Global distribution of the rooting zone water storage capacity reflects plant adaptation to the environment

Abstract

Abstract The rooting zone water storage capacity ( S 0 ) extends from the soil surface to the weathered bedrock (the Critical Zone) and determines land-atmosphere exchange during dry periods. Despite its importance to land-surface modeling, variations of S 0 across space are largely unknown as they cannot be observed directly. We developed a method to diagnose global variations of S 0 from the relationship between vegetation activity (measured by sun-induced fluorescence and by the evaporative fraction) and the cumulative water deficit (CWD). We then show that spatial variations in S 0 can be predicted from the assumption that plants are adapted to sustain CWD extremes occurring with a return period that is related to the life form of dominant plants and the large-scale topographical setting. Predicted biome-level S 0 distributions, translated to an apparent rooting depth ( z r ) by accounting for soil texture, are consistent with observations from a comprehensive z r dataset. Large spatial variations in S 0 across the globe reflect adaptation of z r to the hydroclimate and topography and implies large heterogeneity in the sensitivity of vegetation activity to drought. The magnitude of S 0 inferred for most of the Earth’s vegetated regions and particularly for those with a large seasonality in their hydroclimate indicates an important role for plant access to water stored at depth - beyond the soil layers commonly considered in land-surface models.