Quantifying Dynamic Changes in Stem Storage‐Water Use and Rehydration in Mature Trees: A Study on Pinus sylvestris Trees Exposed to Increased Atmospheric and Soil Drought

ABSTRACT Forests increasingly experience hot droughts, characterized by rising vapour pressure deficits (VPD) and declining soil moisture, highlighting the need to better understand water fluxes within trees. While stomatal conductance and transpiration are well studied, upstream fluxes such as internal stem water storage use and rehydration dynamics remain poorly quantified, despite their importance for maintaining tissue hydration and growth. Here, we quantify storage‐water use and rehydration dynamics by combining high‐resolution sap flow and dendrometer measurements in mature Pinus sylvestris at the VPDrought experiment of the Pfynwald research platform, where rain‐water input and VPD are independently manipulated. Fluxes were analyzed over the 2024 growing season in relation to VPD and soil water potential ( ψ soil ). The relative contribution of internal stem water storage to daily transpiration increased with drought, rising from about 10% under well‐watered conditions to up to 55% under dry conditions, mainly caused by reduced transpiration while storage water use is sustained. Notably, under mild soil drought, high nighttime VPD substantially constrained nighttime tissue rehydration fluxes. Overall, our novel framework highlights the dynamic role of stem water storage in modulating water potentials during drought and demonstrates the importance of explicitly considering temporal stem water storage dynamics when assessing tree water‐use strategies.