Ecography 31: 499508, 2008
doi: 10.1111/j.2008.0906-7590.05331.x
# 2008 The Authors. Journal compilation # 2008 Ecography
Subject Editor: Francisco Pugnaire. Accepted 13 February 2008
Leaf d13C reflects ecosystem patterns and responses of alpine plants
to the environments on the Tibetan Plateau
Minghua Song, Deyu Duan, Hui Chen, Qiwu Hu, Feng Zhang, Xingliang Xu, Yuqiang Tian,
Hua Ouyang and Changhui Peng
M. Song ([email protected]), D. Duan, Q. Hu, F. Zhang, X. Xu, Y. Tian, H. Ouyang and C. Peng, Inst. of Geographical Sciences and
Natural Resources Research, the Chinese Academy of Sciences, Beijing 100101, China. (Present address of C. P.: Inst. des Sciences de
l’Environnement, Dépt des Sciences Biologiques, Univ. du Quebec à Montréal, Case postale 8888, Succursale Centre-Ville, Montréal, QC
H3C 3P8, Canada.) H. Chen, College of Resources and Environment sciences, Hebei Normal Univ., 050016 Shijiazhuang, China.
Leaf d13C is an indicator of water-use efficiency and provides useful information on the carbon and water balance of
plants over longer periods. Variation in leaf d13C between or within species is determined by plant physiological
characteristics and environmental factors. We hypothesized that variation in leaf d13C values among dominant species
reflected ecosystem patterns controlled by large-scale environmental gradients, and that within-species variation indicates
plant adaptability to environmental conditions. To test these hypotheses, we collected leaves of dominant species from six
ecosystems across a horizontal vegetation transect on the Tibetan Plateau, as well as leaves of Kobresia pygmaea
(herbaceous) throughout its distribution and leaves of two coniferous tree species (Picea crassifolia, Abies fabri) along an
elevation gradient throughout their distribution in the Qilian Mountains and Gongga Mountains, respectively. Leaf d13C
of dominant species in the six ecosystems differed significantly, with values for evergreen coniferous Bevergreen
broadleaved treeBalpine shrub Bsedges graminoid Bxeromorphs. Leaf d13C values of the dominant species and of K.
pygmaea were negatively correlated with annual precipitation along a water gradient, but leaf d13C of A. fabri was not
significantly correlated with precipitation in habitats without water-stress. This confirms that variation of d13C between
or within species reflects plant responses to environmental conditions. Leaf d13C of the dominant species also reflected
water patterns on the Tibetan Plateau, providing evidence that precipitation plays a primary role in controlling ecosystem
changes from southeast to northwest on the Tibetan Plateau.
Leaf 13C discrimination by plant leaves is negatively and
linearly correlated with the ratio of intercellular to ambient
CO2 concentration (ci to ca), which reflects the balance
between stomatal conductance and the photosynthetic
capacity (Farquhar et al. 1989). Therefore, measurements
of leaf d13C ratios provide useful information on the
integrated carbon and water balance of plants over long
periods. Numerous studies have confirmed that d13C values
can be used as a proxy for water-use efficiency of C3 plants
based on a positive correlation between plant leaf d13C and
water-use efficiency (Francey and Farquhar 1982, Silim
et al. 2001, Warren et al. 2001, Wang et al. 2003). It also
has been widely used as a means of exploring climate change
and carbon cycling in terrestrial ecosystems (Hobson and
Wassenaar 1999, Hultine and Marshall 2000, Su et al.
2000, Wang et al. 2003) and plant physiology (Wildy et al.
2004). Leaf d13C values differ significantly among plant
species under a given set of environmental conditions, and
especially among plant life forms (Körner et al. 1986,
Brooks et al. 1997), and with variation among plant species appears to be determined mainly by plant genetic
characteristics (Körner et al. 1988). In addition, environmental factors also modify the isotopic composition of
plant tissues through their influence on leaf conductance,
photosynthetic rate, or both parameters simultaneously.
