Seasonal changes in the water use strategies of three co-occurring desert shrubs
Yu Wu1,2, Hai Zhou1,2, Xin-Jun Zheng1, Yan Li1* and Li-Song Tang1
1
State Key Lab of Oasis and Desert Ecology, Xinjiang Institute of Ecology and Geography,
Chinese Academy of Sciences, Urumqi, Xinjiang, China
2
University of Chinese Academy of Sciences, Beijing, China
Corresponding author: Dr. Yan Li
E-mail: [email protected]
Tel: +86 991 7885415
Fax: +86 991 7885320
This article has been accepted for publication and undergone full peer review but has not
been through the copyediting, typesetting, pagination and proofreading process which may
lead to differences between this version and the Version of Record. Please cite this article
as doi: 10.1002/hyp.10114
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Abstract
Water is a major limiting factor in desert ecosystems. In order to learn how plants cope
with changes in water resources over time and space, it is important to understand plant-water
relations in desert region. Using the oxygen isotopic tracing method, our study clarified the
seasonal changes in the water use strategies of three co-occurring desert shrubs. During the
2012 growing season, δ18O values were measured for xylem sap, the soil water in different
soil layers between 0 and 300 cm depth and groundwater. Based on the similarities in δ18O
values for the soil water in each layer, three potential water sources were identified: shallow
soil water, middle soil water and deep soil water. Then we calculated the percentage
utilization of potential water sources by each species in each season using the linear mixing
model. The results showed that the δ18O values of the three species showed a clear seasonal
pattern. Reaumuria songarica used shallow soil water when shallow layer was relatively wet
in spring, but mostly took up middle soil water in summer and autumn. Nitraria tangutorum
mainly utilized shallow and middle soil water in spring, but mostly absorbed deep soil water
in summer and autumn. Tamarix ramosissima utilized the three water sources evenly in
spring and primarily relied on deep soil water in summer and autumn. R. songarica and N.
tangutorum responded quickly to large rainfall pulses during droughts. Differential root
systems of the three species resulted in different seasonal water use strategies when the three
competed for water.
KEY WORDS seasonal change; water use strategies; water resource; co-occurring desert
shrubs; northwest China
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Introduction
Water determines a great many ecosystem processes in arid and semiarid regions, such
as carbon storage, species survivorship, competition and primary production (Briggs and
Knapp, 1995; Tian et al., 1999; Fay et al., 2003; Bunker et al., 2005). As the main water input
in desert ecosystems (Noy-Meir, 1973), precipitation dominates water availability or water
resources for plants. However, due to the uneven distribution of precipitation (Loik et al.,
2004), soil moisture in arid ecosystems is extremely variable both in space and time
(Schwinning and Ehleringer, 2001). There are prevalent periodic or chronic water deficits for
some desert plants. To survive and meet metabolism or growth requirements, perennial plants
must regulate their water use and their roots have to acquire the remaining soil moisture
through extended drought periods (Ehleringer et al., 1991; Dawson, 1993). Therefore,
precipitation seasonality influences the water use patterns of dominant species as well as the
composition and structure of plant communities.
It has been suggested that the exploitation of spatially and/or temporally distinct zones
of soil moisture by plants allows the coexistence of different life-forms (Noy-Meir, 1973).
For example, most grasses are opportunistic, utilizing the short-term availability of water in
the upper soil layers, while shrubs rely on a deeper soil water resource that is more stable
over the long term (Soriano and Sala, 1983). This concept coincided with the vertical
distribution of roots in different plants (Dodd et al., 1998). But root presence per se may not
be a reliable indicator of actual water uptake dynamics (Ehleringer and Dawson, 1992),
which is dependent on root activity in soil areas where moisture is available (Donovan and
Ehleringer, 1994), especially over short time scales (e.g. a season). For some plants, the water
source may change with the growth stage or with the water availability over different seasons.
During establishment, small streamside trees depend on stream water in the upper layers.
Once established, mature individuals use a deeper water source (Dawson and Ehleringer,
1991). In the dry season, Banksia prionotes derives the majority of the water it needs from
deeper sources; while in the wet season, most of the water is derived from shallower sources
(Dawson and Pate, 1996). The water use pattern of Juniperus ashei in Edwards Plateau of
USA exhibited a similar switch in water sources (McCole and Stern, 2007). Therefore,
explorations of seasonal water use strategies by different plants are necessary in order to
improve understanding of plant-water relations and water balances in these ecosystems.
Stable isotope methods offer effective means of identifying water sources utilized by
plants (Flanagan et al., 1992; Schwinning et al., 2005; Duan et al., 2008; Liu et al., 2010).
