Implications of Global Climate
Change for Biogeographic
Patterns in the Greater
Yellowstone Ecosystem
WILLIAM H. ROMME
Biology Department
Fort Lewis College
Durango, CO 81301, U.S.A.
MONICA G. TURNER
Environmental Sciences Division
Oak Ridge National Laboratory
Oak Ridge, TN 37831-6038, U.S.A.
Abstract: Projected changes in global climate have substantial
ramifications for biological diversity and the management
of natural areas. We explored the potential implications
of global climate change for biogeographic patterns in
the Greater Yellowstone Ecosystem by using a conceptual
model to compare three likely climate scenarios: (1) warmer
and drier than the present; (2) warmer and drier, but with a
compensating increase in plant water use efficiency; and (3)
warmer and wetter than the present. The logical consequences
of each scenario are projectedfor several species and
community types chosen to represent a range of local climate
conditions and biotic responses in the Greater Yellowstone
Ecosystem. The upper and lower timberline appear to
be particularly sensitive to climate change. The upper timberline
is likely to migrate upward in elevation in response
to temperature changes, whereas the lower treeline may retreat
under drier conditions or move down slope under wetter
conditions. In all scenarios, the extent of alpine vegetation
in the ecosystem decreased. Climate-induced changes in
the fire regime in the Greater Yellowstone Ecosystem would
probably have substantial consequences for the extent and
age-class distribution of forest communities. Alterations in
the distribution and extent of grassland communities would
affect the populations of large ungulates. Our analyses suggest
directions for establishing long-term measurements for
the early detection of responses to climate change.
Resumen: Los proyectados cambios globales del clima
tienen ramificaciones substanciales para la diversidad biol6gica
y el manejo de las areas naturales. Exploramos las
implicaciones potenciales de los cambios globales del clima
en lospatrones biogeogrdficos del ecosistema de Greater Yellowstone
utilizando un modelo conceptual para comparar
tres posibles escenarios climaticos: (1) mas caliente y seco
que el actual; (2) mcis calientey seco pero con un incremento
compensatorio en la eficiencia del uso de las plantas de
agua; y (3) mas caliente y mas humedo que el actual. Las
consecuencias l6gicas de cada caso son proyectadas para
varias especies y tipos de comunidad escogidos para representar
un rango de condiciones climaticas locales y de respuestas
bi6ticas en el ecosistema de Greater Yellowstone. El
limite superior e inferior de presencia de arboles parece ser
particularmente sensitivo a los cambios climaticos. Es
posible que el limite superior migre hacia altas elevaciones
en respuesta a los cambios de temperatura mientras que el
inferior puede retractarse en condiciones mcis secas o moverse
ladera abajo en condiciones mas ht*medas. En todos los
escenarios climaticos analizados, se disminuy6 la extensi6n
de la vegetacion alpina del ecosistema de Greater Yellowstone.
Los cambios climaticos inducidos con regimen de incendios
en el ecosistema de Greater Yellowstone seguramente
tendrdn consecuencias substanciales en la extensi6n
y la distribuci6n de edad-clase de las comunidades de
bosque. Alteraciones en la distribuci6n y la extensi6n de
comunidades de pastizales afectard a la poblacion de los
grandes ungulados. Nuestro andlisis sugiere directrices para
el establecimiento de medidas a largo plazo para detectar
tempranamente las respuestas a los cambios climaticos.
Paper submittedJuly 1, 1990; revised manuscript acceptedDecember
I11, 1990.
373
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374 Implications of Climate Change Romme & Turner
Introduction
A changing environment presents challenges to the conservation
of biological diversity and the management of
natural areas. The challenges arise in part from the tremendous
uncertainty about the direction and magnitude
of future environmental changes. For example, climate
is a central controlling factor in all ecosystems and
is projected to change dramatically in some regions during
the next century. Global climate change would have
far-reaching ramifications for natural areas (Peters &
Darling 1985; Tangley 1988; Davis 1989a; Graham et al.
1990; Davis & Zabinski, in press), but the magnitude,
rate, and spatiotemporal characteristics of potential
changes in temperature and precipitation regimes are
not well understood. In the face of environmental
change, effective nature reserve management must be
based on (1) an understanding of the major biotic and
abiotic controls on the ecological components and processes
of the area, (2) an expectation of how these controls
may change, either through natural events or human
actions, and (3) an evaluation of the probable
effects of these changes on the biota in the reserve (Golley
1984). In this paper, we explore potential implications
of global climate change for biological diversity in
Yellowstone National Park (YNP) and the Greater Yellowstone
Ecosystem (GYE), which includes YNP, Grand
Teton National Park, and portions of surrounding national
forests and other lands (Clark & Harvey 1988).
From paleoecological reconstructions in the GYE, we
know that major changes in regional climate and vegetation
have occurred in the past. During the most recent
glacial period (ca. 20,000 to 16,000 yrs ago), the upper
timberline in this part of the Rocky Mountains apparently
was 600 to 1200 m lower than today, and most of
the Yellowstone Plateau was glaciated (Barnosky et al.