These factors include precipitation (Stewart et al. 1995,
Warren et al. 2001, Van de Water et al. 2002), temperature
(Leavitt and Long 1982, Panek and Waring 1995, 1997),
soil water content (Ehleringer and Cooper 1988, Stewart
et al. 1995, Korol et al. 1999), irradiance (Leavitt and Long
1991), soil nitrogen availability (Guehl et al. 1994), and
atmospheric CO2 concentration (Beerling and Woodward
1995, Ehleringer and Cerling 1995, Beerling 1997).
Among these factors, precipitation and temperature are
typically most important since these aspects of climate
directly or indirectly influence plant growth. Körner et al.
(1988, 1991) used carbon isotope ratios to examine the
effects of altitudinal and latitudinal gradients on plant
physiological processes for a broad spectrum of plants across
the globe, and found that d13C values changed with both
altitude and latitude. Troughton and Card (1975) found
little variation in C-isotope ratio for plants grown at a range
499
of temperatures from 15 to 358C, whereas Panek and
Waring (1995) reported that low temperature constrained
stomatal conductance, resulting in higher foliar d13C values.
In different studies, the relationship between leaf d13C and
precipitation has been reported to be a negative correlation
(Stewart et al. 1995, Anderson et al. 1996, Leffler and
Evans 1999, Korol et al. 1999, Miller et al. 2001, Leffler
and Enquist 2002) or a positive correlation (Read and
Farquhar 1991, Guo and Xie 2006), or without any
significant correlation (Schulze et al. 1996a,b, 1998).
Thus, it seems that there is a threshold value of annual
precipitation above which additional precipitation has little
impact on leaf d13C (Leffler and Enquist 2002).
To date, there have been no consistent conclusions from
extensive studies of variations in leaf d13C as a function of
plant species physiological characteristics and environmental factors. In stead, previous studies have focused on
different taxonomic group units (i.e. between species or
within species), and have used different environmental
gradients at a range of scales (regional or local). Few studies
have simultaneously analyzed the intra- and inter-species
variation in leaf d13C along environmental gradients at
regional or local scales. In the present study, we set out to
investigate whether the same mechanism that governs
variation in leaf d13C existed within and between species
along environmental gradients at regional and local scales.
We also attempted to identify the plant characteristics and
environmental factors that contributed most strongly to the
variation in plant leaf d13C.
The Tibetan Plateau provides a natural laboratory for
simultaneously studying patterns of plant water-use efficiency under different climatic conditions along horizontal
and vertical transects. The plateau covers ca 2.5 million km
and has an average altitude of 4000 m. The spatial
differentiation among physico-geographical regions on the
plateau is determined mainly by its topographic configuration and atmospheric circulation patterns. The climate is
warm and humid in the southeast, and cold and arid in the
northwest (Zheng 1996). Annual precipitation decreases
gradually from the southeast to the northwest. Along these
temperature and precipitation gradients, vegetation types
change gradually from marine humid montane (tropical
seasonal and rain forest, warm-temperate broad leaved
evergreen forest, temperate deciduous forest, and coniferous
forest) in the southeastern to continental semiarid montane
(temperate shrubland or meadow, temperate steppe, alpine
meadow or shrubland, and alpine steppe) in the middle
region to continental arid montane (temperate desert,
alpine desert, and ice or polar desert) in the northwestern
region (Ni 2000) (Fig. 1). Under the control of the
plateau’s climate and complex topography, the actual
vegetation patterns are strongly three-dimensional, with
vertical and horizontal zonation of vegetation types (CAS
1980).
In this study, we measured differences in leaf d13C values
between and within a range of species along horizontal and
vertical transects at both regional and local scales. The first
study examined the leaf d13C values of the dominant species
in different ecosystems on the Tibetan Plateau, and tested
the hypothesis that leaf d13C of the dominant species in
ecosystems would reflect changes in environmental conditions that corresponded to the patterns of vegetation
distributions from northwest to southeast across the plateau
at a large geographical scales especially in regions where
marked environmental gradients controlled the distribution
of ecosystem types. The second study was carried out at the
within species level. In this study, we measured leaf d13C of
Kobresia pygmaea, the dominant species of alpine meadow,
throughout its distribution from northwest to southeast
along the horizontal transect on the plateau. In addition, we
collected leaves from two coniferous tree species Picea
crassifolia and Abies fabri, which were the dominant tree
species in the Qilian Mountains and the Gongga Mountains, respectively. We analyzed their leaf d13C values to test
Figure 1. Patterns of ecosystems on the Tibetan Plateau and locations of the sampling sites.
