Last year, 37 per cent of Mediterranean and central European forests were affected by tree browning.
Droughts and heatwaves are causing Europe’s forests to turn brown in the summer months.
Last year, 37 per cent of Mediterranean and central European forests were affected, a new study reveals.
Researchers from ETH Zurich University and the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) have been examining the phenomenon over the past 21 years.
The results, published in the journal Biogeosciences, show that summer forest browning is spreading across Europe.
What’s behind summer forest browning in Europe?
Using high-resolution satellite data, researchers identified periods where areas of forest were not as green as they should be during summer.
While drought played a central role in the browning of forests, its effects were not immediate.
Researchers noted a “legacy effect” of intense and persistent dry periods, meaning that the ability of trees to survive heat and drought depends not only on the current weather but on that of previous months and years too.
Delving into the history of low-greenness events, researchers identified weather signals that affected trees years down the line. In particular, frequent periods of little rain over the course of two to three years were a precursor for summer browning.
Frequent periods of high temperatures for at least two years in temperate zones also had a significant impact.
“Prior to low forest greenness in central Europe, we usually observed two dry, hot summers in a row,” says Mauro Hermann, ETH doctoral student and lead author of the study.
Drought also fosters bark beetle and fungal infestations, as well as forest fires - all of which may indirectly contribute to browning - the researchers note.
Which parts of Europe are worst affected by forest browning?
The Mediterranean has been suffering from summer forest browning since the early 2000s.
In recent years, the problem has spread to temperate forests in central Europe.
With 2022 the continent’s hottest summer on record, Europe experienced its most extensive browning yet with more than a third of forests affected in these regions.
This is “far more than any other event in the past two decades,” according to Hermann.
What does this browning mean for Europe’s forests?
Reduced greenness is a sign of reduced vitality and increased stress in forests. It also indicates forest dieback.
In the past, hot, dry summers were less frequent in Europe. Following the record heatwave in 2003, the colour of Europe’s forests was hardly affected.
But since 2018, repeated large-scale droughts and high temperatures have led to extensive browning.
With a repeat of 2022’s record-breakingly hot and dry summer on the cards this year, forest browning could become even more widespread in Europe.
Germany and Switzerland’s spruces and beech trees, in particular, withered prematurely, as entire forests buckled under constant heat and drought stress.
Since weather might not be the only contributor to the phenomenon, the researchers emphasise that it cannot be used to predict the future - but it might offer clues.
“Targeted monitoring of the weather conditions over several seasons could provide valuable information as to whether premature leaf discolouration is likely to occur the following summer,” says Thomas Wohlgemuth, head of the Forest Dynamics research unit at WSL and co-author of the study.
Forest management could help curb forest browning as temperatures rise, he adds.