1987:312). As global temperatures increased and glaciers
retreated at the end of the Pleistocene (ca. 14,000
to 13,000 yrs ago in Yellowstone; Barnosky et al.
1987:299), the upper timberline shifted upward, and
coniferous forests became established on the Yellowstone
Plateau (Baker 1970, 1976, 1986; Waddington &
Wright 1974). The early Holocene (ca. 10,000 to 4,000
yrs ago) was a period of maximum warmth in the Yellowstone
region, and some species such as Douglas-fir
(Pseudotsuga menziesii) apparently grew at higher elevations
than today (Barnosky et al. 1987:302). The climate
became somewhat cooler and possibly wetter in
the mid-Holocene, and the lower timberline in the eastern
portion of the GYE moved downward about 5400 to
4400 yrs ago (Reider et al. 1988).
As a result of anthropogenic increases in carbon dioxide
and other greenhouse gases, another episode of
global climate change is expected in the coming century
(Bolin et al. 1986). The climate change may be of
a magnitude similar to that at the end of the Pleistocene,
but the projected rate is likely to be an order of magnitude
greater than changes that occurred in the past.
Current simulations using the general circulation models
(GCMs) (e.g., Hansen et al. 1988; Schlesinger & Zhao
1988; Wetherald & Manabe 1988) project an average
rise in global temperature ranging from 1.5 to 4.5 C,
with greater warming in winter than summer and increased
warming with increased latitude (Dickinson
1986). Increased precipitation at high latitudes and decreased
summer precipitation and soil moisture at middle
latitudes of the northern hemisphere are also predicted.
These projections are at the global scale; the
magnitude and even the directions of climate change
will probably differ regionally.
Projected Climate Scenarios in the GYE
In the Rocky Mountains, regional projections of climate
change based on GCM simulations suggest elevated temperatures,
reduced winter precipitation, earlier spring
runoff, and elevational shifts in vegetation zones (Neilson
et al. 1989). In Figure 1, we summarize the potential
effects of these changes on the GYE. All of the general
circulation models project increased temperatures in
response to elevated concentrations of atmospheric
CO2 and other greenhouse gases, and warmer temperatures
will lead to higher potential evapotranspiration
(PET). Thus, the Greater Yellowstone Ecosystem would
be subjected to higher temperatures and greater evaporative
demands than at present. There is considerable
uncertainty about other effects of climate change (Lamb
1987; Cushman & Spring, in press). The GCMs cannot
reliably predict regional precipitation patterns; thus,
rainfall may increase, decrease, or remain the same in
the GYE (Cushman et al. 1988; Neilson et al. 1989). In
addition, elevated atmospheric CO2 may have direct effects
on vegetation. For example, water use efficiency
(W[UE) may increase with elevated C02, but the magnitude
and duration of this increase are unknown
(Norby & O'Neill 1989). Thus, the warmer temperatures
and the rise in potential evapotranspiration would
increase plant water stress unless compensated for by
increased precipitation or enhanced water use efficiency.
Finally, atmospheric CO2 enrichment could
stimulate increased primary productivity, although this
potential increase may not be realized because other
constraints (e.g., drought stress or nutrients) may be
more limiting (Strain & Bazzaz 1983; Oechel & Reichers
1986).
We explore the possible implications of these
changes and their uncertainties by creating three climate
scenarios that encompass the range of conditions
likely to exist by the end of the next century:
1. A warm, dry scenario, in which temperature and
PET increase, precipitation decreases or remains
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Romme & Turner Implications of Climate Change 375
and trace gases g p~InreiiasedInrese
Decreased
precipitation
-orIag
ncreased i U
Small increase in WUE
Figure 1. Potential effects of projected climate change in the Greater Yellowstone Ecosystem. PET = potential
evapotranspiration; WUE = plant water use efficiency.
unchanged, and WUE increases only slightly; plants
are subjected to elevated temperatures, C02, and
drought stress.
2. An intermediate scenario, in which temperature and
PET increase, precipitation decreases or remains
unchanged, and WUE increases sufficiently to compensate
for elevated PET; plants are subjected to
elevated temperatures and CO2 but there is no
change in drought stress.
3. A warm, wet scenario, in which temperatures and
CO2 increase, precipitation increases, and WUE
increases significantly; plants are subjected to elevated
temperatures and C02, but to reduced
drought stress.
For each scenario, we projected the probable effects
on several species and community types chosen to represent
a range of local climate conditions and biotic
responses in the GYE. Community types included alpine
communities above the upper timberline, nonforest
communities below the lower timberline, and the forested
zone. Species included whitebark pine (Pinus albicaulis),
an important tree near upper timberline;
Douglas-fir (Pseudotsuga menziesii), which grows near
lower timberline; and elk (Cervis elaphus) and bison
(Bison bison), which generally live at higher elevations
during summer but migrate to lower elevations in win-
ter.
We emphasize that the ecological changes we described
in this paper are projections, not predictions.
