![]() ![]() 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. Dryland ecosystems tend to operate at low θ crit and show adaptation to water deficits. 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. We find systematic differences of these three variables across biomes. Three canonical variables describe how EF is controlled by SM: the maximum EF (EF max), θ crit, and slope (S) between EF and SM. Here, we use global eddy covariance observations to quantify θ crit and evaporative fraction (EF) regimes. Knowledge of the spatial distribution of θ crit is crucial for future projections of climate and water resources. Plant water stress occurs at the point when soil moisture (SM) limits transpiration, defining a critical SM threshold (θ crit). ![]()
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