Water Availability and Hydraulic Strategies Control Leaf Thermoregulation and Damage During Heat Stress and Recovery in Temperate Tree Species

Heatwaves are intensifying worldwide, often coinciding with high vapour pressure deficit (VPD) and soil drought. Yet, how temperate tree species tolerate these combined stresses remains elusive. Using single-tree gas-exchange chambers, we examined the response of leaf gas exchange and thermoregulation of three broadleaved tree species to a stepwise increase in air temperature (25°C-45°C) along with VPD (2.5-7.9 kPa) under well-watered and moderate drought conditions, followed by a 2-day recovery at 25°C. Across species, heat stress increased transpiration but reduced net assimilation, leading to sharp declines in water-use efficiency. Alongside, leaf cooling intensified, driving leaf temperatures below air temperature. Under moderate drought, restricted stomatal conductance reduced transpirational cooling, causing leaf temperatures to exceed air temperature and increasing heat-related damage. Species differed markedly in their heat responses along the isohydric-anisohydric spectrum. Isohydric Acer platanoides showed early stomatal closure, limited cooling, severe leaf damage, and poor recovery. Anisohydric Quercus robur maintained gas exchange, exhibited minimal leaf damage and recovered rapidly, while Fagus sylvatica showed intermediate responses. Together, these results highlight that the ability of temperate trees to withstand future heatwaves will depend on the interplay between water availability, transpirational cooling, as well as species-specific hydraulic strategy and heat tolerance.