Dr. Martina Hobi

Rates of tree mortality are increasing globally, with implications for forests and climate. Yet, how and why these trends vary globally remain unknown. Developing a comprehensive assessment of global tree mortality will require systematically integrating data from ground-based long-term forest monitoring with large-scale remote sensing. We surveyed the metadata from 466 865 forest monitoring plots across 89 countries and five continents using questionnaires and discuss the potential to use these to estimate tree mortality trends globally. Our survey shows that the area monitored has increased steadily since 1960, but we also identify many regions with limited ground-based information on tree mortality. The integration of existing ground-based forest inventories with remote sensing and modelling can potentially fill those gaps, but this requires development of technical solutions and agreements that enable seamless flows of information from the field to global assessments of tree mortality. A truly global monitoring effort should promote fair and equitable collaborations, transferring funding to and empowering scientists from less wealthy regions. Increasing interest in forests as a natural climate solution, the advancement of new technologies and world-wide connectivity means that now a global monitoring system of tree mortality is not just urgently needed but also possible.

Forests can contribute to climate change mitigation by sequestering carbon when management intensity is reduced. However, there is high uncertainty regarding biomass dynamics in temperate forests after the cessation of management. We used forest inventory data from an extensive network of 224 plots in 37 natural forest reserves (NFR) covering a wide environmental gradient with mean annual temperatures ranging from 1 to 10.4 °C and mean annual precipitation ranging from 901 to 2317 mm. Inventories had been conducted approximately every 10 years during the last 60 years. We used mixed effect models to (i) analyse biomass development, (ii) assess the role of time since the cessation of management (TSCM) and (iii) disentangle the environmental and forest structural drivers of biomass change. After the cessation of management and in the absence of high-severity natural disturbances, biomass accumulated gradually along a saturation curve. There were large differences in biomass among reserves and plots, with values ranging from 101 Mg ha⁻¹ to 851.2 Mg ha⁻¹, with a median of 362.1 Mg ha⁻¹ (SD = 122.5 Mg ha⁻¹). The biomass curve did not yet tend towards an equilibrium, most likely because the majority of the NFRs do not exceed 100 years of TSCM. Compared to higher elevations, forests at lower, warmer sites showed a larger total biomass and higher rates of biomass accumulation. We found a reduction by 148 Mg ha⁻¹ of biomass per 1000 m of elevation gain. The strongest positive rate of change (>8 Mg ha⁻¹ year⁻¹) was found in forests with high basal area (>60 m² ha⁻¹) and medium to high levels of tree density (1500 to 2000 stems ha⁻¹). Overall, most reserves have not reached a biomass equilibrium yet and continue to act as carbon sinks in tree biomass. This highlights the carbon sequestration capacity of forest reserves and their role as carbon pools.

With ongoing global warming, increasing water deficits promote physiological stress on forest ecosystems with negative impacts on tree growth, vitality, and survival. How individual tree species will react to increased drought stress is therefore a key research question to address for carbon accounting and the development of climate change mitigation strategies. Recent tree-ring studies have shown that trees at higher latitudes will benefit from warmer temperatures, yet this is likely highly species-dependent and less well-known for more temperate tree species. Using a unique pan-European tree-ring network of 26,430 European beech (Fagus sylvatica L.) trees from 2118 sites, we applied a linear mixed-effects modeling framework to (i) explain variation in climate-dependent growth and (ii) project growth for the near future (2021–2050) across the entire distribution of beech. We modeled the spatial pattern of radial growth responses to annually varying climate as a function of mean climate conditions (mean annual temperature, mean annual climatic water balance, and continentality). Over the calibration period (1952–2011), the model yielded high regional explanatory power (R² = 0.38–0.72). Considering a moderate climate change scenario (CMIP6 SSP2-4.5), beech growth is projected to decrease in the future across most of its distribution range. In particular, projected growth decreases by 12%–18% (interquartile range) in northwestern Central Europe and by 11%–21% in the Mediterranean region. In contrast, climate-driven growth increases are limited to around 13% of the current occurrence, where the historical mean annual temperature was below ~6°C. More specifically, the model predicts a 3%–24% growth increase in the high-elevation clusters of the Alps and Carpathian Arc. Notably, we find little potential for future growth increases (−10 to +2%) at the poleward leading edge in southern Scandinavia. Because in this region beech growth is found to be primarily water-limited, a northward shift in its distributional range will be constrained by water availability.

