Water-use efficiency in response to simulated increasing precipitation in a temperate desert ecosystem of Xinjiang, China

Water-use efficiency（WUE） is a key plant functional trait that plays a central role in the global cycles of water and carbon. Although increasing precipitation may cause vegetation changes, few studies have explored the linkage between alteration in vegetation and WUE. Here, we analyzed the responses of leaf WUE, ecosystem carbon and water exchanges, ecosystem WUE, and plant community composition changes under normal conditions and also under extra 15% or 30% increases in annual precipitation in a temperate desert ecosystem of Xinjiang, China. We found that leaf WUE and ecosystem WUE showed inconsistent responses to increasing precipitation. Leaf WUE consistently decreased as precipitation increased. By contrast, the responses of the ecosystem WUE to increasing precipitation are different in different precipitation regimes： increasing by 33.9% in the wet year（i.e., the normal precipitation years）and decreasing by 4.1% in the dry year when the precipitation was about 30% less than that in the wet year.We systematically assessed the herbaceous community dynamics, community composition, and vegetation coverage to explain the responses of ecosystem WUE, and found that the between-year discrepancy in ecosystem WUE was consistent with the extent to which plant biomass was stimulated by the increase in precipitation. Although there was no change in the relative significance of ephemerals in the plant community, its greater overall plant biomass drove an increased ecosystem WUE under the conditions of increasing precipitation in 2011. However, the slight increase in plant biomass exerted no significant effect on ecosystem WUE in 2012. Our findings suggest that an alteration in the dominant species in this plant community can induce a shift in the carbon-and water-based economics of desert ecosystems.

Differential responses of carbon and water vapor fluxes to climate among evergreen needleleaf forests in the USA

Introduction:Understanding the differences in carbon and water vapor fluxes of spatially distributed evergreen needleleaf forests(ENFs)is crucial for accurately estimating regional or global carbon and water budgets and when predicting the responses of ENFs to current and future climate.Methods:We compared the fluxes of ten AmeriFlux ENF sites to investigate cross-site variability in net ecosystem exchange of carbon(NEE),gross primary production(GPP),and evapotranspiration(ET).We used wavelet cross-correlation analysis to examine responses of NEE and ET to common climatic drivers over multiple timescales and also determined optimum values of air temperature(T_(a))and vapor pressure deficit(VPD)for NEE and ET.Results:We found larger differences in the NEE spectra than in the ET spectra across sites,demonstrating that spatial(site-to-site)variability was larger for NEE than for ET.The NEE and ET were decoupled differently across ENF sites because the wavelet cospectra between ET and climate variables were similar at all sites,while the wavelet cospectra between NEE and climate variables were higher(i.e.,closer coupling between NEE and climatic drivers)in semi-arid and Mediterranean sites than in other sites.Ecosystem water use efficiency(EWUE)based on annual GPP/ET ranged from 1.3±0.18 to 4.08±0.62 g C mm^(−1) ET,while EWUE based on annual net ecosystem production(NEP)/ET ranged from 0.06±0.04 to 1.02±0.16 g C mm^(−1) ET)among ENFs.Responses of NEE and ET to T_(a) varied across climatic zones.In particular,for ENF sites in semi-arid and Mediterranean climates,the maximum NEE and ET occurred at lower ranges of T_(a) than in sites with warm and humid summers.The optimum T_(a) and VPD values were higher for ET than for NEE,and ET was less sensitive to high values of T_(a) and VPD.Conclusions:Large spatial variability in carbon and water vapor fluxes among ENFs and large variations in responses of NEE and ET to major climate variables among climatic zones necessitate sub-plant functional type parameterization based on climatic zones to better represent climate sensitivity of ENFs and to reduce uncertainty in model predictions.