Carbon-water coupling and its relationship with environmental and biological factors in a planted Caragana liouana shrub community in desert steppe, northwest China

The carbon and water cycle, an important biophysical process of terrestrial ecosystems, may be changed by anthropogenic revegetation in arid and semiarid areas. However, there is still a lack of understanding of the mechanism of carbon and water coupling in intrinsic ecosystems in the context of human activities. Based on the CO2 and H2O flux measurements of the desert steppe with the planted shrub Caragana liouana, this study explores the carbon and water flux coupling of the ecosystem by analyzing the variations in gross primary productivity (GPP), evapotranspiration (ET), and water use efficiency (WUE) and discussing the driving mechanism of biological factors. The seasonal variation in climate factors induced a periodic variation pattern of biophysical traits and carbon and water fluxes. The GPP and ET fluctuated in season, but the WUE was relatively stable in the growing season. The GPP, ET, and WUE were significantly driven by global radiation (Rg), temperature (Ta and Ts), water vapor pressure deficit (VPD), leaf area index (LAI), and plant water stress index (PWSI). However, Rg, temperature, and PWSI were the most important factors regulating WUE. Rg and temperature directly affected WUE with a positive effect but indirectly inhibited WUE by rising PWSI. Plant water stress inhibited photosynthesis and transpiration of the planted shrub community in desert steppe. When the plant water stress breaks a threshold (PWSI >0.54), the WUE will decrease since the GPP respond more quickly to the plant water stress than ET. Our findings suggest that policies related to large-scale carbon sequestration initiatives under afforestation must first fully consider the statuses of water consumption and WUE.