Plane Trees Cool Cities Even in Extreme Heat

(CONNECT) Even in extreme heat of over 39 degrees, plane trees evaporate water and cool their surroundings. These are the findings of a study by the Swiss Federal Institute for Forest, Snow and Landscape Research(WSL), based in Birmensdorf, and the Swiss Federal Institute of Technology Lausanne (EPFL). According to a press release, the study was carried out in spring and summer 2023 in Lancy in the canton of Geneva. This experienced two heatwaves of up to almost 40 degrees during the measurement period.

The researchers measured the sap flow on eight sycamore tree trunks. This allows conclusions to be drawn about the amount of water evaporated and thus about the cooling capacity of the trees. The water flow increased despite the increasing heat and dry air. The researchers suspect that deep-lying water reserves in the ground made the increasing water flow possible.

The study disproves the assumption that trees close their leaf pores above a temperature of 30 to 35 degrees in order to prevent water loss, meaning that the cooling effect is lost. Previous models that can be used to determine the future distribution of heat in cities would therefore be inaccurate, according to the press release.

The lasting cooling effect of the plane trees is good news for the urban climate. “Days with temperatures above 30 degrees Celsius are becoming more frequent,” says Bachofen.

“Obviously, we have not yet fully understood how trees react to extreme conditions,” explains study leader Dr. Christoph Bachofen. In future, the researchers want to measure the transpiration behavior of other urban trees in extreme heat. They want to find out which tree species can cope with heat and best fulfill their cooling function in cities. ce/yes