Ecol Res (2005) 20: 167–176
DOI 10.1007/s11284-004-0026-5
O R I GI N A L A R T IC L E
Shiping Chen Æ Yongfei Bai Æ Guanghui Lin
Xingguo Han
Variations in life-form composition and foliar carbon isotope discrimination among eight plant communities under different soil moisture
conditions in the Xilin River Basin, Inner Mongolia, China
Received: 21 November 2003 / Accepted: 29 September 2004 / Published online: 14 January 2005
The Ecological Society of Japan 2005
Abstract Water is one of the key limiting factors for the
survival and growth of plant species in arid and semi-arid
steppe regions. Different plant functional groups (PFGs)
based on life-forms differ in their strategies to cope with
limited water availability. The foliar carbon isotope discrimination (D) value provides an integrated measurement of internal plant physiological and external
environmental properties affecting photosynthetic gas
exchange over the time interval when the carbon was
fixed. In this study, we surveyed the composition and D
values of various life-forms (shrubs, sub-shrubs, perennial grasses, perennial forbs and annuals) in eight different plant communities along a soil moisture gradient in
the Xilin River Basin, Inner Mongolia, China. Our results
showed that: (1) life-forms occurred variously in eight
steppe communities with different soil moisture status; (2)
in wetter habitats, forbs were more abundant and accounted for the majority of aboveground biomass,
whereas grasses became more important in drier habitats.
Shrubs and sub-shrubs increased with decreasing soil
water availability and their relative biomass rapidly increased in degraded steppe and sand dune communities.
(3) The numerical order of the mean D values of life-forms
is as follows: perennial grasses (15.86&) < shrubs
(16.10&) < perennial forbs (16.45&)=annuals
(16.41&) < sub-shrubs (17.55&), reflecting their differences in water use efficiencies. The significant differences
in the D values among these life-forms suggested that
life-form-based PFGs not only represent a morphological
S. Chen Æ Y. Bai Æ G. Lin Æ X. Han (&)
Laboratory of Quantitative Vegetation Ecology,
Institute of Botany, The Chinese Academy of Sciences,
20 Nanxincun, Xiangshan, Beijing, 100093,
People’s Republic of China
E-mail: [email protected]
Tel.: +86-10-82595771
Fax: +86-10-82595771
G. Lin
Department of Global Ecology,
Carnegie Institution of Washington,
Stanford, CA 94305, USA
classification of these plants, but could also represent
a functional group integrating different physiological
processes such as water use strategies, which may
partially explain the differences in PFG composition and
competitive ability of co-existing species along environmental gradients in the Xilin River Basin.
Keywords Grassland Æ Life-forms Æ Soil moisture
gradient Æ Stable isotope Æ Water use efficiency
Introduction
Plant functional groups (PFGs) can be defined as sets of
plants showing similar responses to environmental
conditions and having similar effects on important ecosystem processes (Walker 1992; Noble and Gitay 1996;
Sandra and Marcelo 1997). Many ecologists agree that
complex community structures can be simplified by
categorizing species into PFGs based on suites of correlated traits (Lavorel et al. 1997; Smith et al. 1997). One
important goal of PFG analysis is to use this information on correlated traits to predict the response of vegetation to environmental change, instead of detailed
information on each species (Dyer et al. 2001). The most
common and simplest method for classifying plants is by
life-form. Plant functional types based on life-forms,
e.g., shrubs, grasses and forbs, represent the main aspects of ecosystem functioning (Aguiar et al. 1996). In
this study, we grouped plant species into shrubs, subshrubs, perennial grasses, perennial forbs, and annuals
based on their life-forms.
Ecosystems in arid and semi-arid regions of the world
differ substantially in species composition but not in the
types of life-forms (Sala et al. 1997). Results from a 24year observational study in the Xilin River Basin showed
that there were significant compensatory interactions in
terms of biomass production among major functional
groups, such as perennial grasses and perennial forbs
(Bai et al. 2004). A transect study on 16 steppe com-
168
munities along the gradients of precipitation and temperature in the Xilin River Basin also indicated that the
relative abundance of perennial grasses increased with a
decrease in mean annual precipitation, whereas that of
perennial forbs showed a reverse trend (Bai et al. 2002).
Because of drought and human activities (e.g., overgrazing), shrubs and sub-shrubs markedly increased
their proportions in many degrading grasslands in the
Xilin River Basin (Tong et al. 2002; Xiong et al. 2003).
Overall, these previous studies provided important
information on the compositional change of life-forms
as affected by precipitation and temperature, as well as
overgrazing, but the underlying mechanisms have yet to
be elucidated.
