Wetter- und Klimaextreme und Trockenheit ¶

Als Folge des Klimawandels profitieren temperaturlimitierte Gebirgswälder von höheren Bodentemperaturen, was das Wachstum der Fichte (Picea abies) begünstigt. In der subalpinen Höhenstufe führte dies zu einer verstärkten Ausdehnung der Fichtenkronen und damit zu verschmälerten Gassen zwischen Rotten. Weil die Kronen der Randbäume stärker wachsen als erwartet, reicht der Gassenabstand im Untersuchungsgebiet vermutlich nicht aus, um die grünen Kronenränder der Rotten längerfristig zu erhalten. Die vorliegende Arbeit untersuchte zwölf Rotten in einem Schutzwald in der Surselva. Dabei wurden sechs Rotten nach klassischem Rottenmanagement bewirtschaftet, während die anderen sechs Rotten im Jahr 2012 durchforstet wurden. Ziel der Arbeit war es, mittels dendrochronologischer Analysen den Einfluss unterschiedlicher Bewirtschaftungsarten (durchforstet / nicht durchforstet) sowie Positionen (Rand/Zentrum) auf den Basalflächenzuwachs der Bäume zu quantifizieren und deren Wachstumsreaktion auf extreme Trockenheit darzustellen. Die durchforsteten Rotten sind durch einen stärkeren Basalflächenzuwachs sowie eine grössere Wachstumsresistenz gegen Trockenheit, insbesondere bei den Randbäumen charakterisiert. Der Waldbestand der untersuchten Fläche befindet sich derzeit in einem stabilen Zustand. Doch unter Einfluss des Klimawandels wird künftig nicht nur die jährliche Durchschnittstemperatur weiter steigen, sondern auch die Gefährdung durch biotische und abiotische Störungen zunehmen. Insbesondere auf sehr trockenheitslimitierten Standorten könnte eine Durchforstung im jungen Bestandalter eine Möglichkeit sein, die Anfälligkeit der Fichte gegenüber Trockenheit zu verringern.

Cluster management in a changing climate: possibility of thinning. In the context of climate change, mountain forests constrained by temperature limitations are experiencing higher soil temperatures, promoting the growth of spruce (Picea abies). In the subalpine altitudinal zone, this has led to an increased expansion of the spruce crowns, resulting in narrower corridors between the clusters. The vigorous growth of edge trees' crowns exceeds expectations, suggesting that the corridor spacing in the study area may not adequately maintain the green crowns of the edge trees in the long term. To investigate the influence of different management types (thinned vs. unthinned) and positions (edge vs. centre) on tree growth and response to drought, we conducted dendrochronological analyses on twelve clusters in a protective forest in the Surselva region. Six clusters followed traditional cluster management practices, while the remaining six were thinned in 2012. The thinned clusters showed higher basalarea increments as well as greater growth resistance to drought, particularly among edge trees. Today, the forest is in stable conditions. However, with the ongoing influence of climate change, mountain forests will not only experience a rise in average annual temperatures but also face an increasing threat from both biotic and abiotic disturbances. Thinning at a young stand age may offer a strategic approach to reduce the vulnerability of spruce to drought, especially in areas limited by drought.

Study region: The European alpine space.
Study focus: Despite the advancements in hydrological modeling, there is a lack of comparative studies quantifying the performance differences between local and large-scale models, particularly in the context of drought and low flow assessment. This study addresses this gap by designing a framework to evaluate the European Flood Awareness System (EFAS), a continental system, in simulating low flow events across 101 alpine stations from 1999–2018, with detailed comparisons to two local systems — GR6J in France and PREVAH in Switzerland — at 34 of these stations.
New hydrological insights for the region: The results show that the local systems, PREVAH and GR6J, generally outperform the continental system EFAS for almost all stations. The performance gap between the systems increases as low flow conditions intensify, highlighting the importance of local systems for extreme low flow events. Despite EFAS not being specifically set up for low flows, it has shown an overall acceptable performance compared to the local hydrological systems, especially at locations that are not calibrated within the EFAS system, indicating its potential in ungauged areas. This study lays a foundation for understanding how a continental hydrological system like EFAS can complement local systems or fill the gap when a local system is unavailable to provide reliable predictions of low flow conditions.

