Tree Responses to Heat and Drought: Implications for Urban Cooling

Cities worldwide are becoming increasingly hotter due to ongoing urbanization, rising global temperatures, and more frequent heatwaves, which all intensify the urban heat island (UHI) effect. Urban trees can help mitigate this effect through transpirational cooling, but the cooling performance varies by species and remains poorly understood, making it challenging for city planners to optimise vegetation strategies. Additionally, heightened heat exposure is pushing urban trees closer to their thermal limits, leading to more frequent heat-related mortality. As a result, tree planting initiatives aimed at combating the UHI effect are being compromised. Urban green space managers urgently need better guidance on which tree species can both effectively cool cities and withstand future climate conditions.

Urban tree resilience to heat and their cooling effect on the city microclimate are tightly interlinked through transpiration, which lowers both leaf surface temperatures and the surrounding air. However, the degree to which different tree species cool their leaves remains largely unknown, particularly since heat stress can limit transpiration and, in turn, reduce a tree’s self-cooling capacity. Species with differing water-use strategies are likely to vary in their ability to sustain leaf cooling during hot, dry conditions. Additionally, while trees have intrinsic heat tolerance, the specific temperature thresholds for most species are still unclear. 

We measure tree transpiration throughout the growing season and calculate its cooling effect, regularly assess tree water stress and leaf responses to heat and drought, and monitor the microclimate in and outside the tree canopy. By comparing species with contrasting functional traits (e.g., diffuse-porous vs. ring-porous) growing on the EPFL campus we want to find out which trees are dealing best with hot droughts during the summer while efficiently cooling their environment. The project will monitor trees and the urban microclimate during four years, providing the opportunity to capture potential extreme weather conditions (e.g. heatwaves) and understand the interactions during these. 

We welcome Master’s students to contact us regarding thesis opportunities on this topic.