temperature
change (ECHAM4/OPYC3) results in the lowCushion
forb, lichen, and moss tundra
Graminoid and forb tundra
Prostrate dwarf-shrub tundra
Erect dwarf-shrub tundra
Low- and high-shrub tundra
Cold evergreen needleleaf forest
Cold deciduous forest
Cool evergreen needleleaf forest
Cool mixed forest
Cool–temperate evergreen needleleaf and mixed forest
Temperate evergreen needleleaf forest
Temperate deciduous broadleaf forest
Temperate grassland and xerophytic shrubland
Barren
Ice
Fig. 7.32. Potential vegetation distribution in 2090–2100 as
projected by the BIOME4 model driven with output from the
HadCM2GSa1 model forced with the IS92a emissions scenario
(Kaplan et al., 2003).
Chapter 7 • ArcticTundra and Polar Desert Ecosystems 323
est overall C gain, while the scenario with the lowest
projected temperature change (CGCM2) results in the
highest C gain.
The current estimated circumpolar emissions of CH4 are
in the range of 20 to 60Tg CH4/yr.These have a significant
potential for feedback to a changing climate. Largescale
CH4 flux models are currently not as advanced as
general carbon-cycle models and few allow for projections
of future change based on climate change scenarios.
Early attempts to assess and model tundra CH4
emissions driven by climate change all indicated a potential
increase in emissions (ChristensenT. and Cox, 1995;
Harriss et al., 1993; Roulet et al., 1992) but more
recent improved mechanistic models (Granberg et al.,
2001;Walter and Heimann, 2000) have not yet been followed
up by full coupling to GCM projections to assess
future circumpolar CH4 emissions.Another critical factor
is the geographic extent of wetlands and how these
may change in the future.There is, however, little doubt
that with climate scenarios of warming and wetting of
arctic soils, CH4 emissions are very likely to increase,
while with warming and drying, emissions are likely to
change little or decline relative to current emissions.
Lakes and streams cover large portions of many arctic
landscapes and, due to low evapotranspiration, runoff is
a major component of arctic water budgets.These surface
freshwaters contain large amounts of dissolved
organic and inorganic C that is carried into them by soil
and groundwater flow from the terrestrial portions of
their watersheds (Kling et al., 1992, 2000).The inorganic
C is largely CO2 produced by soil and root respiration.
Organic C concentrations in soil water, groundwater,
and surface waters are typically several times
greater than inorganic C concentrations and are a major
source of respiratory CO2 produced in lakes and
streams, thus adding to their already high dissolved
inorganic C content.
Because the dissolved CO2 in surface waters is typically
supersaturated with respect to the atmosphere, and the
surface area and flow of freshwater is large, surface
waters of arctic landscapes emit large amounts of CO2
to the atmosphere (Kling et al., 1991; section 8.4.4.4).
Estimates of CO2 emissions from surface waters are as
large as 20 to 25% of gross landscape CO2 fixation and
thus may be a major component of landscape C balance
that is not accounted for in studies that include terrestriBox
7.1.The Lund-Potsdam-Jena model
The Lund-Potsdam-Jena (LPJ) dynamic global vegetation model combines process-based, large-scale representations
of terrestrial vegetation dynamics and land–atmosphere carbon and water exchanges in a modular framework.
Features include feedback through canopy conductance between photosynthesis and transpiration, and
interactive coupling between these rapid processes and other ecosystem processes including resource competition,
tissue turnover, population dynamics, soil organic matter and litter dynamics, and fire disturbance.Ten plant
functional types (PFTs) are differentiated by physiological, morphological, phenological, bioclimatic, and fireresponse
attributes. Resource competition and differential responses to fire between PFTs influence their relative
fractional cover from year to year. Photosynthesis, evapotranspiration, and soil water dynamics are modeled on a
daily time step, while vegetation structure and PFT population densities are updated annually.
Within the biosphere model (Sitch et al., 2003), the raw GCM (CGCM2, GFDL-R30_c, HadCM3, and ECHAM4/
OPYC3 forced with the B2 emissions scenario) climatologies for 1900 to 2100 were not used directly.The climate
simulated by present-day GCMs is not spatially detailed enough to directly drive a biosphere model, therefore the
anomaly approach was used.The data were downscaled from the GCM-specific grid onto a grid with 0.5º resolution.
Climate anomalies projected by the GCMs were normalized to the 1961–1990 observed average monthly
Climatic Research Unit climatology (CRU CL 1.0: New et al., 1999).
HadCM3 GFDL-R30_c CGCM2 ECHAM4/OPYC3
Fig. 7.33. Carbon storage anomalies between 1960 and 2080 projected by the LPJ model using climate scenarios generated
by four of the five ACIA-designated models (Sitch et al., 2003).
Anomaly
(kg C/m2)
5
2
1
0
-1
-2
-5
-15
324 Arctic Climate Impact Assessment
al CO2 fluxes only. Similar large CO2 losses also occur in
freshwaters of boreal, temperate, and tropical landscapes
(Cole et al., 1994), but they are generally not considered
in landscape-level C budgets.At present, little is
known about controls over these CO2 losses or how they
might change with changes in climate or water balance.
Attempts to measure the losses directly have yielded
inconsistent results (Eugster et al., 2003).
7.5.4.2. Energy and water exchange
Many of the likely changes in water and energy
exchange that occur in response to projected future
warming are likely to act as a positive feedback to
warming. Earlier disappearance of snow from the tundra
is very likely to lead to a decline in albedo and an
increase in regional warming (Bonan et al., 1992;
Thomas and Rowntree, 1992). Similarly, an expansion
of forest will lead to a reduction in albedo, because
trees mask a snow-covered surface. In areas where forest
expansion occurs, this is very likely to lead to significant
heating of the lower atmosphere (sections 7.4.2.3 and
7.5.2). Paleoclimate modeling experiments have shown
that the northward movement of the treeline 6000 years
BP accounted for half of the climatic warming that
occurred at that time (Foley et al., 1994).Although the
current arctic treeline appears relatively stable or
retreating in some areas of human impact (Callaghan et
al., 2002a;Vlassova, 2002; section 7.5.3.2), any future
northward advance of the treeline is likely to contribute
to regional warming, while treeline retreat is likely to
contribute to regional cooling, particularly in late spring
due to the large differences in albedo between snowcovered
tundra and adjacent forest.
A positive feedback (leading to increased warming) of
displacement of tundra by trees and shrubs could possibly
offset the negative feedback (leading to cooling) due
to increased C sequestration at the local level (Harding
et al., 2002), but the climate forcing by energy and
water exchange operates primarily at the regional scale,
where the energy exchange occurs, whereas the negative
feedback due to sequestration of atmospheric C is likely
to vary between regions and contribute to warming
through changes in the globally mixed pool of atmospheric
CO2. Models suggest that forests in the eastern
Canadian Arctic would have a net negative feedback
through sequestration of C whereas forests in arctic
Russia would have a net positive feedback to climate
through decreased albedo (Betts R., 2000; Betts A. and
Ball, 1997).This complex balance between opposing
feedbacks indicates that encouraging forest to displace
tundra as an appropriate strategy to mitigate global climate
change should take into account the local feedback.
An important contributing factor to the effect of vegetation
change on albedo is the characteristics of the plant
canopy in terms of canopy height relative to snow
height, leaf duration, and leaf optical properties.
