Variations In Water Use Efficiency And Stoichiometry Along A Gradient Of Aridity In Plants

The effects of global changes such as drought stress on ecosystem and the responses of ecosystem are high research priorities. Anthropogenic factors that cause global warming and water shortage have strong influence on community structure and stability of ecosystem. The effects of long-term water shortage on community structure, population distribution and individual metabolism need further research. In this study, our objectives were to interpret the variability of the traits of plant functional groups and community responses to aridity gradients based on variations in water use efficiency, individual traits and stoichiometry.First, we analyzed the relationship between foliar stable carbon isotopes (δ13C) and scaling exponent (a). Accumulating evidence has shown that a is not a constant. Debate continues over what determines the variation in a. We found that a changes with aridity gradients and co-varies with water use efficiency. By measuring foliar stable carbon isotope, plant morphological traits (root-shoot and height-radius ratios) and a along an aridity gradient from eastern to western China, we confirmed that the variation in a was accounted for by changes in plant morphological traits which are adaptive strategies for enhancing water use efficiency during drought stress. This information can be crucial for understanding and predicting community and ecosystem processes.Second, the variation in foliar stable carbon isotope signature (δ13C) of different plant functional groups (PFGs) and its relationship with environmental factors in China were investigated in this study. There were some significant, but small differences in813C among PFGs categorized by life-form (<1‰). The trees (-26.78%o) and shrubs (-26.89‰) had similar mean δ13C, both significantly higher than herbs (-27.49‰). The evergreen shrubs (-25.82‰) had significantly higher mean δ13C than deciduous shrubs (-26.92‰). The perennial herbs (-26.83%o) had significantly higher mean δ13C than annual herbs (-27.10‰). The grasses (-26.46‰) had significantly higher mean δ13C than forbs (-26.96‰). For pooled data,δ13C was significantly and negatively correlated with the mean annual precipitation (MAP) and the mean annual temperature (MAT), while significantly and positively correlated with the latitude and the altitude. There was a threshold value of MAP along the gradients, and813C had no significant change with higher values. The δ13C of PFGs changed with altitude differently, suggesting that increases in δ13C with altitude cannot be generalized. Differences in δ13C between PFGs were generally much smaller than1‰and therefore insignificant. In contrast, MAP and MAT had relatively large effects on δI3C (more than4%o between extremes). The δ13C of some PFGs responded to environmental gradients in the same manner, while their ’rates’of change were significantly different in some cases. This kind of information could help predict potential changes in the distribution of PFGs in response to future climate changes.At the end, nitrogen (N) and phosphorus (P) concentrations and N:P ratios between leaf and root of Nitraria tangutorum along aridity gradients were studied. Our analysis indicated that the means of both leaf N and P in April were higher than the global and Chinese average values, while in August being lower than these reported average values. N. tangutorum was relatively limited by N in April (mean leaf N:P ratios=11.13) and by P in August (mean leaf N:P ratios=38.78). N and P in both leaf and root were highly correlated across sampling sites. Increase in leaf N and P was accompanied by increases in the N and P of root in April along aridity gradients. Leaf N:P ratios were invariant along aridity gradients, while root N:P ratios increased with increasing aridity gradients in April. We suggested that leaf N:P ratios could indicate nutrient status of different plant growth stages, while root N:P ratios could show us whether the soil nutrient become deficient.Our research explained the mechanism of varied a along aridity gradients, which related to WUE and stoichiometry. The fact that variations in WUE of different plant functional groups will change community structure support the hypothesis that a changes with WUE. Individual traits, WUE and stoichiometry changed with aridity gradients, which showed that community structure is controlled by individual behavior and function indirectly or directly due to changing with drought stress.