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Im Mai 2020 legte die EU-Kommission eine Biodiversitätsstrategie und anschliessend daran eine neue Waldstrategie vor. Deren Ziel ist es, die Resilienz von Wäldern in EU-Ländern zu erhöhen und damit langfristig die Multifunktionalität von Wäldern zu sichern. Ein übergeordnetes Konzept in der Waldstrategie bezüglich der Bewirtschaftung von Wäldern ausserhalb von Schutzgebieten lautet «Closer-to-Nature Forest Management»: Damit soll vermittelt werden, dass die Bewirtschaftung von Wäldern verbessert werden muss, wo immer es nötig und sinnvoll ist. Um diesen Begriff verständlicher zu machen, wurde ein Bericht von Wissenschaftlerinnen und Wissenschaftlern aus ganz Europa erarbeitet und 2022 vom Europäischen Forstinstitut (EFI) publiziert.

Same as other parts of Europe, the inner-Alpine Swiss Rhône valley has been increasingly affected by Scots pine (Pinus sylvestris L.) dieback events since the 1990s. Such events were not confined to years of extreme heat and drought across Switzerland and Europe such as 2003 or 2018, and the severity and frequency of sudden tree mortality varied on relatively small spatial scales. Which are the relevant parameters that changed in time and which factors triggered these dieback events?We found that sudden mortality events occurred exclusively after periods of below average precipitation between July and September. During this time of the year, soil moisture regularly drops to a minimum while the atmospheric water demand is high. Further factors such as insect infestation or spring frost may increase the magnitude of tree mortality, but they were neither a required contributor nor were they found to trigger dieback events. Consequently, the region with lowest summer precipitation within the Swiss Rhône valley outlines the area most affected by Scots pine dieback.However, the amount and frequency of the highly variable summer precipitation did not decrease since the 1980s, but the atmospheric water demand in spring and summer increased continuously. As a result of the higher water loss to the atmosphere, the period of low soil moisture has been prolongated and intensified. Therefore, Scots pines have become more dependent on (temporary) water stress relief by precipitation events during mid and late summer.Many Scots pine died (most likely due to hydraulic failure) within months following severe summer water stress. The effects of such periods appeared faster on tree crown defoliation (i.e., the proportion of needles that should be present on a tree, but which have been lost) than on mortality, as some trees died only after a year or two. We found that these Scots pines exceeded a defoliation threshold of about 75 % and were unable to recover. In such strongly defoliated trees, stress-related metabolites increase in needles, but get depleted in roots, indicating that mortality is linked to belowground carbon starvation negatively affecting functions central for tree survival.

Disturbances are increasing in frequency and severity in many regions of the world, affecting the dynamics and provision of ecosystem services of entire forest ecosystems. Higher tree mortality is often observed following such events, generating large amounts of deadwood. The latter is commonly salvaged logged to reduce economic losses, facilitate forest recovery and lower the susceptibility to secondary disturbance agents. However, the long-term and indirect effects of deadwood on forest dynamics and future ecosystem services are still debated. Here, we examined the role of deadwood in natural hazard processes and forest dynamics following disturbance. We focused primarily on the protective function against gravitational natural hazards (snow avalanches and rockfall) and the stand dynamics in mountain forests in central Europe. In the first years following disturbance and in different environmental contexts deadwood offered protection against snow avalanches, as no avalanche release was recorded within several observed windthrown areas. Additionally, deadwood acted as efficient barrier against rockfall, reducing both run-out distance and the number of rocks surpassing the deadwood section. In the longer term, deadwood often served as an important seedbed for tree regeneration, thus, promoting uneven-aged forest structures, forest resilience and the overall provision of ecosystem services. A more comprehensive understanding of the role that deadwood can play in disturbed forests in various environmental contexts will allow us to better identify situations where deadwood can be an asset when left on site and, thus, support the management of disturbed forests.

Forest dieback in Europe has recently intensified and has become more extensive. This dieback is strongly influenced by meteorological variations of temperature, T2m, and precipitation, P, and can be monitored with forest greenness. This study quantitatively investigates the 3-year meteorological history preceding events of reduced forest greenness in Europe's temperate and Mediterranean biome with a systematic approach. A specific focus lies in the timing of unusually persistent and unusually strong anomalies of T2m and P, as well as their relation to synoptic weather systems. A pragmatic approach based on remote sensing observations of the normalized difference vegetation index (NDVI) serves to identify low-forest-NDVI events at the 50km scale in Europe in June to August 2002-2022. We quantify the impact of the hottest summer on record in Europe in 2022, which, according to our criteria, negatively affected 37% of temperate and Mediterranean forest regions, and thereby reduced forest greenness more extensively than any other summer in 2002-2022. The low-NDVI events occurred in particularly dry and hot summers, but their meteorological histories also featured significant anomalies further in the past, with clear differences between the temperate and Mediterranean biome. A key feature is the anomalous accumulation of dry periods (i.e., periods with a P deficit) over the preceding 26 and 34 months in the temperate and Mediterranean biome, respectively. In the temperate biome only, T2m was anomalously persistent during almost the same 26-month period and featured distinctive peaks late in the past three growing seasons. While anomalously strong hot-dry conditions were characteristic of temperate low-NDVI events already in the previous summer, we find hardly any other systematic meteorological precursor in the Mediterranean prior to the event year. The identified dry periods went along with reduced cyclone activity in the Mediterranean and positive anticyclone frequency in the temperate biome. The occurrence of these two weather systems is locally more nuanced, showing, e.g., consistently increased and decreased cyclone frequency over western and northern Europe, respectively, in all event summers. Finally, the systematic meteorological histories are useful to test whether locally observed meteorological impacts, e.g., structural overshoot, systematically influenced the investigated events. In summary, systematic investigations of the multi-annual meteorological history provided clear evidence of how surface weather and synoptic-scale weather systems over up to 3 years can negatively impact European forest greenness. The observation of the record-extensive low-NDVI event in the summer of 2022 underlines that understanding the forest-meteorology interaction is of particular relevance for forest dieback in a changing climate., Biogeosciences, 20 (6), ISSN:1726-4170