Water is the most limiting resource for plant growth
and community productivity in most terrestrial ecosystems, especially in arid and semi-arid regions (Huxman
et al. 2004). Different PFGs in terms of life-forms differ in
their adaptive strategies to limited water availability
(Aguiar et al. 1996), which would affect the competitive
advantages among PFGs and then determine the composition and structure of plant communities. Water use
efficiency (WUE), i.e., the amount of carbon biomass
produced per unit water transpired by the plant, is one
such trait indicating plant productivity when water resources are scarce (Wright et al. 1988). Foliar carbon
isotope discrimination (D) has been developed as a
valuable tool for long-term estimates of WUE, because
the D value is partly determined by Ci/Ca, the ratio of
CO2 concentrations in the leaf intercellular spaces to that
in the atmosphere, which is in turn related to WUE
(Farquhar et al. 1982, 1989; Farquhar and Richards
1984). The Ci/Ca ratio differs among plants because of
the variation in stomatal opening (affecting the supply
rate of CO2), and the variation in the chloroplast demand
for CO2 (Ehleringer et al. 1992). Therefore, the foliar D
value provides an integrated measurement of internal
plant physiological and external environmental properties affecting photosynthetic gas exchange over the time
interval when the carbon was fixed (Smedley et al. 1991).
Cohen (1970) predicted that the water-use pattern would
become more conservative during drought. Thus, the
foliar D value could be used to compare the relative WUE
of different native species to assess relative drought tolerance (Ehleringer et al. 1992; Matzner et al. 2001). The D
value approach is therefore ideal for addressing
functional diversity in complex ecosystems, and for
distinguishing functional groups of species especially in
arid environments (Brooks et al. 1997).
We surveyed the composition of the five life-forms and
measured the foliar D values of representative plants in
eight different plant communities along a soil moisture
gradient in the Xilin River Basin, Inner Mongolia, China.
Our objectives were: (1) to survey if there are compositional changes in life-form functional groups in communities with different soil moisture and land-use types; (2)
to evaluate whether any difference in the foliar D values
(an indicator of Ci/Ca and WUE) exists among these lifeforms; (3) to further explore the adaptive strategies of
different life-forms in response to different habitat qualities, especially different soil moisture regimes.
Materials and methods
Study area
The Xilin River Basin (4326¢–4429¢N, 11532¢–
11712¢E) is located in the typical steppe zone of the
Inner Mongolia Plateau. Topographically, this region,
covering an area of about 10,000 km2, declines gradually
from east (the highest elevation 1,505 m) to west (the
lowest elevation 902 m) (Chen 1988). The Xilin River
Basin has a semi-arid, continental, temperate steppe
climate with dry springs and moist summers. The annual
mean temperature increases from southeast to northwest, ranging from 0.5 to 2.1C (Chen 1988). Total annual precipitation decreases gradually from 400 mm in
the southeast to 250 mm in the northwest (Chen 1988).
More than 70% of annual precipitation occurs during
May–August. Chestnut and dark chestnut soils are the
zonal soil types (Wang and Cai 1988). Our study was
conducted in the Inner Mongolia Grassland Ecosystem
Research Station (IMGERS), the Chinese Academy of
Sciences, which is located in the middle reach of the
Xilin River.
Selection of study sites
Eight study sites were selected representing typical plant
communities adjacent to IMGERS (Table 1). Because
the eight communities were located in a relatively small
area, within a distance of less than 25 km, they presumably were subjected to similar climatic conditions,
such as temperature and precipitation. They differed in
floristic composition and soil water status and represented a soil moisture gradient depending on their relative elevation and soil types. Moreover, sites 4, 6, and 7
present three steppe communities with different land-use
types. Site 4 is a typical Leymus chinensis steppe community, which was fenced in 1979 by IMGERS as a
long-term study plot; site 6 was a degraded L. chinensis
steppe community prior to being fenced in 1983; and site
7 was a severely degraded steppe community due to
long-term free grazing. More detailed information about
the eight communities is given in Table 1.
Survey on vegetation composition
A survey on vegetation composition of the eight sites
was conducted from 10–12 August 2001, when peak
biomass occurs based on our 20-year biomass record of
this area by IMGERS (Bai and Chen 2000). In each
community, ten sampling plots (1·1 m quadrats) were
investigated, with the exception of site 1 (a swamp
meadow) in which only five plots were surveyed. Within
each plot, the aboveground part of each species was
harvested and taken back to the laboratory for analysis.
