Stomatal decoupling during heatwaves does not alter leaf cooling or recovery in Quercus ilex under long-term experimental drought.

Heatwaves and droughts are intensifying, increasing atmospheric evaporative demand and threatening forest functioning. Rising temperatures drive strong declines in photosynthesis and stomatal conductance, yet some trees maintain transpiration during extreme heat via a phenomenon termed stomatal decoupling. However, its occurrence and significance under long-term drought remain poorly understood. We investigated leaf physiology and canopy temperature in a Mediterranean Quercus ilex forest exposed to more than 20 years of experimental rain exclusion, including during an intense heatwave (>40°C). Despite similar soil moisture, experimentally droughted trees experienced lower leaf water potential during the summer. Photosynthesis (Anet) and stomatal conductance (gs) were similar across treatments early in the season. During the heatwave, Anet declined to zero while gs stayed positive, demonstrating strong stomatal decoupling in both drought histories. This decoupling only marginally reduced canopy temperature, which exceeded air temperature by up to 10°C. While foliar thermal tolerance thresholds (>53°C) were not surpassed, extensive canopy damage was observed, especially in experimentally droughted trees, and reduced post-heatwave physiological recovery. Overall, Q. ilex exhibited stomatal decoupling under extreme heat, but this mechanism provided limited evaporative cooling and did not prevent canopy scorching. Long-term drought mainly influenced recovery capacity rather than the occurrence of decoupling itself.