500
2608.63
1345.15
1698.36
2303.03
1804.35
450.94
41.9
340.08
454.0
495.8
551.5
1932.0
6.55
4.57
0.20
9.28
9.83
4.2
Golmud
Wudao liang
Nagqu
Lhasa
Nyingchi
Gongga Mountain
36824?N,
35813?N,
31832?N,
29841?N,
29813?N,
29836?N,
94850?E
9385?E
9282?E
91819?E
94814?E
101849?E
2973
4613
4095
3674
2958
3628
Alpine desert
Alpine steppe
Alpine meadow
Alpine shrub
Evergreen broadleaved forest
Evergreen coniferous forest
Nitraria tcmgutorum Bobr
Stipa purpurea Griseb
Kobresia pygmaea Clarke
Salix rehderiana Schneid
Quercus aquifolioides Rehd. et Wils.
Abies fabri
Annual
Potential evapotranspir
precipitation (mm)
ation (mm)
Mean temperature (8C)
Dominant species
Vegetation types
Our first study was carried out along horizontal vegetation
transect on the Tibetan Plateau, which covers over 2000 km
from Geermu to Gongga Mountain on the Tibetan Plateau
(29813?36824?N and 91819?101849?E) (Fig. l). Along
this geographical range, there is a strong latitudinal gradient
in annual precipitation. Rainfall decreases gradually from
southeast to northwest owing to influence of the monsoon
from the Indian Ocean. Annual precipitation is 2500
mm in the south and decreases to B50 mm in the north of
the transect, and 7090% of the precipitation is concentrated during the summer (from June to September). Along
the same transect, potential evapotranspiration increases
from ca 451 mm in the south to ca 2609 mm in the north.
Mean annual temperature ranged from 4.57 to 9.838C
(Table 1). Associated with this climatic gradient, there are
distinct changes in the composition and structure of the
vegetation. The main vegetation types are sub-mountain
evergreen coniferous forest, subtropical evergreen broadleaved forest, alpine shrub, alpine steppes, alpine meadows
and alpine desert. The dominant species are Abies fabri in
the evergreen coniferous forest, Quercus aquifolioides in the
evergreen broadleaved forest, Salix rehderiana in the alpine
shrubland, Stipa purpurea in the alpine steppe, Kobresia
pygmaea in the alpine meadow, and Nitraria tangutorum in
the alpine desert.
Our second study was conducted along two vertical
transects: one in the Qilian Mountains Water Conservation
Forest Ecosystem Research Station, which is part of the
Chinese Forest Ecosystem Research Network (the Qilian
station, CFERN), and the other at the Gongga Alpine
Ecosystem Observation and Experiment Station, which is
part of the Chinese Ecosystem Research Network (the
Gongga station, CERN) (Fig. 1). The Qilian station was
located in the northeast of the Tibetan Plateau (38824?N,
100817?E). The altitude of the study area ranges from 2100
to 3800 m, with montane forest dominated by Picea
crassifolia, distributed approximately from 2500 to
3300 m. The study area is characterized by cold and
semi-arid climate. The mean annual air temperature is
0.58C with maximum of 31.38C in July and minimum of
24.68C in January. The annual mean precipitation is
368 mm, of which ca 80% falls during the growing season
from May to September (Ge et al. 2005). The lowest
elevation colonized by Picea crassifolia is significantly
limited by low soil water content (Zhang et al. 2001).
The Gongga station was located in the southeast of the
Table 1. Description of sampling location, vegetation and climate characteristics.
Study site and target species
Latitude and longitude Elevation (m)
Materials and methods
Study site
the hypothesis that within species variation in leaf d13C
indicates plant adaptation to the environmental conditions
that controlled the plant species distribution throughout the
study area. By integrating the results of these studies, we
attempted to determine whether variations in plant leaf
d13C between and within species reflect the same mechanisms in that govern plant responses to environmental
gradients, and where plant characteristics and environmental conditions both affected the observed variations in plant
leaf d13C.