Mountain forests have a substantial protective function in preventing natural hazards. Rates of deadwood production have already increased and are predicted to rise further, due to natural disturbances. In particular, higher windthrow event frequencies are expected, primarily due to the emerging even-aged forest stands in alpine regions combined with climate change. Here, we quantified the rockfall protection effect of mountain forests with and without deadwood in unprecedented detail. Repetitive experiments were conducted in which the two most important rock shapes from a hazard potential point of view and masses of 200 kg up to 3200 kg were considered. Based on a multi-camera setup, pre-and post-experimentally retrieved high- resolution lidar data, and rock data measured in situ, we completely reconstructed 63 trajectories. Every parameter of interest describing the rockfall kinematics was retrieved for each trajectory. A total of 164 tree impacts and 55 deadwood impacts were observed, and the currently applied energy absorption curves – partially only derived theoretically – could consequently be corroborated or even expanded to a greater absorption performance of certain species than hitherto assumed. Standing trees in general and deadwood, in particular, were found to strongly impede the notorious lateral spreading of platy rocks. Platy rocks featured a shorter mean run-out distance than their compact counterparts of similar weight, even in the absence of deadwood. These results indicate that the higher hazard potential of platy rocks compared with more compact rocks, previously postulated for open field terrain, applies less to forested areas. Lastly, reproducing the experimental setting showcases how complex forest states can be treated within rockfall simulations. Overall, the results of this study highlight the importance of incorporating horizontal forest structures that are as accurate as possible into simulations in order to obtain realistic deposition patterns.

Wälder sind zunehmend von Sturmschäden betroffen. In einem Projekt der Eidgenössischen Forschungsanstalt WSL wurde untersucht, wie sich Windwürfe auf die Kohlenstoffspeicherung im Waldboden auswirken. Hoch gelegene Nadelwälder sind besonders anfällig für Kohlenstoffverluste aus dem Boden. Im Unterschied zu Wäldern tieferer Lagen werden dort grössere Mengen an Kohlenstoff in rein organischen, mächtigen Humusauflagen gespeichert. Eine Stabilisierung durch mineralische Interaktionen fehlt weitgehend, was die Humusauflagen empfindlich macht für Kohlenstoffverluste durch störungsbedingte Veränderungen des Bestandesklimas. Die langsame Baumverjüngung und die geringen Streueinträge in den Boden bremsen den Humusaufbau nach einer Störung zusätzlich. Eine weitere Zunahme von Windwürfen könnte die Bodenkohlenstoffvorräte in Bergwäldern drastisch reduzieren, was negative Auswirkungen auf das Klima hätte. Darüber hinaus könnten störungsbedingte Humusverluste die Bodenqualität nachhaltig verschlechtern.

The intensity and frequency of droughts events are projected to increase in future with expected adverse effects for forests. Thus, information on the dynamics of tree water uptake from different soil layers during and after drought is crucial. We applied an in situ water isotopologue monitoring system to determine the oxygen isotope composition in soil and xylem water of European beech with a 2-h resolution together with measurements of soil water content, transpiration and tree water deficit. Using a Bayesian isotope mixing model, we inferred the relative and absolute contribution of water from four different soil layers to tree water use. Beech took up more than 50% of its water from the uppermost 5 cm soil layer at the beginning of the 2018 drought, but then reduced absolute water uptake from the drying topsoil by 84%. The trees were not able to quantitatively compensate for restricted topsoil water availability by additional uptake from deeper soil layers, which is related to the fine root depth distribution. Absolute water uptake from the topsoil was restored to pre-drought levels within 3 wk after rewetting. These uptake patterns help to explain both the drought sensitivity of beech and its high recovery potential after drought release.

