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Published online 5 July 2005
Published in J Environ Qual 34:1311-1318 (2005)
DOI: 10.2134/jeq2004.0352
© 2005 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
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TECHNICAL REPORTS

Plant and Environment Interactions

Quantification of Intrinsic Water Use Efficiency along a Moisture Gradient in Northeastern China

Mei Yua,b,*, Yichun Xiec and Xinshi Zhanga

a Laboratory of Quantitative Vegetation Ecology, Institute of Botany, The Chinese Academy of Sciences, Beijing 100093, P.R. China
b Department of Environmental Sciences, University of Virginia, Charlottesville, VA 22904
c Institute for Geospatial Research and Education, Eastern Michigan University, Ypsilanti, MI 48197

* Corresponding author (meiyu{at}ibcas.ac.cn)

Received for publication September 13, 2004.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Water use efficiency (WUE) is an important ecophysiological characteristic of plants, especially in semiarid and arid regions. At the scale of community or ecosystem, WUE is difficult to quantify because the amount of water used per unit dry mass production is a function of microclimatic variables and species composition. In this study, we analyzed corrected intrinsic water use efficiency (IWUEs) of grass and shrub species along the western segment of the Northeast China Transect (NECT) and the relationship between IWUEs and mean annual rainfall, habitat degradation status, vegetation type, and plant functional type (C3 versus C4) at 22 survey sites. Site intrinsic water use efficiency (IWUEv) and its relationship with the aforementioned site variables were analyzed based on species frequencies at each site. First, it was concluded that photosynthetic pathway played a very important role in determining species IWUEs. Mean IWUEs for C4 species was approximately double that of C3 species. Second, mean annual rainfall, vegetation type, and site degradation status significantly affected IWUEs (p < 0.01). Mean IWUEs at degraded sites was twice as high as that at nondegraded sites. The mean IWUEs for meadows was significantly higher than those for other vegetation types (p < 0.05). Third, the frequency of occurrence of C4 plants explained 36% of the variance in IWUEv across the survey sites. The mean frequency of C4 occurrence at degraded sites was more than double that at nondegraded sites. Consequently, mean IWUEv at degraded sites was more than double that at nondegraded sites. Dominant C4 species in saline–alkaline areas tended to have higher intrinsic WUE than dominant C4 species in sandy shrub communities.

Abbreviations: IWUE, intrinsic water use efficiency • IWUEs, corrected intrinsic water use efficiency • IWUEv, site intrinsic water use efficiency • NECT, Northeast China Transect • WUE, water use efficiency


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
WATER LOSS in terrestrial plants often exceeds carbon gained via photosynthesis by three orders of magnitude (Farquhar et al., 1989; Marshall and Zhang, 1994). Understanding the water use efficiency of plants in semiarid and arid environments is of particular interest due to the low water availability in these habitats. Water use efficiency (WUE), the instantaneous ratio of leaf net carbon assimilation rate over transpiration rate, has been reported to be different among plants with different photosynthetic pathways (C3, C4, or CAM) (Ehleringer and Osmond, 1989; Marshall and Zhang, 1994), plants growing in different habitats (Ehleringer and Cooper, 1988; Garten and Taylor, 1992; Meinzer et al., 1992a, 1992b; Ares and Fownes, 1999), different leaf types (Marshall and Zhang, 1994; Damesin et al., 1997), different organs of plants (Comstock and Ehleringer, 1992), and different aged plants (Cavender-Bares and Bazzaz, 2000). In contrast to WUE, the intrinsic water use efficiency (IWUE), defined as the ratio of net assimilation rate to stomatal conductance, is thought to be less tightly coupled to the instantaneous environmental temperature and atmospheric humidity and thus more closely reflects plant physiological properties (Comstock and Ehleringer, 1992; Jones, 1992).

Modeling the response of plant growth to global climate change requires the quantification of WUE characteristics of plant species and their relationships to the climatic factors. Studies on water and carbon balances in plants in semiarid and arid environments requires the quantification of water use efficiency at the community or ecosystem level, which depends on the WUE of each species and thus on species composition. Species composition, a measure of ecosystem structure, is largely affected by the successional status and/or level of disturbance of the ecosystem (National Research Council Committee on Scholarly Communication with the People's Republic of China, 1992; Redmann et al., 1995), and thus the degradation state of the site should be considered an important variable when determining WUE at the community or ecosystem level.