The greatest changes in albedo are likely to occur after
increases in area relative to tundra vegetation of the following
vegetation types (in decreasing order of effect):
dark, evergreen boreal trees such as pine and spruce,
deciduous conifer trees such as larch, deciduous
angiosperm trees such as birch, and low shrubs such as
willows and dwarf birch.
The vegetation changes projected to occur in northern
Alaska in response to climatic warming are calculated to
increase summer heating of the atmosphere by 3.7W/m2
(Chapin F. et al., 2000a).This warming is equivalent to
the unit-area effect of a doubling of atmospheric CO2
concentrations or a 2% increase in the solar constant
(i.e., the difference that caused a switch from a glacial to
an interglacial climate), two forcings that are known to
have large climatic effects (Kattenberg et al., 1996).
Regional climate simulations suggested that a conversion
from moist tussock tundra to shrub tundra would cause a
1.5 to 3.5 ºC increase in mean July temperature on the
Alaskan North Slope, reflecting greater sensible heat
fluxes to the atmosphere from the shrub-dominated
ecosystem.Thus vegetation changes of the sort that have
recently been observed (Sturm et al., 2001b) are very
likely to have large positive feedbacks to regional
warming if the increased shrub cover is extensive.
This vegetation–climate feedback requires only modest
increases in shrub density to enhance sensible heat flux
(McFadden et al., 1998).
The transition from tundra to forest also affects evapotranspiration
and the water storage capacity of the biosphere,
such that freshwater runoff via rivers to the Arctic
Ocean could possibly decrease (Harding et al., 2002).
Other human activities also have impacts on the local
climate of the forest tundra. Deforestation, as a result of
industrial activities or forestry, increases wind speeds;
pollution leads to earlier snowmelt and increased temperatures,
and the northward extension of farming and
settlements in general induces permafrost thawing
(Vlassova, 2002).
7.5.5. Summary
Biological and physical processes in the Arctic operate at
various temporal and spatial scales to affect large-scale
feedbacks and interactions with the earth system.There
are four main potential feedback mechanisms between
the impacts of climate change on the Arctic and the
global climate system: albedo, greenhouse gas emissions
or uptake by ecosystems, greenhouse gas emissions from
methane hydrates, and freshwater fluxes that affect the
thermohaline circulation.All of these feedbacks are controlled
to some extent by changes in ecosystem distribution
and character and particularly by large-scale movement
of vegetation zones. However, it is difficult to
assess the consequences of the interacting feedbacks or
even of individual feedbacks.
There are currently too few full annual measurements
available to give a solid answer to the question as to
whether the circumpolar Arctic is an atmospheric source
Chapter 7 • ArcticTundra and Polar Desert Ecosystems 325
or a sink of CO2 at the landscape scale (Box 7.2).
Indications are, however, that source areas currently
exceed sink areas. Measurements of CH4 sources are
also inadequate, but the available information indicates
emissions at the landscape level that are of great importance
to the total greenhouse gas balance of the circumpolar
north. In addition to the effect of greenhouse
gases, the energy and water balances of arctic landscapes
encompass important feedback mechanisms in a changing
climate. Increasing density and spatial expansion of
the vegetation cover will reduce albedo and cause more
energy to be absorbed on the ground; this effect is likely
to exceed the negative feedback of increased C sequestration
via greater primary productivity.The degradation
of permafrost has complex consequences. In areas of discontinuous
permafrost, warming is very likely to lead to
a complete loss of the permafrost. Depending on local
hydrological conditions, this may in turn lead to a wetting
or drying of the environment with subsequent
implications for greenhouse gas fluxes. Models projecting
vegetation change in response to scenarios of future
climate change indicate a 7 to 18% decrease in the area
occupied by polar desert and a 4 to 11% increase in the
area occupied by taiga over the next 80 years.This is
projected to lead to increased carbon storage over this
same period due to productivity being stimulated more
than respiration. However, the balance depends on the
degree of projected warming.With greater temperature
increases, heterotrophic respiration is stimulated more
and the projected carbon gain is less.Very few models
are available for projections of future CH4 emissions,
but these emissions will be extremely important to the
total greenhouse gas balance and functioning of the
circumpolar Arctic.
Box 7.2.Will the Arctic become a source of carbon, or remain a sink?
There is not yet a definitive answer to this question, although past opinions favored the hypothesis that the
tundra will switch from being a sink for carbon during recent millennia to becoming a source under future
warming scenarios, based mainly on the response of increased soil respiration to warming relative to increases in
photosynthesis. At a recent meeting of experts on carbon dynamics in the Arctic, the following authoritative
consensus statements were made.
Consensus statements on arctic carbon source/sink functioning from the Synthesis Workshop on “Current and
future status of carbon storage and ecosystem–atmosphere exchange in the circumpolar north: Processes,
budgets and projections”, Skogar, Iceland, 21 June 2003.
• The available carbon flux data show large interannual variability. Arctic terrestrial ecosystems are a
patchwork, with some regions being sources of carbon to the atmosphere (mostly dry and mesic
ecosystems) and some regions being sinks (mostly wet tundra). Current indications are that source areas
exceed sink areas.
• The available data indicate that when considering both CO2 and CH4, the Arctic is a source of radiative forcing.
• Contrary to the data from ground-based measurements, current global carbon models indicate that the
Arctic is a small carbon sink.This apparent discrepancy is, however, within the uncertainty range of the data
and model outputs.
• The recent remote-sensing record indicates a greening of the Arctic, suggesting increased photosynthetic
activity and net primary productivity, but does not address the belowground processes (i.e., respiration).
• Projections from global models of vegetation and soil responses to climate change suggest that enhanced
vegetation production will exceed increases in decomposition, thereby resulting in net carbon sequestration.
However, there are large uncertainties, including the response of heterotrophic respiration, nutrient cycling,
permafrost dynamics, land-cover change, and scaling issues.
• Experimental data indicate that warming enhances carbon loss under dry soil conditions and enhances
carbon sequestration under wet soil conditions.Thus, future responses to regional warming will depend
substantially on changes in soil moisture.
• Recent observations indicate that cold-season greenhouse gas emissions contribute substantially to the
annual budget.
The new, but uncertain, insights from these statements are that the Arctic is already a source of carbon and
radiative forcing, and that it is likely to become a weak sink of carbon during future warming. Other uncertainties,
not addressed in the consensus statements, include:
• potential complex flux patterns over long time periods during which acclimation of carbon fluxes to
warming leads to transient trends within millennia-long smoothing of shorter-term dynamics;
• probable increased physical and biotic disturbance during warming that might increase carbon emissions,
particularly if carbon storage shifts from tundra soils to invading forests; and
• various feedbacks between changing vegetation, soil temperatures, quality and quantity of litter, and
biodiversity of decomposer organisms.
Many uncertainties remain, and it should be noted that the carbon considered in this discussion is that in the
active layer of the soil and in vegetation, and not that trapped in permafrost and methane hydrates.
326 Arctic Climate Impact Assessment
Table 7.14. Summary baseline information for the four ACIA regions.