Storms represent a major disturbance factor in forest ecosystems, but the effects of windthrows on soil organic carbon (SOC) stocks are poorly quantified. Here, we assessed the SOC stocks of windthrown forests at 19 sites across Switzerland spanning an elevation gradient from 420 to 1550 m, encompassing a strong climatic gradient. Results show that the effect size of disturbance on SOC stocks increases with the size of the initial SOC stocks. The largest windthrow-induced SOC losses of up to 29 t C ha−1 occurred in high-elevation forests with a harsh climate developing thick organic layers. In contrast, SOC stocks of low-elevation forests with thin organic layers were hardly affected. A mineralization study further revealed high elevation forests to store higher amounts of easily mineralizable C in thick organic layers that got lost following windthrow. These findings are supported by a meta-analysis of available windthrow studies, showing an increase of storm-induced SOC losses with the size of the initial SOC stocks. Modelling simulations further indicate longer-lasting SOC losses and a slower recovery of SOC stocks after windthrow at high compared to low elevations, due to a slower regeneration of mountain forests and associated lower C inputs into soils in a harsh climate. Upscaling the experimental findings/observed patterns by linking them to a data base of Swiss forest soils shows a total SOC loss of ∼0.4 Mt. C for the whole forested area of Switzerland after two major storm events, counteracting the forest net carbon sink of decades. Our study provides strong evidence that the vulnerability of SOC stocks to windthrow is particularly high in forests featuring thick and slowly forming organic layers, such as mountain soils. Thus, the risk of losing SOC to more frequent windthrows in mountain forests strongly limits their potential to mitigate climate change., Science of The Total Environment, 857, ISSN:0048-9697, ISSN:1879-1026

Verfrühter Fruchtabwurf in Schweizer Buchenbeständen im Hitze- und Trockensommer 2018 Die Rotbuche (Fagus sylvatica) zeigt mit ihrer Fortpflanzungsstrategie ein klares Mastverhalten, das heisst, sie produziert nur in einzelnen Jahren sehr grosse Mengen an Blüten und Früchten (Bucheckern). Bisher wurde angenommen, dass die Buche zu den Baumarten gehört, die bei erfolgter Bestäubung in aller Regel auch Früchte produziert. Im Schweizer Hitze- und Trockensommer 2018 reagierte die Buchen an einigen Standorten aber anders: Nach einer sehr starken Blüte entwickelten sie nur wenige Früchte, oder sie warfen die unvollständig entwickelten Früchte bereits Mitte des Sommers ab. Die Analyse von Messdaten der letzten 15 bis 19 Jahre von drei Buchenuntersuchungsflächen der Langfristigen Waldökosystem-Forschung (LWF) zeigt, dass starke Hitzewellen und langanhaltende Trockenheit während der Fruchtentwicklung zu einem Abbruch der Fruchtproduktion führen können. Diese extremen Wetterverhältnisse funktionieren somit als «Umweltveto». In Jahren mit vergleichbaren Bedingungen während der Fruchtentwicklung waren die Durchschnittstemperaturen im Sommer um 1.5 °C höher, und der Niederschlag war um 45% niedriger als im langjährigen Mittel. Der angenommene Zweijahreszyklus der Buchenblüte kann somit nicht nur durch ungünstige Wetterbedingungen während der Bestäubung, sondern auch während der Fruchtentwicklung unterbrochen werden.

Recent forest decline in Europe is strongly influenced by meteorological conditions imposed by seasonal variations of temperature, T2m, and precipitation, P, and can be monitored with forest greenness. This study quantitatively investigates anomalous characteristics of the three-year meteorological storyline preceding events of reduced forest greenness in Europe's temperate and Mediterranean biome in the phase space of seasonal-mean anomalies of (T2m, P). A specific focus is on the amplitude, persistence, and co-variability of these anomalies. A pragmatic approach based on remote sensing observations of normalized difference vegetation index NDVI serves to identify low forest NDVI events at the 50 km scale in Europe in June to August 2000–2020. An independent forest disturbance data set is used to qualitatively validate the identified more than 1'500 low NDVI events. These events occur in summers with particularly dry and hot conditions but their meteorological storylines feature significant anomalies during multiple seasons preceding the events, with clear differences between the two biomes. In the Mediterranean biome, the anomalously dry conditions persist over more than 1.5 y prior to the events, whereas T2m is anomalously warm only during the last 0.4 y. In contrast, in the temperate biome, T2m is anomalously large during most of the 2.5 y prior to the events and, most interestingly, the autumn/winter preceding the events is characterized by anomalously wet and warm conditions. These anomalies potentially induce a negative legacy on the following summer drought. The seasonal-mean anomalies of P are strongly determined by synoptic-scale weather systems, such that long dry periods are characterized by a deficit of cyclones and an excess of anticyclones. A final analysis investigates the peculiarities of low NDVI events that occur in two consecutive summers and the potential role of drought legacy effects. In the temperate biome, the second event summer of an event sequence has less hot and less dry anomalies than the first one and than during a single event. In summary, detailed investigations of the multi-annual meteorological storyline of low forest NDVI events provided clear evidence that anomalies of surface weather and synoptic-scale weather systems over time periods of up to 2.5 y can negatively impact European forest activity, with important differences between the temperate and Mediterranean biomes.

How deadwood mitigates rockfall hazard in mountain forests is a key scientific question to understand the influence of climate induced disturbances on the protective capacity of mountain forests. To address this question both experimental quantification combined with numerical process modelling are needed. Modelling provides detailed insights into the rock-deadwood interaction and therefore can be used to develop effective forest management strategies. Here, we introduce an automatic deadwood generator (ADG) to assess the impact of fresh woody storm debris on the protective capacity of a forest stand against rockfall. The creation of deadwood scenarios allows us to directly quantify their mitigation potential. To demonstrate the functionality of the proposed ADG method, we compare genuine deadwood log patterns, their effective height, and ruggedness at two natural windthrow areas at Lake Klöntal, Switzerland, to their generated counterparts. We perform rockfall simulations for the time a) before, b) directly after and c) 10 years after the storm. The results are compared to scenario d) a complete clearing of the thrown wood, in other words a no forest scenario. We showcase an integration of deadwood in rockfall simulations with realistic, deadwood configurations alongside with a DBH- and rot fungi dependent maximal deadwood breaking energy. Our results confirm the mitigation effect of deadwood significantly reducing the jump heights and velocities for 400 kg rocks. Our modelling results suggest that even after a decade, deadwood has a stronger protective effect against rockfall compared to standing trees. An ADG can contribute to the decision making in forest and deadwood management after disturbances.