Previous studies suggested that the isotopic fractionation of water occurs during phase
transitions, but not during advective flow of water (Dawson and Ehleringer, 1991; Ehleringer
and Dawson, 1992; Dodd et al., 1998; Dawson et al., 2002). There is no isotopic
fractionation during water uptake by roots and transport from root to shoot while isotopic
fractionation indeed occurs in leaves during transpiration (Dawson and Ehleringer, 1991;
Ehleringer and Dawson, 1992; Dawson et al., 2002). Coupled with the seasonal variation in
the isotopic characteristics of precipitation (Dansgaard, 1964), the natural composition of
hydrogen and oxygen isotopes in different water sources provides a non-artificial label for
plant water sources. Thus, the isotopic composition of xylem water is a mixture of different
water sources, reflecting the various zone(s) and depth(s) from which the plant is currently
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extracting soil water (Ehleringer and Dawson, 1992). By comparing and analyzing the
isotopic characteristics of xylem waters and those of potential water sources (e.g. rain water,
soil water and groundwater), a linear mixing model could be used to calculate the percentage
use of each potential water source by plants (White et al., 1985; Phillips and Gregg, 2003;
Phillips et al., 2005).
Although the perennial woody life form is often distinguished as a group for comparison
with other life forms, there is a great deal of variation within shrubs with regards to seasonal
water stress and rooting depth and distribution (Donovan and Ehleringer, 1994). Tamarix
ramosissima, Nitraria tangutorum and Reaumuria songarica are co-occurring dominant
shrubs in the Junggar basin, northwest China. Previous studies have focused on the
physiological response of their twigs to rainfall (Xu and Li, 2006; Li, 2010). The rooting
patterns of the three shrubs have also been investigated to elucidate the water use strategies
during drought periods (Sun and Yu, 1992; Xu and Li, 2009). However, the dynamics of
water use by roots throughout an entire growing season remain largely unexplored. The
objective of our research was to investigate the seasonal changes in the source water used by
these co-occurring shrubs that have different root systems in a desert habitat.
Materials and Methods
Site and species description
The study was conducted at Fukang Station of Desert Ecology, Chinese Academy of
Sciences, located on the southeast edge of Gurbantunggut Desert (44°17′N, 87°56′E, 475 m
a.s.l.). It has a typically temperate continental arid climate and in the hot dry summer, the
highest temperature is above 35°C, but during the winter, the average snow depth is 20~30
cm, with stable snow cover lasting for 100~150 d (Zhou et al., 2009). The annual mean
temperature is 6.6°C and the annual mean precipitation is about 160 mm. The soil type at this
site is silty clay-loam with a high salinity and the upper 1 m of soil is extremely dry due to
the high pan-evaporation (about 2000 mm per year). The groundwater table depth is about 5
m.
The profiles of three co-occurring shrubs are shown in Table 1. Previous excavations in
this area revealed that the tap root of N. tangutorum was no more than 2 m in depth. The
absorbing root area of N. tangutorum was recorded by Sun and Yu (1992). Root information
for T. ramosissima and R. songarica was reported by Xu and Li (2009) who conducted
experiments in the same habitat.
Sample collection
From March to October of 2012, suberized twigs (diameter 0.3~0.5 cm, length 3~5 cm)
from the plants were sampled at half-monthly intervals for xylem water. For each species,
four replicate samples were cut from four individuals. We decorticated twigs gently and
quickly when they were cut. To prevent changes in the isotopic values through evaporation,
all samples were immediately placed into screw-cap glass vials sealed by parafilm and stored
in a freezer prior to water extraction and isotopic analysis. The first samples were taken on
day of year (DOY) 88, and the last sampling was on DOY 291.
The soil samples were collected using a hand auger that soil cores were taken next to the
sampled plants on the same days as the plants were sampled (also at half-monthly intervals).
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The total depth augered was 0~300 cm. Soil samples between 0 and 100 cm were obtained at
10 cm intervals and those between 100 and 300 cm were obtained at 20 cm intervals. Four
replicates for each layer were sampled from four independent cores. Then soil samples were
divided into two parts: one was sealed in glass vials and frozen until they were needed for
isotopic analysis of soil water and the other was sealed in soil tins for subsequent
measurement on gravimetric soil water content (SWC, %), which were obtained with the
oven-drying method.
Groundwater samples were collected from a nearby well every month and were sealed
and stored at 2°C. We collected rainwater from twelve precipitation events. Precipitation
samples were collected from standard rain gauge immediately after each individual rainfall
event and were filtered using a 0.22 μm filter. Each sample was pipetted into a small vial
sealed by parafilm and refrigerated at 2°C until isotope analysis. Precipitations and
temperatures were recorded by a weather station near the site.
