Due to periods of drought and heat, European forests tend to turn brown during the summer months. A new study has revealed that last year, 37% of forests in the Mediterranean and Central Europe were affected by this situation.
Researchers from ETH Zurich and the Swiss Federal Institute for Forest, Snow and Landscape Research (Swiss Federal Institute for Forest, Snow and Landscape Research, WSL) have studied this phenomenon over a 21-year period.
The results of the study, which have been published in the journal Biogeosciences, indicate that forest browning in summer is spreading across Europe.
What causes forest browning in Europe?
Using high-resolution satellite data, the researchers identified periods where forest areas in summer were not as green as they should be.
Although drought had the most significant impact on forest browning, its effect was not immediate. The researchers found a "legacy effect" of intense and prolonged drought periods, meaning that the ability of trees to survive heat and drought depends not only on current conditions but also on weather experienced in previous months and years.
By studying the history of browning incidents, researchers identified weather patterns that affected trees for several consecutive years. Firstly, frequent periods of low precipitation (over two to three years) were a precursor to summer browning. Secondly, frequent periods of high temperatures (over at least two years) in temperate zones also had a significant impact.
“Before forest browning in Central Europe, we usually observed two hot and dry summers in a row,” says ETH doctoral student and the study’s lead author, Mauro Hermanis.
Drought also encourages bark beetle and fungal infestations, as well as forest fires – all of which can indirectly contribute to browning, the researchers note.
What does browning mean for European forests?
Reduced forest greenness indicates a decline in vitality and increased stress. It also points to forest dieback. In the past, hot and dry summers were observed less frequently in Europe. Following the record-breaking heatwave of 2003, there was almost no change in the colour of European forests. However, since 2018, repeated long periods of drought and high temperatures have caused widespread browning.
If the record-breaking hot and dry summer of 2022 repeats, forest browning in Europe could become even more widespread. Spruce and beech trees in Germany and Switzerland are dying prematurely, as the forests have been severely affected by prolonged heat and drought stress.
As weather may not be the only driver of browning, the researchers emphasise that it cannot be used to predict the future, although it could provide clues.
“Monitoring weather conditions over several seasons could provide valuable information on whether premature leaf discolouration is likely next summer,” explains Thomas Wohlgemuth (Thomas Wohlgemuth), head of the Forest Dynamics research unit at WSL and co-author of the study.
He adds that forest management could help limit forest browning as temperatures rise.