City trees provide cooling, even in extreme heat

Even during heat waves with temperatures of over 39°C, plane trees continue to evaporate large amounts of water and help cool the surrounding area. This was demonstrated by researchers from WSL and EPFL thanks to measurements in the Geneva suburb of Lancy. They used sensors to record the sap flow in the trunks, which allowed conclusions to be drawn about the amount of water evaporated and thus the cooling capacity. The results refute earlier assumptions that trees close their leaf pores so much at around 30–35°C that the water flow and cooling capacity are significantly reduced. The next step is to find out how other tree species react to extreme heat. Cooling capacity is one of the tasks performed by urban trees – it will become increasingly important in the future.