In addition, to investigate the responses of the water use patterns to the rainfall pulses in
different seasons, we tracked natural rainfall events. Plant twigs were sampled on 1, 2, 3, 5, 8
days after 5.6 mm spring rainfall (on DOY 141) and 7 mm autumn rainfall (on DOY 266),
respectively. Likewise, when an 11.9 mm summer rainfall occurred on DOY 216, we sampled
plant twigs on the same day (0), and then sampled again 1, 2, 4, 7 and 17 days after the
precipitation event. Meanwhile, in order to obtain the soil water content and isotopic values
of the soil water within the 0~100 cm layer, four soil sample replicates were collected at 10
cm intervals near the sampled plants. All samples were immediately stored in glass vials and
were kept frozen until needed for isotopic analysis.
Analyses and calculation
Hydrogen isotope fractionation has been observed during water uptake in some
xerophytes (Ellsworth and Williams, 2007). Therefore, we chose 18O as a tracer. Xylem water
and soil water were extracted using a cryogenic vacuum distillation extraction line and the
extracted water was stored in sealed glass vials at 2°C. Then the oxygen isotopic composition
of the water was determined by a liquid water isotope analyzer (LWIA, DLT-100, Los Gatos
Research Inc., Mountain View, CA, USA). The oxygen isotopic composition can be
expressed as:
δ18O = (Rsample / Rstandard –1) × 1000‰
where RSample and RStandard are the oxygen stable isotopic composition (18O/16O molar
ratio) of the sample and the standard water (Standard Mean Ocean Water, SMOW),
respectively. To eliminate the effect of methanol and ethanol contamination, δ18O values for
the xylem water were corrected by a standard curve.
Creation of standard curves
The standard curves were created by engineers from Los Gatos followed Schultz et al.
(2011). The procedures were as follows:
Deionized water (DI) (simplicity UV, Millipore Inc., Milford, MA, USA) were spiked
with varying concentration methanol or ethanol (99.9% chromatographic pure).
Concentration gradient for methanol (μL·L-1): 0, 10, 20, 30, 40, 60, 80, 100, 120, 140, 160,
200, 240, 280, 320, 350, 380, 400, 420, 450, 480, 500, 520, 550, 580, 600, 620, 640, 660,
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680, 700, 720, 740, 760, 780 and 800. Concentration gradient for ethanol (mL·L-1): 0, 2, 6,
10, 15, 20, 25, 30, 35 and 40. Three repetitions were done for each concentration.
The δ18O values of above solution were measured by an isotope ratio infrared
spectroscopy (IRIS) analyzer — the Liquid Water Isotope Analyzer. With the Spectral
Contamination Identifier (LWIA-SCI), a post-processing software, narrow-band metric (NB)
represented methanol contamination, and broad-band metric (BB) represented ethanol
contamination. The Relationships between NB, BB and the offsets of δ18O values were
obtained:
∆δ18O(y) ~ lnNB(x): y = 0.1455 x2 + 0.0255x + 0.329, R2 = 0.9946 (Figure 1a)
∆δ18O(y) ~ BB(x): y = -9.1411x + 9.1269, R2 = 0.8962 (Figure 1b)
The methanol resulted in more positive isotope values, and the offsets should be
subtracted from the original isotope values. The ethanol resulted in more negative δ18O
values, and the offsets should be added to the original δ18O values. The main contaminant in
our samples was methanol which is within the range of 10~280 μL·L-1.
Comparison between IRIS and IRMS
The δ18O values from IRIS analysis were compared with corresponding data obtained
from an Isotope Ratio Mass Spectrometry (IRMS) (Finnigan MAT 253, Thermo finnigan,
Bremen, Germany). The xylem water of T. ramosissima, N. tangutorum, R. songarica was
extracted by cryogenic vacuum distillation. There were 12 samples for each species. All
samples were filtered by 0.22 μm filter. Each sample was divided into two sub-samples and
δ18O values were measured by IRIS and IRMS, respectively. IRIS data (before and after
correction) were compared with IRMS data in Figure 2. Without correction, the absolute
value of mean difference in δ18O between IRIS data and IRMS data was (1.95 ± 0.22)‰.
After correction, the absolute value of mean difference in δ18O between IRIS data and IRMS
data was (0.47 ± 0.03)‰. Overall, the corrections effectively eliminated the discrepancies in
δ18O values between IRIS and IRMS method.
