This year, the first trees were already turning brown at the end of August, several weeks earlier than usual. Repeated droughts and heat waves are putting them to the test.
Trees are turning brown earlier in the Jura mountains and on the western side of the Central Plateau, noted the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) on Monday. Early summer precipitation was below average.
The species most affected appear to be beech and hornbeam. Beech trees had already suffered significant damage from the droughts and heatwaves of 2018 and 2022. South of the Alps, the phenomenon mainly concerns lime and birch.
Lime and hornbeam trees have produced particularly large numbers of seeds this year, making them more vulnerable to drought. In addition, the ozone dose exceeds tolerable limits.
Many trees have turned brown without going through the usual stages of autumn coloration. This is problematic, as the coloring process normally allows trees to store nutrients.
While the phenomenon is not new, it is likely to become more pronounced as a result of global warming, WSL points out. It is also possible that some trees are keeping their foliage green for longer due to mild temperatures. These contrasts highlight the stress on forests.
It is still too early to say whether this phenomenon is affecting the whole of Europe.