501
Tibetan Plateau (29836?N, 101853?E). The study area was
characterized by a subtropical monsoon climate. The
evergreen coniferous forest is dominated by Abies fabri,
which is distributed from 2700 to 3600 m elevation. The
mean annual air temperature is 4.28C with a maximum of
23.38C in July and a minimum of 14.08C in January.
The annual mean precipitation is 1932 mm (Shen et al.
2001).
Sampling and data collection
We extensively investigated leaf d13C of the dominant
species in six ecosystem types along the horizontal transect
in August 2005. We established six sampling plots (each
100 100 m2), each uniform in species composition and
cover and representing a single vegetation type along the
transect (Table 1). These plots have experienced little
impact from human activities. In each plot, we sampled
only the dominant species. The number of species sampled
ranged from two to eight per plot. We sampled three to five
individuals per species.
Leaves of K. pygmaea were collected at 25 sites throughout their distribution areas from north to south in the
Tibetan Plateau. At each site, we sampled three to five
individuals from an area of ca 1 ha. All the leaf samples were
dried and ground for carbon isotope analysis.
In the Qilian Mountains, we established seven sampling
along an altitude gradient from 2550 to 3350 m (at
elevation of 2550, 2750, 2950, 3100, 3200, 3280, and
3350 m). The samples were collected at the end of July
2005, during the peak growing period for P. crassifolia. At
each site, we selected four to five P. crassifolia individuals,
and collected the sunlit current-year needles for carbon
isotope analysis.
In the Gongga Mountains, we established six sampling
sites along an altitude gradient from 2700 to 3600 m (at
elevation of 2700, 2900, 3100, 3300, 3500, and 3350 m).
The samples were collected in August 2004. We selected
three A. fabri and collected the sunlit current-year needles
for carbon isotope analysis.
The leaves were oven-dried for 48 h at 708C and then
ground into fine powder using an automatic ball mill. The
stable carbon isotope composition was measured using a
mass spectrometer (Finnegan MAT 253, Environmental
Isotopic Laboratory, Inst. of Geographic Sciences and
Natural Resources Research, CAS). The d13C value was
expressed as d13C (Rsample/Rstandard-1) 1000˜,
where R is the ratio of 13C to 12C in the samples and
standard, respectively. The standard that we used is the
carbon dioxide obtained from PDB, a limestone from Pee
Dee Belmenite formation in South Carolina (USA), which
was provided by the International Atomic Energy Agency
(IAEA), Vienna, Austria.
Climatic data such as the annual mean temperature and
annual precipitation at each sampling site were derived
from the PRISM (parameter-elevation regressions on
independent slopes model) climate model developed by
the Oregon Space Climate Research Center (Daly et al.
1994, 2000). All the data were calibrated using meteorological data recorded by local weather stations as input for
the model. Potential evapotranspiration and air pressure at
502
each sampling site were derived from meteorological data
recorded by local observation stations.
Data analysis
We use one-way ANOVA to analyze the differences in leaf
d13C among the dominant species, among the K. pygmaea
sampling sites along the transect throughout its entire
distribution area, and among P. crassifolia and A. fabri
populations at different positions along the altitude
gradient. Linear and non-linear regression models were
used to analyze the relationship between leaf d13C and
environmental factors. We use correlate analysis to analyze
the relationship between leaf d13C of Picea crassifolia and
environmental factors (elevation, temperature, precipitation
and soil water content, air pressure). All the analyses were
performed using the SPSS 10.0 software package (SPSS
Chicago).
Results
Variation in leaf d13C among the dominant species
and their relationship to climatic factors
The dominant species in each ecosystem (Table 1) were
broadly classified as evergreen coniferous trees, evergreen
broadleaved trees, alpine shrubs, alpine meadow vegetation
(mostly sedge), alpine steppe vegetation (mostly graminaceous species), and alpine desert vegetation (mostly xeromorphy). Plants in these different groups showed distinctly
different leaf d13C values (F 6.18, pB0.001), with an
average value of 26.03˜. The mean d13C value was
lowest for evergreen coniferous trees (29.16˜). The
highest value was for the xeromorphy ( 24.63˜). There
were no significant differences in leaf d13C between the
sedge and the graminaceous species, which are the dominant species of the alpine meadow and alpine steppe
respectively. The differences in leaf d13C among the other
groups were all significant (Fig. 2). Leaf d13C values of the
dominant species were negatively correlated with annual
precipitation (Fig. 3A) and mean annual temperature (Fig.
