Background/Question/Methods High temperatures can induce thermal stress in trees, although species may vary in their ability to acclimate their leaf thermoregulatory traits and their photosynthetic thermal tolerances. In this study, we tested the hypotheses that (1) leaves of adult trees in warmer growing conditions acclimate their thermoregulatory traits to regulate leaf temperatures, (2) leaves acclimate their photosynthetic thermal tolerances such that they are positively correlated with leaf temperature, but plasticity of both thermoregulatory traits and thermal tolerances will vary between species and will be generally insufficient to maintain constant thermal safety margins (TSMs; thermal tolerance - leaf temperature); and (3) species with broader thermal niche breadths have greater acclimatory abilities. We sampled leaves from ≥15 individuals from each of seven tree species occurring across Miami’s Urban Heat Island (Florida, USA), capturing a ~5°C range in air temperatures. We measured suites of the leaves’ thermoregulatory traits to estimate the leaf temperatures using a leaf energy balance model, and we measured the photosynthetic heat tolerances of each focal tree. We also calculated the thermal niche breadth of each species based on their occurrence records. Results/Conclusions We found that trees are generally more homeothermic than expected by chance due to acclimation of their leaf thermoregulatory traits but that this acclimation is not sufficient to offset elevated air temperatures. Furthermore, the leaf thermal tolerances of individuals did not acclimate to warmer temperatures. Consequently, TSMs get narrower for plants in hotter areas. Finally, we found limited support for our hypothesis that species with broader thermal niches are better at acclimating to maintain TSMs. These findings suggest that thermal niche specialists, including many lowland tropical tree species, could be at a heightened risk of thermal stress compared to their climate generalist counterparts as global temperatures continue to rise, potentially threatening the persistence of these species and/or decreasing their ability to provide valuable ecosystem services such as carbon sequestration.