The Northeast China Transect (NECT), established by the International Geosphere and Biosphere Program (IGBP), is along a rainfall gradient centered at 43.5° N and extending 1400 km from 132° E to 108° E (International Geosphere and Biosphere Program, 1995; Gao and Zhang, 1997; Gao and Yu, 1998). Annual precipitation decreases sharply from around 800 mm in the east to less than 100 mm in the west. Vegetation along the transect changes from dark conifer forests, conifer–broadleaved mixed forests, deciduous broadleaved forests, woodlands and shrublands in the east, to meadow steppe and croplands in the middle, and typical steppe and desert steppe in the west primarily as a result of the rainfall gradient (Gao and Zhang, 1997). Functional types of plant species, jointly determined by life form and carbon pathway, also vary along the rainfall gradient (Jiang et al., 1999). Due to rapid population increase and economic development, the transect area has been subjected to increasing influence from intensified human activities, such as cultivation and overgrazing (Yu et al., 2004). Progressive degradation has been reported in the semiarid and arid grasslands along the transect over the past 30 yr. Even though water balance has been recognized as the most important driver of the ecosystem structural and functional changes along the NECT, quantification of water use efficiency at different scales, the most critical task of the transect research, has not been well studied.

Our objectives were to (i) analyze the intrinsic WUE of plant species of grass and shrub communities along the NECT in relation to species photosynthetic pathways, mean annual rainfall, habitat degradation status, and vegetation type; and (ii) quantify the effects of site variables (abundance of C4 species, rainfall, habitat degradation status, and community type) on site intrinsic WUE of grassland and shrub communities along the NECT. The following hypotheses were also tested: (i) intrinsic WUE of species are strongly correlated with habitat dryness; (ii) water supply regulates intrinsic WUE of broadly distributed species; and (iii) C4 species are more abundant in saline, disturbed, dry, or hot environments (Marshall and Zhang, 1994; Redmann et al., 1995; Epstein et al., 1997; Ares and Fownes, 1999; Epstein et al., 2002). This study provides the basis for future research on plant ecophysiological models, dynamic vegetation models at the landscape or regional scale, and studies on carbon and water balances in this area.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Field Experiments
The field survey was conducted from 14 July to 2 Aug. 1997 along the NECT from Hunchun, Jilin Province, the border of China, Russia, and North Korea to Erlianhaote, Inner Mongolia Autonomous Region on the border of China and Mongolia. A total of 34 sites, at an average distance of 49 km apart, were surveyed along the NECT. Detailed information of the survey sites was given in Jiang et al. (1999). Because this study focused on the arid and semiarid regions of the transect, 12 sites within the forests, woodlands, and croplands in the semihumid eastern part of the transect were excluded from the analysis. The remaining 22 sites were located in the meadow, meadow steppe, typical steppe, desert steppe, and sandy shrub communities (Table 1). The geographical coordinates, elevation, vegetation type, and status of degradation at each site were recorded as habitat information (S. Liu, personal communication, 1997). Sampling quadrats (1-m2) were used in 10 replicates to survey the frequencies of the species occurrence at 18 sites out of the 22 sites according to the standard methods of survey, observation, and analysis of Chinese terrestrial plant communities (Dong, 1996). At each site, the net photosynthetic rate, transpiration rate, and stomatal conductance of each species were determined by measuring the current foliage at the top of the adult plants in three replicates using an LCA4 Portable Photosynthetic System (ADC, Hoddesdon, England). Leaf and air temperature, incident photosynthetic photon flux density, relative humidity, atmospheric pressure, and atmospheric and intercellular CO2 partial pressures were measured simultaneously.


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  		Table 1. Description of the sampling sites along the Northeast China Transect (NECT).

 
Intrinsic Water Use Efficiency Indices for Species and Sites
According to Comstock and Ehleringer (1992), instantaneous intrinsic water use efficiency for a specific species (IWUE, mmol mol–1) is calculated as:

[1]
where A is net leaf assimilation rate (µmol CO2 m–2 s–1), gs is stomatal conductance (mmol H2O m–2 s–1), Ca and Ci are atmospheric and intercellular CO2 concentrations (mmol mol–1), respectively, E is leaf transpiration rate (mmol H2O m–2 s–1), and {Delta}{epsilon} is the dimensionless vapor pressure deficit. The term {Delta}{epsilon} is defined as:

[2]
where ei and ea denote absolute intercellular and ambient water vapor pressures (kPa), respectively, and P is atmospheric pressure (kPa).