Our present understanding of the impending climate
changes and of the biotic and abiotic constraints on the
distribution and abundance of species and communities
in the Greater Yellowstone Ecosystem are still too rudimentary
to permit confident predictions. Nevertheless,
we think that this exercise has heuristic value in
that it helps delimit the range and magnitude of possible
changes in the system. It also may suggest areas where
research and monitoring programs may be most urgent
if managers are to deal effectively with the potential
climate changes that we face in the next century.
(1) The Warm, Dry Scenario
The possible consequences of warmer, drier conditions,
with minimal physiological compensation, are summarized
in Figure 2. An important direct effect of elevated
summer temperatures would be a lengthening of the
growing season at high elevations. Because low temperatures
and a short growing season apparently are the
primary constraints on tree growth at the upper timberline
in the Rocky Mountains (Daubenmire 1954; Krebs
1985), it is likely that the upper timberline would shift
to a higher elevation. Interpretations of the generalcirculation-model
predictions suggest an elevational displacement
of temperature zones of 462 to 1155 m
(Neilson et al. 1989). This would be comparable to the
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376 Implications of Climate Change Romme & Turner
WARM, DRY SCENARIO (No. 1)
Increased Increased No change or Small increase
temperature PET reduced precipitation in WUE
Increased growing Increased drought stress
season at upper at lower timberline
timberline
Upward shift of lower
[ t Te ofue]timberline
Upward shift of uppei
timberlineil1
l ~~~~~~~~~Increase in area of
fl LP t ~~~~~low elevation nonforest
fl t ~~~~~Decreas e in total forested communities; upward
Contraction of area; increased fire shift of sagebrushalpine
communities frequency; shift to grassland communities;
predominantly younger semi-desert vegetation
age classes. more common at lower
elevations
Decreased area and Decreased area and increased Increased ungulate winter
increased habitat habitat fragmentation for obligate range within Parks and
fragmentation for obligate old-growth forest species (e.g., Forests; increased area and
alpine species (e. g., rosy marten, goshawk, orchids, twin- connectivity of habitat for
finch, water pipit, arctic flower); some local extinctions; nonforest species; increased
gentian, alpine chaenac- decrease in whitebark pine; component of semi-desert
tis); some local extinctions increase in Douglas-fir. flora, fauna, and communities.
Figure 2. Potential effects on the Greater Yellowstone Ecosystem of a climate that is warmer and drier than the
present climate (scenario 1).
upward shift in species distributions of 400 to 1000 m
that may have occurred in Yellowstone during the Sangamonian
interglacial period some 127,000 yrs ago
(Baker 1986:728). For the projections in this paper, we
use a conservative estimate of 460 m. The upper timberline
in the GYE at present is around 2900 m (Patten
1963; Waddington & Wright 1974; Despain, 1991);
thus, global warming could move the upper timberline
to around 3360 m or higher.
A higher upper timberline would mean a substantially
smaller alpine zone. Indeed, the alpine zone could disappear
completely within the borders of Yellowstone
National Park, where the highest point (Eagle Peak)
reaches only 3440 m. There are higher mountains
nearby, in the Absaroka, Teton, and Wind River ranges,
where an alpine zone would persist. Nevertheless, the
habitat area for obligate alpine species would be reduced
and fragmented throughout the Greater Yellowstone
Ecosystem (Figs. 5 and 6). This in turn could lead
to local extinctions of some alpine species and communities
(MacArthur & Wilson 1967; Diamond 1975; Soule
et al. 1979; Wilcox 1980). Examples of species restricted
to the alpine zone include the arctic gentian
(Gentiana algida), alpine chaenactis (Chaenactis alpinus),
rosy finches (Leucosticte spp.), and water pipit
(Anthus spinoletta).
The lower timberline in the Rocky Mountains appears
to be controlled mainly by summer drought stress
(Daubenmire 1943; Krebs 1985). Increased potential
evapotranspiration without compensating increases in
precipitation or water use efficiency would result in
accentuated drought stress at lower elevations and an
upward shift in the lower timberline of 460 m or more
(Neilson et al. 1989).
With a comparable upward shift in both the upper
and lower timberlines, the total forested area would become
smaller than at present, because there is less land
area available at progressively higher elevations (Figs. 5
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Romme & Turner Implications of Climate Change 377
INTERMEDIATE SCENARIO (No. 2)
[ Increased Increased No change or Large increase
[temperature PET reduced precipitation in WUE
[ Increased growing No change in drought
season at upper stress at lower timberline
timberline I
No change in lower
timberline
Upward shift of uppe
timberline
Increase in total forested No change in area of
Conracionof area; increase in firelo-evtn
alpine communities frequency; shift to pre- | nlow-elevationml
dominantly younger agenofrscmuite
classes.
Decreased area and No change or reduced area of No change in area of winter
increased habitat habitat for obligate old-growth ungulate range but increased
fragmentation for obligate forest species (e.g., marten, survival in milder winters;
alpine species (e. g., rosy goshawk, orchids, twinflower); no change in amount of habitat
finch, water pipit, arctic some local extinctions; decrease for nonforest species, but
gentian, alpine chaenac- in whitebark pine; increase in compositional changes in
tis); some local extinctions. Douglas-fir. communities may occur.