169
Table 1 General condition of the study sites in the Xilin River Basin, Inner Mongolia
Plot
Vegetation
Number type
Location
1
Swamp meadow
N 4337.460¢ E 11640.347¢ 1,150
2
Saline meadow
N 4344.925¢ E 11640.629¢ 1,190
3
Meadow steppe
N 4329.418¢ E 11649.643¢ 1,380
4
5
6
Typical steppe
Typical steppe
Restored
degraded steppe
Degraded steppe
N 4332.895¢ E 11640.708¢ 1,250
N 4332.322¢ E 11633.117¢ 1,180
N 4335.748¢ E 11644.419¢ 1,210
Fixed sand
dune complex
N 4339.189¢ E 11639.892¢ 1,220
7
8
Altitude Soil type
(m)
N 4337.967¢ E 11639.397¢ 1,180
Fresh plant samples were oven-dried at 70C for about
48 h to constant weight, and then weighed. At each site,
plant species were grouped based on their life-forms into
shrubs, sub-shrubs, perennial grasses, perennial forbs,
and annuals according to life-form classification in Flora
of Inner Mongolia (Editorial Committee of Flora of Inner Mongolia 1994). The relative biomass of each lifeform was determined based on the ratio of the biomass
of each life-form to the total biomass of the community.
The relative abundance of life-forms was obtained by
calculating the ratio of the number of species in each lifeform to the total number of species in the community.
Plant and soil sampling
From 23–25 August 2001, plant leaf samples for carbon
isotope composition analyses were collected along the
transect adjacent to the plots where the vegetation survey was conducted. Based on the results of vegetation
composition survey, at each site 6–12 plant species which
were dominant or common species in the communities
were sampled. Subordinate species that only contributed
to a minor proportion of the total community biomass
were not considered in this study. At each site, we first
identified five sampling quadrats. Then, we collected the
fully expanded, well-sunlit leaves from at least 10 individuals/ramet of each plant species in each quadrat to
make one composite sample for carbon isotope composition analysis. Leaf samples were dried at 70C to
constant weight, and then ground to 80 mesh for carbon
isotope composition analysis. At the same time, some
leaves of each species were collected for determination of
leaf water content (LWC). Leaf water content was calculated by the following equation:
LWC ð%Þ ¼
fresh leaf weight dry leaf weight
100:
fresh leaf weight
At each site, soil cores of different depths (0–20, 20–
40, 40–60 cm) were collected using a soil-sampling gauge
Land-use type
Swamp
meadow soil
Dominant species
Fenced plot
Blysmus sinocompressus,
Carex spp.,
Agrostideae spp., etc.
Saline meadow soil Grazed pasture Achnatherum splendens,
Leymus chinensis
Dark chestnut soil Mowing field
Filifolium sibiricum,
Stipa baicalensis
Chestnut soil
Fenced plot
L. chinensis, Stipa grandis
Chestnut soil
Fenced plot
S. grandis, L. chinensis
Chestnut soil
Fenced plot
Caragana microphylla,
L. chinensis
Chestnut soil
Heavily
Artemisia frigida,
grazed pasture Potentilla acaulis
Sandy soil
Fenced plot
Ulmus pumila,
Armeniaca sibirica,
Cleistogenes polyphylla
with a diameter of 5 cm, and 15 replicates were sampled.
Soil water content (SWC) was determined by calculating
the difference between moist and oven-dried (105C to
constant weight) samples according to the following
equation:
SWC ð%Þ ¼
wet soil weight dry soil weight
100:
dry soil weight
Foliar carbon isotope analyses
Foliar carbon isotope ratios were measured by a mass
spectrometer (MAT251, Finnigan MAT, Bremen, Germany) in the Center of Soil and Environment Analysis,
Institute of Soil Sciences, the Chinese Academy of
Sciences. The stable isotopic ratios of the leaves were
expressed as follows:
d13 Cl ð0=00 Þ ¼
ð13 C=12 CÞl ð13 C=12 CÞs
ð13 C=12 CÞs
1000;
where d13Cl is the leaf d13C value, (13C/12C)l and
(13C/12C)s are the carbon abundance ratios of the leaf
and the PeeDee Belemnite (PDB) standard, respectively.
The stable isotopic discrimination of C3 plants (D) was
calculated as follows:
Dð0=00 Þ ¼
d13 Ca d13 Cl
;
1 þ d13 Cl
where d13Ca is the d13C value of air.
Previous studies suggested that C4 plants can be
used as a proxy for the carbon isotope composition
of atmospheric CO2, because the carbon isotopic
composition of C4 plants is insensitive to conditions in
the physical environment and would remain constant
during C4 photosynthesis, that is, the carbon isotope
composition of C4 tissues reflects the atmospheric signal
without physiological variations (Marino and McElroy
170
1991; Marino et al. 1992; Buchmann et al. 1996). In
general, the isotopic composition of carbon in C4 plants
(d13C C4) is related to d13C a by a simple transfer function of the form:
d13 CC4 ¼ d13 Ca D4 ;
where D4 represents the characteristic difference between
d13Ca and d13C C4, about 4.4& for most C4 plants but
there are exceptions (Marino et al. 1992; Buchmann
et al. 1996).
