Hydraulic stress diminishes acclimation of leaf thermoregulation in European trees exposed to hot-drought.

Hydraulic status plays a large role in leaf thermoregulation, which is important for maintaining leaves below thermal thresholds during heatwaves. Still, little is known about how acclimation to warming and drought may affect trees’ abilities to avoid critical temperature thresholds. Using a five-year open-top chamber experiment, we studied the single and interactive effects of heat and soil drought on leaf temperature regulation, heat tolerance, hydraulic status, and gas exchange in two temperate tree species with contrasting water management strategies: common beech (Fagus sylvatica) and pubescent oak (Quercus pubescens). Drought-exposed trees were less homeothermic than control and warmed trees, leading to larger leaf-to-air temperature differentials and greater leaf temperature maxima, especially in hot-drought conditions. During the peak summer heat (ambient temperature reaching > 40°C), gas exchange and hydraulic safety margins in drought-exposed trees (including with added warming) were strongly reduced, particularly in beech, compared to the control and heat exposure alone. Indeed, drought induced extreme hydraulic stress, which limited the trees' ability to preserve thermal safety margins. Consequently, despite acclimation of heat tolerance to leaf temperature maxima, drought, and especially hot drought, led to narrower (even negative) leaf thermal safety margins, widespread leaf scorching, and early senescence. Our results show that while thermoregulation acclimates to increased temperatures, drought remains the dominant driver of canopy damage, which may be exacerbated when combined with heat waves.