Über die Invasivität der Douglasie und ihre Auswirkungen auf Boden und Biodiversität In der Diskussion urn die Aufrechterhaltung der Ökosysternleistungen von Wäldern irn Klirnawandel stehen dern Nutzen von gebietsfrernden Baurnarten ernsthafte Bedenken des Naturschutzes gegenüber. Die Frage, ob eine Baurnart invasiv ist oder nicht, bringt die beiden Positionen auf den Punkt. Der vorliegende Artikel überprüft den Invasivitätsstatus für die Gewähnliche Douglasie (Pseudotsuga menziesii [Mirb.] Franco), die als Zukunftsbaurnart gilt. Es wird dabei von der in der Schweiz gültigen Definition ausgegangen, wonach sich ausbreitende gebietsfrernde Arten invasiv sind, wenn sie die Biodiversität und Ökosysternleistungen beeinträchtigen oder Mensch und Urnwelt gefährden. Angesichts fehlender Hinweise auf eine selbstständige Ausbreitung der Baurnart kann die Douglasie in der Schweiz nicht als invasiv bezeichnet werden. Vergleichsstudien in Mitteleuropa belegen, dass der Anbau der Douglasie nicht nachteilig auf die Bodeneigenschaften wirkt. Irn Vergleich zu Reinund Mischbeständen aus Eiche, Buche und Fichte bieten jedoch Douglasienreinbestände ungünstigere Lebensräurne für Tier- und Pilzarten, was sich in geringeren Artenzahlen ausdrückt. Für Gefässpflanzen und Moose sind die Unterschiede stark von der Bestandesstruktur abhängig und daher steuerbar. Wir folgern daher, dass die Douglasie in naturnah bestockten Wäldern beigernischt werden kann, wobei rnagliche Auswirkungen auf die Biodiversität kontinuierlich beobachtet werden rnüssen.

Die gebietsfremde Douglasie geniesst einen hervorragenden Ruf in der mitteleuropäischen Forstwirtschaft und wird deshalb in unseren Nachbarländern Deutschland und Frankreich forstlich stark genutzt. In der Schweiz gilt sie wegen ihrer Resistenz gegen Sommertrockenheit als Zukunftsbaum mit einem breiten Standortsspektrum unter heutigen klimatischen Verhältnissen. Das Wachstumsoptimum wird sich aber im Rahmen des Klimawandels in Höhenlagen oberhalb von 1000 m ü.M. verschieben. Den Chancen der forstlichen Nutzung stehen die Risiken gegenüber, bei hohen Anteilen der Douglasie insbesondere die Auswirkungen auf die Biodiversität. Die Effekte in Reinbeständen der Douglasie in Mitteleuropa zeigen in einer Literaturstudie mehrheitlich eine deutliche Reduktion der Artenvielfalt bei Pilzen, Arthropoden und Vögeln. Geringer sind die Effekte, wenn die Douglasie beigemischt ist. Es besteht beträchtlicher Forschungsbedarf, ab welchem Mischungsgrad der Douglasie negative Auswirkungen entstehen.

Significance Forests are experiencing growing risks of drought-induced mortality in a warming world. Yet, ecosystem dynamics following drought mortality remain unknown, representing a major limitation to our understanding of the ecological consequences of climate change. We provide an emerging picture of postdrought ecological trajectories based on field indicators of forest dynamics. Replacement patterns following mortality indicate limited short-term persistence of predrought dominant tree species, highlighting the potential for major ecosystem reorganization in the coming decades. The great variability of the observed dynamics within and among species reinforces the primary influence of drought characteristics and ecosystem legacies, modulated by land use, management, and past disturbances, on ongoing drought-related species turnover and their potential implications for future forest biodiversity and ecosystem services., Forest vulnerability to drought is expected to increase under anthropogenic climate change, and drought-induced mortality and community dynamics following drought have major ecological and societal impacts. Here, we show that tree mortality concomitant with drought has led to short-term (mean 5 y, range 1 to 23 y after mortality) vegetation-type conversion in multiple biomes across the world (131 sites). Self-replacement of the dominant tree species was only prevalent in 21% of the examined cases and forests and woodlands shifted to nonwoody vegetation in 10% of them. The ultimate temporal persistence of such changes remains unknown but, given the key role of biological legacies in long-term ecological succession, this emerging picture of postdrought ecological trajectories highlights the potential for major ecosystem reorganization in the coming decades. Community changes were less pronounced under wetter postmortality conditions. Replacement was also influenced by management intensity, and postdrought shrub dominance was higher when pathogens acted as codrivers of tree mortality. Early change in community composition indicates that forests dominated by mesic species generally shifted toward more xeric communities, with replacing tree and shrub species exhibiting drier bioclimatic optima and distribution ranges. However, shifts toward more mesic communities also occurred and multiple pathways of forest replacement were observed for some species. Drought characteristics, species-specific environmental preferences, plant traits, and ecosystem legacies govern postdrought species turnover and subsequent ecological trajectories, with potential far-reaching implications for forest biodiversity and ecosystem services.

