Leaf temperatures exceed thermal heat tolerances for a community of eastern North America hardwood trees.

Changing climates are creating more intense and frequent high-temperature events that could disrupt forest communities. In temperate forests, we have a relatively limited understanding of how trees are impacted by heat events, hindering our ability to predict the impacts of future heat waves. We conducted a community-level assessment of thermal safety margins in 11 hardwood tree species native to eastern North America. We used chlorophyll fluorescence to determine the critical heat tolerance of photosystem II (PSII) across 2 years in central Tennessee, USA. We focus on the temperature at which PSII first starts to decline (T _crit) as this is the temperature where membranes become unstable, resulting in permanent damage to these tissues. T _crit varied within the season and between years, being higher in July than June and in 2022 than 2023. T _crit also varied among species with species like Ulmus rubra and Ostrya virginiana showing consistently lower heat tolerances. When compared to the record high temperature for our study site, 10 of 11 species would have experienced heat stress during at least one sample period. When compared to current year high temperatures, the risk was variable and lower across all species and sample periods. However, we found that leaf temperatures often exceeded air temperatures many species were likely heat stressed as heat tolerances were often below species-specific leaf temperatures. Indeed, four species were potentially heat stressed during every sample period. Our data highlights the importance of using leaf temperature, not air temperature to assess thermal safety margins and that community-wide stress may already occur under extreme heat conditions. As climate change intensifies, leaf temperatures will likely approach critical thresholds that lead to damage across the tree community. Understanding species-specific responses to heat stress is essential to predicting future forest dynamics and ecosystem functioning.