When trees run on savings: Stem water storage and rehydration as a missing link in drought responses

Hotter droughts in European forests increasingly combine declining soil moisture with rising atmospheric demand, raising fundamental questions about how trees sustain transpiration while avoiding embolism-induced mortality under drought stress. While stomatal regulation and transpiration responses are well documented, the role of upstream, within-tree water fluxes, particularly the use and replenishment of internal stem water storage, represent an emerging research frontier.Here, we present high-temporal resolution observations of stem water storage use and rehydration dynamics in mature Pinus sylvestris, combining sap-flow and dendrometer measurements from the VPDrought experiment at the Pfynwald research platform in the dry inner-Alpine Rhône valley of Switzerland. By independently manipulating soil moisture and vapour pressure deficit (VPD), this experiment allows us to disentangle atmospheric and soil controls on internal tree water fluxes.We show that under drought, trees increasingly “run on savings”: the contribution of stem water storage to daily transpiration rises sharply from approximately ~5% under well-watered soil conditions to up to ~40% under dry soil conditions, when transpiration declines but storage water use persists. In parallel, the replenishment of stem storage-water reserves through water flow into the stem declines with decreasing soil water potential. Notably, even under mild soil drought, elevated VPD substantially constrains nighttime rehydration of stem storage-water reserves.The findings we present emphasize stem water storage as a dynamic and drought-responsive component of tree-water use. Accounting for both the mobilization and rehydration of internal water reserves is essential for understanding how trees buffer hydraulic stress during drought and enhance model representations of plant-water interactions under increasingly frequent hotter droughts.