A model for leaf temperature decoupling from air temperature

Leaf temperature (Tleaf) influences rates of respiration, photosynthesis, and transpiration. The local slope of the relationship between Tleaf and Tair, β, describes leaf thermal responses. A range of values have been observed, with β <1 indicating limited homeothermy where Tleaf increases at a lower rate than Tair, β = 1 indicating poikilothermy where Tleaf tracks Tair, and β >1 indicating megathermy where Tleaf increasingly exceeds Tair. However, theory for variation in β has not been developed. Here we derive an equation for β that predicts how it varies with multiple trait and microenvironment variables. The approach also predicts how maintenance of Tleaf away from lethally high values may help explain regulation of stomatal conductance (gS). The work delineates contexts in which each class of leaf thermal response is expected and develops concepts for predicting leaf responses to thermally extreme environments.