Trees in Cities Provide Cooling even in Extreme Heat

(EPFL, June 30, 2025)

Urban trees fight off heatwaves by increasing water evaporation, cooling their surroundings effectively. Researchers from the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) and EPFL, led by Dr. Christoph Bachofen, found that even during extreme heat above 39 °C, city plane trees significantly boost water evaporation, cooling nearby areas. This discovery overturns previous assumptions and underscores urban trees' role in making cities more bearable as global temperatures climb. In spring 2023, the team equipped eight trees in Lancy, Geneva, with devices to monitor sap flow, particularly during record-breaking heatwaves. This method measured the cooling effect of the trees through water evaporation. The findings encourage urban planners to choose tree species that withstand pollution and road salt and cool cities during heatwaves, offering a natural way to fight urban heat islands. This research provides essential guidance on enhancing urban forestry for better climate resilience.