Acclimation to High Vapor Pressure Deficit in Warmer Air Can Reduce Tree Vulnerability to Drought-Induced Mortality

Climate change imposes new constraints on tree survival, emphasising two key parameters: the vapour pressure deficit (VPD) and air temperature. Yet, no study has experimentally evaluated drought-induced tree mortality risk following acclimation to elevated temperatures with low or high VPD. Three tree species of contrasting temperature and drought tolerances (Prunus mahaleb, Quercus robur, and Populus nigra) underwent a growing season of acclimation to elevated temperature and/or VPD, and a lethal drought the following year until stem hydraulic failure was confirmed through micro-CT. Our mechanistic approach to assess temperature and VPD acclimation impacts on drought-induced mortality includes tracking stomatal conductance (g_s), minimum stomatal conductance (g_min), total leaf area (LA_tot), water potential at turgor loss point (Ψ_TLP), and estimating the time to hydraulic failure using modelling. Acclimation to elevated VPD and temperature accelerated stomatal closure, reduced g_min, and raised Ψ_TLP. In contrast, while high temperature reduced g_min, it also increased LA_tot and height. Consequently, hydraulic failure occurred faster in high-temperature-acclimated trees, while it was generally delayed by adding higher VPD. Our findings highlight that the balancing effects of temperature-driven leaf area expansion, which accelerate mortality, and VPD-driven acclimation in stomatal sensitivity, counteract each other, stabilising the timing of mortality.