Risiken für die Gesellschaft, die aufgrund einer Kombination von meteorologischen Extremereignissen ausgelöst
und durch Prozesskaskaden verstärkt werden, sind schwierig zu analysieren und noch kaum erforscht, insbesondere auch im Kontext von Klimaanpassung. In diesem Beitrag stellen wir die Ergebnisse einer Fallstudie vor, deren Ziel es war, Wissenslücken und «blinde Flecken» in Bezug auf kombinierte Klimarisiken zu schliessen. In einer semiquantitativen Analyse wurde als Ausgangslage die Kumulation von zwei aufeinanderfolgenden sehr trockenen und warmen Jahreszeiten angenommen und die möglichen Auswirkungen auf den Schutzwald im Misox (GR) untersucht. In einer umfangreichen Befragung schätzten 29 Fachpersonen aus Praxis, Forschung und Verwaltung die Wahrscheinlichkeit für einen Verlust der Schutzfunktion in verschiedenen Gefährdungsszenarien ein. Die Ergebnisse zeigen, dass einzelne Gefahren, die durch das Extremereignis ausgelöst werden können (z.B. Trockenheit, Borkenkäferbefall, Sturm), die Schutzfunktion des Waldes wahrscheinlich nicht beeinträchtigen. In Szenarien, in denen Gefahren kombiniert auftreten, erwarten die meisten Fachpersonen zumindest einen teilweisen Verlust der Schutzfunktion. Sie favorisieren Massnahmen, welche die Waldstruktur durch Verbesserung der Verjüngung stärken, insbesondere eine Reduktion des Wildbestandes. Um die Widerstandsfähigkeit der alpinen Schutzwälder gegenüber dem Klimawandel zu gewährleisten, ist es von entscheidender Bedeutung, dass Anpassungsplanung und Präventionsmassnahmen das Risiko kombinierter Extremereignisse und der damit verbundenen Gefahren berücksichtigen, welche die Wälder durch kumulative und kaskadenartige Effekte bedrohen können.

Risks triggered by cumulating meteorological extreme events which exacerbate through process cascades are challenging to evaluate, barely studied, and insufficiently considered for climate adaptation. Here, we present the findings of a case study which was conducted in the Valle Mesolcina (GR) to re-spond to knowledge gaps and ‘blind spots’ related to combined climate risks. A semi-quantitative analysis was conducted to assess the potential impact of two consecutive very dry and warm seasons on the protective forest. In a compre-hensive survey, 29 experts from practice and administration estimated the likelihood for a loss of protective function under various hazard scenarios. Results indicate that, individu-ally, threats to the forest which can be induced by the extreme event such as drought mortality, bark beetle infestationand windstorm are unlikely to cause significant harm to the protective function of the forest. However, in scenarios where these effects are combined, most experts expect at least a partial loss of the protective function. To ensure resilience of alpine protection forests against climate change, it is vital that adaptation planning and preventative measures consider the risk of combined extreme events and related hazards that may threaten the forests via cumulative and cascading effects.

Protective forests provide an indispensable ecosystem service by protecting people and infrastructure against natural hazards and, therefore, serve a vital role in managing associated risks. Forests growing in avalanche terrain can inhibit avalanche formation by stabilizing the snowpack and significantly decelerate or stop small to medium size avalanches that have released within forests or closely above the treeline. However, forests are increasingly affected by global change, including climate change, more frequent and severe natural disturbances, and shifts in land use, affecting the longterm and sustainable provision of their protective services. To improve our understanding of the various impacts that global change has on avalanche protective forests, we summarized the current knowledge based on a systematic literature review following the PRISMA (Preferred Reporting Items for Systematic reviews and Meta-Analyses) method. This resulted in 36 peer-reviewed, English publications, which specifically addressed changes in forests’ protective effects against snow avalanches under global change. The geographic distribution of the studies’ locations reflects the long tradition and institutional embedding of protective forest management in the European Alps. Overall, most of the reviewed studies report at least partly negative impacts of global change on the protective effects of forests against snow avalanches, especially under exacerbating climate change scenarios. A decline in forest growing stock due to more frequent and severe disturbances, drought-related mortality or growth decrease, as well as due to land-use change or intensive management is expected to reduce the protective effect in the long-term. However, climate change-related growth increases at high elevations, re-/afforestation as well as post-disturbance legacies can positively affect avalanche protective forests. Most of the reviewed studies were case studies using indicators of forest structure, while quantitative hazard and risk assessments were less common. Studies often used forest simulation models or process-based avalanche models, but never combined the two. More empirical studies and monitoring efforts, consistent indicators across different case studies as well as modeling approaches linking forest structure to hazard and risk are needed for a better understanding of changes in avalanche protective forests to support a pro-active and sustainable management of these extremely valuable Nature-based Solutions under global change.

Runoff from glacierised Andean river basins is essential for sustaining the livelihoods of millions of people. By running a high-resolution climate model over the two most glacierised regions of Peru we unravel past climatic trends in precipitation and temperature. Future changes are determined from an ensemble of statistically downscaled global climate models. Projections under the high emissions scenario suggest substantial increases in temperature of 3.6 °C and 4.1 °C in the two regions, accompanied by a 12% precipitation increase by the late 21st century. Crucially, significant increases in precipitation extremes (around 75% for total precipitation on very wet days) occur together with an intensification of meteorological droughts caused by increased evapotranspiration. Despite higher precipitation, glacier mass losses are enhanced under both the highest emission and stabilization emission scenarios. Our modelling provides a new projection of combined and contrasting risks, in a region already experiencing rapid environmental change.