Tree cores are a highly valuable scientific resource. Annual growth ring data can, for example, improve our understanding of climate change impacts and effects of environmental pollution, allow for better annual estimations of tree growth patterns and carbon storage, and help quantify dynamics and changes in forest ecosystems. The value of coring trees for research has been weighted against concerns around the potential harm coring might cause to trees. To date, there is indeed limited research accurately quantifying the potential effects of coring on tree growth and only a handful of studies assessing its influence on mortality. Consequently, many European long-term forest inventorying and monitoring programs are concerned that tree coring might bias the repeated tree measurements in permanent plots, which they rely on for assessments of states and changes of forests. In this study, we assessed the effects of tree coring on the growth and mortality of three widespread European tree species approximately 10 years after they were cored. We used repeated tree measurements from permanent research sites in Switzerland and Ukraine. In Switzerland, we assessed 35 cored and 159 uncored Norway spruce (Picea abies) trees as well as 147 cored and 332 uncored silver fir (Abies alba) trees. In Ukraine, we assessed 348 cored and 6'611 uncored European beech (Fagus sylvatica) trees. We found no statistical evidence that coring negatively affected the growth or mortality of the three tree species assessed. Although we cannot rule out subtle effects on tree health and wood quality, our findings do not provide any evidence that coring affects or biases repeated measurements (such as DBH measures and recording of mortality) performed on the investigated tree species. Tree coring could therefore be considered more often for routine incorporation, particularly in long-term forest inventorying and monitoring programs and initiatives.

Les forêts primaires sont rares en Europe occidentale, en particulier en Suisse, du fait d'un long historique de pratiques agricoles et de gestion forestière. Afin de favoriser les processus naturels, des réserves forestières sans intervention humaine ont été créées. Le Parc naturel du Jorat, établi en 2021, en est un exemple. Un inventaire réalisé durant l'hiver 2021/22 a permis de
documenter l'état initial des forêts du parc. Les résultats révèlent une forêt encore fortement influencée par la gestion passée, avec une prédominance de conifères et une faible quantité de bois mort au sol par rapport à d'autres réserves suisses. A l'avenir, la forêt devrait évoluer vers un état plus naturel, avec une composition en espèces plus naturelle et des quantités plus importantes de bois mort.

Primary forests are rare in Western Europe, particularly Switzerland, due to a long history of agricultural practices and forest management. In order to promote natural processes, forest reserves without human intervention have been created. The Jorat Nature Park, established in 2021, is an example. An inventory carried out during the winter of 2021/22 made it possible to document the initial state of the park's forests. The results reveal a forest still strongly influenced by past management, with a predominance of conifers and a low quantity of dead wood on the ground compared to other Swiss reserves. In the future, the forest is expected to evolve towards a more natural state, with a more natural species composition and greater amounts of deadwood.
 

Gebirgswälder bieten ein breites Spektrum an Waldleistungen, stehen aber aufgrund des globalen Wandels vor grossen Herausforderungen. Aufgrund zunehmender natürlicher Störungen sind gute lanungsgrundlagen zur Priorisierung der Bewirtschaftung von fichtendominierten Gebirgswäldern besonders wichtig. Dafür erstellen wir flächendeckende, interaktive Karten zur Waldstruktur und Störungsanfälligkeit der Wälder. Die Karten repräsentieren den aktuellen Waldzustand und weisen die Anfälligkeit der Schutzwälder gegen verschiedene Störungen aus. Kombiniert mit Informationen zur Schutzfunktion und Grundlagen bezüglich waldbaulicher und erntetechnischer Optimierung, dienen sie der Priorisierung von forstlichen Eingriffen in fichtendominierten Gebirgswäldern.