The impacts of climate extremes on forest ecosystems are still poorly understood but important for predicting carbon and water cycle feedbacks to climate. Despite evidences of the different climatic thresholds of tree carbon source and sink activities, implementations of mechanistic growth components into dynamic global vegetation models (DGVM) remain challenged by lacking observational data into the most important terrestrial biological sink, i.e.; into the cambial zone of tree stems.In this study, we aim to provide a framework for mechanistic understanding of drought impacts on carbon and water dynamics based on accurate analyses of the physiological processes that indirectly regulate these budgets. We quantified the drought impact and resilience of intra-annual carbon sequestration and water use in four mature Norway spruces from a Swiss subalpine site by comparing high-resolution growth (i.e., xylogenesis and wood anatomy) and physiological (i.e., stable carbon isotope ratios) data from an exceptional dry summer (year 2015) with those from a regular growing season (year 2014).Our observations described the cascade of impacts from leaf physiology to cambial activity during and after a 41-day period of physiological water deficit. During water deficit, all wood formation processes were strongly reduced diminishing carbon sequestration by 67% despite a 11% increased water-use efficiency. However, with the recovery of the positive hydric state in the stem, we observed a fast recovery of the rates of the different cell formation phases at the expenses of the accumulated assimilates produced during the drought event.Our results clarify how the interaction between source and sink is modulated via external environmental factors and provide evidence that, under specific circumstances, tree growth can be extremely resilient. These novel findings should provide a framework to improve sink model components in DGVMs and consequently help to bridge understanding of carbon and water fluxes between atmosphere and forest ecosystems.

Summer droughts strongly affect soil organic carbon (SOC) cycling, but net effects on SOC storage are unclear as drought affects both C inputs and outputs from soils. Here, we explored the overlooked role of soil fauna on SOC storage in forests, hypothesizing that soil fauna is particularly drought-sensitive, thereby reducing litter incorporation into the mineral soil and, eventually, long-term SOC storage.In a drought-prone pine forest (Switzerland), we performed a large-scale irrigation experiment for 17 years and assessed its impact on vertical SOC distribution and composition. We also examined litter decomposition of dominant tree species using litterbags of different mesh sizes and determined soil fauna abundance and community composition.Long-term irrigation resulted in a C loss in the organic layers (-1.0 kg C m-2) and a comparable C gain in the mineral soil (+0.8 kg C m-2) in the first decade of irrigation, and thus did not affect total SOC stocks. Irrigation increased the mass loss of Quercus pubescens and Viburnum lantana leaf litter more strongly when meso- and macrofauna were included (+215%) compared to excluded (+44%). The enhanced faunal-mediated litter decomposition was paralleled by a many-fold increase in the abundance of meso- and macrofauna during irrigation. Moreover, irrigation led to a shift in Acari and Collembola community composition, with irrigation characterized by the presence of drought-sensitive species. In comparison, microbial SOC mineralization was less responsive to lower soil moisture. Our results suggest that the vertical redistribution of SOC with irrigation was mainly driven by litter incorporation through meso- and macrofauna, here accelerated by irrigation and suppressed by naturally occurring summer droughts.Our study shows that soil fauna is highly sensitive to natural drought reducing the incorporation of C from organic layers to the mineral soil. In the longer term, this potentially affects SOC storage, decreasing the C stored in mineral soil. Therefore, soil fauna plays a key but so far largely overlooked role in shaping SOC responses to drought.

Productivity of Europe’s temperate forest regions is strongly influenced by meteorological conditions and their interannual variability, including seasonal variations of temperature (T2m) and precipitation (P) and short-term extreme events. Understanding the occurrence of individual events of reduced forest productivity therefore requires knowledge at the interface of atmospheric science and forest ecology. While numerous recent studies focused on the forest ecosystem perspective of such events, the preceding evolution of basic meteorological parameters such as T2m and P has not yet been analyzed systematically. The purpose of this study is thus to provide an atmospheric science perspective on such events in Europe in 2000–2020. To this end, we examine the meteorological evolution in the year prior to events of reduced forest productivity in June–August (JJA), by considering the trajectory of normalized 90-day T2m and P anomalies from the ERA5 reanalysis in their phase space. The events have been identified based on normalized difference vegetation index anomalies (NDVI') at 0.05° resolution. Furthermore, a pragmatic approach is developed to coarse-grain the events to the 0.5° scale of ERA5.From a set of event characteristics we identify different event types. One event type (Fdrop) is found to feature a pronounced NDVI' drop over JJA, while a second type (Flow) exhibits consistently negative NDVI' throughout JJA. Well-known forest damage occurrences are identified as Fdrop events, e.g., in the Balkans in 2000, in central Europe in 2003, and in northern Europe in 2018. Wide-spread Flow events are identified for the first time in 2014. They occur most prominently in 2019 and 2020 in regions already affected by Fdrop events in 2018. The multi-seasonal meteorological history of Fdrop events in the T2m–P phase space is characterized by exceptionally dry conditions already in the early growing season and increasingly warm summer conditions. In some contrast, Flow events emerge after two consecutively hot-dry summers. During these events, anomalies of T2m and P are less extreme than during Fdrop events, suggesting the involvement of forest legacy and secondary disturbances (e.g., fire, insects).In future work, we will validate the reduced forest productivity events with already existing datasets of forest disturbances in Europe and expand the analysis of these events’ meteorological history more quantitatively. Also, we aim at further refining the event classification according to the spatiotemporal variability of their meteorological history to better understand the meteorological impact on forest productivity in Europe.