3B), but were not significantly correlated with elevation or
air pressure (p 0.1).
Within species variation of leaf d13C and their
relationship to climatic factors
Leaf d13C values of K. pygmaea differed significantly along
the transect through the Tibetan Plateau (F 3.94, p B
0.01), with an average values of 25.47˜. The maximum
value ( 24.39˜) occurred in the Wudaoliang alpine
steppe where the climate is cold and arid. The minimum
value ( 26.81˜) occurred in the Linzhi forest ecosystem,
where the climate is warm and moist. Leaf d13C values of K.
pygmaea were negatively correlated with annual precipitation and annual mean temperature (Fig. 4A, B), but have
no significant correlations with elevation and air pressure
(p 0.1).
–24
–24
Leaf δ13C values (‰)
Leaf δ13C values (‰)
–22
e
–26
d
d
c
b
–28
–25
–26
–27
y = –0.0043x - 23.34
(R2 = 0.38, p<0.001)
a
–30
Ec
Eb
Sh
Se
Gr
(A)
Xe
–28
Plant life forms
300
400
Figure 2. Leaf d C values for the dominant plant life forms in
ecosystems of the Tibetan Plateau. Ec, evergreen coniferous; Eb,
evergreen broad hard-leaf; Sh, shrubs; Se, sedges; Gr, graminoids;
Xe, xeromorph. Bar labeled with different letters differ significantly in their leaf d13C values (pB0.05).
Leaf δ13C values (‰)
600
700
800
–24
Leaf δ13C values (‰)
–20
500
Precipitation (mm)
13
–25
–26
–27
y = –0.19x - 25.88
–22
(R2 = 0.45, p<0.001)
(B)
–24
–28
–7
y = –1.28Ln(x) - 18.55
(R2 = 0.76, p<0.0001)
–3
–1
1
3
Temperature (°C)
–26
Figure 4. Relationship between leaf d13C values of Kobresia
pygmaea and (A) annual precipitation and (B) mean annual
temperature.
–28
–30
(A)
–32
0
500
1000
1500
2000
2500
3000
Precipitation (mm)
–20
y = –0.14x - 25.75
(R2 = 0.14, p<0.0001)
–22
Leaf δ13C values (‰)
–5
–24
–26
–28
–30
(B)
–32
–5
–2.5
0
2.5
5
7.5
10
Temperature (°C)
Figure 3. Relationship between leaf d13C values of the dominant
species and (A) annual precipitation and (B) mean annual
temperature.
Leaf d13C of P. crassifolia differed significantly among
the elevations (F4.73, pB0.004), with average of
24.69˜. The maximum value (23.87˜) occurred at
the lowermost elevation (2550 m), and the minimum value
(25.23˜) occurred at an elevation of 3100 m. Leaf d13C
showed variable trends with respect to elevations. It
decreased with increasing elevation between 2550 and
3100 m, with a rate decrease equal to 2.47˜ km 1, and
increased with increasing elevation from 3100 to 3350 m, at
a mean rate of increase of 3.59˜ km 1. In contrast, the
rate of increase was thus 1.45 times higher than the rate of
decrease. The variation in leaf d13C of P. crassifolia along
the elevation gradient could be simulated using a secondorder quadratic curve (R 0.93, pB0.01) (Fig. 5A). The
relationship between leaf d13C of P. crassifolia and annual
precipitation and mean annual temperature can be quantified using the second-order quadratic curve, respectively
(Fig. 5B, C) (R2 0.90, p B0.001; R2 0.93, pB0.001).
The variation in leaf d13C of A. fabri does not
significantly along the elevations (F 0.95, p 0.05).
The d13C value ranged from 30.10 to 28.49˜, with
a mean value of 29.16˜. There were no significant
correlations between leaf d13C of A. fabri and annual
precipitation (Fig. 6A), mean annual temperature (Fig. 6B),
elevation and air pressure respectively.
