Even high-altitude forests in Switzerland are now weakened by frequent droughts, making natural hazards more dangerous
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Due to global warming, Swiss forests are now at a tipping point where summer droughts are overwhelming trees’ tolerance and resilience, scientists have shown. That bodes ill for people in the valleys because forests form their first line of defense against avalanches, rockfalls, landslides and floods.
“Here we show that it no longer holds true, as was until recently the case, that trees growing at the highest altitudes are mostly shielded from the worst effect of summer droughts,” said Dr. Estelle Noyer, a researcher at Bern University of Applied Sciences and the first author of a new study in Frontiers in Forests and Global Change.
“Even as high as at 2,200 meters (7,220 feet) altitude, the number of trees per stand is reduced due to the decline or death of large, dominant trees in particular. This causes a progressive decline of these precious forests which protect against natural hazards.”
On a slope to drought
Even before climate change began to make itself felt, forests were vulnerable to summer drought. Historically, forests in low-altitude vegetation belts like the “colline” belt (between 282 meters (925 feet) and 1,184 meters (3,880 feet) above sea level) and the “montane” belt (600 meters (1,970 feet) to 1,648 meters (5,410 feet)) were the most prone. Cold temperatures and long-lasting snow cover kept soils moist and limited trees’ demand for water in the highest, “subalpine” belt, between 1,235 meters (4,050 feet) and 2,219 meters (7,280 feet).
Subalpine trees can also suffer from drought, but it was previously thought that higher temperatures due to global warming gave them a net growth boost, helping them recover faster. Over recent decades, however, rapid warming at intermediate and high altitudes has made trees “thirstier.” At the same time, a drop in precipitation and the retreat of glaciers have cut the water supply to soils, making droughts more frequent.
Noyer and colleagues wanted to know whether the altitudes at which drought stress commonly strikes in Swiss protective forests have shifted over the past four decades and whether this is affecting the resilience of the canopy and the makeup of forest stands. They analyzed records gathered between 1983 and 2022 by the Swiss national forest inventory (NFI), which lists all stands across a nationwide grid.
Foresters regularly measured the diameter at breast height of all trees across hundreds of representative 200-square-meter plots. Noyer and colleagues focused on protective forest stands exclusively in areas that hadn’t been damaged by fires or undergone interventions like selective logging. For each of five time periods within the NFI, they looked at 891 to 1,543 tree stands at altitudes between 282 meters (925 feet) and 2,219 meters (7,280 feet).
Growing problems
The results showed that the percentage of declining stands increased over time in every belt, but especially at lower altitudes. The strongest decline was in the colline belt, where the share of protective stands classified as “declining” increased from 11.1% in 1983 to 30.4% by 2022. In the lower montane belt, it rose from 17.4% to 21.4% but stayed approximately constant at around 10.8% in the higher montane belt.
In the highest belts, the “subalpine inferior” (1,208 meters (3,960 feet) to 1,897 meters (6,220 feet)) and “subalpine superior” (1,752 meters (5,750 feet) to 2,219 meters (7,280 feet)), the percentage of declining stands dipped to its lowest values, between 10.3% and 15.3%, from the mid-1990s to the early 2000s before rising sharply to 18.8% in the years since then.
The density of trees decreased in all belts over time. As expected, declining stands typically displayed the highest drought indices and the strongest drop in tree basal area, indicating more severe drought stress and shrinking tree cover. But the historically strong link between higher altitude and less severe drought stress was no longer visible after 2017.
The authors concluded that the composition of stands in Swiss protective forests changed over time as droughts selectively killed the largest and most vulnerable trees, even at the greatest heights. As a result, today’s stands appear to contain more resilient tree species and size classes, though they may be weakened when droughts occur in quick succession, as is increasingly common.
Noyer and colleagues believe there is a pressing need for adaptive forestry strategies, not only in Switzerland but throughout European mountain ranges.
“The critical open questions center on how rapidly forest species composition will change, and how foresters can make forests more resilient by promoting multi-layered and diverse stands. Our ongoing follow-up study is expected to yield actionable spatial data to help foresters achieve this,” said Noyer.
More information
Four decades of drought impacts on Swiss protective forests: a vanishing altitudinal gradient, Frontiers in Forests and Global Change (2026). DOI: 10.3389/ffgc.2026.1933304
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Even high-altitude forests in Switzerland are now weakened by frequent droughts, making natural hazards more dangerous (2026, October 1)
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