Drought alters carbon (C) allocation within trees, thereby impairing tree growth. Recovery of root and leaf functioning and prioritized C supply to sink tissues after drought may compensate for drought -induced reduction of assimilation and growth. It remains unclear if C allocation to sink tissues during and following drought is controlled by altered sink metabolic activities or by the availability of new assimilates. Understanding such mechanisms is required to predict forests' resilience to a changing climate. We investigated the impact of drought and drought release on C allocation in a 100-y-old Scots pine forest. We applied (CO2)-C-13 pulse labeling to naturally dry control and long-term irrigated trees and tracked the fate of the label in aboveand belowground C pools and fluxes. Allocation of new assimilates belowground was ca. 53% lower under nonirrigated conditions. A short rainfall event, which led to a temporary increase in the soil water content (SWC) in the topsoil, strongly increased the amounts of C transported belowground in the nonirrigated plots to values comparable to those in the irrigated plots. This switch in allocation patterns was congruent with a tipping point at around 15% SWC in the response of the respiratory activity of soil microbes. These results indicate that the metabolic sink activity in the rhizosphere and its modulation by soil moisture can drive C allocation within adult trees and ecosystems. Even a subtle increase in soil moisture can lead to a rapid recovery of belowground functions that in turn affects the direction of C transport in trees.

Erkenntnisse aus der Trockenheit 2018 für die zukünftige Waldentwicklung Eine ausgeprägte Niederschlagsarmut im Sommer und Herbst des Jahres 2018 führte in grossen Teilen der Schweiz, insbesondere im nordöstlichen Mittelland, zu einer verbreiteten und lang anhaltenden Trockenheit. Die unmittelbaren und mittelfristigen Auswirkungen des trockenen Sommers 2018 auf Wachstum, Schädlingsund Krankheitsbefall und letztlich auf das Überleben vieler Bäume waren von bisher kaum beobachtetem Ausmass im Schweizer Wald. Nicht nur die Fichte, sondern auch die Buche und Mischwälder waren betroffen. Dabei wurden regionale und lokale Unterschiede festgestellt, die sich auch in der räumlichen Ausprägung des Niederschlags und der Bodentrockenheit manifestierten. Die teilweise massiven Auswirkungen führen vor Augen, dass Extremereignisse wie das Trockenjahr 2018 eine zentrale Rolle bei der klimabedingten Änderung der Waldzusammensetzung spielen können. Aus wissenschaftlicher Sicht gilt es daher, so viele Erkenntnisse und Lehren wie möglich aus diesen Extremereignissen zu ziehen, um adäquate, zukunftsgerichtete Waldbewirtschaftungskonzepte entwickeln zu können. Eine Forschungsinitiative der Eidgenössischen Forschungsanstalt für Wald, Schnee und Landschaft wurde genau mit diesem Ziel sehr kurzfristig im Laufe des Sommers 2018 lanciert. Der vorliegende Beitrag fasst wichtige Erkenntnisse im Hinblick auf die zukünftige Waldentwicklung zusammen.

Since the 1990s the invasive fungus Hymenoscyphus fraxineus has caused severe crown dieback and high mortality rates in Fraxinus excelsior in Europe. In addition to a strong genetic control of tolerance to the fungus, previous studies have found landscape heterogeneity to be an additional driver of variability in the severity of dieback symptoms. However, apart from climatic conditions related to heat and humidity influencing fungal infection success, the mechanistic understanding of why smaller or slower-growing trees are more susceptible to dieback remains less well understood. Here, we analyzed three stands in Switzerland with a unique setting of 8 years of data availability of intra-annual diameter growth and annual crown health assessments. We complemented this by ring width and quantitative wood anatomical measurements extending back before the monitoring started to investigate if wood anatomical adjustments can help better explain the size-related dieback phenomenon. We found that slower-growing trees or trees with smaller crowns already before the arrival of the fungus were more susceptible to dieback and mortality. Defoliation directly reduced growth as well as maximum earlywood vessel size, and the positive relationship between vessel size and growth rate caused a positive feedback amplifying and accelerating crown dieback. Measured non-structural carbohydrate (NSC) concentrations in the outermost five rings did not significantly vary between healthy and weakened trees, which translate into large differences in absolute available amount of NSCs. Thus, we hypothesize that a lack of NSCs (mainly sugars) leads to lower turgor pressure and smaller earlywood vessels in the following year. This might impede efficient water transport and photosynthesis, and be responsible for stronger symptoms of dieback and higher mortality rates in smaller and slower-growing trees.

