From a temperature of about 30 degrees, trees close the gap openings of their leaves. This avoids evaporating too much survival water. However, plane trees are an exception: Under certain conditions, they still release water over the leaves at temperatures above 39 degrees, as researchers have now found. With the evaporating water, the trees can cool down the surroundings over heat. This knowledge could help to better adapt cities to the ever warmer climate.
Trees not only donate shade: Via the stomata in their leaves, they also lose water that cools down the ambient air when evaporating. However, if it gets too hot – at air temperatures from around 30 to 35 degrees – trees close their gap openings to prevent them from losing too much water. The environment then cools less down in heat waves. This can become a problem, especially in cities where the air between sealed areas and dense buildings are strongly heated up.
Evaporation also at 39 degrees
Now the random find of a team led by Christoph Bachofen from the Federal Research Institute for Forest, Snow and Landscape is turning these assumptions upside down. The researchers have examined how plane trees behave at extreme temperatures. To do this, they installed sensors on eight plane trees in a place in the Geneva suburb of Lancy in Switzerland. The sensors measured the juice flow in the tree trunks and gave information about the evaporated amount of water. In addition, they measure both how much water is available in the plane trees before sunrise and at noon, as well as the temperature in the tree tops and on the pavement in the shade under the trees.
During the examination between May and October 2023, there were two heat waves with temperatures of up to almost 39 degrees. According to the common assumptions, the leaf stomata of the plane trees examined should have closed during this time. But the measurement data was surprising: Even at temperatures above 35 degrees, the eight plane trees still evaporate water. The juice flow and the loss of temperature due to evaporation even increased with increasing temperature. The temperature in the treetops was 0.86 degrees cooler than in the air.
Platans are actually considered a hydrostable tree species – that is, they close their folding openings early on during dry stress to protect themselves from water loss. But although the plane trees examined during the heat wave suffered from drought stress in August, they continued to evaporate water over the stomata and did not suffer any damage. This shows that not all tree species set their leaf evaporation at 30 degrees and more – at least planes form an exception to the rule under certain circumstances. “Obviously we have not yet fully understood how trees react to extreme conditions,” explains Bachofen.
Are plane trees of the city tree of the future?
But why did the plane trees react differently than expected in this case? The research team suspects that the groundwater near the surface at the location of the trees ensured that the plane trees have had enough water. “At drier locations, heat waves can go hand in hand with droughts in summer, which causes additional stress and possibly restricts evaporation,” explain the researchers. Accordingly, plane trees could react differently at drier locations than in the experiment in Geneva. “Nevertheless, we were able to show that current models may significantly underestimate the cooling in cities, since they do not reproduce the reactions of the folding openings to high temperatures,” explain the researchers.
In the future, other tree species could be examined in this way to find out whether you can also cool down the surroundings during heat waves if you have enough water. From this, research -based recommendations for urban planners and gardeners could derive which tree species you should plant in order to cool down cities in increasing warming through climate change.
Source: Federal Research Institute for Forest, Snow and Landscape; Specialist articles: Urban Forestry & Urban Gardening, DOI: 10.1016/J.FUG.2025.128819