503
–28
y = 7E-06x2 - 0.04x + 36.23
(R 2 = 0.94, p<0.0001)
–23.5
–28.5
Leaf δ13C values (‰)
Leaf δ13C values (‰)
–23.0
–24.0
–24.5
–25.0
–25.5
–29
–29.5
–30
(A)
–26.0
2500
2750
3000
3250
(A)
–30.5
1500
3500
Elevation (m)
–28
–23.0
y = 0.0002x2 - 0.18x + 15.41
(R2 = 0.90, p<0.0001)
–23.5
Leaf δ13C values (‰)
Leaf δ13C values (‰)
1750
2250
2000
Precipitation (mm)
–24.0
–24.5
–25.0
–25.5
2500
(B)
–28.5
–29
–29.5
–30
(B)
–26.0
350
400
450
500
Precipitation (mm)
550
0
y = 0.21x2 + 0.82x - 24.40
(R 2 = 0.93, p<0.0001)
–23.5
Leaf δ13C values (‰)
1.5
3
Temperature (ºC)
4.5
6
Figure 6. Variation in leaf d13C of Abies fabri as a function of (A)
annual precipitation and (B) mean annual temperature.
–23.0
–24.0
–24.5
–25.0
–25.5
(C)
–26.0
–5
–4
–3
–2
–1
Temperature (°C)
0
1
Figure 5. Relationship between leaf d13C of Picea crassifolia and
(A) elevations, (B) annual precipitation, and (C) mean annual
temperature.
Discussion
Plant leaf d13C values on the Tibetan Plateau confirmed our
hypothesis that variations in leaf d13C between species and
within species reflected the same mechanisms that underlie
the adaptation of plants to environmental gradients.
Variations in leaf d13C among dominant species reflected
504
–30.5
water gradients which play a key role in controlling
ecosystem distribution patterns at large and horizontal
geographical scales. In addition, variations in leaf d13C
within species indicated plant adaptation to the environmental gradients that control their distribution throughout
the entire distribution areas in which the species can be
found. Leaf d13C value of the dominant species and K.
pygmaea were both negatively correlated with annual
precipitation and mean annual temperature. The same
mechanisms thus underlie the variations in leaf d13C within
a dominant species as well as within a species along a water
gradient. Intra- and inter-specific variations in d13C have
previously been documented along naturally occurring
environmental gradients such as moisture availability and
elevation, demonstrating patterns of greater water-use
efficiency with decreased moisture availability (Dawson
and Ehleringer 1993, Zhang and Marshall 1995, Panek and
Waring 1997, Cordell et al. 1998, Sandquist and Ehleringer
2003). The physiological response of these plants illustrates
a strategy to minimize water loss while maintaining carbon
assimilation.
Reduced moisture availability may also lead to selection
for plant species that can limit their water loss and maintain
a more favorable water balance in dry environments
(Lambrecht and Dawson 2007). Dominant species are the
species most capable of adapting to local environmental
conditions, and are thus able to make full use of local
resources. Along water gradients, different species become
dominant based on their ability to adapt to local water
conditions at a given point along the gradient. And the
replacement of dominant species along our transect
reflected the large-scale changes in water conditions from
the southeast to northwest of the plateau, which also
corresponds to the ecosystem patterns along this transect.
Different species have their own optimal breadth in water
use. When the water conditions are unsuitable for growth,
the species will become less abundant, and other species that
better adapt to the water conditions will become dominant
instead. Our data, although not explicitly designed to test
for the replacement among the dominant species in the six
ecosystems, suggest that the dominant species have different
breadth in physiological responses to changes in broad-scale
habitat variables, and they would be substituted when
habitat variables go beyond their adaptability. Leaf d13C of
the dominate species in the six ecosystems reflected the
water gradient that controls ecosystem distributions at large
geographical scales along our transect through the Tibetan
Plateau. Our results thus provide evidence that water
gradient is the primary factor that control the distributions
of ecosystems on the Tibetan Plateau. For species distributed across wide areas that encompass a range of varying
environmental conditions, plants that adapt to these habitat
conditions must be strongly adaptive. Simultaneously
acting, antagonistic influences on species distribution,
such as drought and cold lead to increased variation in
d13C within species. For example, the observed variation in
leaf d13C of K. pygmaca reflected the adaptation of this
species to changes in precipitation and temperature along
the transect.