In this study, d13C values of two C4 species, Cleistogenes squarrosa (a perennial bunchgrass) and Kochia
prostrata (a sub-shrub), were measured. C. squarrosa is
distributed more widely than K. prostrata and was found
in six of the eight sites. So, we used the d13C values of C.
squarrosa collected from different sites to calculate the
d13C of air at different sites. Because C4 species were
absent at sites 1 and 3, the d13Ca values from the nearest
site having an almost identical water condition were
used to estimate the ambient d13C values. We further
defined the D value of each life-form as the mean D value
of all C3 species in the same life-form.
Statistical analysis
Statistical tests were performed using SPSS for Windows
Version 10.0 (SPSS Inc., Chicago, 1999). The effects
of site and soil layer on SWC and those of site and
life-form type on the D value were tested by two-way
ANOVA (P<0.05). the same method was also used to
test the effect of site and life-form on relative biomass
and abundance of different life-forms. We used the LSD
method (P<0.05) to make multiple comparisons of
Fig. 1 Variations in a soil water content, b species richness and
biomass, c relative biomass of life-forms, d relative abundance of
life-forms at different sites. S Shrubs, SS sub-shrubs, PF perennial
forbs, PG perennial grasses, A annuals
means for relative abundance, relative biomass, and D of
different life-forms. The same method was also used to
distinguish the significance of the means of relative
abundance, relative biomass, and D of each life-form at
different sites. Linear regression was made between D
values and the LWC of different C3 species, mean D
value and biomass of communities at different sites,
mean D value and LWC of communities at different
sites, respectively. The significances of regression coefficients were test by ANOVA at the P<0.05 level.
Results
Changes in soil water content of different communities
Soil water content (SWC) of site 1 was a swamp and
undoubtedly had significantly higher (P<0.0001) SWC
(up to 135.3, 145.3, and 90.8%) at depths of 0–20, 20–40,
and 40–60 cm, respectively) than other sites. For all
sites, SWC gradually decreased in three soil layers
(Fig. 1a). Both community type (site) and soil depth had
significant effects on SWC (Table 2). On the whole, sites
1–8 could represent a soil moisture gradient from extremely wet to dry.
Variation in life-form composition of different
communities of the Xilin River Basin
For the 75 sampling quadrats that we surveyed, 139
species were encountered in the 8 study sites in the Xilin
River Basin. Among them, 89 species were perennial
forbs and 23 species were annuals, together they accounted for over 80% of the total species, while shrubs
and semi-shrubs only represented less than 10% of the
species found (Fig. 2a). Perennial grasses only accounted for a little over 10% of the number of species,
171
Table 2 Two-way ANOVA for the effects of soil layer and plot site
on soil water content
Source of variation
df
Mean square
F
P
Soil layer
Site
Soil layer · site
Error
2
7
14
325
2,329
48,718
983
279
8.3
174.5
3.5
–
<0.0001
<0.0001
<0.0001
–
but in terms of the mean biomass, they were the highest
(Fig. 2b).
Along the soil moisture gradient, biomass and species
richness of different communities and the composition
of life-forms changed greatly (Fig. 1b–d). Both the
characteristics of life-forms and soil moisture had significant effects on community composition in terms of
relative biomass and relative abundance (P<0.0001;
Table 3). Perennial grasses and forbs were the two most
important life-forms over the sites but they had distinctly different preferential habitats. In the typical
steppe (sites 4 and 5), perennial grasses were the dominant life-forms and made up 82 and 95% of the total
community biomass, respectively. The life-forms in saline meadow (site 2) also mainly consisted of perennial
grasses. However, perennial forbs were more abundant
than other life-forms in the meadow community (site 3).
The relative biomass and species richness of shrubs/subshrubs significantly increased with decreasing soil water
availability and became the dominant life-form in the
sand dune community (P<0.05; site 8). Human activity
(mainly overgrazing) also showed obvious effects on the
composition of life-forms. In degraded steppe (site 7),
the relative biomass of perennial grasses significantly
Table 3 Results of two-way ANOVA showing the effects of lifeforms and sites on relative abundance and relative biomass
Source
Relative
biomass
df
Mean square F
Life-form
4 18,052
Site
7
797
Life-form · site 21 6,439
Error
309
101
Relative
Life-form
4 17,583
abundance Site
7
73
Life-form · site 21 2,346
Error
309
44
179
7.9
64
–
398
1.7
53
–
P
<0.0001
<0.0001
<0.0001
–
<0.0001
0.118
<0.0001
–
decreased to 28% of the total community biomass due
to long-term overgrazing, whereas shrubs/sub-shrubs
rapidly increased up to 40% of the total community
biomass. In the restoring steppe (site 6), the relative
biomass and relative abundance of sub-shrubs markedly
decreased and that of perennial grasses greatly increased
after 18-year restoration (P<0.05). Being ephemeral
plant species and opportunists (e.g., Corispermum spp.,
Odontites serotina, etc.), annuals appeared in almost all
communities although they made up only a small proportion.
