Plane trees cool cities even in extreme heat
Plane trees evaporate water even at over 39 degrees and cool their surroundings. This has been shown in a study by the Swiss Federal Institute for Forest, Snow and Landscape Research and the Swiss Federal Institute of Technology in Lausanne. The cooling effect of trees therefore lasts above a temperature of 30 to 35 degrees.
Even in extreme heat of over 39 degrees, plane trees evaporate water and cool their surroundings. These are the findings of a study conducted 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 conducted 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 the cooling capacity of the trees. The water flow increased despite increasing heat and dry air. The researchers assume that deep-lying water reserves in the soil enabled the increasing water flow.
The study disproves the assumption that trees close their leaf pores from a temperature of 30 to 35 degrees in order to prevent water loss, meaning that the cooling effect is lost. Previous models used to determine the future distribution of heat in cities would therefore be inaccurate, according to the press release.
The persistent cooling effect of the plane trees is good news for the urban climate. “Days with temperatures above 30 degrees Celsius are becoming more frequent,” explains study leader Dr Christoph Bachofen.
“Obviously, we have not yet fully understood how trees react to extreme conditions,” says Bachofen. In future, the researchers want to measure the transpiration behaviour of other urban trees in extreme heat. In this way, they hope to find out which tree species are able to cope with heat and best fulfil their cooling function in cities.