Region 1 Region 2 Region 3 Region 4
Arctic Europe,East
Greenland,European
Russian North,and
NorthAtlantic
Central Siberia Chukotka, Bering
Sea,Alaska, and
western Arctic
Canada
Northeast Canada,
Labrador Sea, Davis
Strait, and West
Greenland
Projected environmental changes
Mean Annual Temperaturea (ºC)
Baseline 1981–2000 -17 to 16 -8 to 4 -8 to 12 -20 to 12
2020 (change from baseline) -1 to 3 0 to 2.5 -0.5 to 3.0 -1 to 4
2050 (change from baseline) -0.5 to 4.0 0.5 to 4.0 0 to 4 0 to 7
2080 (change from baseline) 1 to 7 1 to 6 0.5 to 6 0 to 8
Precipitationa (mm/month)
Baseline 1981–2000 10 to 150 10 to 70 10 to 150 5 to 130
2020 (change from baseline) -10 to 12 -8 to 5 -10 to 8 -10 to 20
2050 (change from baseline) -20 to 20 -2 to 5 -4 to 12 -8 to 35
2080 (change from baseline) -20 to 25 0 to 10 -3 to 12 -15 to 35
Increase in UV-B radiation levels from 1979–1992 baseline (%)b
Mean for 2010–2020 0 to 10 0 to 8 0 to 2 0 to 14
Mean for 2040–2050 0 to 2 0 to 2 0 to 2 0 to 2
Change in albedoc (due to vegetation change)
2050 -0.10 to 0.05 -0.050 to 0.025 -0.10 to 0.025 -0.050 to 0.025
2080 -0.10 to 0.01 -0.050 to 0.025 -0.10 to 0.025 -0.10 to 0.025
Ecosystem processes projected by the LPJ modeld
NPP (Pg C/yr) See Table 7.13 for the arctic total
1960s 1.2 4.0 4.6 1.5
2080s 1.8 6.5 7.5 3.8
Change (%) 46.3 62.3 63.8 144.4
NEP: Change in C storage 1960–2080 (Pg C) See Table 7.13 for the arctic total
Vegetation 0.2 1.7 2.5 1.3
Soil -0.1 0.5 1.9 4.7
Litter -0.02 0.5 1.8 3.4
Total 0.04 (-0.7 to 0.8) 2.8 (-0.9 to 7.1) 6.2 (4.1 to 9.5) 9.3 (6.5 to 14.0)
Landscape processes projected by the LPJ modeld
Increase in taiga area (%)e,f
1960–2020 1.1 (-1.1 to 2.3) 6.1 (4.2 to 8.6) 4.2 (1.9 to 5.7) 3.7 (2.9 to 4.4)
1960–2050 3.2 (1.6 to 4.3) 9.4 (7.5 to 10.2) 8.2 (6.5 to 9.7) 5.0 (3.8 to 6.2)
1960–2080 5.0 (3.7 to 5.9) 13.7 (11.1 to 17.3) 11.9 (9.8 to 15.1) 9.5 (6.8 to 12.5)
Change in polar desert area (%)e,g
1960–2020 -2.3 (-3.1 to -1.1) -6.9 (-11.0 to -3.9) -5.3 (-10.8 to -2.4) -12.7 (-23.0 to -7.2)
1960–2050 -3.5 (-4.8 to -2.7) -9.9 (-13.2 to -7.7) -11.0 (-15.7 to -8.9) -23.6 (-33.5 to -16.3)
1960–2080 -4.2 (-5.9 to -3.2) -11.4 (-14.6 to -10) -13.2 (-16.6 to -11.4) -35.6 (-47.8 to -25.9)
Biodiversity
Rare endemic vascular plant speciesh
2 18 69 8
Threatened vascular plant species (occurring at a
single unprotected location in each region)h
1 4 11 0
Threatened animal speciesi
2 4 6 1
afrom the ACIA-designated model simulations; bfrom Taalas et al. (2000); cBetts R. (2001) using IS92a emissions scenario; daverages and ranges (in parentheses) of the
responses of the LPJ model (Sitch et al., 2003) driven with outputs from four different climate models (CGCM2, GFDL-R30_c, HadCM3, ECHAM4/OPYC3) forced with the
B2 emissions scenario (section 4.4); eonly a proxy as the change is derived from functional characteristics of the vegetation simulated by the model rather than projections
of specific vegetation composition per se. For a proper vegetation distribution estimate, it would be more appropriate to use a coupled biogeography and biogeochemistry
model such as BIOME4; fbased on the percentage increase in woody plants simulated by LPJ; gbased on the percentage reduction in bare ground simulated by LPJ; hfrom
Talbot et al. (1999); iIUCN (2003).
Chapter 7 • ArcticTundra and Polar Desert Ecosystems 327
7.6. Synthesis: Scenarios of projected
changes in the four ACIA regions for
2020, 2050, and 2080
This synthesis draws on information in this chapter that
can be assessed within the four ACIA regions. Most of
the information is therefore based on model output.
Details of the regions and the logic determining them
are presented in section 18.3, while details of the models
that generate the UV-B radiation and climate scenarios
are presented in sections 5.6 and 4.2.7, respectively.
Many of the details relating to vegetation and carbon
dynamics are derived specifically for this section from
the LPJ model (Sitch et al., 2003), details of which are
presented in Box 7.1. Other aspects of the assessment
that cannot currently be divided into the ACIA regions
are summarized within and at the end of the various sections
of this chapter.
7.6.1. Environmental characteristics
The four ACIA regions (section 18.3) differ greatly in
their geography and climatology, which leads to variation
in future possibilities for the relocation of species and
ecosystems, and differences in scenarios of future changes
in climate and UV-B radiation levels (Table 7.14).
Geographically, Region 4 has a far greater extent of land
at high latitudes compared with the other regions.This is
likely to support northward migration of arctic biota
even if the Canadian Archipelago and the glacial landscape
of Greenland together with lack of suitable soils
will, to some extent, pose barriers to migration.
Relatively narrow tundra zones in some parts of Regions
1 and 3 could possibly, with sea-level rise and northward
boreal forest expansion, disappear and forest reach the
shore of the Arctic Ocean (Fig. 7.32). Region 1 contains
the relatively isolated high-arctic islands of Svalbard, and
the islands of Iceland and the Faroe Islands, that are likely
to experience delayed immigration of southern
species during warming. Both Iceland and the Faroe
Islands have equivocal positions within classifications of
the Arctic: the northern part of Iceland and the alpine
zones of the Faroe Islands (Fossa, 2003) have the
strongest arctic characteristics and climate warming is
likely to lead to altitudinal displacement of tundra-like
vegetation in both areas, and displacement from the
northern coastal area in Iceland.The imbalance of
species loss and replacement by species migrating more
slowly to islands is projected to lead to an initial loss in
diversity (Heide-Jørgensen and Johnsen, 1998).
The scenarios of temperature change generated by the
five ACIA-designated models show complex temporal
patterns, in some areas shifting from initial cooling to
substantial warming.The data used for the modeling of
vegetation zone displacement and carbon storage used a
different baseline period (1961–1990) than that used for
the ACIA-designated model projections (1981–2000) and
excluded output from the CSM_1.4 model. In addition,
the output from the LPJ model is provided for 2100,
rather than the 2071–2090 time slice used in the ACIA
scenarios (section 7.5). It is therefore difficult to compare
the results, even though both approaches had four
GCMs in common and used the same emissions scenario.
Changes in UV-B radiation levels are projected to vary
among regions, but only over the next 20 years.