An obvious discrepancy in leaf d13C values appears in
the dominant species in different ecosystems along the
transect across the Tibetan Plateau. This discrepancy relates
to plant life forms. In this study, the roots of the herbaceous
species (S. purpurea in the alpine steppe plot and K.
pygmacea in the alpine meadow) were concentrated in the
upper 30 cm of the soil. This indicates that these species
mainly rely on unstable sources of near-surface soil water,
which is derived primarily from precipitation (Duan et al.
2008). In comparison, the alpine shrubs (N. tangutorum
and S. rehderiana) and the forest species (Q. aquifolioides)
have deeper root systems. They mainly use groundwater or
a mixture of near-surface soil water and groundwater, which
are relatively steady and reliable sources of water in our
study area (Duan et al. 2008). Leaf d13C values for those
species with deeper roots and more reliable water sources
were often more negative which indicates lower water-use
efficiency and increased stomatal conductance.
Different modes of water use influenced by the
distribution of plant roots, might be responsible for the
leaf d13C discrepancy (Dawson and Ehleringer 1991, 1998,
Pennuelas et al. 1999). The availability of water determines
whether plants will suffer from water stress when local
precipitation patterns change, and this is a very important
consideration in understanding future vegetation changes.
Brooks et al. (1997) examined leaf d13C values of the
dominant species in three boreal forest ecosystem where
different moisture gradient occur, and revealed distinct
differences in carbon isotope discrimination. Stewart et al.
(1995) examined how d13C changed in plants along a
rainfall gradient, and found that the d13C values of a
community reflected the longer-term mean precipitation for
a given site. Lloyd and Farquhar (1994) suggested that
carbon isotope discrimination varied significantly among
vegetation types on a global basis. Our investigation of
ecosystem patterns on the Tibetan Plateau indicated that
horizontal changes in vegetation types depended more upon
water supply than on temperature. Annual precipitation
decreases gradually from the southeast to the northwest on
the Tibetan Plateau because of topographic features and
characteristics of the region’s atmospheric circulation
(Zheng et al. 1979). Our studies measured the variation
in leaf d13C of dominant plants along a horizontal
vegetation transect, and the results showed that water status
played the most important role in controlling the patterns
of ecosystem distributions on the Tibetan Plateau.
Species adaptability to environmental conditions determines the scope of their distribution. The distribution of K.
pygmacea was limited simltaneously by temperature and
precipitation. The optimal distribution areas for survival of
K. pygmacea are sunny mountain slopes because the species
prefer a dry habitat with strong illumination and high
evapotranspiration. However, previous studies showed that
the niche width of K. pygmaca was broad, with tolerance for
a wide range of soil water potentials and illumination (Zhou
2001), which suggests K. pygmacea has strong adaptability
to these habitat conditions, and a wider resource-utilization
spectrum. Our result showed that leaf d13C of K. pygmacea
was varied with changes in broad-scale habitat variables (i.e.
temperature and precipitation). This provides evidence that
K. pygmacea was able to improve its water-use efficiency to
adapt to both low temperatures and low soil water levels on
the Tibetan Plateau.
Our results also showed that the leaf d13C values of the
two coniferous trees we studies on the Tibetan Plateau
differed significantly. The mean d13C value of P. crassifolia
was 4.47˜ higher than that of A. fabri. Picca crassifolia
inhabited sites with a cold and semiarid climate, whereas A.
fabri grew in a warm and humid environment. The
relationship between the leaf d13C of P. crassifolia and
elevation, altitude and annual precipitation and mean
annual temperature formed a concave-up parabola. These
patterns might reflect the adaptation of this species to the
local cold and semi-arid climatic conditions that are
common in the northeast plateau. We separated these
data from inflexion of parabola into two groups, and one
group corresponding to elevation from 2550 to 3100 m,
and the other group corresponding to elevation from 3100
to 3350 m. For each group, we made correlate analysis
using leaf d13C values and environmental factors (elevation,
temperature, precipitation, soil water content, and air
pressure). The strongest negative correlation between leaf
d13C of P. crassifolia and precipitation (r 0.884, p B
0.01) as well as soil water content (r 0.891, pB0.01)
at the lower distribution (elevation from 2550 to 3100 m)
and the strongest negative correlation between leaf d13C of
P. crassifolia and temperature (r 0.968, p B0.01) at
the upper distribution (elevation from 3100 to 3350 m)
suggest that P. crassifolia would suffer from drought stress at
its lower distribution boundary, whereas at its upper
505
boundary, it would sustain low temperature stress. For
example, the mean soil water content (30 cm depth) was
only 19.9% at their lowermost limit of 2550 m for this
species during the growing season, and mean soil temperature (10 cm) at the treeline (3280 m) in July was 5.748C.