To calculate the amounts of the different water sources used by each species at each
sampling time, the isotopic values for the xylem water were compared with those of potential
water sources using the IsoSource model (Phillips and Gregg, 2003). Potential water sources
were the soil waters within the different layers that had been collected on the same day as the
plants were sampled. Three potential water sources were used in our research (see below),
source increment was defined as 1% and mass balance tolerance was defined as 0.1‰. First,
we obtained the percentage utilization of the different water sources by each species on each
sampling day and then calculated the mean and the possible range of water utilization in each
season. Due to its root limitation, R. songarica could not access the deep water source
directly, so we quantified the percentage utilization of the shallow and middle soil water
sources by R. songarica using a two-compartment linear mixing model (White et al., 1985).
For the rain pulses study, data input to the IsoSource model have two origins: 0~100 cm
δ18O values of soil water and xylem water were sampled on the day of concern, while the
δ18O values of 100~300 cm were taken as the nearest monitored values, which were obtained
at half-monthly intervals. During the experimental period, the soil water content and the δ18O
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values of soil water below 100 cm were almost unaffected by a single rainfall pulse (see
Figure 5).
Data analysis
Multiple comparison of δ18O values for the soil water from all individual layers and the
groundwater were processed by Fisher’s least significant difference method (LSD) in the
one-way ANOVA (seasonal changes) module in SPSS 13.0. The significance level was P <
0.05.
Based on the similarities in δ18O values for the soil water in each layer, we divided the
soil profile into three major sections (0~50 cm, 50~180 cm and 180~300 cm). Accordingly,
three potential water sources were identified as follows:
(1) shallow soil water (0~50 cm), δ18O values varied significantly with season and depth;
(2) middle soil water (50~180 cm), δ18O values decreased with depth, but showed no
apparent seasonal changes and
(3) deep soil water (180~300 cm), δ18O values were relatively stable, were very similar to
those of the groundwater and showed no significant variations for depth and season.
Results
δ18O values for rain water and precipitation amount
The average oxygen isotope ratios (δ18O) of rain water and individual rainfall amount
from February to October of 2012 are shown in Figure 3. The δ18O values for rain water
varied considerably among events, ranging between -22.8‰ and 1.7‰, and showed
significant seasonal variation. Due to the temperature effect (Dansgaard, 1964), the δ18O
values for rain water were lower in spring (March~May) and autumn (September~October)
and higher in summer (June~August) (Figure 3).
In our study region, the total rainfall from February to October 2012 was 94.6 mm. Most
events were less than 5 mm, especially in summer, and 68% of rainfall frequency was less
than 1 mm. The largest event was 11.9 mm, which occurred on DOY 216.
δ18O values for soil water and soil water content
The δ18O values for soil water and soil water content (SWC) in each layer in the 0~300
cm soil profile are displayed in Figure 4. Surface soil waters have highly variable δ18O values
due to inputs from rains with variable δ18O signatures and evaporative enrichment. The
Fisher’s multiple comparison found that in the upper 50 cm layers, the δ18O values for soil
waters were significantly higher in summer and autumn than in spring (P < 0.018, Figure 4a),
and dramatically dropped as the depth increased. The δ18O values for soil waters below 50 cm
showed no apparent seasonal variation (0.09 < P < 0.885), and progressively declined with
increasing depth. Groundwater had constant δ18O values, at -11.7 ± 0.1‰ (mean ± SE), over
the whole growing season. In addition, the δ18O values of soil waters from 180 to 300 cm
were indistinguishable from that of groundwater (0.057 < P < 0.767 for spring; 0.126 < P <
0.838 for summer and 0.444 < P < 0.979 for autumn).
Although there were subtle differences in SWC between the seasons, the general trend
for SWC was similar at three seasons (Figure 4b). In the top 50 cm of the soil profile, SWC
fluctuated with the seasons. The SWC at 0~20 cm depth was higher in spring, but was less
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than 5% in the summer and autumn (Figure 4b). In the middle part of the soil profile, SWC
increased slightly with depth. Between 180 and 300 cm, SWC exhibited no obviously
seasonal changes, but increased sharply from 8% at 180 cm depth, peaked at 23% at 220 cm
depth and then decreased steadily.
The responses of 0~100 cm SWC and δ18O values of soil water to rainfall pulses are
shown in Figure 5. Both SWC and δ18O values of soil water within 0~10 cm layer were
significantly affected by rainfall pulses and evaporation. However, those within 10~50 cm
were only marginally affected and those of 50~100 cm were almost uninfluenced.
Seasonal changes in the δ18O values of xylem water
Figure 6 shows the seasonal changes in δ18O values of xylem water for the three shrubs.
All the δ18O values for R. songarica were higher than those of the other two, except in the
early spring and in the last data sets. The average δ18O values for R. songarica, N.
tangutorum and T. ramosissima were –6.8 ± 0.5‰, –9.5 ± 0.4‰ and –10.5 ± 0.4‰,
respectively (Figure 6, P < 0.001, n = 14).