By 2050, recovery of the stratospheric ozone layer is
projected to reduce UV-B radiation to relatively low levels
above present-day levels, with no differences among
the ACIA regions. Of course, this recovery depends
entirely on the success of management and regulation of
ozone-depleting substances. In the near future, however,
increases in UV-B radiation levels are projected to be
greatest in Region 4, followed by Regions 1 and 2
(Taalas et al., 2000).
Active-layer thickness is projected to increase by 20 to
60% by 2071–2100 (compared to the IPCC baseline,
1961–1990).The greatest percentage increases are projected
to occur in northern Siberia and the interior of
the Alaska–Yukon area. In general, the greatest relative
changes in the active layer are projected to occur in
those regions where the active layer is presently shallow
(section 6.6.1.3). Degradation of continuous permafrost
to discontinuous permafrost and the disappearance of
discontinuous permafrost are projected to occur at the
southern boundaries of all of the ACIA regions.
7.6.2.Vegetation zones and carbon balance
Region 1 (Arctic Europe, East Greenland, European
Russian North, and North Atlantic) includes many higharctic
areas but these are separated from terrestrial
ecosystems at lower latitudes by barriers of open sea.
The possibilities for future species relocation are limited,
even though moderate warming is projected here
(Table 7.14). In contrast, Region 2 (Central Siberia) has
continuous landmasses from the tropics to the high
Arctic.This region is currently warming, and scenarios
project that future warming will be greater here than
elsewhere.The possibilities for responses in ecosystem
distribution, structure, and C balance are therefore considerable.This
is illustrated by large projected increases
in taiga that displaces tundra in particular, and also in
projected decreases in polar desert that is displaced to
some extent by northward movement of the tundra
(Table 7.14).There is also a projected northward displacement
and reduction in prostrate dwarf-shrub tundra,
particularly inYakutsia and theTaymir Peninsula,
together with a displacement of erect dwarf-shrub tundra
from much of the Russian Arctic by low- and highshrub
tundra that is projected to expand markedly there
(Figs. 7.2 and 7.32). Region 3 (Chukotka, Bering Sea,
Alaska, and the western Canadian Arctic) has little higharctic
area and a large maritime influence. Increases in
temperature and precipitation are projected to be moderate,
as are changes in vegetation (Table 7.14). Region
4 (Northeast Canada, Labrador Sea, Davis Strait, and
West Greenland) is a region of fragmented landmasses
that are often extensively glaciated or have recently
328 Arctic Climate Impact Assessment
become deglaciated.This area has experienced recent
cooling, but a warming trend is projected to occur
between now and 2100. Increases in temperature and
precipitation are projected to lead to relatively small
increases in taiga (compared with other regions) but a
particularly large (~36%) loss of polar desert by 2080.
In terms of C storage, all regions are projected to accumulate
C, largely because of the replacement of bare
ground by tundra. Consequently, the greatest carbon
gain is projected to occur in Region 4 (Table 7.14;
Fig. 7.33). In contrast, the smallest gains – but still gains
– are projected to occur in Region 1, which has the
smallest projected increase in temperature.
7.6.3. Biodiversity
Biodiversity is affected by habitat fragmentation.
Scenarios of the impacts of projected human infrastructure
development on arctic flora and fauna suggest that
in the Arctic, these impacts are likely to extend for 4 to
10 km away from the infrastructure (Nellemann et al.,
2001).This is a much wider zone of influence than in
other regions of the earth. Nellemann et al. (2001) calculated
that 50 to 80% of the Arctic is likely to be affected
by infrastructure development by 2050. Of course,
infrastructure development varies among the ACIA
regions and this remains to be characterized. However,
threats to flora and fauna are likely to be increased by
the additive or possibly interactive effects of infrastructure
development and climate change.
The number of rare endemic vascular plant species in
the Arctic varies greatly between the regions (Table
7.14). Region 1 has relatively little land mass and supports
only two of the rare endemic vascular plant
species. Region 4, which encompasses a significant proportion
of the high Arctic, contains 8% of the species
and Region 2 contains 18%. In contrast, over 70% of the
rare endemic vascular plant species are found in Beringia
(Region 3).Twenty-four species are found onWrangel
Island (Talbot et al., 1999).A recent modification
(Talbot and Murray, 2001) of the list of threatened arctic
plant species (Talbot et al., 1999) adds a further 63 plant
species, but data have not yet been compiled on their
distributions within the ACIA regions.AlthoughTable
7.14 shows clear regional differences in the distribution
of rare and endemic plant species, and also a surprisingly
high number of these species, it should be noted that the
taxonomic treatment of species is likely to vary from
region to region and there is uncertainty about the taxonomic
status of some of the species.
It is not clear to what extent the rarity of the species
listed inTables 7.7 and 7.14 will be affected by climate
change, as many other factors determine rarity.
However, species concentrated in small areas such as
Wrangel Island are particularly at risk from any future
climate warming and species invasion.
The likely impacts of climate change on biodiversity in
terms of threatened species require new concepts of
“threatened species” and “protection” of currently perStrict
nature reserve, wilderness
area, or national park
Natural monument or
habitat/species management area
Protected landscape/seascape or
managed resource protected area
Areas with projected future
changes in vegetation
Regions:
1. Arctic Europe, East
Greenland, European
Russian North, North
Atlantic
2. Central Siberia
3. Chukotka, Bering Sea,
Alaska, western Arctic
Canada
4. Northeast Canada, Labrador
Sea, Davis Strait,West
Greenland
11
2
3
2
4
3
4
Fig. 7.34. Existing protected areas in the Arctic
(CAFF, 2001) overlain with projected changes in
vegetation (derived from Kaplan et al. (2003);
Figs. 7.2 and 7.32).
Chapter 7 • ArcticTundra and Polar Desert Ecosystems 329
ceived threatened species (Fig. 7.34; Chapter 11).
The numbers of species currently perceived as threatened
vary between regions. Region 3 contains significantly
more rare plant species and threatened animal and plant
species than other regions.Although temperature and
precipitation changes are likely to be less in this region
than in others, the vulnerability of the biodiversity of this
area is likely to be considerable. Northward expansion of
dwarf-shrub and tree-dominated vegetation into an area
such asWrangel Island that is rich in rare endemic
species could possibly result in the loss of many plant
species.Although some of these might not be considered
vulnerable because they are currently in “protected”
areas, this protection is against local human activities
(e.g., hunting, infrastructure development, etc.) and protection
cannot extend to changes in climate and UV-B
radiation levels. It is possible that some plant species,
particularly outliers of more southerly distributions,
might experience population expansion or reproduction
and recruitment to populations, leading to initial expansion
in response to warming. However, displacement of
herbaceous species by woody immigrants is possible in
the long term in mesic areas. In contrast to the possibility
that some threatened species will proliferate in a warmer
climate, some currently widespread species are very likely
to become less abundant and even “threatened”.
The greatest long-term threat to arctic diversity is the
loss of arctic habitat (section 7.2.6). In locations where
the tundra zone is narrow, boreal forest moves northward,
and the ocean moves southward due to sea-level
rise, there is very likely to be, over a period of centuries,
a loss of arctic ecosystems and the species that
characterize them.
