Dr. Peter Bebi

• The proportion of protective forest with little regeneration has risen again in the past decade as protective forests have become denser and browsing by wild ungulates has remained high.
• According to the National Forest Inventory, 44% of Swiss forest is classed as protective forest, meaning that it protects people and physical assets from natural hazards such as landslides, avalanches, rockfall or debris flows.
• Increasing disturbances due to climate change will reduce the protective effect of forests in some areas. In order to minimise such adverse effects, the resilience of protective forests must be enhanced.

• La part de forêts protectrices avec peu de régénération a encore augmenté durant la dernière décennie du fait de leur densification et d'un abroutissement
toujours intense des ongulés.
• Selon l'Inventaire forestier national (IFN), 44 % des forêts suisses sont des forêts protectrices. Elles protègent la population et les infrastructures contre les dangers naturels tels que glissements de terrain, avalanches, chutes de pierres ou laves torrentielles.
• Par endroits, les perturbations accrues causées par les changements climatiques affaibliront leur effet protecteur. Pour limiter au maximum leur impact, la résilience de ces forêts doit être augmentée.

• Nell'ultimo decennio, i boschi di protezione con scarsa rinnovazione sono nuovamente aumentati perché sono diventati più fitti e il brucamento da parte degli ungulati è rimasto a un livello elevato.
• Secondo l'Inventario forestale nazionale il 44 per cento della superficie forestale è costituita da boschi di protezione che proteggono le persone e i beni materiali da pericoli naturali come scivolamenti, valanghe, cadute di massi o colate di detriti.
• Il crescente impatto del cambiamento climatico ridurrà in parte l'azione protettiva dei boschi. Per contenere il
più possibile questi effetti negativi, occorre incrementare la resilienza dei boschi di protezione.

• In der letzten Dekade hat der Anteil des Schutzwaldes mit wenig Verjüngung erneut zugenommen, weil die Schutzwälder dichter geworden sind und der Verbiss durch Schalenwild auf hohem Niveau verharrte.
• 44 % des Schweizer Waldes sind gemäss Landesforstinventar Schutzwald. Er schützt Menschen und Sachwerte vor Naturgefahren wie Rutschungen, Lawinen, Steinschlag oder Murgängen.
• Die zunehmenden Störungen aufgrund des Klimawandels werden die Schutzwirkung des Waldes gebietsweise mindern. Um solche Beeinträchtigungen möglichst gering zu halten, muss die Resilienz der Schutzwälder erhöht werden.

Sustainable regeneration of silver fir (Abies alba Mill.) is important for maintaining and improving the future resilience and protective capacity of mountain forests in the southern Swiss Alps. However, increasing densities of wild ungulates, such as red and roe deer, can hamper the regeneration success of silver fir. A comprehensive understanding of forest–wildlife interactions is essential to plan concrete measures for improving the adaptation potential of mountain forests. Here, we monitored the dynamics of natural silver fir regeneration in mountain protective forests. This study encompassed 30 unfenced permanent plots across two sites, Lostallo and Soazza, located in Valle Mesolcina in the southern Swiss Alps. These sites were selected for their comparable ecological characteristics related to silver fir regeneration, yet contrasting densities of mature, seed-bearing silver fir trees. We recorded the number of seedlings (<10 cm height) and saplings (10–200 cm height), and repeatedly assessed height growth, frequency, and intensity of ungulate browsing on saplings in the summers 2016, 2019, and 2022. In 2022, we additionally recorded the presence of browsing on other tree species in the unfenced plots and the height growth increments of silver fir saplings in pre-existing fences. On average, 509 silver fir seedlings/ha were present in the study area, but there was a lower density of saplings (382 saplings/ha). Between 2016 and 2022, the browsing frequency on silver fir saplings increased from 33 per cent to nearly 100 per cent, accompanied by a significant rise in browsing intensity. Furthermore, sapling height growth from 2019 to 2022 was significantly reduced or even negative in the unfenced plots as compared to the growth of saplings in the fenced plots. The observed seedling density indicated a good potential for natural regeneration of silver fir, irrespective of the differences in the density of seed-bearing trees between the two sites. However, the increased ungulate browsing impaired successful regeneration in the considered mountain forests in the southern Swiss Alps. Thus, without an effective control of ungulate browsing, silver fir may not be able to substantially contribute to the transition to more resilient and future-proof mountain forests.