The δ18O values for xylem water within R. songarica showed a clearly seasonal
variation. In spring, late summer and early autumn, δ18O values were close to that of rain
water, but they significantly dropped between mid-June and mid-July (Figure 6). N.
tangutorum showed a very similar pattern, with δ18O value fluctuations declining when
entering summer. It should be noted that between mid-June and early August, δ18O values for
N. tangutorum coincided with those for T. ramosissima and were very similar to that of
groundwater (Figure 6). For T. ramosissima, the δ18O values ranged between –11.6 ± 0.1‰
and –6.6 ± 0.4‰. When entering summer, the δ18O values continuously declined (became
more negative). Over the course of the whole summer to early autumn, the xylem water in T.
ramosissima had fairly stable δ18O values, which were similar to that of groundwater (Figure
6).
Seasonal changes in water use strategies among the three shrubs
For T. ramosissima, the contributions made by the three sources were relatively equal in
spring, and the ranges were 0~81%, 0~82% and 0~73% for shallow, middle and deep soil
water, respectively. Thereafter, T. ramosissima exploited the deeper water sources. In summer,
the deep soil water accounted for 74~100% (Figure 7a). In autumn, the utilization of middle
soil water increased slightly, but that of deep soil water was still up to 43~99% (Figure 7a).
During the experimental period, the contribution of middle soil water for N. tangutorum
could not be adequately resolved using stable isotope with possible contributions ranging
from 0 to 100% (0~97% in spring and summer, 0~100% in autumn) (Figure 7b). The relative
contribution of shallow soil water varied in wide ranges in spring (0~78%), but not wide in
summer and autumn (Figure 7b). The contribution of deep soil water varied in a narrow range
in spring, but then ranges became considerably wide in summer (0~99%) and autumn
(0~87%) (Figure 7b).
In spring, the contribution of shallow and middle soil water was nearly equal
(approximately 50% from each layer respectively) for R. songarica (Figure 7c). While in
summer and autumn, R. songarica mainly derived water from middle soil water (>75%), with
less water from shallow soil (Figure 7c).
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Var iations in water use patterns after rainfall pulses
To further investigate the effects of a rainfall pulse on the water use patterns of the three
shrubs, we traced the changes in δ18O values of xylem water for 17 days after a natural, large
event in summer. For T. ramosissima, the contribution of the deep soil water was almost up to
100%, and the contribution from shallow or middle soil were negligible both before and after
rainfall (Figure 8a). This indicated that T. ramosissima did not use rain water in the shallow
or middle soil layers. In contrast, N. tangutorum showed a quick response to the rain pulse.
On the 2nd day after rainfall, the percentage of shallow and middle soil water increased from
nearly zero to 0~29% and 0~95%, respectively. Meanwhile, the contribution percentage of
deep soil water decreased from 90~98% to 5~72% (Figure 8b). R. songarica responded even
quicker to this precipitation. On the 1st day after rainfall, the contribution by shallow soil
water immediately increased from 31.2 ± 4.3% to 71.5 ± 5.6%. Thereafter, the proportion of
shallow soil water gradually declined, whereas that of middle soil water gradually increased
(Figure 8c).
For the 5.6 mm spring rainfall, the three species did not response significantly (Figure 9).
For the 7 mm autumn rainfall pulse, the responses of T. ramosissima and R. songarica were
similar with those to the summer rainfall pulse (Figure 10a and c), while the contribution
range of middle soil water by N. tangutorum was not stable as it did in summer pulse (Figure
10b).
Discussion
The plant root system type is fundamental in determining the water-use strategy of
desert shrubs and the physiological responses of a plant to an occasional rainfall pulse (Xu
and Li, 2006). This study demonstrated the water use strategies among deep and
shallow-rooted shrubs (Xu and Li, 2006). In the current study, we focused on the water use
dynamics over the different seasons and following large rainfall pulses and have shown how
co-occurring shrubs are well-adapted to utilizing different water sources in a desert
environment.
Root distributions and water use strategies by the three shrubs
Previous excavation studies showed that the vertical distribution of root systems of the
three species differed significantly. For T. ramosissima, 70% of the total absorbing roots
occurred in the 200~310 cm soil layer, but the 0~60 cm soil layer contained less than 5% of
the roots (Figure 11a). The absorbing root area of N. tangutorum in each soil layer has not
been investigated yet, and we failed to collect details of that from published literatures. Based
on our knowledge, N. tangutorum has a dimorphic root system with horizontal roots
extending to 4~6 m and a tap root of which could reach 2 m. However, more than 2/3 of total
root area was restricted to the top 40 cm of the soil profile (Sun and Yu, 1992). For R.
songarica, the root depth was less than 1 m, with more than 90% of total absorbing root area
occurring between 0 and 60 cm (Figure 11b).
