Impacts of Extreme Droughts on Ecosystem Water Use Efficiency Diverge between Forest and Grassland

Ecosystem-scale water use efficiency (WUE), as a quantification of the coupling between carbon and water cycle, determines whether vegetation could survive under severe drought stress. Nevertheless, how extreme droughts affect ecosystem-scale WUE and its difference among regions and biomes are still poorly understood. In this study, using data-oriented gridded products of gross primary productivity (GPP) and evapotranspiration (ET), we assessed the ecosystem WUE change during extreme drought years over China, and further compared drought impacts on WUE between forest and grassland ecosystems. We found a spatial heterogeneity of WUE change in response to extreme droughts across regions. Particularly, grassland WUE was substantially reduced concurrent with suppressed photosynthesis, while most of forest exhibited slightly decreased or even increased WUE under extreme droughts. In addition, we demonstrated that drought characteristics and environmental conditions modulated drought impacts on WUE. Temperature anomalies during droughts and leaf area were found to be the predominant factors driving WUE change for both forest and grassland. With increasing occurrence of compound dry and hot extremes, our results, therefore, would be an insightful supplement to the current understanding of the influence of extreme events on terrestrial ecosystems.