Die prognostizierte Zunahme von extremen Störungsereignissen wie Borkenkäferbefall und Windwurf im Kontext des Klimawandels stellt die Schweizer Waldbewirtschaftung vor grosse Herausforderungen. Besonders betroffen sind fichtendominierte Gebirgswälder, die grossflächig verbreitet sind und häufig eine zentrale Schutzfunktion gegenüber Naturgefahren erfüllen. Die WSL erarbeitet zusammen mit der Praxis Grundlagen für die Priorisierung der Bewirtschaftung dieser Wälder unter dem Einfluss von extremen Störungen. Dazu werden räumliche Daten zu Waldstruktur, Standort, Störungsanfälligkeit und Naturgefahren auf Landschaftsebene mit waldbaulichen, planerischen und ökonomischen Aspekten verknüpft und in einem interaktiven Dashboard visualisiert. Dieser inter- und transdisziplinäre Ansatz wird hier anhand eines Fallbeispiels im Schilstal (Kanton St. Gallen) vorgestellt.

Management of spruce-dominated mountain forests in the context of extreme disturbances
The predicted increase in extreme disturbance events, such as windthrow and bark beetle infestations in the context of climate change poses major challenges for Swiss forest management. Mountain forests dominated by Norway spruce, which are both widespread and often fulfil an essential protective function against natural hazards, are particularly at risk. In response, the WSL is collaborating with practitioners to develop a framework for prioritising the management of these forests under the influence of extreme disturbances. This involves integrating spatial data on forest structure, site conditions, susceptibility to disturbance and natural hazards at the landscape level with silvicultural, planning and economic aspects. These insights are visualised through an interactive dashboard. This interand transdisciplinary approach is presented here with a case study from the Schilstal in the Canton of St. Gallen.

Gebirgswälder erfüllen zahlreiche Funktionen. Sie sind Lebensraum für Pflanzen- und Tierarten, tragen zur Kohlenstoffspeicherung bei, laden zur Erholung ein und liefern Bau- und Energieholz. Unverzichtbar ist ihre Rolle als natürlicher Schutz vor Steinschlag, Lawinen und Erdrutschen. Seit dem Forstpolizeigesetz von 1876, das den Grundsatz der Nachhaltigkeit einführte, haben Fläche und Dichte der Bergwälder zugenommen. Heute sind diese Wälder im Rahmen des globalen Wandels einer neuen Dynamik unterworfen, in welcher wärmere Temperaturen, Dürren und Winterstürme, aber auch Holzpreise grosse Herausforderungen darstellen, die Waldfunktionen zu erhalten.

Forests are under pressure and going through rapid changes. However, current inventorying and monitoring (IM) programs are often either disjointed, too narrow in their scope and/or do not operate at fine enough temporal resolutions, which may hinder scientific understanding, the timely supply of information, fast decision making, and may result in the sub-optimal use of resources. For these reasons, there is an urgent need for Advanced Forest Inventorying and Monitoring (AIM) programs to (i) achieve expanded relevance (by augmenting data/information across ecosystem properties and trophic levels), (ii) have increased temporal resolution (by tailored data collection frequency), and (iii) make use of technological advances (by incorporating novel tools and technologies). The Advanced Inventorying and Monitoring for Swiss Forests (SwissAIM) initiative was launched in 2020 to address these needs. SwissAIM builds upon the foundation offered by the existing programs (e.g., national forest inventory, long-term forest ecosystem research, biodiversity monitoring). It aims to offer a collaborative and adaptive framework to enable integrated data collection, evaluation, interpretation, analysis, and modeling. Ideally, it will result in a more responsive system with respect to current and predicted biotic/abiotic stressors that will challenge Swiss forests. Developing such a system implies identifying the information needs of different stakeholders (e.g., science, policy, practice), related technical requirements, and governance frameworks. Here, we present (i) the main features of the SwissAIM initiative (vision, scientific questions and variables, governance and engagement), (ii) the main outcomes of the participatory design process (measurements, sampling, and plot design), (iii) the potential transferability of AIM initiatives outside Switzerland (timing, relevance, practicability), and (iv) the key messages that emerged (i.e., need for advancement, integration and transdisciplinarity, statistical underpinning). Since similar needs related to forest inventorying and monitoring are emerging throughout Europe and elsewhere, the objective of this opinion paper is to share our experience and promote a dialog with those interested in developing AIM initiatives in other countries and regions.

