Thermal optimum of photosynthesis is controlled by stomatal conductance and does not acclimate across an urban thermal gradient in six subtropical tree species.

Modelling the response of plants to climate change is limited by our incomplete understanding of the component processes of photosynthesis and their temperature responses within and among species. For ≥20 individuals, each of six common subtropical tree species occurring across steep urban thermal gradients in Miami, Florida, USA, we determined rates of net photosynthesis (A net ), maximum RuBP carboxylation, maximum RuBP regeneration and stomatal conductance, and modelled the optimum temperature (T opt ) and process rate of each parameter to address two questions: (1) Do the T opt of A net (T optA ) and the maximum A net (A opt ) of subtropical trees reflect acclimation to elevated growth temperatures? And (2) What limits A net in subtropical trees? Against expectations, we did not find significant acclimation of T optA , A opt  or the T opt of any of the underlying photosynthetic parameters to growth temperature in any of the focal species. Model selection for the single best predictor of A net both across leaf temperatures and at T optA revealed that the A net of most trees was best predicted by stomatal conductance. Our findings are in accord with those of previous studies, especially in the tropics, that have identified stomatal conductance to be the most important factor limiting A net , rather than biochemical thermal responses.