Folia Geobotanica 38: 357­366, 2003
THE CALCAREOUS RIDDLE: WHY ARE THERE SO MANY
CALCIPHILOUS SPECIES IN THE CENTRAL EUROPEAN
FLORA?
Jörg Ewald
Fachhochschule Weihenstephan, University of Applied Sciences, Department of Forest Science and Forestry,
D-85350 Freising, Germany; e-mail joerg.ewald@fh-weihenstephan.de
Abstract: The pool of the Central European flora consists of a majority of vascular plant taxa that are restricted
to very base rich and calcareous soils. Ellenberg indicator values for Germany indicate that this floristic pattern is
one of the potentially most powerful determinants of the richness of modern temperate plant communities.
Considering the example of the forest flora, which, as the putative natural core of the species pool, exhibits the
same skew, it is shown that neither the frequency of suitable soil types nor other correlated ecological factors can
explain this striking pattern. Also, the ramification of higher taxa offers no indication of higher evolution speeds
in calciphilous plants. As an alternative, it is hypothesized that Pleistocene range contractions have caused the
extinction of more acidophilous than calciphilous species, because acid soils were much rarer when refugial
areas were at their minimum. If this is correct, one of the most significant ecological patterns in the contemporary
distribution of plant diversity must be regarded as a result of ecological drift imposed by a historical bottleneck.
Keywords: Biodiversity, Indicator plants, Soil acidity, Species pool, Species richness
Nomenclature: ELLENBERG et al. (1991).
INTRODUCTION
The dichotomy of calciphilous versus acidophilous plant species has long been recognized
as a striking feature of European vegetation (KERNER VON MARILAUN 1896, SCHIMPER
1903). Although its ecophysiological background is not entirely understood (KINZEL 1982,
LARCHER 1994), the importance of pH and the concentrations of "base" (Ca, Mg, K, Na)
versus "acid" cations (H, Al, Fe, Mn) in soil solutions for the composition of plant
communities is undoubted (e. g. GÖNNERT 1989, HAKES 1994, FALKENGREN-GRERUP et al.
1995, ELLENBERG 1996, TYLER 1999). It is also a widespread notion that temperate and arctic
plant communities of neutral soils tend to possess higher species densities than those of acid
soils (GRIME 1979, GRUBB 1987, PEET & CHRISTENSEN 1988, GOUGH et al. 2000). This may
be ultimately due to a reservoir (GRIME 1979) or species pool effect (ZOBEL et al. 1998), i.e.,
to a disparity in the number of calciphilous versus acidopilous species on the broad
geographic scale of temperate floras (PÄRTEL 2002). A query of the list of indicator plants for
soil reaction compiled by ELLENBERG et al. (1991) demonstrates that markedly basiphilous
and calciphilous plants constitute more than 50% of the German vascular flora (Fig. 1).
Uneven distribution of broad-scale richness along a particular environmental gradient and
among the corresponding habitat types (g-diversity sensu WHITTAKER 1960) can be
explained in several ways:
(1) The species richness of a habitat type increases with its areal extent (ROSENZWEIG
1995). If the species-area relationship causes the calciphyte-acidophyte disparity, neutral and
alkaline soils should be more widespread than acid soils.
(2) Species richness is a result of speciation and extinction (RICKLEFS 1987). If base-rich
environments have higher evolution rates (IVERSEN 1958, GRIME 1979), calciphilous plants
should be part of more ramified evolutionary lineages, i.e., they should tend to be found in the
more species-rich genera and families.
(3) Finally, the increase in richness with pH could be confounded with other environmental
gradients. If the disparity results from such hidden drivers, calciphilous plants should also be
members of other particularly diverse indicator groups, e.g. for low nitrogen and high light
availability (Fig. 1).
This paper presents the calciphyte/acidophyte disparity in the Central European flora in
quantitative detail, assesses available evidence for the validity of explanations and attempts to
draw the attention of contemporary biodiversity research to a fascinating and, as I believe,
largely unresolved phenomenon.
MATERIAL AND METHODS
Ellenberg's list contains 2726 Central European plant taxa (mostly species, in a few cases
species groups) and lists indicator values of soil reaction (R-values) derived on the basis of a
large body of phytosociological field data and pH measurements (ELLENBERG et al. 1991).
Species optima are expressed by ordinal numbers between 1 and 9 (x for indifferent species).