Succès mitigé malgré un poteltial élevé: une étude de plusieurs années dans les forêts de protection du Val Mesolcina (GR) montre que la régénération naturelle du sapin est possible, mais que l'abroutissement important par le gibier empêche les plantules de croître.

Wenig Erfolg trotz hohem Potenzial: Eine mehrjährige Studie zeigt, dass die natürliche Verjüngung der Weisstanne in den Schutzwäldern in Misox (GR) grundsätzlich möglich ist, dass aber starker Wildverbiss einen erfolgreichen Aufwuchs verhindert.

Qua o là sta amo ün tal eroe plain fideltà sün seis post, cun büst sfrattamà, e stenda il bratsch our da spelm e grava, sco üna sgür pronta pel cumbat, o ch'el sta ingramüschà sco ün vegliet cha la burasca da la vita ha fat sgobar sün sias crotschas.

Understanding the dynamics of treeline ecotones under global change requires long-term ecological and environmental data. The Stillberg ecological treeline research site in the Swiss Alps was established in 1975 by planting 92,000 seedlings of Larix decidua, Pinus cembra and Pinus mugo ssp. uncinata, and has been continuously monitored since then. Here, we present curated long-term data acquired over almost 50 years at the Stillberg site, and we synthesise the major research findings. The long-term datasets comprise 6.5 million ecological and environmental records from the 40-year afforestation experiment, as well as from a 9-year free-air CO₂ enrichment experiment crossed with a 6-year soil warming experiment, a 12-year nutrient addition experiment and an 8-year multifactorial tree seedling recruitment experiment. Our datasets further include 38 million records of 25 meteorological parameters measured at an hourly resolution from 1975 to 1996, and at a 10 min resolution since 1997. We provide all datasets and the corresponding metadata as open research data. Almost five decades of research in this treeline ecotone showed high mortality after tree establishment that was closely related to microclimatic variability. The two Pinus species survived at a much lower rate than L. decidua, due to indirect pathogen interactions. Furthermore, CO₂ enrichment only increased growth of L. decidua, while warming increased growth of P. mugo ssp. uncinata and two Vaccinium shrub species. Enhanced nutrient availability stimulated growth in tree and understorey shrub species. In addition, soil warming and CO₂ enrichment stimulated microbial activity and decreased soil carbon stocks. These findings improve our understanding of ecological processes in the treeline ecotone under global change and confirm the importance of tree growth and establishment limitations. The enhanced availability and quality of these long-term data are expected to foster whole-system approaches and transdisciplinary research syntheses, supporting the development of effective global change adaptation strategies.

A continuous upward shift of the upper treeline is observed in many mountain regions worldwide. Such forest expansion is causing significant impacts on ecosystem services, especially as it may reduce avalanche hazards. Forests mitigate the release of snow avalanches mainly through snowpack stabilization. Trees prevent the creation of continuous weak layers thus decreasing the probability of slab release. Forest density, size of forest gaps, tree height and tree species are key factors to estimate the forest protective effect against snow avalanches. Field observations indicate that a certain tree height is crucial for the prevention of avalanche release in young forest stands and that tree height should be at least one-and-a-half to two times taller than the snow cover, with required tree height increasing with slope inclination. In this study, we analysed the relationship between avalanche occurrence, tree height and snow height within the long-term afforestation research area Stillberg near Davos, Switzerland. A decreasing number of avalanche releases was observed in relation to changes in tree height in the afforestation during the last 49 years. Furthermore, the spatial distribution of avalanche release areas shifted from covering most of the studied area of Stillberg to three local avalanche release areas within gullies. These gullies were characterized by lower tree densities but larger tree heights and larger snow accumulation. The analysis of all avalanches at Stillberg indicated that the ratio of tree height to snow height had contrasting effects on the release of loose snow avalanches and slab avalanches; a high ratio hindered the formation of slab avalanches, but under certain conditions some loose snow avalanches released. Areas prone to loose snow avalanches typically have few or small Larix and lack evergreen trees. Overall, the Stillberg afforestation after almost 50 years, together with local temporary protection measures for the gullies could be effective in mitigating avalanche activity.

