Changes in bur oak leaf temperature, critical temperature, and thermal safety margin under elevated temperature and CO2

Rising global temperatures are expected to increase heat stress for plants, which may threaten forests and the ecosystems they support. At the same time, atmospheric carbon dioxide (CO₂) is increasing, which can influence how plants regulate water loss and temperature. Together, these changes create uncertainty about how plants will respond to future climate conditions. This study examined how warming and elevated CO₂ affect leaf temperature and plant heat tolerance in bur oak, a widespread North American tree species. We grew young trees under controlled conditions with different temperature and CO₂ levels over a 10-week period. We measured how hot leaves became and how much heat they could tolerate before experiencing damage. We found that plants were able to adjust to warmer conditions by increasing their heat tolerance over time. This adjustment helped offset rising leaf temperatures, so plants generally maintained a margin of safety from heat damage. However, warming still reduced this safety margin compared to current conditions. Elevated CO₂ had smaller and less consistent effects and did not strongly increase heat stress. Overall, these results suggest that bur oak has some capacity to cope with short-term warming, but increasing temperatures remain the main factor pushing plants closer to their heat limits. Understanding these responses can help improve predictions of how forests will respond to climate change and inform strategies for managing ecosystems under future conditions.