Each R-value corresponds to a certain amplitude on the pH gradient, as shown in Fig. 2. For
simplicity's sake species will be juxtaposed as "acidophilous" (R = 1­6) and "calciphilous"
(R = 7­9) throughout this paper. ELLENBERG et al. (1991) describe species with R-value 7 as
"indicators of moderately acid to moderately basic conditions, entirely absent from strongly
acid soils", those of R-value 9 as "indicators of bases and lime, exclusively on calcareous
soils" and indicators 8 as "intermediate between 7 and 9, i.e., mostly indicating the presence
of lime". Calciphilous plants in the sense used here should therefore rarely find suitable
habitats on soils with pH < 5 (see Fig. 2).
I screened the data base of ELLENBERG et al. (1991) for the numbers of species assigned to
reaction values 1 to 9. Separate assessments were done for each of 8 plant formations
distinguished by ELLENBERG et al. (1991). The forest formation (not including deciduous
scrub communities) was further subdivided into phytosociological classes, of which the
character species were assessed for their preference on the soil reaction gradient.
The areal extent of forest soils of different acidity in Germany was estimated from the
frequency of measured pH values in federal forest soil monitoring (ANONYMOUS 1997). The
distribution of habitats suitable for natural vegetation types was assessed by analyzing the
spatial extent of units in the map of potential natural vegetation in Germany (BOHN et al.
2003; author's interpretations with respect to soil acidity in mapping units are available on
request).
Based on the Ellenberg list I calculated the number of species per genus and per family as
indices of taxonomic ramification. Differences in the values of these indices taken by
358 J. Ewald
acidophilous and calciphilous species
were assessed by non-parametric
Mann-Whitney U-tests.
Systematic differences between
acidophilous and calciphilous plant
species regarding other indicator
values were detected by comparing
both groups by Mann-Whitney U-tests.
RESULTS
pH-dependence of species
richness across formations
Of the 2726 vascular plant species
listed in ELLENBERG et al. (1991) 86%
are regarded as indicators of the acidity
status of soils. Species richness is
distributed highly unevenly among
reaction indicator groups (Fig. 1):
R-values 7 and 8 contribute far more
species than any other class.
Consequently 55% of all species in the
German vascular flora ­ or 64% of all
reaction indicators ­ prefer to grow on
calcareous or at least very base-rich
soil.
ELLENBERG et al. (1991) assign
2503 or 92% of the species to one of 8
broad vegetation formations, among
which overall species diversity is
unevenly distributed. The most
diverse formations are anthropogenic
heaths and meadows (23%) and
ruderal vegetation (22%), followed by
alpine talus and meadows (13%),
deciduous forest and scrub (12%) and
freshwater and bogs (12%). Only
small fractions of the vascular flora
prefer natural tall herb communities
(6%), coniferous forests (3%) or saline
habitats (3%). Calciphytic richness
(Fig. 3, left) is basically proportional
to the overall g-diversity of
formations. The floras of halophytic,
Calcareous riddle 359
0
10
20
30
40
50
60
70
80
90
100
1 2 3 4 5 6 7 8 9 >9
indicator value
cumulative%ofindicatorspecies
N
F
T
R
L
K
Fig. 1. Richness of indicator groups in the German flora;
above: histogramme of indicators for soil reaction; below:
cumulative curves summarizing the g-diversity in indicator
groups for nitrogen (N), moisture (F), temperature (T),
reaction (R), light (L) and continentality (K).
371
36
100
141
171 161
236
648 632
230
0
100
200
300
400
500
600
700
800
indifferent
1
2
3
4
5
6
7
8
9
Ellenberg's R-value
#vascularplantspecies
0
10
20
30
40
50
60
70
80
90
100
2 3 4 5 6 7 8 9
0
10
20
30
40
50
60
70
80
90
100
2 3 4 5 6 7 8 9
9
8
7
6
5
4
3
2
1
%offorestarea
pH at soil depth
0-10 cm
amplitude
R-Values
cumulative
frequency of
soils
Fig. 2. Amplitudes of reaction indicator species on the pH
gradient as verbally defined by ELLENBERG et al. (1991);
cumulative frequency of pH in forest soil monitoring (mineral
soil, ANONYMOUS 1997).
360 J. Ewald
Table 1. Species richness of the Central European forest flora and its distribution across indicator values.
x ­ species indifferent towards R, S ­ # species total, F ­ % of forest flora, C ­ % calciphilous.