The marked differences in rooting patterns among the three shrubs resulted in the spatial
separation of water acquisition in this desert community. Shallow-rooted R. songarica
showed great ability in exploiting water available in shallow and middle soil layers (Figure 7c)
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and were more sensitive to the rainfall pulses during drought (Figure 8c and 10c). However,
the high percentage utilization of shallow soil water derived from rainfall could not be
maintained by R. songarica and the fraction made up by middle soil water rose quickly
following infiltration (Figure 8c and 10c).
Due to its dimorphic rooting habit, N. tangutorum was able to flexibly shift between
possible water sources. In summer, N. tangutorum used deep soil water as its major water
source (Figure 7b), but following the large rain event, it responded rapidly by taking up more
shallow and middle soil waters (Figure 8b). This is probably due to a trade-off between the
activity of shallow and deep roots (Williams and Ehleringer, 2000). It may not be
advantageous for perennials to maintain active roots in upper soil layers over the whole
summer (Ehleringer and Dawson, 1992), but it is beneficial to develop new feeding roots or
activate surface root activities after a large rainfall event (Dawson and Pate, 1996).
Because the antecedent soil water conditions were fine in spring (Figure 5a), the three
species did not show markedly responses to the spring rainfall pulse (Figure 9). In summer
and autumn, 0~10 cm SWC peaked on the 1st day after rainfall events (Figure 5c and e), and
the amounts of the shallow soil water utilization by R. songarica and N. tangutorum reached
their highest values on day 1 or 2 after rainfall (Figure 8b and c, Figure 10b and c). This was
similar to the dominant species in the Ordos Plateau of northwestern China (Cheng et al.,
2006). The 50~100 cm SWC did not significantly increase after rainfall pulses (Figure 5c and
e). This indicated that the water use from this layer likely did not change either, although the
possible ranges of middle soil water used by N. tangutorum were highly varying as given by
IsoSource (Figure 8b, Figure 10b). The xylem water (the water use by the plant) was likely a
mixture of deep soil water (the source before the rain) and surface water which was
significantly increased after rain (Figure 5c and e). Namely, water use dynamics was
determined by root activity rather than root presence.
Comparatively, although the roots of T. ramosissima were distributed continuously
throughout the soil profile, it seemed that the most active sites for water absorption were
limited to the deeper soil layers. This may be because the shallow water is not a reliable water
source for T. ramosissima, while deep soil water, which is derived from groundwater,
represents a stable water source. When the deep water source was utilized, the variable
shallow soil water was left almost unexploited by T. ramosissima in the drought period
(Figure 7a). Thus T. ramosissima did not take up any pulse water, even after the large rain
event during summer (Figure 8a). These results implied that the shallow water source may be
of limited importance to the long-term water balance of T. ramosissima. In addition, the water
use patterns of T. ramosissima and R. songarica fit well with previous research on the
physiological responses of their twigs to rainfall (Xu and Li, 2006). Transpiration, leaf water
potential and water-use efficiency in T. ramosissima were stable during the drought period,
while R. songarica responded strongly to rain pulses, in terms of leaf water potential and
transpiration.
Seasonal competition for water among the three co-occurring shrubs
It is well documented that competition for water in deserts influences species
interactions and community dynamics (Fowler, 1986). The different types of rooting patterns
among species may be adaptations for minimizing competition for water during prolonged
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drought conditions (Dawson and Pate, 1996).
The three shrubs exhibited seasonal inter-specific competition for water. At the
beginning and end of the growing season, the three species tended to compete for middle soil
water. On the first and last sampling day, δ18O values for the three species were very similar
(for the first sampling, –8.1 ± 0.4‰, P = 0.335 and for the last sampling, –9.6 ± 0.1‰, P =
0.967, Figure 6) to those of the corresponding middle soil water values. This was probably
due to snow water recharging in spring (Zhou et al., 2009) and continuous medium rains
infiltrating the soil in autumn (Figure 3), resulting in large amounts of soil water being stored
in the middle layers during these two seasons. In the dry summer, with declining water
availability in the shallow and middle soil layers, both N. tangutorum and T. ramosissima
competed for the deep water source (Figure 7a and b). Their δ18O values coincided for a long
time and were similar to that of the groundwater (Figure 6). The deep water source was
beyond the reach of R. songarica, and the high soil temperatures (Ma et al., 2012) may have
inhibited its surface root activity (Williams and Ehleringer, 2000). Therefore, R. songarica
mainly utilized the middle soil water during summer (Figure 7c).