7.7. Uncertainties and recommendations
7.7.1. Uncertainties
Current understanding of ecological processes and
changes driven by climate and UV-B radiation is strong
in some geographic areas and in some disciplines, but is
weak in others.Although the ability to make projections
has recently increased dramatically with increased
research effort in the Arctic and the introduction of
new technologies, current understanding is constrained
by various uncertainties.This section focuses on these
uncertainties and recommends ways in which they can
be reduced.
7.7.1.1. Uncertainties due to methodologies
and conceptual frameworks
Methods of projecting impacts on species and
ecosystems
Each method has advantages and strengths and has led to
the important and extensive current knowledge base.
However, each method also has uncertainties that need
to be identified so that methods can be refined and
uncertainties quantified.
The use of paleo analogues to infer future changes underrepresents
the differences between past changes and
those likely to occur in the future due to differences in
the starting state of the environment and biota, and the
different nature of past and likely future changes. Major
differences include anthropogenic effects (e.g., extent of
land-use change, current and future stratospheric ozone
depletion, and trans-boundary pollution) that are probably
unprecedented.
Using geographic analogues can indicate where communities
and species should be in a warmer world, but they
do not indicate at what rate species can relocate or if
new barriers to dispersal such as fragmented habitats
will prevent potential distributions from being achieved.
Observations and monitoring provide essential data about
changes as they occur, and can be used to test hypotheses
and model projections, but they have little predictive
power in a time of changing climate during which many
biotic responses are nonlinear.
Experiments that simulate future environmental conditions
(e.g., CO2 concentrations, UV-B radiation levels,
temperature, precipitation, snow depth and duration,
etc.) all have artifacts, despite attempts to minimize
them. It is difficult in field experiments to include simulations
of all likely eventualities: in warming experiments,
it is very difficult to identify separate effects of
seasonal warming and extreme events because most
experiments are small in spatial extent, and are shortterm
in the context of the life cycles of arctic plants and
animals. It is also difficult to identify the complex interactions
among all the co-occurring environmental
change variables, and ecological processes determined
from experiments in one geographic area may not relate
sufficiently to other areas because of different ecological
conditions and histories.
Indigenous knowledge, although a valuable contributor to
current understanding of ecological changes (section
7.7.2.2), is more qualitative than quantitative, and often
characterized by relatively coarse measures (i.e., monthly
and seasonal change rather than daily or weekly).
The information available is sometimes limited to phenomena
that fall within the cycle of subsistence resource
use, and is more likely to be diachronic (long time series
of local information) and not synchronic (simultaneously
observed). It is often difficult to assign particular environmental
changes to individual changes in biota, to
determine mechanisms of change, and distinguish climate-related
change from other changes occurring in the
environment. Indigenous knowledge is variable between
and within communities, and interpretation and verification
processes are as important as collection and documentation.
It is a knowledge system.
Uncertainties can be reduced when information from several
methods converges.This chapter accepts all methodologies,
knowing their limitations, and qualifies the information
presented by the methodologies used to obtain it.
330 Arctic Climate Impact Assessment
Measuring primary production and controlling
factors
Key unknowns about primary productivity in the Arctic
include root production and turnover and belowground
allocation processes in general, including allocation to
mycorrhizae and exudation.Also poorly understood are
long-term (multi-year to multi-decade) interactions
between the carbon cycle and nutrient cycles, in which
relatively slow changes in soil processes and nutrient
availability interact with relatively rapid changes in
photosynthesis in response to climate change. One
major unknown is the control on dispersal, establishment,
and rate of change in abundance of species and
functional types that are more productive than current
arctic species but are not now present or common in
arctic vegetation (e.g., trees and tall shrubs).
There are two major approaches to assessing NEP: classic
biomass weighing and year-round CO2 flux recording,
but these approaches are not always compatible.
One particular gap in current estimates is the lateral
transport of organic C from one ecosystem to another.
The two methodologies give opposite results when
accounting for the input of allochthonous (produced
outside) organic matter to a particular ecosystem: CO2
flux measurement gives negative NEP due to increased
CO2 emission from soil to atmosphere, while weighing
gives a higher accumulation of organic C in the soil.
In addition, current estimates of buried C released to
ecosystems due to thermokarst and soil erosion
(Figs. 7.21 and 7.22) are poor.
Difficulties in studying microorganisms
Understanding of microbes that are critically important
in many ecosystem processes is limited. Knowledge of
microbial diversity and function has been strongly constrained
by lack of development in methodology and
conceptual frameworks.
Bacteria and even the more advanced microscopic yeast
and fungi cannot be characterized by visual observation
alone due to their very simple shapes (rods, spheres, filaments).Typically,
microbial strains must be cultivated in
pure culture to reveal their various functional features,
and an appreciable amount of laboratory work is
required to differentiate a microbe from close relatives.
Only a small fraction of soil microorganisms are able to
grow on artificial laboratory media, and less than 1% of
the cells observed with a microscope form colonies on
the plate.The main reasons for this “Great Plate Count
Anomaly” (Staley and Konopka, 1985) include the metabolic
stress of the “famine-to-feast” transition occurring
when cells are brought from soil to artificial, nutrientrich
media; inadequacy of cultivation conditions compared
with the natural environment; and metabiotic
interactions/cooperation in natural communities that are
broken after cells have been separated by plating
(Panikov, 1995).This technical problem has resulted in
an underestimation of diversity in natural habitats.
Fortunately, new cultivation approaches are being developed
that are helping to overcome this problem (Staley
and Gosink, 1999). However, it is not presently possible
to make a fair comparison between the numbers of
species of animals and plants versus bacteria given that
these groups are defined differently (Staley, 1997).
Lack of feasible technologies to measure microbial
population dynamics in the field has lead to the use of
microbial biomass, which is convenient but a poor
predictor of productivity and C and nutrient turnover.
More sensitive measurements of microbial activity need
to be developed for field application.
Incomplete databases
Length of time series of data: Although many long time
series of relevant data (e.g., species performance and
phenology) exist, most information relates to short
time series.This is a particular problem in the Arctic
where complex population dynamics (e.g., cycles) need
to be understood over periods long enough to allow
trends to be separated from underlying natural dynamics.
Observations of trace gas emissions also require
annual observations over time periods long enough to
encompass significant climate variability. Experiments
are usually too brief to capture stable responses to environmental
manipulations and to avoid artifacts that are
disturbance responses. Long time series of data are also
necessary in order to identify extreme events and nonlinear
system changes.
Geographic coverage and spatial scaling:The ecosystems and
environments in the Arctic are surprisingly variable yet
generalizations to the circumpolar Arctic are often made
from a few plot-level studies. Sometimes, particular
experiments (e.g., CO2 concentration and UV-B radiation
manipulations) or observations are restricted to a
few square meters of tundra at just one or two sites.
Uncertainties due to generalizing and scaling up are thus
significant.The IBP and the ITEX are exceptional examples
of how standardized experiments and observations
can be implemented throughout the Arctic.
Coverage of species and taxa: Chapin F. and Shaver (1985a)
and others have demonstrated the individualistic
responses of species to experimental environmental
manipulations, including climate, while Dormann and
Woodin (2002) have shown the inadequacy of the concept
of “plant functional types” in generalizing plant
responses to such experiments.An approach has to be
developed to measure responses of a relevant range of
species to changes in climate, and particularly UV-B
radiation levels. Plants studied in the UV-B radiation
manipulation experiments were generally at their northernmost
distributional limits and well adapted to high
UV-B radiation levels characteristic of southern parts of
their ranges. Greater responses would be expected from
species at their southern distributional limits where
increased UV-B radiation would exceed levels in the
plants’ recent “memory”.