R-value
x 1 2 3 4 5 6 7 8 9 S F C
Tree species 11 1 1 5 10 8 1 37 10 24
Understorey species
Erico-Pinetea 3 3 10 8 24 6 88
Pulsatillo-Pinetea 1 2 2 1 6 2 50
Vaccinio-Piceetea 5 4 14 8 3 3 1 1 39 10 3
Coniferous forest 8 4 14 8 3 4 3 4 12 9 69 18 36
Salicetea 1 2 5 8 2 88
Alnetea 1 1 5 6 3 1 17 4 6
Quercetea 4 4 5 2 15 4 0
Querco-Fagetea 8 3 5 17 34 73 46 2 188 49 64
Quercetea, Querco-Fagetea 8 4 7 10 19 34 73 46 2 203 52 60
Deciduous forest 9 4 8 15 25 38 76 51 2 228 59 57
Other sciophytes 18 3 7 5 2 3 7 8 53 14 28
Understorey total 35 4 21 23 23 31 44 87 71 11 350 90 48
Forest total 46 4 21 24 24 36 54 95 72 11 387 100 46
0 100 200 300 400 500 600 700
Halophytic
Coniferous
forest
Tall herb
vegetation
Freshwater
and bogs
Deciduous
forest and
scrub
Talus and
alpine
meadow
Ruderal
Anthropogenic
heaths and
meadows
# species
0 100 200 300 400
Coniferous
forest
Halophytic
Tall herb
vegetation
Freshwater
and bogs
Deciduous
forest and
scrub
Talus and
alpine
meadow
Anthropogenic
heaths and
meadows
Ruderal
# species R > 6
0 20 40 60 80 100
Coniferous
forest
Anthropogenic
heaths and
meadows
Freshwater
and bogs
Deciduous
forest and
scrub
Tall herb
vegetation
Ruderal
Talus and
alpine
meadow
Halophytic
% species R > 6
Fig. 3. Distribution of overall and calciphytic richness (species with R indicator values > 6) across broadly
defined vegetation formations; left ­ absolute numbers of species; centre ­ absolute numbers of calciphilous
species; right ­ proportion of calciphilous species
alpine, ruderal, tall herb and deciduous
forest communities consist of a
majority of calciphilous species
(Fig. 3, right). The deciduous forest
formation is in this respect (61%
calciphilous species) more or less
representative of the total flora, while
coniferous forest species are largely
acidophytes.
pH-dependence of species
richness in forests
Tree species, that make up only 10%
of the forest flora (Tab. 1), were most
often judged as indifferent towards soil
reaction or as moderately basiphytic by
ELLENBERG et al. (1991). By far the
most listed forest species are
understorey species (Fig. 4)
characteristic of the classes Quercetea
robori-petraeae and Querco-Fagetea
(52%), which according to
OBERDORFER (1992) comprise the
temperate forests of Fagus, Quercus,
Acer, Fraxinus, Carpinus, Ulmus and
Tilia, that form the potential natural
vegetation of large parts of Central
Europe (BOHN et al. 2003).
Calciphilous species make up 60% of their character species. Acidophytic spruce
(Vaccinio-Piceetea) and oak forests (Quercetea robori-petraeae) are the only forest types that
contain a majority of understorey species with low R-values. Character species of alder
swamps (Alnetea glutinosae) have mostly intermediate R-values.
Frequency of acid versus calcareous forest sites
The results of federal forest soil monitoring in Germany (ANONYMOUS 1997) reveal that
the present-day areal extent of habitat conditions does not coincide with the distribution of
species richness across the acidity gradient. In a systematic grid of 1650 points 75% of the
forest sites had pH-values of less than 4.46 in the uppermost 10 cm of mineral soil, in 90%
pH-values of less than 5.52 were measured. If we assume the amplitudes in Fig. 1 are correct,
46% of the German forest flora depends on 25% of the forest habitats. It is at a soil depth of
60­90 cm (n = 1194) that the proportion of sites favourable to calciphilous species
approximately matches the contribution of this group to the forest flora (50% of sites with pH
4.66 or above) ­ a zone that is hardly reached by roots of the vast majority of species
Calcareous riddle 361
0
10
20
30
40
50
60
70
80
90
100
R-value
#forestspecies
tree species
understorey of broad amplitude
coniferous forest
deciduous forest
0
10
20
30
40
50
60
70
80
90
100
1 2 3 4 5 6 7 8 9
indicator value
cumulative%indicatorspecies
N
L
T
F
R
K
Fig. 4. Richness of indicator groups in the character species of
German forests; above: histogramme of indicators for soil
reaction; below: cumulative curves summarizing the
g-diversity in indicator groups.
(POLOMSKI & KUHN 1998). In good accordance with the findings of soil monitoring the map
of potential natural vegetation by BOHN & NEUHÄUSL (2000) designates 53% of Germany as
habitat of acidophytic, 30% as habitat of basiphytic and only 7% of calciphytic community
types. Thus in Germany, the richness of reaction indicator plants runs counter to the frequency
of suitable habitats (Fig. 5).