The calculation of IWUE in Eq. [1] and [2] is independent of air humidity, but is dependent on photosynthetic photon flux density (PAR), because A and gs are differently affected by PAR (Thornley and Johnson, 1990; Gao et al., 2002b). Stable isotope studies using {delta}13C have shown that long-term intrinsic water use efficiency was positively correlated with PAR (Zimmerman and Ehleringer, 1990). Since we had to visit two sites per day, one in the morning and the other in the afternoon, we could not maintain PAR at a fixed value during the field study. To make water use efficiency less dependent on incident PAR and thus more "intrinsic" to an individual species, we introduced the following correction to the definition of the instantaneous intrinsic water use efficiency in our analysis with the corrected water use efficiency denoted as IWUEs (mmol mol–1):

[3]
where I is incident PAR (µmol photons m–2 s–1), and b1 and b2 are parameters empirically set to 400 and 800 µmol photons m–2 s–1, respectively (Q. Gao, personal communication, 2003). The factor (I + b2)/(I + b1) is a decreasing function of PAR (I), varying from 1.73 to 1.14 as I increased from 150 to 2500 µmol photons m–2 s–1 (the range in our analysis). Thus, this should cancel most of the increase of A/gs with I, and make IWUEs approximately independent of I.

The calculated IWUEs was averaged across the replicates of each species at each site. In this study, IWUEs was measured at the peak of the growing season (July–August) and was used to represent the typical intrinsic water use efficiency characteristics of each species sampled along the transect.

The water use efficiency characteristic of a site was dependent on both species water use efficiency and species composition. For greater accuracy, the quantification of the water use efficiency of a site should incorporate the green biomass of each species at a site. However, since some of the sites were located within protected areas, we were unable to make biomass measurements. Instead, we used the species water use efficiency and frequency of species occurrence to approximate the site-dependant water use efficiency characteristic, which we called site water use efficiency.

Site water use efficiency, IWUEv (mmol mol–1), was calculated as the sum of the average water use efficiency of each species at a site weighted by its frequency at the site:

[4]
where fs is the frequency of species s at site v. Note subscript s serves two purposes here: as a correction of the earlier definition of instantaneous intrinsic water use efficiency and as an index to species number. Species with C4 photosynthetic pathways were looked up from the available literature (Redmann et al., 1995; Yin and Li, 1997; Yin and Wang, 1997; Pyankov et al., 2000). The total frequency of C4 species for a site was calculated as fs /fs.

All statistical analyses were performed using the SAS statistical package (SAS Institute, 2001). The effects of mean annual rainfall, degradation status, vegetation type, photosynthetic pathway (for IWUEs), and the frequency of the occurrence of C4 plants (for IWUEv) on IWUEs and IWUEv were tested using ANOVAs. Tukey's studentized range (HSD) tests were used to test the differences in the means of IWUEs or IWUEv between nondegraded and degraded sites, and between different vegetation types at the 0.05 significance level. A paired t test was used to test the differences in mean IWUEs between the C3 and C4 groups at a given site. Linear regression was used to analyze the dependence of the IWUEv on the frequency of the occurrence of C4 plants at a site.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Effects of Photosynthetic Pathway, Mean Site Annual Rainfall, Degradation Status, and Vegetation Type on Corrected Intrinsic Water Use Efficiency (IWUEs)
To test the effects of the light intensity correction factor on the calculation of intrinsic water use efficiency, we chose species that had more than 10 records. We found that the coefficients of variation of intrinsic water use efficiency (the standard deviation divided by mean) of 14 species out of the 20 species tested decreased to different levels. Statistical t tests showed that the correction factor decreased the p values and thus increased the statistical significance of the differences of mean IWUEs between C3 and C4 species. The effects of the correction factor were especially evident for the differences in mean IWUEs between the C4 species, flaccid pennisetum (Pennisetum centrasiaticum Tzvel.), and five C3 species, Dahurian asparagus (Asparagus dauricus Link), common sawwort (Serratula centauroides L.), crested wheatgrass [Agropyron cristatum (L.) Gaertn.], prairie sagewort (Artemisia frigida Willd.), and needleleaf sedge (Carex duriuscula C.A. Mey). The P values decreased from 0.41, 0.37, 0.24, 0.22, and 0.13 for t tests without the correction, to 0.13, 0.14, 0.09, 0.07, and 0.05 for t tests with the correction, respectively.