Results from regression analyses indicated that significantly negative relationships existed between the
relative biomass and relative abundance of shrubs and
SWC, respectively (P<0.05 and P<0.01; Table 4).
However, significantly positive relationships were found
between those of perennial forbs and SWC (P=0.05 and
P<0.05). For perennial grasses, only their relative
abundance showed a significantly negative correlation
with SWC (P<0.05), but not for their relative biomass
(P>0.05). No significant relationship existed between
SWC and relative biomass and abundance of sub-shrubs
and annuals (P>0.05).
Relative biomass and abundance of perennial grasses
had significant positive relationships with the total biomass of community (P<0.05), while those of perennial
forbs showed significant positive relationships with
species richness of community (P<0.01; Table 4).
Variations in carbon isotope discrimination
of C3 plants in the Xilin River Basin
Fig. 2 a Number of species and b aboveground biomass of five
different life-forms for all plant samples investigated (139 species)
in the Xilin River Basin. Abbreviations as in Fig. 1
The stable carbon isotopic composition (D) of 304 plant
samples, representing 15 families, 39 genera and 47 C3
plant species from 8 different plant communities, were
determined; the results are compiled in Table 5.
Along the soil moisture gradient, significant differences existed in the mean D values of C3 plants among
different community types (Fig. 3). The mean D value of
C3 plants increased with increasing soil water content
(r=0.866, F=17.974, P=0.005, n=8). Mean D values of
C3 plants also showed significant positive relationships
with mean leaf water content and aboveground biomass
of different communities (P<0.01; Fig. 4).
Different life-forms also showed distinct differences in
carbon isotope discrimination (Fig. 5). Perennial grasses
172
Table 4 Correlation analyses between life-form composition (including relative biomass and relative abundance) and soil water content
(SWC), biomass and species richness of eight different communities
Soil water content
Relative biomass
Relative abundance
a
Sa
SS
PG
PF
A
S
SS
PG
PF
A
Biomass
Species richness
r
F
P
r
F
P
r
F
P
0.913
0.413
0.545
0.700
0.210
0.958
0.406
0.811
0.754
0.317
14.98
1.03
2.54
5.78
0.28
32.98
0.59
11.52
7.89
0.67
0.031b
0.358
0.162
0.053b
0.618
0.010b
0.498
0.015b
0.031b
0.444
0.846
0.170
0.704
0.577
0.693
0.703
0.084
0.808
0.536
0.747
7.55
0.15
5.90
3.00
5.53
2.93
0.03
11.29
2.41
7.59
0.071
0.714
0.050b
0.134
0.57
0.186
0.861
0.015b
0.171
0.033b
0.145
0.167
0.647
0.889
0.118
0.277
0.224
0.882
0.893
0.281
0.06
0.14
4.30
22.70
0.08
0.25
0.26
20.99
23.49
0.51
0.817
0.722
0.083
0.003b
0.783
0.652
0.629
0.004b
0.003b
0.500
S Shrubs, SS sub-shrubs, PG perennial grasses, PF perennial forbs, A annuals
Correlation is significant at P<0.05
b
had the lowest values among all life-forms and subshrubs had the highest. The order of the mean D values
of life-forms was as follows: perennial grasses (15.86&)
< shrubs (16.10&) < perennial forbs (16.45&)=
annuals (16.41&) < sub-shrubs (17.55&).
Both life-form and study-site conditions showed significant effects on the mean D value of each life-form
(Table 6). For a given study site, life-form had different
effects on the D value: there were no differences in the D
value of different life-forms at study sites 1, 4, and 5;
shrubs and/or sub-shrubs showed significantly higher D
values than other life-forms at study sites 6, 7, and 8. As
two dominant life-forms, D values of perennial grasses
and perennial forbs showed different patterns along a
soil water gradient (Table 7). Perennial grasses had
higher D values in sand dunes (site 8) and degraded
steppe (site 7), and lower D values in meadow steppe (site
3), typical steppe (sites 4 and 5) and restoring steppe (site
6). Perennial forbs showed the highest D value in swamp
meadow (site 1), the lowest D value in saline meadow
(site 2), typical steppe (sites 4 and 5), and degraded
steppe (site 7) (Table 7).
