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Swiss study sheds light on how heat affects photosynthesis

beech leaves
Researchers examined beech (pictured), spruce, sessile oak and small-leaved lime trees. Keystone

The capacity of trees to carry out photosynthesis is reduced when temperatures climb above 30°C, according to a study by the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL).

In all species looked at for the study, researchers found that the uptake of CO2, by means of which the tree produces sugar from sunlight during photosynthesis, decreases at temperatures above 30°C, while water loss through transpiration continues to increase.

Inefficient photosynthesis over a long period can seriously compromise the growth, development and adaptability of trees and plants – and ultimately have repercussions for the entire forest ecosystem, according to the WSL.

+ Read more: how Swiss forests are adapting for the future

The scientists also showed that trees reduce their uptake of CO2 even when there is sufficient CO2 in the air. The study, published in the New Phytologist journal, calls into question the traditional hypothesis that reduced photosynthesis at high temperatures is due to reduced availability of CO2.

“This suggests a limitation in the biochemistry of trees from around 30°C onwards,” said Marco Lehmann, head of the study, quoted in a WSL press release. This limitation appears to be due to an alteration in the enzymatic processes involved in photosynthesis, he added.

Experimental installation

In their study, the researchers examined beech, spruce, sessile oak and small-leaved lime trees, all of which reacted in the same way.

According to WSL, the study was enabled by a new experimental facility that enabled the plants to be subjected to different environmental conditions under controlled conditions and to monitor their behaviour by means of gas exchange and isotope measurements.

Adapted from French by DeepL/dos

This news story has been written and carefully fact-checked by an external editorial team. At SWI we select the most relevant news for an international audience and use automatic translation tools such as DeepL to translate it into English. Providing you with automatically translated news gives us the time to write more in-depth articles.  If you want to know more about how we work, have a look here, and if you have feedback on this news story please write to

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