In many regions, drought is suspected to be a cause of Scots pine decline and mortality, but the underlying physiological mechanisms remain unclear. Because of their relationship to ecohydrological processes, δ18O values in tree rings are potentially useful for deciphering long-term physiological responses and tree adaptation to increasing drought. We therefore analyzed both needle- and stem-level isotope fractionations in mature trees exposed to varying water supply. In a first experiment, we investigated seasonal δ18O variations in soil and needle water of Scots pine in a dry inner Alpine valley in Switzerland, comparing drought-stressed trees with trees that were irrigated for more than 10 years. In a second experiment, we analyzed twentieth-century δ18O variations in tree rings of the same forest, including a group of trees that had recently died. We observed less 18O enrichment in needle water of drought-stressed compared with irrigated trees. We applied different isotope fractionation models to explain these results, including the Péclet and the two-pool correction, which considers the ratio of unenriched xylem water in the needles to total needle water. Based on anatomical measurements, we found this ratio to be unchanged in drought-stressed needles, although they were shorter. The observed lower 18O enrichment in needles of stressed trees was therefore likely caused by increased effective path length for water movement within the leaf lamina. In the tree-ring study, we observed lower δ18O values in tree rings of dead trees compared with survivors during several decades prior to their death. These lower values in declining trees are consistent with the lower needle water 18O enrichment observed for drought-stressed compared with irrigated trees, suggesting that this needle-level signal is reflected in the tree rings, although changes in rooting depth could also play a role. Our study demonstrates that long-term effects of drought are reflected in the tree-ring δ18O values, which helps to provide a better understanding of past tree physiological changes of Scots pine.

Promoting tree species diversity is commonly advocated in the anticipation of predicted increases in drought frequency and severity. However, mixing effects on drought responses vary substantially with site conditions and species identity. We combined annually resolved tree-ring data and repeated forest inventory data spanning the last 90 years to examine the effect of species-specific neighbourhood competition on the drought response (resistance, recovery and resilience) of European beech (Fagus sylvatica), silver fir (Abies alba) and Norway spruce (Picea abies) for six drought events that occurred since the 1970s at three sites in Switzerland. We found predominantly weak neighbourhood competition and tree species diversity effects, with significant interspecific influences only for resistance and recovery of beech. These minor neighbourhood effects were outweighed by tree age and size effects. Although age effects depended on species identity and components of resilience, tree size consistently negatively affected all species. Our results emphasize that diversity effects may vary for each given species combination which makes broader conclusions challenging. This is because species interact through their specific set of traits and interactions vary in space and time. Adaptive management strategies are likely to be more effective when they promote more drought-tolerant species and reductions in stand density. Despite the absence of an unequivocal advantage of tree diversity on drought resilience, striving towards species-rich forests nonetheless allows for a risk spreading among multiple species and the reinforced provision of numerous ecosystem services.

Long generation times have been suggested to hamper rapid genetic adaptation of organisms to changing environmental conditions. We examined if environmental memory of the parental Scots pines (Pinus sylvestris L.) drive offspring survival and growth. We used seeds from trees growing under naturally dry conditions (control), irrigated trees (irrigated from 2003 to 2016), and formerly irrigated trees (“irrigation stop”; irrigated from 2003–2013; control condition since 2014). We performed two experiments, one under controlled greenhouse conditions and one at the experimental field site. In the greenhouse, the offspring from control trees exposed regularly to drought were more tolerant to hot–drought conditions than the offspring from irrigated trees and showed lower mortality even though there was no genetic difference. However, under optimal conditions (high water supply and full sunlight), these offspring showed lower growth and were outperformed by the offspring of the irrigated trees. This different offspring growth, with the offspring of the “irrigation‐stop” trees showing intermediate responses, points to the important role of transgenerational memory for the long‐term acclimation of trees. Such memory effects, however, may be overridden by climatic extremes during germination and early growth stages such as the European 2018 mega‐drought that impacted our field experiment., Environmental conditions (natural dry vs. irrigated) experienced by the mother trees of Pinus sylvestris significantly influenced the offspring's growth and survival, indicating that transgenerational environmental memory can shape trees' acclimation potential over the midterm to the long term.

Summer droughts strongly affect soil organic carbon (SOC) cycling, but net effects on SOC storage are unclear as drought affects both C inputs and outputs from soils. Here, we explored the overlooked role of soil fauna on SOC storage in forests, hypothesizing that soil faunal activity is particularly drought-sensitive, thereby reducing litter incorporation into the mineral soil and, eventually, long-term SOC storage. In a drought-prone pine forest (Switzerland), we performed a large-scale irrigation experiment for 17 years and assessed its impact on vertical SOC distribution and composition. We also examined litter mass loss of dominant tree species using different mesh-size litterbags and determined soil fauna abundance and community composition. The 17-year-long irrigation resulted in a C loss in the organic layers (−1.0 kg C m−2) and a comparable C gain in the mineral soil (+0.8 kg C m−2) and thus did not affect total SOC stocks. Irrigation increased the mass loss of Quercus pubescens and Viburnum lantana leaf litter, with greater effect sizes when meso- and macrofauna were included (+215%) than when excluded (+44%). The enhanced faunal-mediated litter mass loss was paralleled by a many-fold increase in the abundance of meso- and macrofauna during irrigation. Moreover, Acari and Collembola community composition shifted, with a higher presence of drought-sensitive species in irrigated soils. In comparison, microbial SOC mineralization was less sensitive to soil moisture. Our results suggest that the vertical redistribution of SOC with irrigation was mainly driven by faunal-mediated litter incorporation, together with increased root C inputs. Our study shows that soil fauna is highly sensitive to natural drought, which leads to a reduced C transfer from organic layers to the mineral soil. In the longer term, this potentially affects SOC storage and, therefore, soil fauna plays a key but so far largely overlooked role in shaping SOC responses to drought., Global Change Biology, 28 (9), ISSN:1354-1013, ISSN:1365-2486

