Exceptional weather conditions over several years are turning forests brown, new research shows.

It was as though autumn had arrived in July. Anyone hiking through the Swiss or German forests in the summer of 2018 could literally see how the hot, dry weather in central Europe was affecting the trees.

Spruces and beech tress, in particular, withered prematurely, their leaves and needles turning brown, with entire forest stands under constant stress. In the Mediterranean region, such large-scale phenomena have already occurred several times since 2003.

Researchers have now systematically examined all low-greenness events in the temperate and Mediterranean forests of Europe over the past 21 years (2002–2022). Their findings appear in the journal Biogeosciences.

In their efforts to study forest browning across Europe, the researchers used high-resolution satellite data to identify events of large-scale reduced forest greenness in summer. Reduced greenness is a sign of reduced vitality and stress in forests and is also used as an indicator of forest dieback.

The findings underline previous observations: summer browning has spread throughout Europe. The Central European (temperate) forests have suffered particularly extensive browning in recent years. The Mediterranean region experienced major events already in the early 2000s.

In their analysis, the researchers also quantified the record summer of 2022 and its impact on European forests for the first time: during the hottest summer since records began, Europe experienced its most extensive browning yet, covering 37% of temperate and Mediterranean forest regions—”far more than any other event in the past two decades,” says lead author Mauro Hermann, a doctoral student in atmospheric dynamics under Heini Wernli, professor of atmospheric physics at ETH Zurich.

This wasn’t actually the goal that the researchers were pursuing. “We wanted to understand how the weather affects forests over a large area during several seasons,” says Wernli, who led the study.

The central role of drought was clear. “However, the connection between forests and weather is far more complex than it might appear at first glance,” he says.

“Not every dry period—even if it is intense and persistent—causes forests to turn brown immediately,” says Hermann, referring to the “legacy effect” that has been observable in our forests for a number of years. How well trees survive heat and drought depends not only on the current weather conditions, but also on those of the months or years before.

This was one reason why researchers were especially keen to look into the meteorological history of low-greenness events. They were aiming to identify characteristic weather patterns that preceded multiple of the investigated events.

Read the full article about forests in Europe by Peter Ruegg-Eth at Futurity.