Climate Trees cool cities better than previously assumed
Plane trees cool their surroundings even at extreme temperatures of over 39 degrees Celsius. This is shown by a new WSL study. It refutes earlier assumptions that the cooling function of trees reaches its limits at 30 to 35 degrees Celsius.
23.06.2025, 09:59
SDA
This is good news for the urban climate, the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) stated in a press release on Monday. Days with temperatures above 30 degrees are becoming more frequent. The task now is to find out which tree species are particularly good at cooling.
Trees in cities cool their surroundings by evaporating water through their leaves. This process works like sweating: Evaporation draws heat from the environment, causing the air temperature to drop.
If leaf temperatures rise above 30 to 35 degrees Celsius, photosynthesis no longer works - the leaf pores close to prevent water loss.
"Not yet fully understood"
In the summer of 2023, the research team from WSL and the Swiss Federal Institute of Technology in Lausanne (EPFL) measured how plane trees behave under heat stress. The measurements on eight trees in the Lausanne suburb of Lancy showed that the water flow in the trunks did not decrease even when it was very hot - on the contrary, evaporation actually increased as temperatures rose.
"Obviously, we have not yet fully understood how trees react to extreme conditions," study leader Christoph Bachofen is quoted as saying in the press release. The researchers suspect that, among other things, deep-seated water reserves in the soil helped the plane trees.
The actual cooling effect of urban trees during heatwaves could therefore be significantly underestimated by current predictions using conventional models, the researchers wrote in the study, which was published in the journal "Urban Forestry & Urban Greening".
