Chronically rising air temperatures and vapor pressure deficit (VPD) is an emerging risk for forests as it pushes trees closer to their thermal limits while also increasing the risk of hydraulic dysfunctions, independently of soil moisture changes. During heatwaves, trees face a dilemma between closing their stomata to prevent embolism and maintaining high transpiration to avoid critical overheating of the canopy. Yet, how trees adjust their gas exchange and thermotolerance during periods of high temperature remains largely unknown. I will discuss recent findings from manipulative experiments in open-top chambers and the field in this presentation. Results suggest that coniferous trees drastically reduce transpiration during hot periods and avoid lethal leaf temperatures because of effective non-evaporative cooling. In contrast, broadleaved trees maintain high transpiration under high air temperatures, which can lead to severe hydraulic damage even without soil drought. These findings provide new insights into one of the most pressing threats associated with climate change: the worsening of heatwaves and how they affect tree physiological functions in broadleaved vs. coniferous trees.