City trees provide cooling even in extreme heat

Plane trees in cities have an important cooling effect even in extreme heat, according to a new study conducted by the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) and EPFL published in Urban Forestry & Urban Greening. The next step is to identify which tree species are particularly effective at cooling.

When the sun is beating down, urban trees cool the surrounding area by evaporating water through their leaves. Given the expected increase in heat waves predicted by climate models, they play a crucial role in keeping cities pleasant places to live. However, trees also reach their limits at some point: if leaf temperatures rise above 30°C to 35°C, photosynthesis no longer works—the leaf pores close to prevent water loss.

The research team investigated how plane trees behave under extreme conditions—with surprising results: even at temperatures above 39°C, the trees continued to evaporate much more water than expected, effectively cooling the surrounding area.

Lead researcher Christoph Bachofen and his team installed measuring devices on eight plane trees in the Geneva suburb of Lancy in spring 2023. The sensors recorded the sap flow in the trunks, which allowed conclusions to be drawn about the amount of water evaporated and thus the cooling capacity. That summer, of all times, Geneva experienced two heat waves with record temperatures of almost 40°C.

How trees react to extreme conditions

Contrary to expectations, the trees did not stop their water flow—on the contrary, it actually increased as the heat rose, even though the air was extremely dry. “We clearly do not yet fully understand how trees react to extreme conditions,” explains Bachofen. The researchers suspect that deep water reserves in the soil helped the plane trees, among other things.

The extent of the water flow was surprising and raises important questions: if trees react differently to heat than previously thought, predictions about their cooling effect are also inaccurate—and the models used to predict future heat distribution in cities would be unreliable.

The fact that trees evaporate a lot of water even in extreme heat is good news for the urban climate. “Days with temperatures above 30°C are becoming more frequent,” says Bachofen.

The next important step for research is now to find out how effectively other tree species transpire in extreme heat. This would enable recommendations to be developed on which species can cope not only with air pollution and road salt, but also with extreme heat. After all, cooling is just one of the many tasks performed by urban trees—but it will become increasingly important in the future.

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