Brown leaves already seen in many Swiss forests

A forest fire warning sign outside Bern in August: heat and drought mean stress for the trees © Keystone / Anthony Anex

An unusually large number of trees in Swiss forests are already showing brown leaves at the end of August.

This was confirmed by ecologist Constantin Zohner on Wednesday when asked by the Swiss News Agency Keystone-SDA.

According to Zohner, the reason for this is the stress to which the trees have been exposed due to heat, drought and high UV radiation. Data on leaf discolouration for 2023 are not yet available, said Zohner, who researches the interactions between plants and their environment at the federal technology institute ETH Zurich. However, there are many trees that show clear damage to their leaves, he said.

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According to Zohner, this is not the normal course of leaf colouration, that is, the yellow or orange colours that colour the foliage in autumn. “Now many leaves are brown, some of them already dead,” Zohner said. This is due to stress damage, he said.

The scientist explains this with the interaction of several factors. On the one hand, heat and drought mean stress for the trees. On the other hand, there have been strong temperature fluctuations this summer. In addition, high UV radiation and thus high ozone levels in late summer lead to more diseases in trees.

Not a new phenomenon

The premature browning of forests in Europe is not a new phenomenon, however. A study published in March in the journal Biogeosciences showed that European forests have browned extensively in the last five years.

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According to the study, the largest browning to date occurred in the summer of 2022: it covered 37% of the temperate and Mediterranean forest regions. How well trees survive heat and drought depends not only on the current weather conditions, but also on those in the months or years before. Drought stress also favours summer browning indirectly, for example by promoting bark beetle and fungal infestation.

If this happens several years in a row, trees can die, according to Zohner. In addition, the trees stop photosynthesis when the leaves die, so they absorb less CO2.

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