Ramification of families and genera
Among the 158 plant families listed in ELLENBERG et al. (1991) the most species rich are
the Asteraceae (340 species, 103 acidophytes/237 calciphytes), Poaceae (225, 72/151) and
Cyperaceae (146, 60/84). Despite the overrepresentation of calciphytes in the two largest
families, calciphilous and acidophilous species do not differ significantly in their tendency to
occur in more species-rich families according to the Mann-Whitney U-test (Tab. 2). Of the
733 genera Carex (103, 47/55), Hieracium (54, 28/26) and Alchemilla (40, 16/19) are the
most ramified and do not contain remarkably high numbers of calciphytes. The U-test detects
a significant tendency for calciphilous species to occur in smaller genera (Tab. 2). The same
tendencies are found for acidophilous and calciphilous forest species.
Relationships between R and other indicator values
When considering the whole flora, the two groups of reaction indicators differ significantly
with respect to all other indicator values (Tab. 2): Calciphilous species tend to have their
optima in habitats enjoying more nitrogen and light, but less moisture. Climatically, they have
a slight preference for warmer sites and their distributions extend further into the continent.
Again, the same trends are found, when reaction indicators occurring in forests are compared.
362 J. Ewald
Table 2. Comparison of reaction indicator species groups with regard to other attributes by Mann-Whitney
U-test. L ­ light, T ­ temperature, K ­ continentality, F ­ moisture, N ­ nitrogen, n ­ # species, U, z ­ test
statistics, P ­ significance level.
acidophilous calciphilous Mann-Whitney test
mean n mean n U z P
All species
Species per family 114 845 116 1510 631778 0.392 0.695
Species per genus 18 845 13 1510 570832 -4.242 0.000
L 7.1 835 7.3 1502 594572 2.080 0.032
T 4.8 750 5.5 1389 390500 9.565 0.000
K 3.7 768 4.2 1378 412980 8.443 0.000
F 5.4 822 4.9 1481 510160 -6.445 0.000
N 3.2 802 4.3 1418 410499 10.899 0.000
Forest species
Species per family 90 158 77 230 17426 -0.686 0.492
Species per genus 17 158 11 230 16077 -1.929 0.053
L 4.9 154 5.3 230 15558 2.019 0.040
T 5.0 121 5.6 207 9025 4.222 0.000
K 3.7 144 4.0 226 13948 2.317 0.016
F 5.7 150 5.0 222 13135 -3.455 0.000
N 3.8 149 4.8 215 10796 5.290 0.000
DISCUSSION
More than half of the German vascular flora belongs to ecological groups favouring
calcareous or very base-rich soils, while a markedly smaller number of species appear to be
adapted to acid conditions. The same skew is found in the vascular floras of Austria, where
KARRER (unpubl.) assigns 54% out of 3228 taxa to R-classes 7­9, of Hungary, where
BORHIDI (1995) put 63% out of 2174 taxa in R-classes 7­9, and of Switzerland, where
LANDOLT (1977) places 64% of 3363 taxa in classes 4 and 5 on his 5-point reaction scale.
With minor exceptions, the disproportion between calciphytes and acidophytes exists across
the major vegetation formations, which makes it unlikely that the proportions are generally
biased by the large number of synanthropic species of mediterranean and Irano-Turanian
origin. I analyzed the forest flora as the putative core of the natural flora in particular detail.
Plant ecologists familiar with the temperate flora will be far from surprised by this pattern
of species richness, and some may even find it uninteresting. In an elegant metaanalysis
PÄRTEL (2002) has shown how far from self-evident it is: Acidophytic communities of the
south of the temperate zone are usually richer in species because the acidophytic species pools
are larger.
The vast majority of contemporary German forest soils are acid and unsuitable habitats for
calciphytes, which is also mirrored in maps of potential natural vegetation. The species-area
relationship can therefore not explain the large pool of calciphilous forest species. Instead, the
majority of forest species appear to depend on quite rare habitats. This raises the question if
species pools are in equilibrium with the environment. In fact, industrial immissions have
recently enforced the acidification of forest soils beyond its natural level (ULRICH & MEYER
1987), but there is yet little evidence for a corresponding shift in floristic composition
(FISCHER 1999).