Figure 3. Potential effects on the Greater Yellowstone Ecosystem of a climate that is warmer and drier than
the present climate, but with a compensating increase in plant water use efficiency (scenario 2).
and 6). An upward shift in the forested zone also would
have a disproportionate effect on the extent of highelevation
forest types. For example, whitebark pine forests
presently occur in a zone from 2600 to 2900 m,
which occupies an area of about 250,000 ha within YNP
and on the adjacent high peaks (Despain, in press; unpublished
YNP data). The upper elevational limit of
whitebark pine apparently is controlled by low temperatures;
the lower limit may be set partly by competition
with other conifers, particularly lodgepole pine. If vegetation
zones shifted by 460 m, then whitebark pine
would be found from about 3060 to 3360 m, with an
area of only 27,000 ha (Figs. 5 and 6). This represents a
90% decrease in available habitat for whitebark pine.
Such a reduction would have additional ramifications,
because this species not only forms a distinctive forest
type at the upper timberline but also is an important
food source for Clark's nutcrackers (Nucifraga columbiana),
red squirrels (Tamiasciurus hudsonicus), and
grizzly bears (Ursus arctos).
Quite a different situation is projected for the lowerelevation
tree species, Douglas-fir. The lower timberline
presently is around 1900 m, and Douglas-fir occurs from
about 1900 to 2200 m (Patten 1963; Waddington &
Wright 1974). A 460-m upward shift in elevational
zones would result in a larger potential range for this
species within the borders of Yellowstone Park since
most of the park area lies above 2000 m (Figs. 5 and 6).
However, Douglas-fir would probably disappear from
lower-elevation areas elsewhere in the GYE, so its regional
abundance would remain the same or decrease.
Furthermore, even within YNP this species might not
occupy all of the sites that were climatically suitable,
because it requires high calcium levels that are available
mainly in soils derived from calcareous substrates
(Patten 1963; Despain, 1991). Most of the higherConservation
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378 Implications of Climate Change Romme & Turner
WARM, WET SCENARIO (No. 3)
Increased Increased Increased precipitation Large increase
temperature ] PET in WUE
Increased growing Reduced water stress at
season at upper lower timberline
timberline
Downward shift of lower
__________ ttimberline
Upward shift of uppe
[ timberline ]
Large increase in total
Contraction of forested area; decrease in Decrease in area of
alpine communities fire frequency; shift to pre- for
_ ] ~~~~~dominantly older age lweeainnn
classes. __l _l
Decreased area and Increased area of habitat for Decrease in area of nonforested
increased habitat old-growth forest species (e.g., winter ungulate range in
fragmentation for obligate marten, goshawk, orchids, twin- protected Parks and Forests,
alpine species (e. g., rosy flower); decrease in whitebark but possibly increased forage
finch, water pipit, arctic pine; increase in Douglas-fir. production ;decrease in habitat
gentian, alpine chaenac- for nonforest species with some
tis); some local extinctions. local extinctions; semi-desert
elements disappear almost entirely.
Figure 4. Potential effects on the Greater Yellowstone Ecosystem of a climate that is warmer and wetter than
the present climate (scenario 3).
elevation areas in YNP are underlain by rhyolite and
other extrusive igneous rocks containing low calcium
levels (Despain, 1991).
The age class distribution throughout the forest zone
could change substantially if the climatic changes also
alter the disturbance regime, particularly the frequency
and severity of fires. The occurrence of extensive, standreplacing
fires in the GYE is controlled largely by
weather conditions; large fires only occur during summers
of below-normal precipitation (Swetnam & Betancourt
1990; Despain, 1991; Turner & Romme, in press;
Renkin & Despain, in prep.). The subalpine forest landscape
of the GYE presently contains a large component
of old-growth stands that exceed 200 years in age
(Romme & Despain 1989). However, if a warmer, drier
climate leads to an increased frequency of severe standreplacing
fires, the landscape could be converted into
one dominated by younger stands, as is the current configuration
in parts of the Canadian Rockies and the subarctic
(e.g., Johnson & Fryer 1987; Tande 1979;Johnson
1979; see summary in Turner and Romme, in press).
The combination of a slightly smaller forest zone and
a shift to predominantly younger forest age classes
means that habitat for old-growth species in the Greater
Yellowstone Ecosystem could become smaller in area
and more fragmented than at present. Old-growth habitat
already is being reduced in portions of the GYE that
are managed for timber production. If changing climatic
conditions and disturbance regimes also reduce oldgrowth
forests in protected parks and wilderness areas,
then some species could be threatened with local extinction.
Examples of old-growth forest species in the
GYE include the northern twinflower (Linnaea borealis),
fairy slipper (Calypso bulbosa), pine marten (Martes
americana), and goshawk (Accipiter gentilis).
With an upward shift in the lower timberline, the area
of low-elevation nonforest vegetation would increase.