Both drought stress (Ehleringer and Cooper 1988, O’Leary
1995, Araus et al. 1997, Van de Water et al. 2002) and low
temperature stress (Panek and Waring 1995) would reduce
the carbon isotope discrimination, leading to higher leaf
d13C. Therefore, the variation in d13C of P. crassifolia along
an elevation gradient would be controlled mainly by two
different climatic factors: lower soil water content at low
distribution area (elevations), and lower temperature at high
elevations. The leaf d13C of A. fabri showed no significant
correlation between leaf d13C and either annual precipitation or mean annual temperature. Abies fabri was found
primarily on warm and humid site in the southeastern
plateau. For instance, at an elevation of 3000 m, the mean
annual air temperature was 4.28C. The corresponding mean
annual precipitation was 1932 mm (Shen et al. 2004).
Variations in leaf d13C of A. fabri were thus not
significantly affected by elevations, air temperature or
precipitation, so these appear to not be limiting factors
for its growth.
Nevertheless, the non-linear response of leaf d13C to
annual precipitation in the coniferous trees that we studied
may also suggest that d13C reflected the water availability
under water stress conditions in dry areas but not wet areas.
This is consistent with the observations by Warren et al.
(2001) who concluded that d13C may only be a useful
indicator of drought stress in seasonally dry climates. In
addition, Leffler and Enquist (2002) suggested that a
threshold value for annual precipitation may exist, above
which additional precipitation has little impact on d13C.
Such threshold relationships in biological systems have also
been reported in other studies of d13C and water availability
(Leffler and Evans 1999, Van de Water et al. 2002).
Similarly, other studies have showed that a dramatic
increase in leaf d13C occurred when precipitation declined
to a certain level (Schulze et al. 1998, Miller et al. 2001).
Our investigation spanned a precipitation gradient from
200 to 2730 mm. It seems that when the precipitation
reached 1450 mm, additional precipitation had little impact
on leaf carbon isotope composition, and leaf d13C value
remained relatively stable (Fig. 3A).
Variation in d13C has also been linked to changes in
temperature (Tans and Mook 1980, Freyer and Belacy
1983). However, the temperature-dependent relationship
varied among studies. Measurements of the d13C value for
plants grown at a range of temperatures from 15 to 358C
showed little variation in their carbon isotope ratio in one
study (Troughton and Card 1975). In contrast, Panek and
Waring (1995) reported that at four sample sites, the sites
with unfavorable (low) temperatures constrained stomatal
conductance, resulting in less-negative foliar d13C. Su et al.
(2000) found that the responses of plant d13C to
temperature variation were species specific along the
transect (Northeast China Transect, NECT) through the
grassland northeastern China. Wang et al. (2002) investigated four C3 plant species (Chenpodium album, Lepidium
506
apetalum, Cirsium leo and Plantago depressa) in northern
China, and found that the overall d13C values were
significantly negatively correlated with mean annual temperature. Despite the significant temperature gradients
shown in our study, and leaf d13C of dominant species
was more strongly correlated with precipitation than with
temperature. This suggested that precipitation was a
primary controlling factor for the distribution of alpine
ecosystem along our horizontal transect through the
Tibetan Plateau.
Acknowledgements We thank Martin Werth (Inst. of Soil Science
and Land Evaluation, Univ. of Hohenheim, Germany) for his
meticulous work in improving the language of our manuscript.
This study was funded by the National Natural Science Foundation of China (NSFC) (Grant no. 30600070), the National Basic
Research Program of China (Grant no. 2005CB422005), and
International Partnership Project ‘‘Human Activities and Ecosystem Changes’’ (Grant no. CXTD-Z2005-1). Our study complied
with the current laws of our country.
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