The difference in the capacity to use rainfall pulses among the three dominant species
might greatly influence the composition and structure of the community. The different sized
rainfall events infiltrated different soil layers. Small rainfall events usually recharge the
shallow soil water, whereas large events can infiltrate deeper layers. Thus, altered
precipitation regimes over long periods determine surface versus deep layer water availability
for plants that co-exist with each other. Based on the possible changes in local precipitation
patterns, we predict that under fixed rainfall amounts, frequently occurring small events
would favor an increase in R. songarica and N. tangutorum, while occasionally large events
will promote T. ramosissima, and will benefit N. tangutorum. From the long term perspective,
the highly opportunistic water-use strategy by N. tangutorum could be put at an advantage
when competition for water occurs within the ecosystem.
Acknowledgements
This research was supported by a grant to Li-Song Tang from the Natural Science Foundation
of China (No. 41171049), a cooperative China-New Zealand research project (No.
2011DFA31070) and the ‘Western Light’ program of the Chinese Academy of Science (No.
XBBS201001). We thank all the staff at the Fukang Station of Desert Ecology for their help
in the laboratory analysis and field sampling. And we thank Jie Ma and Jiang-bo Xie for
reading and improving the manuscript.
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References
Briggs JM, Knapp AK. 1995. Interannual variability in primary production in tallgrass prairie:
climate, soil moisture, topographic position, and fire as determinants of aboveground
biomass. American Journal of Botany 82: 1024–1030.
Bunker DE, DeClerck F, Bradford JC, Colwell RK, Perfecto I, Phillips OL, Sankaran M,
Naeem S. 2005. Species loss and aboveground carbon storage in a tropical forest. Science
310: 1029–1031.
Cheng XL, An SQ, Li B, Chen JQ, Lin GH, Liu YH, Luo YQ, Liu SR. 2006. Summer rain
pulse size and rainwater uptake by three dominant desert plants in a desertified grassland
ecosystem in northwestern China. Plant Ecology 184: 1–12.
Dansgaard W. 1964. Stable isotopes in precipitation. Tellus 16: 436–468.
Dawson TE. 1993. Hydraulic lift and water use by plants: implications for water balance,
performance and plant-plant interactions. Oecologia 95: 565–574.
Dawson TE, Ehleringer JR. 1991. Streamside trees that do not use stream water. Nature 350:
335–337.
Dawson TE, Mambelli S, Plamboeck AH, Templer PH, Tu KP. 2002. Stable isotopes in plant
ecology. Annual Review of Ecology and Systematics 33: 507–559.
Dawson TE, Pate JS. 1996. Seasonal water uptake and movement in root systems of
Australian phraeatophytic plants of dimorphic root morphology: a stable isotope
investigation. Oecologia 107: 13–20.
Dodd MB, Lauenroth WK, Welker JM. 1998. Differential water resource use by herbaceous
and woody plant life-forms in a shortgrass steppe community. Oecologia 117: 504–512.
Donovan LA, Ehleringer JR. 1994. Water Stress and Use of Summer Precipitation in a Great
Basin Shrub Community. Functional Ecology 8: 289–297.
Duan DY, Ouyang H, Song MH, Hu QW. 2008. Water Sources of Dominant Species in Three
Alpine Ecosystems on the Tibetan Plateau, China. Journal of Integrative Plant Biology 50:
257–264.
Ehleringer JR, Dawson TE. 1992. Water uptake by plants: perspectives from stable isotope
composition. Plant, Cell and Environment 15: 1073–1082.
Ehleringer JR, Phillips SL, Schuster WSF, Sandquist DR. 1991. Differential utilization of
summer rains by desert plants. Oecologia 88: 430–434.
Ellsworth PZ, Williams DG. 2007. Hydrogen isotope fractionation during water uptake by
woody xerophytes. Plant and Soil 291: 93–107.
Fay PA, Carlisle JD, Knapp AK, Blair JM, Collins SL. 2003. Productivity responses to
altered rainfall patterns in a C4-dominated grassland. Oecologia 137: 245–251.
Flanagan LB, Ehleringer JR, Marshall JD. 1992. Differential uptake of summer precipitation
among co-occurring trees and shrubs in a pinyon-juniper woodland. Plant, Cell and
Environment 15: 831–836.
Fowler N. 1986. The role of competition in plant communities in arid and semiarid regions.
Annual Review of Ecology and Systematics 17: 89–110.
Li YH. 2010. Response of leaf traits of Nitraria tangutorum Bobr and Nitraria sphaerocarpa
Maxim to increasing water. PhD dissertation, Chinese Academy of Forestry. (in Chinese
with English abstract)
This article is protected by copyright. All rights reserved.