Discussion
Variations in life-form composition in the Xilin
River Basin
In the Xilin River Basin, the composition of life-forms
varied greatly in eight communities with different soil
moisture regimes and land-use types (Fig. 1). As two
dominant life-forms in the Xilin River Basin, perennial
grasses and forbs had distinctly different preferential
habitats: perennial forbs were more abundant and
dominated the aboveground biomass of the total
community in wetter habitats, whereas grasses occupied drier habitats. Bai et al. (2002) reported that the
relative abundance of perennial forbs decreased with
an increase in aridity in the Xilin River Basin, and
that of perennial grasses showed a reverse trend. In
our study, however, no significant relationship was
found between the relative biomass of perennial
grasses and soil water content, which may indicate
that in addition to soil water condition, human
activity (mainly overgrazing) also affects the composition of life-form, especially for the perennial grasses.
Because most perennial grasses were palatable plant
species and were more likely to be grazed by animals
and herbivores (Li 1989), the proportion of grasses
decreased greatly in the degraded and restoring communities (sites 6 and 7), whereas some grazing-tolerant
species such as shrubs and sub-shrubs increased significantly.
In the Inner Mongolia grassland, increase in shrubs
and sub-shrubs in arid and degraded steppe is a common
phenomenon and has received more attention recently
(Wang et al. 1985; Zhao et al. 1988; Xiong et al. 2003).
In our study, shrubs and sub-shrubs significantly increased in degraded and drier communities. Caragana
microphylla, a leguminous shrub, is more abundant in
the degraded steppe of the Xilin River Basin (Xiong
et al. 2003). Increase in shrubby species in degraded
grassland ecosystems has been reported by many
researchers elsewhere (Reynolds et al. 1997; Archer et al.
2000; Van Auken 2000) and this change in the vegetation
structure is often caused by overgrazing (man-made
change) and/or by drought (climate-driven change)
(Breman and de Wit 1983; Graetz 1991). Intensive
grazing altered grass–woody plant interactions in favor
of unpalatable trees and shrubs, resulting in the transformation of grasslands and savannas into shrublands
and woodlands (Schlesinger et al. 1990; Archer 1994;
Aguiar et al. 1996). In this study, both environmental
(soil moisture) and/or anthropogenic (grazing) factors
might have resulted in life-form compositional changes
in the Xilin River Basin.
Life-forms of plants are closely related to functional
traits and represent alternative strategies to deal with the
most frequent constraints on plant growth and survival,
as well as water scarcity (Aguiar et al. 1996). Thus,
changes in the composition of life-forms in different
173
Table 5 Name, life-form, and carbon isotope discrimination values
(D) of C3 plant species in the Xilin River Basin, Inner Mongolia
Species
Achnatherum sibiricum
A. splendens
Afropyron cristatum
Allium ramosum
Anemarrhena asphodeloides
A. sibirica
Artemisia eriopoda
A. frigida
Artemisia pubescens
Caltha palustris
C. microphylla
Carex korshinskyi
Carex spp.
Cicuta virosa
Cirsium esculeutum
Corispermum spp.
Cotoneaster mongolicus
Filifolium sibiricum
Haplophyllum dauricum
Hedysarum fruticosum
var. lignosum
Inula britanica
Iris lactes Pall. var.
chinensis Koidz.
Koeleria cristata
Leontopodium
leontopodioides
L. longifolium
Leymus chinensis
Ligularia sibirica
Odontites serotina
Oxytropis myriophylla
Pedicularis striata
Polygonum divaricatum
P. acaulis
Potentilla anserina
P. tanacetifolia
Psammochloa villosa
Pulsatilla turczaninovii
Ranunculus japonicus
Ribes diacanthum
Scirpus spp.
Serratula centauroides
Spiraea aquilegifolia
Stellera chamaejasme
Stipa grandis
Suaeda corniculata
Thalictrum petaloideum
T. squarrosum
Triglochin maritimum
PFG-based
life-forma
D (&)
Mean
SD
n
PG
PG
PG
PF
PF
S
PF
SS
PF
PF
S
PF
PF
PF
PF
A
S
PF
PF
SS
15.69
16.26
16.57
16.50
14.90
14.49
17.11
17.87
16.66
16.86
16.39
16.12
17.61
17.21
17.49
16.05
15.54
16.83
17.30
17.20
0.21
0.24
0.54
0.41
0.65
0.60
0.22
0.55
0.32
0.47
0.90
0.78
0.33
0.28
0.67
0.25
0.92
0.28
0.37
0.47
5
5
20
5
5
6
5
10
4
3
24
33
3
3
3
9
3
5
5
9
PF
PF
18.25
14.64
0.36
0.37
3
5
PG
PF
15.92
17.01
0.30
0.54
15
5
PF
PG
PF
A
PF
PF
PF
PF
PF
PF
PG
PF
PF
S
PF
PF
S
PF
PG
A
PF
PF
PF
19.29
15.45
18.06
17.66
15.41
17.90
15.95
16.57
17.71
16.03
16.73
15.22
17.04
17.59
17.46
16.51
16.04
14.99
15.26
16.31
17.37
16.31
18.39
0.14
0.39
0.51
0.57
0.54
0.28
0.64
0.28
0.13
0.32
0.46
0.27
0.15
1.45
0.43
0.32
1.58
0.34
0.41
0.53
0.26
0.57
0.25
3
30
3
3
5
5
12
5
3
10
9
5
3
3
3
3
3
13
20
5
5
8
3
Fig. 3 Mean D values of C3 plants at eight sites in the Xilin River
Basin. Different letters represent significant differences among the
means (P<0.05, ANOVA LSD test). Error bars are 1 SE
Fig. 4 Relationships between mean D values with a mean leaf water
content, b aboveground biomass of C3 plants at eight sites in the
Xilin River Basin. Error bars are 1 SE
and sub-shrubs played a more important role in degraded steppe and sand areas.