Animals characteristic of treeless landscapes, such as
pronghorn (Antilocapra americana) and badger
(Taxidea taxus), might become more numerous. Sagebrush-grasslands,
dominated by big sagebrush (Artemisia
tridentata), bluebunch wheatgrass (Agropyron spiConservation
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Romme & Turner Implications of Climate Change 379
catum), and Idaho fescue (Festuca idahoensis),
probably would move to higher elevations than they
presently occupy. At the lowest elevations, sagebrushgrasslands
could be replaced by semidesert vegetation,
characterized by saltbush (Atriplex gardneri) and
greasewood (Sarcobatus vermiculatus). This type of
vegetation presently is rare in Yellowstone Park, where
it is restricted to soils developing in marine shales and
other extremely droughty substrates (Despain, 1991).
With a warmer, drier climate, these xerophytic communities
could expand to occupy a greater variety of sites
at lower elevations, though they may remain limited by
edaphic conditions.
Compositional changes within vegetation zones could
be just as important as the elevational shifts. Species
respond individually to the environmental changes because
of differing physiological tolerances, and altered
competitive interactions between species are likely to
result (Strain & Bazzaz 1983; Davis 1984; Neilson et al.
1989). Thus, community composition and dynamics
could shift. In addition, a relatively small shift in the
seasonality of precipitation could have dramatic effects
on overall vegetation physiognomy in the Greater Yellowstone
Ecosystem even if total precipitation does not
change. Winter precipitation favors woody vegetation,
whereas summer precipitation favors grasses and forbs
(Neilson et al. 1989). In YNP today there is a gradient in
the seasonality of precipitation; western portions of the
Park have more winter precipitation, characteristic of
the Great Basin region, while the eastern parts have
more spring and early summer precipitation, similar to
the Great Plains (Despain 1987).
These kinds of changes could lead to the development
of plant communities in the Greater Yellowstone
Ecosystem quite unlike any that we know today. Paleoecological
reconstructions of eastern North American
vegetation during the last 20,000 years have demonstrated
the widespread occurrence of species assemblages
that have no modern analogues (Solomon &
Webb 1985; Davis 1984, 1989a). Our understanding of
the biotic and abiotic processes that presently control
community structure in the GYE, and of the specific
ways those processes would be altered by global climate
change, are inadequate to permit any specific projections
of how communities may change in the next century.
At present, all we can say is that change in community
structure and composition is likely, and that the
changes could be striking (Davis 1989a).
Another aspect of the warm, dry scenario to be considered
is its implication for the large, free-ranging ungulates
that constitute one of the special features of the
Greater Yellowstone Ecosystem. The total numbers of
elk, bison, and other native ungulates are limited primarily
by the availability of winter forage (Meagher 1973;
Houston 1982). Nonforested areas at low elevations
provide the major winter habitat for these animals.
Milder winters and a larger nonforest area at low elevations
could mean higher populations of ungulates in
the GYE. Of particular significance would be the increased
winter habitat within protected parks, which lie
at relatively high elevations. Large numbers of ungulates
presently travel out of YNP every winter into surrounding
lands where they are subjected to hunting, artificial
feeding, and other kinds of management that complicate
the overall management of these animals (Berger 1991).
With milder winters and more habitat within YNP, a
greater number of animals might remain within the park
boundaries. However, the associated drier conditions
also could depress plant production, and elevated atmospheric
CO2 could alter C/N ratios in plant foliage
(Strain & Bazzaz 1983). The result could be reduced
forage quantity and quality and a lessening of the improvement
in winter ungulate habitat produced by
milder winter weather and a larger winter range.
Ungulates intensively utilize the vegetation in parts of
the GYE, influencing plant growth form and possibly
plant community composition. For example, heavy
browsing appears to prevent regeneration of aspen
(Populus tremuloides) stands except following extensive
fires (Jones 1974; DeByle 1985; Mueggler & Bartos
1977). Changes in ungulate habitat selection or foraging
behavior in response to vegetation changes would further
alter the composition and structure of plant communities
at low elevations in the GYE.
(2) The Intermediate Scenario
In the intermediate scenario, a large, compensating increase
in water use efficiency is hypothesized to accompany
the increased temperature, increased potential
evapotranspiration, and reduced or unchanged precipitation
described above. This combination of climatic
and physiological changes probably would produce the
same effects at high elevations as were just presented for
the warm, dry scenario: length of the growing season
would increase, the upper timberline would move upward,
the alpine zone would be reduced, and local extinction
of some obligate alpine species could occur
(Fig. 3). The range of whitebark pine also would shift to
a higher elevational zone with a substantially smaller
area (Fig. 6).
The position of the lower timberline might not shift,
however, because the effects of higher PET would be
compensated for by increased WUE. Thus, the elevational
range of Douglas-fir could expand, since its upper
limits apparently are determined by low temperatures
and snow depth (Leverenz & Lev 1987), which would
be ameliorated in this scenario, while its lower limits are
set by drought stress, which could remain the same (Fig.
6). As noted earlier, Douglas-fir might not move into all
of the sites that were climatically suitable, because of its
edaphic requirements.
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380 Implications of Climate Change Romme & Turner
4000
3000-
.0
> 2000
1000-
0 2000 4000 6000 8000 10000 12000
Area (square km)
Figure 5. Cumulative area above each elevation
zone within Yellowstone National Park and the adjacent
mountains along its eastern border. (Data
courtesy of the National Park Service.)