The impact of climate extremes on forest ecosystems is poorly understood but important for predicting carbon and water cycle feedbacks to climate. Some knowledge gaps still remain regarding how drought-related adjustments in intra-annual tree-ring characteristics directly impact tree carbon and water use. In this study we quantified the impact of an extreme summer drought on the water-use efficiency and carbon sequestration of four mature Norway spruce trees. We used detailed observations of wood formation (xylogenesis) and intra-annual tree-ring properties (quantitative wood anatomy and stable carbon isotopes) combined with physiological water-stress monitoring. During 41 d of tree water deficit, we observed an enrichment in 13C but a reduction in cell enlargement and wall-thickening processes, which impacted the anatomical characteristics. These adjustments diminished carbon sequestration by 67% despite an 11% increase in water-use efficiency during drought. However, with the resumption of a positive hydric state in the stem, we observed a fast recovery of cell formation rates based on the accumulated assimilates produced during drought. Our findings enhance our understanding of carbon and water fluxes between the atmosphere and forest ecosystems, providing observational evidence on the tree intra-annual carbon sequestration and water-use efficiency dynamics to improve future generations of vegetation models., New Phytologist, 236 (1), ISSN:0028-646X, ISSN:1469-8137

Forests are rockfall-protective ecological infrastructures as a significant amount of kinetic energy is absorbed during consecutive rock-tree impacts. Although many recent works have considered rock impacts with standing trees, the effect of lying deadwood in forests has not yet been considered thoroughly, either experimentally or numerically. Here, we present a complete examination of induced rockfall experiments with sensor-equipped, 45 kg, artificial rocks on a forested area in three different management stages. The trilogy is conducted in a spruce forest stand (i) in its original state of forest; (ii) after a logging operation with fresh, lying deadwood; and (iii) after the removal of the deadwood. The tests allow us to directly quantify the effect of fresh deadwood on overall rockfall risk for the same forest (slope, species) under three different conditions. The study yields quantitative results on the barrier efficiency of the deadwood logs as only 3.6 % of the rocks surpass the deadwood section. The mean run-out distance is reduced by 42 %. Conversely, the run-out distance increases by 17 % when the cleared stand is compared to the original forest. These results quantitatively confirm the benefits of nature-based mitigation measures integrated into forestry practice, whose detailed effect has to be scrutinized for higher rockfall energies. Based on the experimental results, we extended a modern rockfall code by three-dimensional deadwood logs to incorporate such complex but realistic forest boundary conditions., Natural Hazards and Earth System Sciences, 22 (7), ISSN:1561-8633, ISSN:1684-9981

Severe drought events negatively affect tree growth and often cause legacy effects, expressed by smaller tree rings in the post-drought recovery years. While the pattern of reduced tree-ring widths is frequently described the processes underlying such legacy effects, i.e., whether it is due to shorter growth periods or lower growth rates, remains unclear and is investigated in this study. To elucidate these post-drought effects, we examined radial stem growth dynamics monitored with precision band-dendrometers on 144 Douglas fir, Norway spruce and silver fir sample trees distributed along four elevational gradients in the Black Forest (Southwest Germany) during the post-drought years 2019 and 2020. Growth onset of all investigated species occurred between 11 and 24 days significantly earlier in 2020 compared to 2019. Modelling growth onset based on chilling and forcing units and taking the study year into account explained 88-98 % of the variance in the growth onset data. The highly significant effect of the study year (p 0.001) led to the conclusion, that other factors than the prevailing site conditions (chilling and forcing units) must have triggered the earlier growth onset in 2020. On the other hand, for Douglas fir growth rates were significantly higher in 2020 compared to 2019 (2.9 μm d

Forests are rockfall-protective ecological infrastructures, as a significant amount of kinetic energy is absorbed during consecutive rock-tree impacts. Although many recent works have considered rock impacts with standing trees, the effect of lying deadwood in forests has not yet been considered thoroughly, either experimentally or numerically. Here, we present a complete examination of induced rockfall experiments on a forested area in three different management stages. The trilogy is conducted in a spruce forest stand (i) in its original state, (ii) after a logging operation with fresh, lying deadwood and (iii) after the removal of the deadwood. The tests allow us to directly quantify the effect of fresh deadwood on overall rockfall risk for the same forest (slope, species) under three different conditions. The study yields quantitative results on the barrier efficiency of the deadwood logs as only 3.6 % of the rocks surpass the deadwood section. The mean runout distance is reduced by 42 %. Conversely, the runout distance increases by 17 % when the cleared stand is compared to the original forest. These results quantitatively confirm the benefits of nature-based mitigation measures integrated into forestry practice and we show how modern rockfall codes can be extended to incorporate such complex, but realistic forest boundary conditions.