Natural disturbances play an important role in shaping the dynamics of mountain forests, yet their effects on essential ecosystem services, such as protection against natural hazards, can be significant. With the challenges posed by climate change and increasing disturbances, as well as the complexities of salvage logging, there is a growing interest in understanding post-disturbance development in unsalvaged mountain forests, alongside the advancement of decision support systems aimed at ensuring sustained provision of ecosystem services. In this study, we combined a space-for-time substitution approach with long-term monitoring data to evaluate regeneration processes and development of deadwood decay following three distinct windthrow events in Central European mountain forests that were locally unsalvaged (specifically Vaia in 2018, Kyrill in 2007, and Vivian in 1990). Our unique dataset additionally provided insights into the long-term effects of disturbance legacies and tree regeneration on protection against natural hazards. Deadwood cover gradually decreased with time since disturbance, from an average of 50% two years after Vaia to 25% twelve years after Kyrill and 15% thirty years after Vivian. Similarly, deadwood height above ground significantly decreased over time, with median values dropping from 1 to 2 m immediately after the disturbance to 25–30 cm three decades later. The decay stage and diameter of deadwood significantly influenced tree regeneration, with larger diameters of logs and deadwood in more advanced decay stage (especially less solid/soft to very loose stage), facilitating seedling establishment, thus a second wave of tree regeneration. About a quarter of saplings grew on deadwood thirty years after disturbance. The analysis of post-windthrow stand development showed an increase in tree cover and height with time since disturbance, with distinct patterns observed across different windthrow events and sites. Three decades post-disturbance, the number of trees had notably increased, with tree cover reaching 50%. Although Norway spruce remained the dominant species, the forest composition had shifted towards a predominance of broadleaves, particularly evident at lower elevations and areas with moderate browsing pressure. Our findings underscore the critical role of post-disturbance forest recovery and deadwood dynamics in promoting uneven-aged mixed forest structures, thereby enhancing forest regeneration, structural diversity, and protection against natural hazards. Emphasizing the vital importance of retaining deadwood, our study suggests that its role as a valuable substrate for enhancing resilience and ecosystem services is likely to grow in the future.

Europe's semi-natural grasslands support notably high levels of temperate biodiversity across multiple taxonomic groups. However, these ecosystems face unique conservation challenges. Contemporary agricultural practices have replaced historical traditional low-intensity agriculture in many regions, resulting in a spectrum of management intensities within these ecosystems, ranging from highly intensive methods to complete abandonment. Paradoxically, both extremes along this spectrum of management intensity can be detrimental to biodiversity of semi-natural grasslands. Moreover, while anthropogenic climate change is an overarching threat to these ecosystems, rapid changes in land use and its intensity often present more immediate pressures. Often occurring at a faster rate than climate change itself, these land-use changes have the potential to rapidly impact the biodiversity of these grasslands. Here, we divide the ecological processes, threats, and developments to semi-natural grasslands into three sections. First, we examine the different impacts of agricultural intensification and abandonment on these ecosystems, considering their different consequences for biodiversity. Second, we review seminal works on various evidence-based management practices and offer a concise summary that provides support for various conservation and management strategies. However, the socio-economic factors that drive both abandonment and intensification in semi-natural grasslands can also be used to develop solutions through strategic governmental and non-governmental interventions. Accordingly, we conclude with a way forward by providing several key policy recommendations. By synthesizing existing knowledge and identifying research gaps, this essay aims to provide valuable insights for advancing the sustainable management of semi-natural grasslands.

Natürliche Störungen gehören zu den grössten Herausforderungen für die Erhaltung der Schutzfunktionen unserer Wälder. Die Förderung einer möglichst hohen Resilienz gegenüber solchen Störungen wurde in den letzten Jahrzehnten zu einem immer wichtigeren Bestandteil der Schutzwaldbewirtschaftung. In diesem Artikel geben wir einen Überblick zum Einfluss von natürlichen Störungen auf die Schutzfunktion gegenüber Naturgefahren, wobei wir Resultate aus kürzlich an der WSL und der ETH abgeschlossen Dissertationen aufgreifen. Darauf aufbauend und vor dem Hintergrund der erwarteten weiteren Zunahme von Störungen und Extremereignissen diskutieren wir, wie die zukunftige Resilienz von Schutzwälder gefördert werden kann.

Natural disturbances are among the greatest challenges for the management of forests with a protective role against natural hazards. Promoting the highest possible resilience related to natural disturbances has thus become an increasingly important part of mountain forest management in recent decades. In this article, we present results of recently finalized PhD dissertations and provide an overview on the effect of natural disturbances on the protective function against natural hazards. Based on this overview and an expected further increase in the frequency and intensity of extreme events and natural disturbances in mountain forests, we discuss how the resilience of protective forests to future extreme events may be promoted.