With a higher upper timberline and no change in
lower timberline, the total forest area would increase
(Fig. 6). However, the age class distribution of the forests
probably would shift to a predominance of younger
age classes just as in the warm, dry scenario. This is
because the increase in WUE could compensate for
physiological drought stress but would not affect the
expected increase in fire frequency and severity under
warmer and drier climatic conditions. The result of increased
forest area and a shift in age class distribution
could be either no change or a decrease in old-growth
habitat.
The area of nonforest communities at low elevations
would not change in this scenario. However, there
could be dramatic changes in species composition,
since plant species would respond individually to the
effects of CO2 on WUE and net photosynthesis. These
physiological changes would alter species tolerances
and competitive abilities. The area of nonforested ungulate
winter range also would not change in this scenario,
but more of the range could be accessible for longer
periods if winters became milder. The fertilization effect
of elevated CO2 could potentially increase net primary
productivity and forage production, leading to a further
increase in the number of ungulates that could be supported
on the winter range, although soil nutrient limitations
and altered C/N ratios might reduce the importance
of the latter effect (Strain & Bazzaz 1983).
(3) The Warm, Wet Scenario
In this as in the previous scenarios, warmer temperatures
probably would lead to an upward shift in the
upper timberline, a reduction in area of the alpine zone,
and local extinction of some alpine species and communities
(Fig. 4). The range of whitebark pine also would
shift up the mountains and occupy a smaller area (Fig.
6). With increased precipitation accompanying the elevated
temperatures, however, even the remaining subalpine
environment in the GYE could become unsuitable
for this species because of increased competition.
Whitebark pine is near the southern limit of its distribution
in this area (Arno & Hoff 1989). The mechanistic
explanation for its southern range limit has not
been demonstrated, but examination of its range map
shows that it is absent in the central and southern Rocky
Mountains where moisture-laden air masses from the
Gulf of Mexico augment late-summer precipitation (the
"Arizona monsoon"). In fact, the northern limit of the
Arizona monsoon marks the biogeographic boundary
for several plant species (Mitchell 1976; Neilson &
Wullstein 1983). Whitebark pine also is absent in the
wet coastal mountain ranges of the Pacific Northwest
(Arno & Hoff 1989). If whitebark pine presently is restricted,
either physiologically or via competitive interactions,
to the drier summer climates that presently
characterize the northern Rockies, then a climatic shift
to wetter summers in the GYE could result in further
reduction or even local extinction of whitebark pine in
this area.
With increased precipitation and water use efficiency,
drought stress at low elevations would be alleviated, and
the lower timberline could shift down slope to a lower
elevation. The range of Douglas-fir could expand both
upward and downward in this scenario, unless it is limited
by edaphic factors, as discussed above (Fig. 4). The
total forested area would increase as the upper timberline
moved upward and the lower timberline moved
downward (Fig. 6). Pollen data indicate that such an
expansion of subalpine forest both up and down slope in
Central Colorado occurred during a mid-Holocen period
(ca. 7000 to 4000 yrs ago) that apparently was
warmer and wetter than today (Fall 1985). Wetter conditions,
especially in summer, could lead to a decrease
in fire frequency and severity and a shift in forest ageclass
distribution to older age classes. The successional
mosaic of subalpine forests in the GYE could become
more like that seen today in the subalpine landscapes of
the Colorado Rockies, where stands >500 years old are
common (e.g., Peet 1981; Veblen 1986; Aplet et al.
1988; see summary in Turner and Romme, in press).
With increased forest area and a shift to older age
classes, the habitat available for old-growth species
would increase (Fig. 4).
The nonforest area at low elevations would be reduced
if the lower treeline moved down slope. Semidesert
species and communities, presently restricted to
severe substrates at the lowest elevations, could disappear
entirely from YNP. Nonforested winter range for
ungulates would be reduced, which potentially could
lead to smaller numbers of animals on the winter ranges.
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Romme & Turner Implications of Climate Change 381
Alpine Zone
3500 (1,23
3000 - Today
Whitebark Pine Zone
3500
(1, 2,3)
3000 j
oday
Forest Zone
3500
(2)
3000
2500 E
2000 - Today
. 0 (3)
C,
wL Douglas-fir Zone
3000 -
2500 -
2000 - Today (2)
1500 - (3)
1000 Low-elevation
Nonforest Zone
2500 -
2000 Today, (2) (1)
(3)
2000 4000 6000 8000 10,000 12,000
Area (square kilometers)
Figure 6 Present and projected areas and elevational ranges of vegetation zones in Yellowstone National Park
and the adjacent mountains along its eastern border. Scenario (1) is elevated temperature, CO2, and drought
stress. Scenario (2) is elevated temperature, but no change in drought stress. Scenario (3) is elevated temperature
and CO2, but reduced drought stress. See text for details.
However, ungulates are quite adaptable and probably
would increase their use of forests in this situation (Murie
1951; Meagher 1973; Geist 1982; Houston 1982).