a
S Shrubs, SS sub-shrubs, PG perennial grasses, PF perennial
forbs, A annuals
Carbon isotope discrimination of C3 plants
in the Xilin River Basin
communities also correlate with changes in ecosystem
functioning, such as biomass, productivity, and species
richness of the community. Compared with perennial
grasses, perennial forbs were more abundant and contributed more to species richness of the community, but
only accounted for a small proportion of aboveground
biomass (Table 4). The perennial grasses had significant
effects on community biomass and productivity. Shrubs
Our study showed a relatively wide variation in D values
of C3 plants in the Xilin River Basin, ranging from 14.49
to 19.29& (mean 16.63&). Our results are consistent
with a previous study in the same area which showed
that the D values of major C3 plants ranged from 15.77
to 19.53& (mean 17.39&) (Tieszen and Song 1990). A
similar range of D values was also reported in a study on
174
Fig. 5 Mean D values of five life-forms in the Xilin River Basin.
Different letters represent significant differences among the means
(P<0.05, ANOVA LSD test). Error bars are 1 SE. Abbreviations
are the same as in Fig. 1
Table 6 Results of two-way ANOVA showing the effects of lifeforms and sites on mean D values
Source
df
Mean square
F
P
Life-form
Site
Life-form · site
Error
4
7
13
331
7.0
6.6
3.3
0.6
11.6
11.0
5.5
–
<0.0001
<0.0001
<0.0001
–
desert plants along a soil moisture gradient (Ehleringer
and Cooper 1988). The foliar D value reflects the ratio of
intercellular CO2 concentration per atmospheric CO2
concentration (Ci/Ca) which is an important physiological trait related to water use and stomatal behavior
under drought (Farquhar et al. 1989; Williams and
Ehleringer 1996). Generally, lower foliar D indicates
higher WUE and a more conservative water-use pattern
(Ehleringer and Cooper 1988; Farquhar et al. 1989).
Since a 1& change in D is equivalent to approximately a
15 lmol mol1 difference in Ci (Ehleringer and Cooper
1988), a range of about 5& among the C3 species we
analyzed would correspond to a difference of
75 lmol mol1 on average Ci. A decrease in the Ci value
indicates an increase in the stomatal diffusion limitation
to photosynthesis and also an increase in leaf water-use
efficiency. Thus, our D data indicate that there is significant difference in water-use efficiency of C3 species in
the Xilin River Basin.
The Xilin River Basin belongs to a typical semi-arid
grassland area and has an annual average rainfall of
about 350 mm. Thus water availability is one of the
most important limiting factors for plant growth, and
the variation in the competitive ability to take up water
resources among co-existing species could largely
determine the species composition of a community
(Johnson and Asay 1993; Vilà and Sardans 1999). Efficient water-using plants may have a competitive
advantage, allowing them to dominate the vegetation
biomass (Ehleringer 1993b). In our study, with
decreasing soil water availability, C3 plants showed
significantly lower D values and a more efficient wateruse strategy in different community types. Significant
positive relationships between mean D values and the
biomass of the community suggest that a conservative
water-use pattern is very important for a community to
obtain a higher aboveground biomass in the Xilin River
Basin. Our results are consistent with Tsialtas et al.
(2001) and support Tilman’s (1982) theory that competition can be intense in low-production environments,
and that successful competition results from utilizing
scarce resources more efficiently. So, in an environment
where water is scarce, plants may compete effectively by
increasing their potential water-use efficiency as indicated by d13C values, and the relative abundance and
biomass of a particular species in a community are
regulated by the species’ ability to compete for water.