Analysis of variance indicated that photosynthetic pathway significantly affected IWUEs (p < 0.001, Fig. 1) . The mean IWUEs for the C4 species (0.11) was significantly higher than that for the C3 plants (0.051) at HSD test (p < 0.05). Moreover, a paired t test showed that the mean IWUEs for the C4 species was significantly greater than that for the co-occurring C3 species at a given site (p < 0.01, Fig. 2) .



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  		Fig. 1. Corrected intrinsic water use efficiency (IWUEs) for C3 (small dots) and C4 (large dots) species at degraded (filled dots) and nondegraded (open dots) sites in different vegetation types along the mean annual rainfall (mm) gradient of the Northeast China Transect (NECT). Photosynthetic pathway effect on IWUEs was detected by ANOVA (p < 0.001). Vegetation type, degradation status, and mean site annual rainfall all significantly affected IWUEs (p < 0.01 for each).

 


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  		Fig. 2. Mean corrected intrinsic water use efficiency (IWUEs) (+1 SE) averaged across C3 (filled bars) or C4 (open bars) or all species (bars with shadow) at a site, and mean IWUEs (+1 SE) averaged across species in different vegetation type–degradation status combinations. Site information is in Table 1. Paired t tests indicated that mean site IWUEs for C4 species was significantly higher than that for C3 species at the given site (p < 0.01). Open dots denote the mean IWUEs for all species in nondegraded sites within a specific vegetation type, while filled dots indicate the mean IWUEs for species in degraded sites.

 
Mean site annual rainfall, vegetation type, and degradation status were all significant explanatory variables of variations in IWUEs, with p < 0.01 (ANOVA) for each of these factors (Fig. 1 and 2). The mean IWUEs for the degraded sites (0.089) was significantly higher than that for the nondegraded sites (0.039) (p < 0.05, HSD test). Comparisons of mean IWUEs between the different vegetation types revealed that the mean IWUEs for meadow communities (0.16) was significantly higher than that for meadow steppe (0.061), typical steppe (0.028), desert steppe (0.049), and sandy shrub communities (0.052) (p < 0.05, HSD test). Moreover, there were significant differences in the mean IWUEs between meadow steppe and typical steppe communities (p < 0.05, HSD test). A significant interactive effect of vegetation type and degradation status on IWUEs was also detected by ANOVA (p < 0.01). Across the degraded sites, the mean IWUEs for meadow communities was significantly higher than those for other plant communities (p < 0.05, HSD test). The mean IWUEs values were 0.19, 0.07, 0.03, and 0.08 for meadow, meadow steppe, typical steppe, and desert steppe, respectively. However, at the nondegraded sites, the mean IWUEs values were 0.05, 0.05, 0.03, and 0.03 for meadow, meadow steppe, typical steppe, and desert steppe, respectively. These mean IWUEs values were much lower than their counterparts at the degraded sites except for that of the typical steppe communities. Furthermore, the differences of mean IWUEs were significant between meadow communities and desert steppe, between meadow steppe and typical steppe, and between meadow steppe and desert steppe (p < 0.05, HSD test; Fig. 2).

Another test of the relationship between IWUEs and the mean site annual rainfall was conducted for six species that occurred at more than five sites (prairie sagewort, C3; tansyleaf cinquefoil (Potentilla tanacetifolia Willd. ex Schltdl.), C3; Rutheni medick [Melissitus ruthenica (L.) C.W. Chang], C3; Chinese iris [Iris lactea Pall. var. chinensis (Fisch.) Koidz], C4; scabrous cleistogenes [Cleistogenes squarrosa (Trin.) Keng], C4; and needleleaf sedge, C3). However, we did not find significant correlation between the mean annual rainfall and IWUEs for any of these.

Effects of the Frequency of C4 Plants, Mean Site Annual Rainfall, Degradation Status, and Vegetation Type on Site Intrinsic Water Use Efficiency (IWUEv)
The IWUEv was positively correlated to the frequency of occurrence of C4 plants at a site (R2 = 0.36, p < 0.01, Fig. 3) and was affected by a site's degradation status (p = 0.069, ANOVA; Fig. 3). The mean IWUEv value for the degraded sites was 0.088, much higher than that for the nondegraded sites (0.035). At the nondegraded sites, mean IWUEv values (±1 SE) for meadow steppe, typical steppe, and desert steppe were 0.047 (±0.0078), 0.028 (±0.0054), and 0.028 (±0.0052), respectively (Fig. 4) . However, the differences of mean IWUEv were not significant between any two of them (at 0.05 significance level, HSD test).