Chapter 7 • ArcticTundra and Polar Desert Ecosystems 331
Some species and taxonomic groups are particularly difficult
to study, or have little socioeconomic value, and so
are underrepresented in databases. Examples include
mosses, lichens, soil fauna and flora, and microorganisms
(section 7.7.1.1).
Nomenclature and concepts
The restricted use of appropriate language often generates
uncertainties.The nomenclature of vegetation and
plant community types allows changes in the distribution
of these assemblages of species in a changed climate to
be modeled, but constrains understanding of changes in
the structure of the assemblages that are likely to occur
because assemblages of species do not move en bloc.This
problem limits understanding of novel future communities
(Chapin F. and Starfield, 1997) and no-analogue
communities of the past, and emphasizes the uncertainties
due to the inability of quantitative models to project
qualitative changes in systems. Similarly, the concept
of “line” to denote the limit of species distributions
(e.g., treeline) is inadequate to express the gradient of
changes from one zone to another that can occur over
tens of kilometers.
The concept of “species” is particularly difficult in the
context of microorganisms as discussed previously, and
even as applied to flowering plants.The traditional view
that there are few rare and endemic arctic plant species
is challenged by recent studies of the flora ofWrangel
Island and Beringia (Table 7.14) but it is not known to
what degree plant taxonomy is problematic (although
the Pan Arctic Flora project is addressing this problem).
Such problems need to be resolved before the impacts of
climate change on biodiversity can be assessed.
7.7.1.2. Uncertainties due to surprises
Perhaps the only certainty in the assessment of impacts
of changes in climate and UV radiation levels on terrestrial
ecosystems in this chapter is that there will be surprises.
By definition, it is difficult to predict surprises.
However, the possibility that climate cooling will occur
because of a change in thermohaline circulation is potentially
the most dramatic surprise that could occur.
Regional cooling
The potential for a negative feedback arising from an
increased freshwater flux to the Greenland, Icelandic,
and Norwegian seas and the Arctic Basin, leading to a
partial or complete shut down of the thermohaline circulation
of the global oceans, remains an area of considerable
uncertainty (sections 6.8.4 and 9.2.3.4). Such an
event would be likely to lead to marked and rapid regional
cooling in at least northwest Europe.This region at
present enjoys an anomalously warm climate given its latitude
(50º–72º N), enabling agriculture to be practiced
and substantial settlements maintained at far higher latitudes
than in any other arctic or subarctic region. Such
cooling would be likely to qualitatively alter terrestrial
ecosystems (Fossa, 2003), agriculture, and forestry over
very large areas of Fennoscandia and Europe.
Mutations
Mutations are projected to occur as a result of increased
levels of UV radiation and also as a result of aerosols and
volatile chemical mutagens transported to the cool polar
atmosphere from the mid- and low latitudes.The direct
mutagenic effect is probably not strong, especially if the
protecting shielding effects of soil particles and adaptive
mechanisms are taken into account. However, possible
microbial mutants could lead to epidemic outbreaks that
could have profound and unexpected consequences for
the Arctic and elsewhere.
Desertification
Several approaches suggest that climate warming will
lead to an increase in the productivity of arctic vegetation
and long-term net sequestration of CO2. However,
the complex interactions among warming, permafrost
dynamics, hydrology, precipitation, and soil type are
poorly understood. Desertification is a plausible outcome
in some areas where scenarios suggest that permafrost
will thaw, drainage will increase, temperatures will
increase, and precipitation will not increase substantially.
In areas of sandy soil and loess deposition, such as parts
of eastern Siberia, there is a particular risk of desertification.
In the polar deserts, herb barrens, and heaths of
northern Greenland, plant productivity is strongly correlated
with precipitation and increased evapotranspiration
is likely to lead to a similar process (HeideJørgensen
and Johnsen, 1998). Locally, the impacts of
overgrazing and anthropogenic disturbance can accelerate
the process.A clear example of the effect of warming
and drying on the C balance of Alaskan tundra is
provided in section 7.5, and an example of boreal coniferous
forest loss in North America is given in section
14.7.3.1. However, the possible wider geographic scale
of this process is unknown.
Changes in current distributions of widespread
and rare species
Climate change could possibly have counter-intuitive
impacts on species distributions. Currently rare arctic
plant species, particularly those that are northern outliers
of species with more southerly distributions, could
possibly expand during initial phases of climate warming.
In contrast, currently widespread species, particularly
lichens and mosses, could possibly become more
restricted in their abundance during warming. It is necessary
to reassess the concept of “threatened species” in
the context of climate and UV radiation change (see section
7.6 and Chapter 11).
7.7.1.3. Model-related uncertainties
During the IBP period (late 1960s and early 1970s), tundra
research was characterized by extensive field obser-
332 Arctic Climate Impact Assessment
vations but a general lack of modeling capability.
Currently, a technological revolution has stimulated
model generation and remote sensing of ecosystem
change. However, in some cases, validation is insufficient.
Models that project NPP at a global or circumpolar
level are insufficiently validated, as recent measurements
of NPP are rare and restricted to a few localities.The
lack of inter-comparison between models and
existing observations leads to potential projection
errors: modeled displacement of the tundra by the boreal
forest contradicts current observations of the southward
retreat of the treeline in some areas and the expansion
of “pseudotundra” in parts of Russia due to permafrost
degradation, paludification, and human activities.
The climate models used to provide future climate scenarios
for the ACIA were forced with the B2 emissions
scenario (section 4.4.1), but projections based on the A2
emissions scenario have been used to a limited extent as a
plausible alternative (IPCC, 2001; section 1.4.2).The A2
scenario assumes an emphasis on economic development
rather than conservation, while the B2 scenario assumes a
greater emphasis on environmental concerns: each has
considerable uncertainties (section 4.4.1). In this chapter,
projections based mainly on the B2 emissions scenario
were used to model changes in vegetation and carbon
storage. Projections based on the A2 emissions scenario
result in higher temperatures for a particular time period
than those based on the B2 emissions scenario.The
changes projected by models forced with the A2 emissions
scenario for the 2041–2060 and 2071–2090 time
slices occur 5 to 10 and 10 to 20 years earlier, respectively,
compared with projections based on the B2 scenario.
Potential impacts on ecosystems would thus occur faster
should emissions follow the A2 scenario.
The major implication for ecosystems of a faster rate of
temperature change is an increased mismatch between
the rate of habitat change and the rate at which species
can relocate to occupy new habitats in appropriate climate
envelopes.The overall, generalized, difference
between projections based on the B2 and A2 emissions
scenarios is likely to be an increased risk of disturbance
and disease in species that, under projected conditions
based on the A2 scenario, cannot relocate quickly
enough. Projections based on the A2 scenario also imply
an increased mismatch between initial stimulation of soil
respiration and longer-term vegetation feedbacks that
would reduce C fluxes to the atmosphere.
7.7.2. Recommendations to reduce
uncertainties
7.7.2.1.Thematic recommendations and
justification
This section reviews important thematic topics that
require particular research. For each topic, the state of
knowledge and important gaps are summarized, and
recommendations to fill these gaps are suggested
(in italic font).