Variations in carbon isotope discrimination
of different life-forms
Smedley et al. (1991) found that carbon isotope composition was associated with life-form in arid grassland
communities in western North America: perennial
grasses showed lower D values than those of perennial
forbs and perennial species showed lower D values than
those of annual species. Brooks et al. (1997) suggested
Table 7 Mean D values of different life-forms at different sites
Site
1
2
3
4
5
6
7
8
Mean D value (&)
S
SS
PG
–
–
–
–
15.32±0.23
16.10±0.10
17.66±0.13
15.93±1.30
–
–
–
–
–
18.36±0.06
17.38±0.10
17.20±0.16
–
16.01±0.11
15.41±0.10
15.85±0.08
15.67±0.19
15.56±0.13
16.37±0.23
16.73±0.15
Ba
Bc
Aa
Bb
Aa
Ba
Ba
Data in columns are mean ± SE. Capital letters indicate the difference in D value of each life-form among different sites, whereas
lowercase letters represent the difference in D values among lifeforms at the same site. Significant differences among variables were
PF
BCa
Db
CDa
CDa
CDd
ABb
Aab
17.76±0.13
15.23±0.23
16.30±0.15
16.01±0.18
15.76±0.25
16.60±0.11
15.91±0.14
16.35±0.23
A
Aa
Db
BCa
BCDa
BCDa
Bb
CDb
BCDb
17.66±0.33
16.31±0.24
–
–
–
–
–
16.05±0.08
Aa
Ba
Bb
determined by t-test or ANOVA (LSD test, P<0.05). Values with
any letter in common are not significantly different. S, SS, PG, PF
and A represent shrubs, sub-shrubs, perennial grasses, perennial
forbs and annuals, respectively
175
that life-form groupings could be used to integrate plant
responses at the physiological, ecological, and geophysical levels. In our study, perennial grasses showed
higher WUE and more conservative water-use patterns
than other life-forms in the Xilin River Basin, which
might explain why perennial grasses are the dominant
PFG in drier communities of the Xilin River Basin.
We hypothesized that significant differences in D and
WUE among life-forms were likely caused by different
root systems for specific water availabilities. Shrubs and
sub-shrubs generally have deeper root systems than
perennial grasses and forbs in water-limited ecosystems
(Jackson et al. 1996; Schenk and Jackson 2002). Therefore, perennial grasses and forbs use resources mostly
from the upper soil layers, while shrubs/sub-shrubs take
up water mainly from the deeper soil layer (Sala et al.
1997). Because water resources in deeper layers can be
kept more constant, and separation in resource utilization reduced the competition between shrubs/sub-shrubs
and perennial grasses and forbs, shrubs/sub-shrubs
could employ a relatively prodigal water-use pattern (Le
Roux et al. 1995; Gebauer et al. 2002) and contribute
more aboveground biomass in drier habitats such as
site 8.
Moreover, perennials generally had a lower D value
than annuals in our study. This is in agreement with
previously reported results. For example, Ehleringer and
Cooper (1988) found a negative correlation between
plant longevity and carbon isotope discrimination.
Long-lived species have a greater probability of experiencing severe drought and have to employ more conservative water-use patterns to survive a prolonged
period of low soil moisture availability (Schuster et al.
1992; Ehleringer 1993a), thus long-lived species would
have a lower D value.
The patterns of plant community composition are
different along environmental gradients. From a physiological perspective, traits that influence water loss and
the ability to compete for limited soil water should
determine which plant species or life-forms of plants will
occupy given locations along arid gradients (Schuster
et al. 1992). As a whole, the lower D values are typically
found in life-forms exposed to prolonged droughts, or in
long-lived life-forms, which run a higher risk of experiencing serious drought during their lifetime. Short-lived
life-forms and those occupying wetter microhabitats,
such as swamp meadow, tend to have a higher D and
lower WUE under conditions of high resource availability. Their D values therefore indicate a reasonable
overall level of WUE and a survival strategy during
water stress. The significant difference in D values among
life-forms in our study suggests that life-form-based
PFG not only could be used as a morphological classification of plants, but also could represent a functional
group with different physiological processes such as
water-use strategies, which may partially explain the
differences in PFG composition and competitive ability
of co-existing species along environmental gradients in
the Xilin River Basin.
Acknowledgements We thank Dr. Yu Liang, Dr. Jianhui Huang,
and Dr. Linghao Li for helpful comments on an earlier version of
this manuscript. Thanks are also extended to Dr. Quansheng Chen
for providing floral composition data of the swamp. This research
was supported by the National Natural Science Foundation of
China (90211012), the ‘‘100 People’’ project of Chinese Academy
of Sciences to G. Lin, and Key Project of the Chinese Academy of
Sciences’ Knowledge Innovation Program (KSCX1-08).
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