Leaf temperature and its departure from ambient air temperature

Leaf temperature (T_l), the temperature at which leaf-air exchanges of carbon and water occur, varies with ambient air temperature (T_a), regulated by microclimate and species' energy balance traits. Ground and satellite thermal measurements of the T_l-T_a relationship are widely used to infer plants' thermoregulation capacity. On the basis of a global synthesis of observations across diverse climates and biomes, we show that reported thermoregulation patterns vary primarily along temperature gradients. Megathermy (dT_l/dT_a > 1) is particularly prevalent in warm tropical regions and in sun-exposed canopy-top leaves owing to ineffective dissipation of the often excessively accumulated solar radiation, while limited homeothermy (dT_l/dT_a < 1) and poikilothermy (dT_l/dT_a = 1) are reported mostly for cold ecosystems or sub-canopy leaves. Under heat-stressed conditions, some warm-adapted species can abate rapid T_l surge through active stomatal control, unless critical temperature thresholds are exceeded, above which T_l might increase non-linearly as a warning sign of damaging stress. This thermal consideration of stomatal regulation is currently missing in mechanistic models as a source of bias in estimated photosynthetic rates. We highlight the pressing need to develop new stomatal theories that tackle a triple-target optimization between carbon gain, water loss and thermal regulation.