Milder winter temperatures and possible increases in
forage production also would compensate for the reduction
in nonforest habitat for elk and bison. Populations
of obligate grasslands species such as pronghorn and
badger, however, probably would be reduced in the
GYE and some could become locally extinct.
The combination of warmer temperatures, longer
growing seasons, increased precipitation, and elevated
CO2 could produce substantial increases in primary productivity
throughout the vegetation zones of the
Greater Yellowstone Ecosystem. The potential augmenConservation
Biology
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382 Implications of Climate Change Ronume & Turner
tation of productivity might not be realized, however,
because other limiting factors such as nutrients might
become more important under these conditions. Patterns
in herbivory also would undoubtedly be altered,
with probable but presently unpredictable effects on
primary productivity. Again, because of individual plant
species responses to all of the changes in limiting factors,
some dramatic changes in community composition
could occur throughout the vegetation of the GYE.
Discussion
The three climate scenarios share some similarities. The
upper treeline in the GYE is likely to move toward
higher elevations in response to increased temperatures,
regardless of the direction and magnitude of
changes in precipitation and water use efficiency, and
the distribution of Douglas-fir is likely to expand. Concomitantly,
the alpine and whitebark pine zones decrease
in extent and become more fragmented under all
scenarios considered here (Fig. 6). Thus, it appears that,
under these climate scenarios, several or many species
and communities that are restricted to the alpine zone
are likely to become locally extinct within Yellowstone
Park and possibly the Greater Yellowstone Ecosystem
during the next few centuries. However, the total number
of species within YNP and the GYE actually may
change little. Semidesert vegetation, which is currently
rare and restricted to specialized habitats, may expand
in lower-elevation portions of the GYE, especially under
the warm, dry scenario.
The simplistic prospect of a smooth northerly and
upward migration of plant species and communities is
complicated by individual species responses and by the
rate at which climate change may occur (Davis 1989b;
Davis & Zabinski, in press). Neilson et al. (1989:76)
state that thermal regimes could move 4 to 6 degrees
northward within 100 years; this is a rate of 70 to 100
km per decade. Similarly, elevational zones could shift
65 to 90 m per decade (Neilson et al. 1989:56). By the
time a slow-growing tree reaches reproductive age, the
environment may no longer be suitable for seedling survival
(Davis 1989b; Davis & Zabinski, in press). Probably
the species that will track the moving thermal zones
are those with short, rapid life histories, for example,
introduced weeds, or species with a broad distribution
such as lodgepole pine (Strain & Bazzaz 1983). The
species that will respond least effectively are the longlived
species that reproduce late or irregularly and
those with already limited, fragmented distributions, for
example, whitebark pine and alpine species. Competitive
interactions between species also would be complicated
as new species from lower elevational zones
would have to become established in the higher zones
where adults of the formerly dominant species would
still be present even if they were no longer reproducing
locally.
Mature individuals of many long-lived species may
persist in their present locations for as much as decades
even centuries, after the climate becomes unsuitable for
survival of their offspring (Davis 1984; Brubaker 1986).
Plant communities in the GYE might appear to be stable
for a long time, but after a disturbance (such as fire,
insect outbreak, or windstorm) the mature forest community
could be replaced by a completely different
suite of species. Indeed, many of the vegetational
changes caused by climate change would be mediated
by disturbance (Brubaker 1986; Neilson et al. 1989; Sandenberg
et al. 1987), and the disturbance regimes themselves
will be altered by the climatic changes, as noted
in the discussion of the warm, dry, and intermediate
scenarios. There is paleoecological evidence for rapid
changes in community composition following disturbance
of mature communities that had persisted for
some time despite climatic change (Brubaker 1986;
Dunwiddie 1986; Cwynar 1987).
Research and Monitoring Needs
The range of scenarios we presented and their potential
implications suggest some directions for research and
monitoring in the Greater Yellowstone Ecosystem. Because
of the complexity and spatiotemporal scales of
potential ecological responses, it is important to design
long-term measurements creatively so that they are particularly
sensitive to early indications of ecological
change. For example, species or individuals that are near
the limits of their range of tolerance are likely to respond
more rapidly than those that are well within their
physiological range. Our analyses suggest that the location
of the upper and lower timberline in the GYE will
probably respond to any of the likely climate scenarios.
Research in subarctic North America and Europe and in
mountain ranges of the Pacific Northwest and Great Basin
in western North America has shown that timberlines
can respond quickly even to climate changes of the
magnitude observed in the last 100 to 500 years (Davis
1984; Brubaker 1986). Therefore, upper and lower timberlines
should be high-priority sites for research and
monitoring.
A first step in the GYE timberline studies would be to
relate the present location of the upper and lower timberlines
to underlying variables such as geologic substrate,
slope, aspect, and disturbance and land-use history.
This information is needed before we can select
specific sites for monitoring or distinguish between local
and global causes of timberline change. Yellowstone
National Park's geographic information system is an excellent
tool for this analysis within park boundaries, although
adjacent public and private lands must also be
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Romme & Turner Implications of Climate Change 383
evaluated. Research is then needed to clarify the mechanisms
that explain the site-specific composition and
location of timberlines. Differences in soils, local climate,
and species availability contribute to variability
among sites (Krebs 1985). After clarifying the present
timberline patterns and mechanisms, we recommend
the establishment of permanent transects that traverse
upper and lower timberline areas. The transects should
span a gradient from forest interior to nonforest zones.
