Similar patterns of leaf temperatures and thermal acclimation to warming in temperate and tropical tree canopies.

As the global climate warms, a key question is how increased leaf temperatures will affect tree physiology and the coupling between leaf and air temperatures in forests. To explore the impact of increasing temperatures on plant performance in open air, we warmed leaves in the canopy of two mature evergreen forests, a temperate Eucalyptus woodland and a tropical rainforest. The leaf heaters consistently maintained leaves at a target of 4°C above ambient leaf temperatures. Ambient leaf temperatures (Tleaf) were mostly coupled to air temperatures (Tair), but at times leaves could be 8-10°C warmer than ambient air temperatures especially in full sun. At both sites, Tleaf were warmer at higher air temperatures (Tair > 25°C), but cooler at lower Tair, contrary to the 'leaf homeothermy hypothesis'. Warmed leaves showed significantly lower stomatal conductance (-0.05 mol m-2 s-1 or - 43% across species) and net photosynthesis (-3.91 μmol m-2 s-1 or - 39%), with similar rates in leaf respiration rates at a common temperature (no acclimation). Increased canopy leaf temperatures due to future warming should reduce carbon assimilation via reduction in photosynthesis in these forests, which could potentially weaken the land carbon sink in tropical and temperate forests.