Study: trees have major cooling effect even in extreme heat

Even in extreme heat above 39°C, plane trees continue to evaporate large amounts of water, thereby cooling their surroundings, the Swiss study published on Monday found. Keystone-SDA

Plane trees in cities have an important cooling effect even in extreme heat, according to a new study by the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL). This challenges earlier assumptions that the cooling effect of trees reaches its limits at 30-35° degrees Celsius.

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Even in extreme heat above 39°C, plane trees continue to evaporate large amounts of water, thereby cooling their surroundings, the study published on Monday found.

This is good news for urban areas. Days with temperatures above 30°C are becoming more frequent. The next important step is to find out how effectively other tree species transpire in extreme heat.

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 30-35°C 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 Federal Institute of Technology Lausanne (EPFL) measured how plane trees behave under heat stress. The measurements on eight trees in the Geneva suburb of Lancy showed that the water flow in the tree trunks did not decrease even when it was very hot – on the contrary, evaporation actually increased as temperatures rose.

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“Obviously, we have not yet fully understood how trees react to extreme conditions,” said study leader Christoph Bachofen. The researchers suspect that, among other things, deep-lying 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 said in the study, which was published in the journal Urban Forestry & Urban Greening.

Translated from German with DeepL/sb

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