The distribution of calciphytes among genera and families does not indicate particularly
high rates of evolution as compared to acidophytes. This result may be compromised by the
fact that I analyzed only the German flora, which may not be representative of the larger
continental arena of evolution. Furthermore, the evolution of calciphytes may have occurred
quite recently and equally across taxonomic lineages, as suggested by IVERSEN (1958). He
Calcareous riddle 363
pH in 0-10 cm soil depth
0 20 40 60 80
2.5-5.0
5.0-6.5
6.5-8.5
% forest area
Group of pnv-units
0 20 40 60 80
not defined
acidophytic
basiphytic
calciphilous
% potential forest area
Reaction-value
0 100 200 300
indifferent
1-3
4-6
7-9
# forest species
Fig. 5. Frequency of topsoil pH (after federal forest soil monitoring), areal extent of units of potential natural
vegetation (after BOHN & NEUHÄUSL 2000) and richness of reaction indicator groups in forests (after
ELLENBERG et al. 1991).
believed that the warming phases following continental
glaciations with their open woodland communities
thriving on unleached calcareous soil favoured the
immigration and in situ evolution of calciphytes. GRUBB
(1987), however, thought that calcareous habitats had
been more "apparent" throughout the history of temperate
floras, i.e., that the ratio between calcareous and acid
habitats was reversed for much of the past.
I propose to combine Iversen's and Grubb's arguments
with species-area theory and look at the disparity as an
inherited species-area relationship (Fig. 6). The exact
pre-Pleistocene calciphyte/acidophyte ratio is not crucial
for this hypothesis. Nevertheless, it is not unreasonable to
assume that highly leached acid soils prevailed for long
time periods and favoured the evolution of a
predominantly acidophytic flora (t1), i.e., long-term
adaptation proportional to habitat area (ROSENZWEIG
1995). Glaciation (t2) sharply reduced the total habitat for
the European flora causing substantial regional
extinctions (WATTS 1988). Soil paleoecology suggests
that periglacial processes of denudation, solifluction and
loess sedimentation created young calcareous soils at the
expense of mature soils (FIEDLER & HUNGER 1970), thus
causing a disproportionate decline of acid habitats and
their flora. As a result, the postglacial flora (t3) is
selectively impoverished, causing an ecological disparity,
that cannot be explained by the contemporary
environment. Note that the hypothesis rests on the
asymmetry between speciation and extinction: The result
of adaptive radiation is rapidly reversed by extinction,
while the opposite process requires long time periods.
HUBBELL (2001) has recently introduced the term
"ecological drift" for such processes: Although species
extinctions are stochastic, their severity is negatively proportional to the size of the surviving
metacommunity (sum of all individuals of all species) during the bottleneck.
Preferences of plant species for calcareous soils are not independent of responses to other
environmental gradients. Looking at the total flora, this could raise the suspicion that
calciphytic richness really reflects a preference of many species for dry habitats with ample
light supply. However, this can be precluded in forests, where calciphytes predominate over
acidophytes, although which tend to have higher indicator values for nitrogen and
continentality.
In conclusion, this analysis shows that across formation types the Central European species
pool for calcareous habitats is larger than that for the much more widespread acid soils. My
364 J. Ewald
calciphytic
#speciesn
t1 t2 t3
acidophytic
time
habitatareah
t1:
tertiary
t2:
ice age
t3:
postglacial
calcareous
acid
Fig. 6. Palaeoecological model of
habitat area and species richness of
acidophytes and calciphytes. Habitat
areas go through the bottleneck of
glacial refugia and recover to
pre-glaciation levels, whereas species
richness is lost by extinction. The
contemporary calciphytic to
acidophytic richness ratio corresponds
to the calcareous to acid habitat area
ratio in refugia:
h t
h t
n t
n t
c
a
c
a
( )
( )
( )
( )
.2
2
3
3
=
hypothesis that the disparity reflects a species-area relationship of the past, and thus an
example of ecological drift, requires testing by paleoecologists and biogeographers. Modern
tools of systematics, biogeography and plant ecology should also be used to search for
alternative explanations. Species pool size puts an upper bound on the richness, that can be
encountered in individual stands of vegetation (GRIME 1979) and could provide an
explanation for the positive relationship between pH and plant species density throughout the
temperate zone (ERIKSSON 1993, PÄRTEL et al. 1996). Calciphytic a- and g-diversity as one
of the most consistent gradients of biological richness in the temperate biome (PÄRTEL 2002)
deserves much more attention from contemporary students of biodiversity.
Acknowledgements: I would like to thank U. Bohn for supplying GIS statistics on mapping units; thanks also to
G. Karrer for querying his preliminary list of indicator plants and to F. Horváth for querying the Hungarian
database; M. Pärtel provided a helpful peer review of an earlier version of the manuscript.
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Received 8 February 20023, revision received and accepted 21 October 2002
366 J. Ewald