Due to global warming, Swiss forests are now at a tipping point where the tolerance and resilience of trees against summer droughts are being overwhelmed, 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 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 was making itself felt, forests were vulnerable to drought every summer. Historically, forests at low altitude vegetation belts like the ‘colline’ (between 282 and 1,184 meters above sea level) and ‘montane’ (600 to 1,648 meters) belts were the most prone, as cold temperatures and a long-lasting snowpack kept soils moist and limited trees’ demand for water in the highest, the ‘subalpine’ belt between 1,235 and 2,219 meters.
Subalpine trees can also suffer from drought, but it was previously thought that higher temperatures due to global warming are giving these a net growth boost, making them recover faster. But over recent decades, 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 if the altitudes at which drought stress commonly strikes in the Swiss protective forests have shifted over the past four decades, and whether this is impacting the resilience of the canopy and the make-up of forest stands. They analyzed records gathered between 1983 and 2022 by the Swiss national forest inventory (NFI), which lists all stands across a nation-wide grid.
Foresters regularly measured the diameter at breast height of all trees across hundreds of representative 200-square-meter plots. Noyer and colleagues here 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 between 282 and 2,219 meters altitude.
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 seen in the colline belt, where the ‘declining’ share among protective stands increased from 11.1% in 1983 to 30.4% by 2022. In the lower montane belt, this rose from 17.4% to 21.4%, but stayed approximately constant around 10.8% in the higher montane belt. And in the highest belts, the ‘subalpine inferior’ (1,208 to 1,897 meters) and ‘subalpine superior’ (1,752 to 2,219 meters), the percentage of declining stands dipped to its lowest value between 10.3% and 15.3% in the mid-1990s to early 2000s, before rising sharply to 18.8% over 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 a shrinking tree cover. But importantly, the historic 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 the Swiss protective forests changed over time, as the most vulnerable and largest trees were selectively killed off by droughts, even at the greatest heights. As a result, today’s stands appear to be composed of more resilient tree species and size classes – but which may be weakened when drought episodes occur in quick succession, as is increasingly common.
Noyer et al believe that there is a pressing need for adaptive forestry strategies, not only in Switzerland but throughout European 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.
Frontiers in Forests and Global Change
Observational study
Not applicable
Four decades of drought impacts on Swiss protective forests: a vanishing altitudinal gradient
1-Oct-2026
The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest