Towards understanding stomatal decoupling under extreme heat: integrating stomatal kinetics, leaf light responses, and the role of stomatal versus cuticular conductance

· Earth's forests are increasingly exposed to hotter drought events, causing unprecedented dieback of trees and forests. Evidence of stomatal decoupling at high leaf temperatures (T leaf ) is mounting, but the driving mechanisms and variability across species are unknown. · We experimentally imposed heat and drought stress onto two rainforest and two semi-arid tree species in a glasshouse study to better understand accelerated water loss at high T leaf by measuring plant thermal and hydraulic balance. · Stomatal decoupling at high T leaf occurred in two species, including the most anisohydric species, but did not occur in a vulnerable rainforest species. Accelerated leaf water loss occurred predominantly through open stomata, rather than the cuticle, but some species prioritised thermal acclimation over leaf thermoregulation during heat stress. This suggests decoupling occurs only in species with high capacity for transpirational cooling (>5°C), or alternatively, that full sunlight is required for stomatal decoupling. · The maximum rate of light-induced stomatal opening was up to two times faster, while dark-induced stomatal closing was 14 minutes slower on average in leaves at 34°C, relative to 25°C. Stomatal kinetics are an overlooked trait that may help to better predict accelerated water loss under high T leaf across species.