Hot summer colors trees autumn too early


Birmensdorf ZH – Researchers at the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) have studied this year’s premature autumnal coloration of forests. One reason could be the heat wave at the end of August.

According to a media release from the Swiss Federal Institute for Forest, Snow and Landscape Research(WSL), trees at low altitudes or in sun-exposed locations in Switzerland have turned autumn colors far too early this year. A closer look also shows, for example, that many trees have turned brown without going through the typical phases of leaf discoloration in the fall. They cite wind and heat, ozone pollution, spring growth or the amount of seed produced as possible reasons.

North of the Alps, beech trees already suffered severely from drought and heat in 2018 and 2022 and showed premature discoloration then as well. The effects of such extreme events could last for several years. For example, in the Mendrisio region of southern Ticino, most deciduous trees have wilted since the end of August 2022.

Especially in sunny summers, but with more or less regular precipitation as in 2023, natural vegetation develops a number of characteristic symptoms, including reddening, yellowing and browning as in beech or hazel, WSL says. These symptoms can be observed throughout Switzerland this year, he said.

According to WSL, the phenomenon of premature discoloration is not new and always occurs when a severe summer drought is combined with a heat wave, as occurred in 1947, 1976, 2003, 2015, 2018, and 2022. It will probably occur more frequently in the future, he said. To keep an eye on such occurrences, researchers across Switzerland have established networks to monitor tree phenology. They are coordinated by MeteoSuisse, WSL, the Swiss Rangers and Globe via the participatory science platform ce/gba