The stakes are unusually concrete. Protective forests cover roughly 540,000 hectares, about 44 percent of Switzerland’s total forest area, and represent the country’s most important biological defense against natural hazards. Their ability to shield people, villages and infrastructure from avalanches, rockfall, landslides, floods and debris flows depends on structural traits such as stand basal area, stem density and canopy cover. A forest that thins out, loses trees or develops canopy gaps can no longer intercept falling rocks or slow sliding snow with the same reliability. In steep mountain terrain, where these forests are often the only thing standing between a hazard source and a damage potential, structural continuity is not an aesthetic preference but a functional requirement.

To track how these forests have fared, researchers led by Estelle Noyer of Bern University of Applied Sciences, working with colleagues at the Swiss Federal Institute for Forest, Snow and Landscape Research, combined two exceptionally long datasets. The first was the Swiss National Forest Inventory, a systematic permanent sample grid covering the entire country, which has measured every tree with a stem diameter above 12 centimeters on thousands of circular plots since 1983. The second was a satellite-derived vegetation index, the Normalized Difference Moisture Index, computed from Landsat imagery between 1984 and 2022, which captures the moisture status of the forest canopy. By pairing ground-based demographic measurements with canopy-level remote sensing across five altitude belts, from the warm lowland colline zone to the harsh subalpine superior belt above 2,000 meters, the team could follow both the structural evolution of individual forest stands and the moisture resilience of their canopies through four successive inventory periods.

The technical machinery behind the analysis is worth unpacking, because it reveals how drought stress was quantified. The researchers computed a drought stress index from a Thornthwaite-type monthly water balance model, using gridded temperature and precipitation data at 100-meter resolution. The model partitions precipitation into interception and infiltration, estimates potential evapotranspiration from a heat index corrected for sun angle, day length and local topography, and then compares actual evapotranspiration against evaporative demand. The resulting index, expressed as one minus the ratio of supply to demand, rises as soil moisture falls short of what the atmosphere pulls away. Extreme drought episodes were flagged whenever the index exceeded 1.5 standard deviations above each site’s long-term mean. By this measure, the years 2003, 2006, 2015, 2018 and 2022 stand out, with more than 30 percent of all studied stands affected, including those in the highest subalpine zones. Canopy responses to these episodes were then decomposed into the three classic resilience components of the Lloret framework: resistance, or how little the canopy suffered during the drought; recovery, or how quickly it bounced back afterward; and overall resilience, the ratio of post-drought to pre-drought canopy condition.

The headline finding is a progressive disappearance of the altitude gradient in canopy resilience. In the earliest inventory periods, growing stands at low elevations showed higher resistance and resilience than their high-elevation counterparts, exactly as the classical picture predicted. But over the four decades, the significant influence of altitude on the resilience components vanished in a specific sequence: first overall resilience, then resistance, and finally recovery. By the most recent period, no altitude belt could be statistically distinguished from any other in canopy response to extreme drought. The interpretation is sobering. Trees across the entire gradient appear to have converged toward similar levels of drought acclimation, not because lowland forests improved, but because the legacy effects of repeated extreme drought episodes now exceed the recovery capacity of trees everywhere, from the warmest valleys to the coldest ridgelines.

Declining stands, those losing basal area over time, told an even sharper story. These stands showed no altitude influence on any resilience component in any period, suggesting that once fundamental drought resilience thresholds are surpassed, stand decline proceeds largely independently of elevation. Intriguingly, the specific resilience component signaling decline shifted over time. In earlier periods, declining stands were marked by lower resistance, meaning their dominant trees were inherently vulnerable to the initial drought shock. In the most recent period, the marker switched to lower recovery, suggesting a progressive compositional adaptation in which the most drought-vulnerable individuals were filtered out, leaving stands dominated by trees that resist the initial stress but grow progressively fatigued under repeated episodes. This adaptation, however, proved insufficient to prevent continued decline, indicating that the underlying threshold had already been exceeded even as stand composition shifted toward more drought-tolerant trees.

