Critical soil moisture thresholds of plant water stress in terrestrial ecosystems

Plant water stress occurs at the point when soil moisture (SM) limits transpiration, defining a critical SM threshold (θ(crit)). Knowledge of the spatial distribution of θ(crit) is crucial for future projections of climate and water resources. Here, we use global eddy covariance observations to quantify θ(crit) and evaporative fraction (EF) regimes. Three canonical variables describe how EF is controlled by SM: the maximum EF (EF(max)), θ(crit), and slope (S) between EF and SM. We find systematic differences of these three variables across biomes. Variation in θ(crit), S, and EF(max) is mostly explained by soil texture, vapor pressure deficit, and precipitation, respectively, as well as vegetation structure. Dryland ecosystems tend to operate at low θ(crit) and show adaptation to water deficits. The negative relationship between θ(crit) and S indicates that dryland ecosystems minimize θ(crit) through mechanisms of sustained SM extraction and transport by xylem. Our results further suggest an optimal adaptation of local EF-SM response that maximizes growing-season evapotranspiration and photosynthesis.