This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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." 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). 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.
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