Liu WJ, Liu WY, Li PJ, Duan WP, Li HM. 2010. Dry season water uptake by two dominant
canopy tree species in a tropical seasonal rainforest of Xishuangbanna, SW China.
Agricultural and Forest Meteorology 150: 380–388.
Loik ME, Breshears DD, Lauenroth WK, Belnap J. 2004. A multi-scale perspective of water
pulses in dryland ecosystems: climatology and ecohydrology of the western USA.
Oecologia 141: 269–281.
Ma J, Zheng XJ, Li Y. 2012. The response of CO2 flux to rain pulses at a saline desert.
Hydrological Processes 26: 4029–4037.
McCole AA, Stern LA. 2007. Seasonal water use patterns of Juniperus ashei on the Edwards
Plateau, Texas, based on stable isotopes in water. Journal of Hydrology 342: 238–248.
Noy-Meir I. 1973. Desert Ecosystems: Environment and Producers. Annual Review of
Ecology and Systematics 4: 25–51.
Phillips DL, Gregg JW. 2003. Source partitioning using stable isotopes: coping with too
many sources. Oecologia 136: 261–269.
Phillips DL, Newsome SD, Gregg JW. 2005. Combining sources in stable isotope mixing
models: alternative methods. Oecologia 144: 520–527.
Schultz NM, Griffis TJ, Lee XH, Baker JM. 2011. Identification and correction of spectral
contamination in 2H/1H and 18O/16O measured in leaf, stem, and soil water. Rapid
Communications in Mass Spectrometry 25: 3360–3368.
Schwinning S, Ehleringer JR. 2001. Water use trade-offs and optimal adaptations to
pulse-driven arid ecosystems. Journal of Ecology 89: 464–480.
Schwinning S, Starr BI, Ehleringer JR. 2005. Summer and winter drought in a cold desert
ecosystem (Colorado Plateau) part I: effects on soil water and plant water uptake. Journal
of Arid Environment 60: 547–566.
Soriano A, Sala OE. 1983. Ecological strategies in a Patagonian arid steppe. Vegetatio 56:
9–15.
Sun X, Yu Z. 1992. A study on root system of Nitraria tangutorum. Journal of Desert
Research 12: 50–54. (In Chinese with English Abstract)
Tian H, Melillo JM, Kicklighter DW, McGuire AD, Helfrich J. 1999. The sensitivity of
terrestrial carbon storage to historical climate variability and atmospheric CO2 in the
United States. Tellus 51B: 414–452.
White JWC, Cook ER, Lawrence JR, Broecker WS. 1985. The D/H ratios of sap in trees:
implications for water sources and tree ring D/H ratios. Geochimica et Cosmochimica Acta
49: 237–246.
Williams DG, Ehleringer JR. 2000. Intra- and interspecific variation for summer precipitation
use in Pinyon-Juniper woodlands. Ecological Monographs 70: 517–537.
Xu GQ, Li Y. 2009. Root distribution of three co-occurring desert shrubs and their
physiological response to precipitation. Sciences in Cold and Arid Regions 1: 0120–0127.
Xu H, Li Y. 2006. Water-use strategy of three central Asian desert shrubs and their responses
to rain pulse events. Plant and Soil 285: 5–17.
Zhou HF, Li Y, Tang Y, Zhou BJ, Xu HW. 2009. The characteristics of snow-cover and
snowmelt water storage in Gurbantunggut desert. Arid zone research 26: 312–317. (In
Chinese with English Abstract)
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Table 1. Morphological characteristics of the three desert shrubs
Species
Height
Crown diameter
Root depth Absorbing root area
(cm)
(cm)
(cm)
(cm2)
T. ramosissima* 175 ± 13.4
155 ± 17.2
≈ 300
30249.2 ± 34.3
N. tangutorum
78.3 ± 5.8
328.6 ±18.36
≤ 200
10622.09 ± 26.4**
R. songarica*
55 ± 8.2
35 ± 5.6
≤ 100
361.8 ± 19.7
Standard error is provided for the mean.
* Data are cited from Xu and Li (2009).
**Data are cited from Sun and Yu (1992).
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Figure 1. Correction curves
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Figure 2. IRIS vs IRMS
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Figure 3. Precipitation and temperature
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Figure 4. Isotopic values of soil water and soil water content
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Figure 5. Response of soil water to rainfall pulses
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Figure 6. Isotopic values for xylem water
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Figure 7. Seasonal changes in water use
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Figure 8. Variation in water use after summer rainfall pulse
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Figure 9. Variation in water use after spring rainfall pulse
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Figure 10
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Figure 11. Vertical distribution of the absorbing root area
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