Root water uptake depth explains drought response of temperate tree species

Climate change is increasing drought frequency, duration, and severity in large parts of the world, thereby reducing soil water availability and increasing risks for forest decline and mortality. The drought response of a tree can be described by the tree’s 'drought sensitivity', indicating reversible processes such as stomatal responses to soil drying, and 'drought vulnerability', indicating irreversible damages, such as the risk of hydraulic failure. Different tree species differ substantially in their drought sensitivity and vulnerability. Yet, the underlying physiological and morphological mechanisms remain poorly understood. We tested whether species-specific differences in root water uptake depth (RWUD) can explain differences in drought sensitivity and vulnerability of mature trees belonging to nine temperate European tree species. Using a unique six-year dataset (2018–2024) from the Swiss Canopy Crane II site, we quantified drought sensitivity from the response of daily maximum sap flux density to soil drying. We quantified drought vulnerability by calculating hydraulic safety margins of trees relative to species-specific critical xylem hydraulic thresholds. RWUD was estimated from stable water isotopes and analyzed against sensitivity and vulnerability traits.

We show that species differ markedly in both sensitivity and vulnerability. We discuss that these differences are largely determined by variation in the tree's maximum RWUD: shallow-rooted species closed stomata early and rapidly approached hydraulic thresholds during drought, while deep-rooted species sustained transpiration and maintained wide hydraulic safety margins. RWUD alone explained more than 65 % of the interspecific variation in both drought sensitivity and vulnerability. Our results demonstrate that RWUD is a key morphological trait linking belowground water access to aboveground drought physiology. By quantifying this connection in mature trees, our study identifies RWUD as a strong predictor of forest drought resilience and a critical parameter for integrating rooting traits into ecosystem and Earth system models to improve forecasts of forest–climate feedbacks under intensifying drought regimes.