Climate change is intensifying the frequency and severity of heatwaves, droughts, and atmospheric dryness - expressed as rising vapor pressure deficit (VPD) - which increasingly co-occur and pose major challenges to tree survival. These compound stressors can disrupt plant water relations, carbon balance, and thermoregulation, ultimately leading to hydraulic failure or metabolic collapse. Yet, our understanding of how trees physiologically adjust to long-term exposure to such conditions, and the effectiveness of these acclimation strategies in preventing mortality, remains limited. This thesis addresses these gaps by investigating the mechanisms of physiological acclimation and the drivers of mortality under combined heat, drought, and VPD stress in European tree species. Through three complementary experiments, the thesis aimed at (1) investigating the potential damages of using X-ray MicroCT (µCT) to track embolism in living trees, as it stands as the more promising tool for in-vivo monitoring of hydraulic impairments in saplings stems; (2) separating the individual impacts of VPD and temperature on how trees use water, and examining whether prior growth under warmer and drier atmospheric conditions enhances their likelihood of surviving drought; (3) advancing our grasp of how trees endure simultaneous heat and drought stress, with particular focus on the difficulties they face in controlling leaf temperature and fine-tuning gas exchange throughout the day. The findings reveal that while morphological and physiological acclimation occurs, these adjustments often do not suffice to maintain photosynthesis and leaf cooling under extreme conditions. Beech (Fagus sylvatica) and oak (Quercus pubescens) exhibited distinct differences in their capacity for thermoregulation, with beech being more susceptible to overheating and functional decline. Additionally, long-term exposure to high VPD promoted more conservative water use and delayed hydraulic failure, while exposure to heat under low VPD induced growth responses that may increase future vulnerability. Notably, the thesis also validates the use of µCT for embolism monitoring, although care is needed during periods of active growth. Overall, this work highlights the trade-offs trees face between maintaining carbon gain and avoiding hydraulic or thermal damage, and suggests that acclimation does not necessarily confer greater drought resistance during extreme events. By identifying the roles of soft and hard traits in modulating risk, the thesis contributes to a more nuanced understanding of tree mortality in a chronically warmer and drier world. The results of this work will enhance mechanistic vegetation models and support forest managers by providing a robust understanding to promote tree resilience in the face of accelerating climate stress.