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  		Fig. 3. Relationship between site intrinsic water use efficiency (IWUEv) and the frequency of C4 species at a site, and IWUEv along the mean annual rainfall (mm) gradient of the Northeast China Transect (NECT). The IWUEv values for degraded sites are denoted as filled dots; nondegraded sites are open dots.

 


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  		Fig. 4. Mean site intrinsic water use efficiency (IWUEv) (+1 SE) and mean frequency of C4 species (+1 SE) averaged across sites in different vegetation type–degradation status combinations. Open dots denote the mean of nondegraded sites within a specific vegetation type; filled dots denote degraded sites within a vegetation type.

 
The mean of the frequency of C4 plants was significantly higher in the degraded sites (0.44) than that in the nondegraded sites (0.15, p < 0.05, HSD test). At the nondegraded sites, the mean values of the frequency of C4 species were 0.2 for meadow steppe, 0.075 for typical steppe, and 0.14 for desert steppe communities (Fig. 4).


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Photosynthetic pathway is an important determinant of intrinsic water use efficiency at the species level. In our study, the mean IWUEs for C4 species (0.11) was double that of C3 species (0.051), which was consistent with other findings reported in the literature (Ehleringer and Cooper, 1988; Marshall and Zhang, 1994). Because of the strong correlation between mean annual rainfall and vegetation type (R2 = 0.67), we used vegetation type only as an explanatory variable in our analysis as a surrogate for rainfall. Both vegetation type and habitat degradation status significantly affected species IWUEs, supporting the hypothesis that there exists a strong relationship between a species intrinsic water use efficiency and its microhabitat (Ehleringer and Cooper, 1988; Marshall and Zhang, 1994; Ares and Fownes, 1999). Using carbon isotope discrimination analysis, Marshall and Zhang (1994) showed that the intrinsic water use efficiency of either evergreen or deciduous species increased with altitude, which was along a decreasing rainfall gradient, indicating that the intrinsic water use efficiency increased with site dryness in their study. However, in this study, we did not find any significant trend between IWUEs and vegetation type (denoting site dryness) even after separating species into C3 and C4 groups. Comparisons between the mean IWUEs of the different vegetation types revealed that the mean IWUEs for meadows was the highest: 1.5 times higher than meadow steppe, 4.5 times higher than typical steppe, 2 times higher than desert steppe, and 2 times higher than sandy shrubs. The mean IWUEs for meadow steppe was twice that of typical steppe vegetation and also significantly different. These differences in mean IWUEs among vegetation types can be attributed to differences in the species composition among vegetation types and, in particular, differences in the occurrence of species with different carbon pathways. More C4 species with higher intrinsic water use efficiency occurred in the meadow and meadow steppe communities. In the meadow and meadow steppe, C4 species accounted for 49.2 and 31.3% of the total species in these communities, respectively; in the typical steppe, desert steppe, and sandy shrub communities, C4 species accounted for 10.6, 18.2, and 28.9% of the total species, respectively.

Mean species intrinsic water use efficiency at the degraded sites was double that of the nondegraded sites, largely due to the occurrence of more C4 species at the degraded sites. We found that the percentage of C4 species sampled at the degraded sites was 36.8% as compared with 15.5% of the total species sampled at the nondegraded sites. Our results indicated that more C4 species occurred at degraded sites and in meadow and meadow steppe communities. These results are in agreement with the findings of Redmann et al. (1995) who reported that C4 species were most common in meadow steppe, saline grasslands (both meadow sites in our study were saline grasslands), and disturbed habitats.

Ares and Fownes (1999) reported that water supply regulated the intrinsic water use efficiency of koa (Acacia koa Gray) in Hawaii. On the other hand, Palmroth et al. (1999) found no clear climatic effects on the intrinsic water use efficiency (both instantaneous and long-term measurements) of Scots pine (Pinus sylvestris L.) over a range of wet to dry climates. The results from our analysis on the six common species widely distributed along the NECT did not show any significant relationships between IWUEs and the mean site annual rainfall. One possible reason is the large spatial distance of our sampling area, which features great spatial heterogeneity of edaphic factors. For example, soil types changed from saline–alkaline soils in meadow and some meadow steppe sites, to chernozem at other meadow steppe sites, to chestnut soils at sites in typical steppe communities, to brown desert soils in desert steppe and sandy soils in sandy shrub communities. The hydraulic properties and nutrient availability of these different soil types are quite different and may regulate the production of carboxylases for photosynthesis. In addition, the sandy soils have poor water retention so that even under similar rainfall conditions water availability in sandy soils is lower than that in other soil types (Gao et al., 1997, 2002a). Saline–alkaline soils also tend to provide less water to plants due to the high concentration of cations (Gao et al., 1996a, 1996b, 1998).

