Spatial variations in water use efficiency across global terrestrial ecosystems

Ecosystem water use efficiency (WUE) is an important property for understanding the coupling between ecosystem carbon and water cycles and for constraining land models, whereas the mechanism of its spatial variations remains unclear. Here we proposed a framework to decompose ecosystem WUE into component processes, with which the physiological and non-physiological processes controlling ecosystem WUE were included. Further, with the data of measurements by eddy covariance at global Fluxnet sites, we investigated the spatial patterns and determinants of ecosystem WUE with the framework proposed. Our results showed that crop exhibited the highest value of WUE, followed by forest, grassland, wetland, and shrub. The cross-site difference in WUE was mainly determined by that of gross primary productivity (GPP) and the physiological component, i. e., the ratio of GPP to plant transpiration. Spatially, ecosystem WUE showed an overall pattern of increase with air temperature, peaking at ca. 10 degree celsius celcius and decreased thereafter. Mean annual precipitation was positively correlated with WUE when < 1000 mm. Moreover, leaf area index (LAI) and the ratio of ambient to intercellular CO2 concentration at canopy level (C-i/C-a) were the two key factors dominating the spatial ecosystem WUE patterns. LAI affected ecosystem WUE mainly through biophysically controlling the fraction of soil water evaporation. Our findings highlight the importance of LAI and canopy level C-i/C-a on controlling the spatial variations in ecosystem WUE, which warrants more attentions and investigations in ecosystem models.