The structural mechanics of decline are equally revealing. Across all growing stands, the relative stand basal area increment decreased over time, but the driver was not a reduction in average individual tree growth. Instead, the loss was traced to accelerating net stem density losses, with the average net stem density increment falling from 2.84 percent per year in the earliest period to 1.92 percent in the most recent. Drought stress, in other words, reshapes individual tree growth drivers before inducing stand-scale reorganization. The growth dominance coefficient, a metric that quantifies whether growth is allocated proportionally across tree sizes or asymmetrically toward the largest individuals, proved decisive. Growing stands maintained stable, slightly negative coefficients, indicating a structural strategy in which growth is not exclusively allocated to the biggest trees. Declining stands, by contrast, initially exhibited positive coefficients, meaning growth was concentrated in large individuals, a configuration that left them vulnerable when drought arrived, because large trees carry higher water demand and maintenance costs.

One localized exception stands out. In the highest subalpine superior belt, declining stands showed a structural reorganization in the two most recent periods, with growth dominance coefficients dropping to strongly negative values alongside higher average individual tree growth rates. This suggests that in those stands, growth became increasingly reliant on smaller, potentially more drought-tolerant individuals better positioned to maintain growth under repeated stress. Whether this localized response represents a viable long-term recovery pathway or merely a delayed manifestation of the decline trajectory already visible at lower elevations remains an open question. What is clear is that drought stress reshaped the functional relationships between tree size, canopy resilience and individual growth well before any stand-scale structural shift became detectable, a sequential, multi-scale process with direct implications for how forest managers should intervene.

The implications for Switzerland’s hazard defenses are significant. The proportion of declining stands increased over time across all altitude belts, with the strongest and most consistent trend at low elevations, where the colline belt saw declining stands rise from 11.1 percent in the earliest period to 30.4 percent in the most recent. Overall stand basal area increment fell by roughly 30 percent in the colline belt and 18 percent in the subalpine superior belt between the first and last periods. Because stand basal area is a well-established proxy for the protective effect against rockfall, avalanches and shallow landslides, these declines translate directly into a potentially diminishing protective function. The low rate of tree regeneration could critically jeopardize the stability and resilience of these forests in the longer term, a conclusion that aligns with recent simulation-based studies predicting critical declines in stand basal area and protective effect, particularly at lower elevations.

The authors argue that ensuring the long-term protective function of these forests will require proactive, spatially differentiated management. Regular silvicultural interventions can promote regeneration and accelerate the transition toward drought-tolerant tree cohorts, a conclusion supported by the finding that higher initial stem density and lower individual tree diameter favored growth under drier conditions. Prioritizing the maintenance of sufficient stem density and structurally diverse, multi-layered stands would enhance local microclimate buffering under intensifying drought. The study is not without limitations: the water balance model does not explicitly account for snowpack accumulation or delayed snowmelt, potentially overestimating water stress in subalpine zones, and the satellite record carries calibration uncertainties near sensor transitions, most notably around the 2003 Landsat 7 failure. Tree species composition was also not analyzed, leaving open whether the observed compositional adaptation reflects within-species selection or turnover toward more tolerant species. Yet the consistency of the patterns across four decades of inventory data, multiple independent drought episodes and the full altitude gradient lends the central conclusion considerable weight. The historical advantage of altitude against drought is diminishing, and the protective forests of the Swiss Alps, the living infrastructure that has quietly guarded mountain communities for generations, now face a future in which no elevation is safe from the consequences of a drying climate.

Subject of Research: Long-term drought impacts on the structure and resilience of Swiss protective forests along an altitudinal gradient

Article Title: Four decades of drought impacts on Swiss protective forests: a vanishing altitudinal gradient

Article References: Noyer, E., Dorren, L., Allgaier Leuch, B., & Moos, C. (2026). Four decades of drought impacts on Swiss protective forests: a vanishing altitudinal gradient. Frontiers in Forests and Global Change, 9, Article 1933304. https://doi.org/10.3389/ffgc.2026.1933304

Image Credits: AI Generated

DOI: 10.3389/ffgc.2026.1933304

Keywords: Swiss protective forests, drought, altitudinal gradient, forest resilience, NDMI, National Forest Inventory, stand basal area, stem density, tree growth, adaptive forest management, natural hazards, climate change