Tree mortality and seedling establishment would be
measured along these transects at regular time intervals.
These long-term measurements must be coupled with
research designed to understand the underlying mechanisms
of any changes detected in timberline location or
composition. For example, yearly or decadal variation in
tree establishment and mortality could be correlated
with comparably scaled fluctuations in temperature and
precipitation to clarify climatic constraints on tree
growth and survival.
Another early indicator of global climate change may
be alterations in the frequency and severity of disturbances,
which may be the proximal cause of substantial
changes in the structure and function of natural landscapes.
Given the importance of fire in the Greater Yellowstone
Ecosystem, particular emphasis should continue
to be placed on increasing our ability to predict
the occurrence and effects of fire. Postfire succession
should be monitored following the 1988 fires and after
future fires, especially in areas near the upper and lower
timberlines. Monitoring of disturbances also must be
coupled with research directed at understanding the
mechanisms of postdisturbance change and the individualistic
responses of species to disturbances of differing
magnitudes and severities. For example, the widespread
and abundant establishment of aspen seedlings following
the 1988 fires was surprising (Despain & Renkin,
unpubl. ms.). It was previously thought that aspen seedlings
could not become established under current climatic
conditions in the northern Rocky Mountains. We
do not yet know whether this aspen establishment was
a normal response to large-scale fires or whether it heralds
a fundamental change in postfire succession in the
GYE since the last extensive fires that occurred during
the eighteenth and nineteenth centuries. The survival of
these aspen seedlings under a variety of site conditions
must be studied. Additional research should be initiated
to understand the potential limits of aspen distribution,
abundance, and reproduction in the Greater Yellowstone
Ecosystem in response to local climate, soils, herbivory,
disturbance, and competitive interactions with
other plants.
Long-term measurements of net primary production
and community composition would be most appropriate
in the lower-elevation sagebrush-grasslands. The
grasses and shrubs are likely to show more rapid
changes in productivity and composition in response to
climate than the subalpine forests. The grasslands also
are influenced by native ungulates, so research into vegetation-climate-herbivore
interactions should continue.
The use of remotely sensed data and simulation models
to complement long-term field measurements is desirable.
The detection of broad-scale ecological changes
is complicated by practical limitations such as the difficulty
of replicating extensive experiments or sampling
regimes and extrapolating local data to an entire region
(Turner et al. 1989). Remote sensing holds great promise
for gathering synoptic data on vegetation composition,
structure, and processes. Landsat imagery has been
used to estimate productivity in the low-elevation grasslands
(Merrill et al. 1988), and repeated observations
would permit the detection of directional changes over
large areas. Models are of particular importance because
of the time lags in forest responses. In addition, simulation
models can be used to explore hypotheses about
ecological responses, identify sensitive parameters, and
extrapolate from fine scales to the region. For example,
Pastor and Post (1988) explored the impact of climate
change on mature forests in eastern North America by
linking an individual-based stand simulation model with
an ecosystem model. The nitrogen cycle and water balance
were modeled explicitly, and a variety of soils
were simulated. Results illustrated how broad-scale soil
heterogeneity influenced forest ecosystem responses to
climate over long time periods. Similarly, models would
be needed to predict landscape changes in response to
altered fire frequency and severity over long time scales
in the GYE under varying climate regimes. Fire regimes
are sensitive to the average climate over decades to
centuries, as well as the interannual variability of water
balance (Clark 1990). Thus, a model linking meteorological
conditions, fire initiation and spread, and plant
reestablishment could be extremely helpful in projecting
long-term landscape dynamics in the GYE.
Although the inevitability of global climate change is
not assured, the potential implications are of sufficient
magnitude that it would be foolish to ignore them. The
conservation of biological diversity in extensive natural
areas such as the Greater Yellowstone Ecosystem will
become increasingly difficult as the broad-scale constraints
on the biota undergo changes that are more
rapid than those experienced in the past. Explorations
of scenarios such as those we have presented can provide
useful heuristic tools to increase our understanding
of ecological dynamics in response to climate change,
and can stimulate discussion regarding the strategies appropriate
for maintaining biological diversity in the face
of environmental change.
Acknowledgments
We thank Jay E. Anderson, Joel Berger, David Challinor,
Virginia H. Dale, Richard Marsten, Ronald P. Neilson,
Conservation Biology
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384 Implications of Climate Change Ronume & Turner
Robert V. O'Neill, James Schmidt, and an anonymous
reviewer for providing comments on this manuscript.
This research was funded by the Ecological Research
Division, Office of Health and Environmental Research,
U.S. Department of Energy, under contract no. DEAC05-840R2
1400 with Martin Marietta Energy Systems,
Inc.; and by the Ecology Program, National Science
Foundation, through grant no. BSR-8408181. Publication
No. 3616 of the Environmental Sciences Division,
ORNL.
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