Not only do the numbers of C3 and C4 species differ between sites, but frequencies of C3 and C4 groups also vary. By analyzing species frequency we found that the frequency of C4 plants at the degraded sites was double or even triple that of nondegraded sites within a specific vegetation type (Fig. 4). The frequency of C4 plants was greater than 50% in the degraded sites with meadow and sandy shrub vegetation, which might imply a high tolerance of C4 species to adverse environmental conditions of high salinity and drought. With their high intrinsic water use efficiency, it is likely that C4 species out-competed other co-occurring C3 species during the successional history of a site and therefore increased the water use efficiencies of the communities in these habitats. At the nondegraded sites, we observed that the frequency of C4 plants was higher in meadow steppe than that in adjacent typical steppe communities that had less annual rainfall, which might be related to the highly saline and alkaline soils in these meadow steppe areas (Gao et al., 1996a, 1996b, 1998). The frequency of C4 plants was higher in desert steppe than that in typical steppe (i.e., it increased with site dryness), a result in agreement with findings of other authors (Redmann et al., 1995; Epstein et al., 1997, 2002).

Because species composition and frequency were different between nondegraded and degraded sites (National Research Council Committee on Scholarly Communication with the People's Republic of China, 1992; Redmann et al., 1995), it was necessary to consider IWUE at the community and ecosystem level for our analysis, which depended on both species IWUE and species composition and frequency. In this study, we found that the frequency of C4 plants could explain 36% of variance in IWUEv and that the mean IWUEv at the degraded sites was more than double that found at the nondegraded sites, following the same pattern as the frequency of C4 plants. However, although the frequency of C4 plants in the sandy shrub communities (degraded) was high (Fig. 4), the mean IWUEv was relatively low compared with other degraded sites with a similar frequency of C4 plants (Fig. 4). One reason for this was that C4 species in the sandy shrub vegetation had lower intrinsic water use efficiencies than C4 species in the saline–alkaline sites (Fig. 1 and 2, P41 and P42 were sites in sandy shrub vegetation). African pricklegrass [Crypsis aculeata (L.) Aiton], puncturevine (Tribulus terrestris L.), and alkali belvedere [Kochia sieversiana (Pall.) C.A. Mey.] were the dominant C4 species in the saline–alkaline area, whereas scabrous cleistogenes, green bristlegrass [Setaria viridis (L.) Beauv.], and slender Russian thistle (Salsola collina Pall.) were the dominant C4 species in the sandy shrub vegetation. Measurements on long-term intrinsic water use efficiency revealed that the {delta}13C ({per thousand}) values of African pricklegrass, puncturevine, and alkali belvedere growing in saline–alkaline soils were –12.8, –11.6, and –12.5, respectively (Redmann et al., 1995; Tang and Liu, 2001). In contrast, the sandy-shrub C4 species, scabrous cleistogenes, green bristlegrass, and slender Russian thistle, had {delta}13C ({per thousand}) values of –16.4, –17.9, and –13.1, respectively (Redmann et al., 1995; Pyankov et al., 2000). Thus Ci values for C4 species growing in saline–alkaline areas are likely to be lower than those for C4 species in the sandy shrub vegetation, and the intrinsic water use efficiencies greater.

In summary, the level of degradation of a habitat was an important determinant of the IWUEs, frequency of C4 species at a site, and IWUEv, and thus is a critical indicator of ecosystem structure and functional dynamics along the NECT.

This study was based on instantaneous intrinsic water use efficiencies measured at the peak of the growing season; future studies could combine the results from this study with more extensive work on carbon isotope discrimination that provides a long-term measure of water use efficiency. More information about biomass and LAI will be needed to incorporate information on community and ecosystem level water use efficiencies into landscape or regional level models on carbon and water balances.


   ACKNOWLEDGMENTS
 
The authors thank Prof. M. Dong, Prof. G.M. Jiang, and Dr. H.P. Tang for the assistance in the field measurements, Prof. Q. Gao for the help in the statistical analysis, and Dr. R.E. Sherman for English wording. Support was provided by the Ministry of Science and Technology of China under Grant G2000018605 and the National Science Foundation of China under Grants 90202008 and 90211002.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 





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