Mechanisms of species responses to changes in
climate and UV-B radiation levels. Changes in
microbe, animal, and plant populations are triggered by
trends in climate and UV radiation levels exceeding
thresholds, and by extreme events, particularly during
winter. However, information is uneven and dominated
by trends in summer climate. Appropriate scenarios of
extreme events are required,as is deployment of long-term
experiments simulating extreme events and future winter
processes in particular.A better understanding of thresholds
relevant to biological processes is also required.
Biodiversity changes. Some groups of species are
very likely to be at risk from climate change impacts,
and the biodiversity of particular geographic areas such
as Beringia are at particular risk. It is not known if currently
threatened species might proliferate under future
warming, nor which currently widespread species might
decrease in abundance. The nature of threats to species,
including microbes,must be reassessed using long-term climate
and UV-B radiation change simulation experiments.There is
also a need to identify and monitor currently widespread species
that are likely to decline under climate change,and to redefine
conservation and protection in the context of climate and UV
radiation change.
Relocation of species.The dominant response of current
arctic species to climate change, as in the past, is
very likely to be relocation rather than adaptation.
Relocation possibilities are very likely to vary according
to region and geographic barriers. Some changes are
already occurring. However, knowledge of rates of relocation,
impact of geographic barriers, and current
changes is poor. There is a need to measure and project rates
of species migration by combining paleo-ecological information
with observations from indigenous knowledge,environmental
and biodiversity monitoring,and experimental manipulations of
environment and species.
Vegetation zone redistribution. Forest is very likely
to replace a significant proportion of the tundra and is
very likely to have a great effect on species composition.
However, several processes (including land use and
permafrost dynamics) are expected to modify the modeled
response of vegetation redistribution related to
warming. Models of climate,hydrology (permafrost),ecosystems,and
land use need to be developed and linked.These models
need to be based on improved information about the current
boundaries of major vegetation zones,defined and recorded
using standardized protocols.
Carbon sinks and sources in the Arctic. Current
models suggest that arctic vegetation and active-layer
soils will be a sink for C in the long term because of the
northward movement of vegetation zones that are more
productive than those they displace. Model output needs
to be reconciled with observations that tundra areas that
are C sources currently exceed those that are C sinks,
although the measurements of circumpolar C balance
are very incomplete.To what extent disturbance will
reduce the C sink strength of the Arctic is also unknown.
Chapter 7 • ArcticTundra and Polar Desert Ecosystems 333
There is a need to establish long-term,annual C monitoring
throughout the Arctic;to develop models capable of scaling
ecosystem processes from plot experiments to landscapes;to
develop observatories,experiments,and models to relate disturbance
such as desertification to C dynamics;and to improve the
geographic balance of observations by increasing high-arctic
measurements.There is also a need to combine estimates of
ecosystem carbon flux with estimates of C flux from thawing
permafrost and methane hydrates.
Ultraviolet-B radiation and CO2 impacts.
Enhanced CO2 concentrations and UV-B radiation levels
have subtle but long term impacts on ecosystem
processes that reduce nutrient cycling and have the
potential to decrease productivity. However, these are
generalizations from very few plot-scale experiments,
and it is difficult to understand impacts that include
large herbivores and shrubs. There is a need for long-term
experiments on CO2 and UV-B radiation effects interacting
with climate in a range of arctic ecosystems; short-term experiments
stimulating repeated episodes of high UV radiation
exposure; long-term experiments that determine the consequences
of high CO2 concentrations and UV-B radiation levels
for herbivores; and short-term screening trials to identify the
sensitivity of a wide range of species, including soil microbes,
to current and projected UV-B radiation levels.
Local and regional feedbacks. Displacement of tundra
by forest is very likely to lead to a decrease in albedo
with a potential for local warming, whereas C sequestration
is likely to increase with potential impacts on global
concentrations of greenhouse gases. However, the timing
of the processes and the balance between the processes
are very uncertain. How local factors such as land use,
disturbance, tree type, and possible desertification will
affect the balance is also uncertain.There is a need for
long-term and annual empirical measurements,analysis of past
remotely sensed images,and collection of new images together
with the development and application of new models that
include land use,disturbance,and permafrost dynamics.
7.7.2.2. Recommendations for future
approaches to research and monitoring
No single research or monitoring approach is adequate,
and confidence is increased when results from different
approaches converge (section 7.7.1.1). Current
approaches should be maintained, and new approaches
and even paradigms (e.g., when defining “threatened
species” and “protected areas”) developed. Some important
approaches are highlighted below.
Reducing uncertainty by increasing and
extending the use of indigenous knowledge
Arctic indigenous peoples retain strong ties to the land
through subsistence economies and they are “active participants”
in ecosystems (sensu Bielawski, 1997). Unlike a
scientist, a hunter is not bound in his observations by a
project timeline, budget, seasonality, or logistical constraint
(Krupnik, 2000; Riedlinger and Berkes, 2001).
Subsistence activities occur on a daily basis, year after
year, and throughout the winter period when many scientists
are south in home institutions. Indigenous peoples
of the Arctic therefore possess a substantial body of
knowledge and expertise related to both biological and
environmental phenomena. Such local expertise can
highlight qualitative changes in the environment and
provide pictures of regional variability across the Arctic
that are difficult to capture using coarser-scale models.
This chapter presents some of the first efforts at linking
western science and indigenous knowledge to expand
the range of approaches that inform this assessment.
However, the potential is far greater, including, for
example, local-scale expertise, information on climate
history, generation of research hypotheses, community
monitoring, and community adaptation (Riedlinger and
Berkes, 2001).
Monitoring
Long-term environmental and biological monitoring
have been undervalued but are becoming increasingly
necessary to detect change, to validate model projections
and results from experiments, and to substantiate measurements
made from remote sensing. Present monitoring
programs and initiatives are too scarce and are scattered
randomly. Data from the Arctic on many topics are often
not based on organized monitoring schemes, are geographically
biased, and are not long-term enough to
detect changes in species ranges, natural habitats, animal
population cycles, vegetation distribution, and C balance.
More networks of standardized, long-term monitoring
sites are required to better represent environmental and
ecosystem variability in the Arctic and particularly sensitive
habitats. Because there are interactions among many
co-varying environmental variables, monitoring programs
should be integrated. Observatories should have
the ability to facilitate campaigns to validate output from
models or ground-truth observations from remote sensing.There
should be collaboration with indigenous and
other local peoples’ monitoring networks where relevant.
It would be advantageous to create a decentralized
and distributed, ideally web-based, meta-database from
the monitoring and campaign results, including relevant
indigenous knowledge.
Monitoring also requires institutions, not necessarily
sited in the Arctic, to process remotely sensed data.
Much information from satellite and aerial photographs
exists already on vegetation change, such as treeline displacement,
and on disturbances such as reindeer/caribou
overgrazing and insect outbreaks. However, relatively little
of this information has been extracted and analyzed.
Monitoring C fluxes has gained increased significance
since the signing of the Kyoto Protocol. Past temporal
and spatial scales of measurement used to directly measure
C flux have been a poor match for the larger scale of
arctic ecosystem modeling and extrapolation. It remains
a challenge to determine if flux measurements and
334 Arctic Climate Impact Assessment
model output are complementary.The technological
difficulties in extrapolating many non-linear, complex,
interacting factors that comprise fluxes at hundreds to
thousands of square kilometers over time, space, and
levels of biological and environmental organization in
the Arctic have been significant (Oechel et al., 2000b,
1998). Research is needed to better understand how the
complex system behaves at the meter scale related to
larger spatial scales that can be efficiently modeled and
evaluated at the regional and circumpolar scale.To do
this, extensive long-term and year-round eddy covariance
sites and other long-term flux sites, including
repeated aircraft flux measurements and remote sensing
(Oechel et al., 2000a), provide the basis for estimating
circumpolar net ecosystem CO2 exchanges. Currently,
the circumpolar Arctic is disproportionately covered by
current and recent measurements, with Canadian and
high-arctic regions particularly poorly represented.
Long-term and year-round approach to
observations and experiments
Many observations and experiments are short term
(<5 years) and they are biased toward the summer period
often because researchers have commitments to institutions
outside the Arctic during winter. However, throughout
this assessment it has become clear that long-term
and year-round measurements and experiments are
essential to understand the slow and complex responses
of arctic organisms and ecosystems to climate change.
Long-term (>10 years) observations and experiments
are required in order to enable transient responses to be
separated from possible equilibrium responses; increase
the chances that disturbances, extreme events, and significant
interannual variation in weather are included in
the observations; and allow possible thresholds for
responses to be experienced.
Year-round observations are necessary to understand the
importance of winter processes in determining the survival
of arctic species and the function of arctic ecosystems.
Such observations are necessary to recognize the
projected amplification of climate warming in winter
and to redress the current experimental bias toward
summer-only warming. For microbes, it is particularly
important to understand changes in winter respiration
and nutrient mobilization during freeze–thaw events in
spring and late autumn.
It is important to improve the appropriateness of the timing
of observations and experiments. For example, current
information about the impacts of increased UV-B
radiation levels is mainly derived from general summer
enhancements or filtration of UV-B radiation, although
future increases in UV-B radiation levels are likely to be
highest in spring and during specific stratospheric ozone
depletion events.The frequency of observations should
be fitted to the rate of change of the species or processes
of interest, for example, decadal measurements may suffice
for some variables such as treeline movement.
Increasing the complexity and scale of
environmental and ecosystem manipulation
experiments
Single-factor manipulation experiments now have limited
applicability because it is clear that there are many
interactive effects among co-occurring environmental
change variables.There is need for well-designed, large,
mutifactorial environmental (e.g., climate, UV-B radiation
levels, and CO2 concentrations) and ecosystem
(e.g., species removal and addition) manipulation experiments
that are long-term and seek to understand annual,
seasonal, and event-based impacts of changing environments.The
complexity of appropriate treatments and
timescales is vast and the spatial scale is also a significant
challenge, as it is important to have manipulations that
can be related to larger plants (e.g., trees, shrubs) and
animals (e.g., reindeer/caribou).
Assessing the impacts of cooling on ecosystems
Scenarios of increased temperature dominate the
approaches to projecting responses of ecosystems to
future climate. However, cooling in some areas remains
a possibility.As the impacts of cooling on terrestrial
ecosystems and their services to people are likely to be
far more dramatic than the impacts of warming, it is
timely to reassess the probabilities of cooling projected
by GCMs and the appropriateness of assessing cooling
impacts on ecosystems.
Modeling responses of arctic ecosystems to
climate and UV-B radiation change and
communicating results at appropriate
geographic scales
High-resolution models are needed at the landscape
scale for a range of landscape types that are projected to
experience different future envelopes of climate and
UV-B radiation levels. Modeling at the landscape scale
will simulate local changes that relate to plot-scale
experiments and can be validated by results of experiments
and field observations.Visualization of model
results presented at the landscape scale will also enhance
the understanding of the changes and their implications
by local residents and decision-makers.A particular challenge
is to provide scenarios for changes in climate and
UV-B radiation levels at the scale of tens of meters.
7.7.2.3. Funding requirements
It is inappropriate here to comment on levels of funding
required to fulfill the recommendations discussed above.
However, it is appropriate to highlight two essential
aspects of funding.
First, current short-term funding is inappropriate to
support research into long-term processes such as
ecosystem responses to climate change and UV-B
radiation impacts. A stable commitment to long-term
funding is necessary.
Chapter 7 • ArcticTundra and Polar Desert Ecosystems 335
Second, funding possibilities that are restricted to
single nations, or at best a few nations, make it
extremely difficult to implement coordinated research
that covers the variability in ecosystems and projected
climate change throughout the circumpolar north, even
though the instruments for coordination exist (e.g.,
within the International Arctic Sciences Committee,
the International Council of Scientific Unions, and the
International Geosphere–Biosphere Programme).
Limitation of international funding possibilities leads to
geographic biases and gaps in important information.
Circumpolar funding is required so that coordinated projects
can operate at geographically appropriate sites over the same
time periods.
Acknowledgements
Many individuals and organizations kindly contributed information to
this assessment and we are grateful to all. In particular, we thank the
Steering Committees of the IASC projects on theTundra-Taiga Interface
and Feedback from ArcticTerrestrial Ecosystems. ITEX participants also
gave support.We thank Dr.Anders Michelsen, University of
Copenhagen, for information on arctic mycorrhizal associations and Jed
Kaplan (MPI Jena) for providing unpublished estimates of the past and
potential future extent of tundra derived from simulations made using
the BIOME 4 model.We are grateful to Håkan Samuelsson and Chris
Callaghan for help with some of the figures.Terry Callaghan and
Margareta Johansson are very grateful to the Swedish Environmental
Protection Agency (Naturvårdsverket) for funding to allow them to contribute
to the ACIA process. Participantion of K. Laine and E.Taulavuori
have been facilitated by financial support from the Academy of Finland
andThule Institute, University of Oulu. Phycology research in the
Arctic by J. Elster and colleagues has been sponsored by two grants; the
Natural Environment Research Council (LSF-82/2002), and the Grant
Agency of the Ministry of Education of the Czech Republic (KONTAKT
- ME 576).We are grateful to A. Katenin for providing unpublished data
andT. Polozova for providing part of Figure 7.3. Finally, we are grateful
to the ACIA leadership, integration team, and chapter liason Pål
Prestrud for guidance and encouragement.
Personal communications and
unpublished data
Callaghan,T.V.Abisko Scientific Research Station, Sweden.
Clemmensen, K.E. and A. Michelsen, 2004. Institute of Biology,
University of Copenhagen.
Flanagan, P.W., 2004. Global Environmental Enterprises, Inc., Chester,
Virginia.
Friborg,T., 2005. Institute of Geography, University of Copenhagen,
Denmark.
Henttonen, H., 2004.Thule Insitute, University of Oulu, Finland.
Jonasson, S. and A. Michelsen, 2005. Institute of Biology, University of
Copenhagen.
Kaplan, J., 2002. Canadian Centre for Climate Modelling and Analysis.
Katenin,A., 2004. Komorov Botanical Institute, St. Petersburg, Russia.
Meltofte, H., 2004. National Environment Research Institute, Denmark.
Nutti, E., 2004. Karesuando, Sweden.
Polozova,T., 2005. Komorov Botanical Institute, St. Petersburg, Russia.
Svoboda, J. Department of Botany, University ofToronto, Canada.
Taulavuori, K., E.Taulavuori and K. Laine, 2004.Thule Institute,
University of Oulu, Finland.
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