Ziel des adaptiven Waldbaus ist es, die Waldbewirtschaftung nach Bedarf an die sich ändernden Umweltbedingungen anzupassen, damit Waldleistungen und Biodiversität angesichts der durch den Klimawandel immer häufigeren und auch extremeren Störungsereignisse erhalten bleiben. Dieser Artikel fasst die Kernaussagen der in dieser Schwerpunktausgabe vorgestellten Beispiele aus Forschung und Praxis zum aktuellen Stand des adaptiven Waldbaus zusammen. Und er zeigt auf, dass der adaptive Waldbau in der Praxis voranschreitet. Gleichzeitig müssen sowohl Praxis als auch Forschung weiter und zusammen daran arbeiten, bestehende Unsicherheiten im Hinblick auf die zukünftige Klimaentwicklung und die Eignung verschiedener waldbaulicher Anpassungsmassnahmen zu reduzieren.

The objective of adaptive forest management is a directed adjustment of silvicultural measures wherever necessary to ensure provision of forest services and biodiversity in the face of more frequent and more intense disturbance events due to climate change in the future. This paper synthesises the main points of the examples from practitioners and scientists that are presented in this special issue and shows that there is good progress towards the adaptation of forest stands to climate change. At the same time, forest practitioners and scientists need to further collaborate to reduce existing uncertainties regarding the future development of the climate and the effectiveness of specific silvicultural adaption measures.

1. Tree regeneration is a key demographic process influencing long-term forest dynamics. It is driven by climate, disturbances, biotic factors and their interactions. Thus, predictions of tree regeneration are challenging due to complex feedbacks along the wide climatic gradients covered by most tree species. The stress gradient hypothesis (SGH) provides a framework for assessing such feedbacks across species ranges, suggesting that competition between trees is more frequent under favourable conditions, whereas reduced competition (i.e. positive interactions) is more likely under climatic stress. Moreover, tree life-history strategies (LHS) may shed light on how and whether the SGH explains regeneration of different tree species.
2. To address these topics, we developed statistical models based on >50,000 recruitment events observed for 24 tree species in an extensive permanent plot network (6540 plots from 299 unmanaged European temperate, boreal and subalpine forests) covering a wide climatic gradient.
3. We found that the effects of Leaf Area Index (as a proxy for competition) on tree recruitment changed along climatic gradients but in a species-specific manner. Competition predominates, with its intensity decreasing under stressful conditions for most species, as predicted by the SGH. However, positive interactions were only evident for a few species. Additionally, the ability of the SGH to explain patterns of competition and positive interactions across the gradients differed among species, with some differences and exceptions that may be related to varying LHS.
4. Synthesis. Our study shows that competition between trees toward climatic stress decreases systematically but depends on species stress tolerance to climate and shade. These findings explain within- and between-species differences in tree recruitment patterns in European temperate forests. Moreover, our findings imply that projections of forest dynamics along wide climatic gradients and under climate change must accommodate both competition and positive interactions, as they strongly affect rates of community turnover.

While enhanced tree growth over the last decades has been reported in forests across the globe, it remains unclear whether it drives persistent biomass increases of forest stands, particularly in mature forests. Enhanced tree growth and stand-level biomass are often linked with a simultaneous increase in density-driven mortality and a reduction in tree longevity. Identifying empirical evidence regarding the balance between these processes is challenging due to the confounding effects of stand history, management, and environmental changes. Here, we investigate the link between growth and biomass via the negative relationship between average tree size and stand density (tree number per area). We find increasing stand density for a given mean tree size in unmanaged closed-canopy forests in Switzerland over the past six decades and a positive relationship between tree growth and stand density across forest plots - qualitatively consistent with our simulations using a mechanistic, cohort-resolving ecosystem model (BiomeE). Model simulations show that, in the absence of other disturbances, enhanced tree growth persistently increases biomass stocks despite simultaneous decreases in carbon residence time and tree longevity. However, the magnitude of simulated biomass changes for a given growth enhancement critically depends on the shape of the mortality functions. Our analyses reconcile reports of growth-induced reductions of tree longevity with model predictions of persistent biomass increases, and with our finding of trends toward denser forests in response to growth - also in mature stands.

Tree replacement patterns strongly shape species coexistence and dominance in forest ecosystems. In mixed forests subject to a small-scale disturbance regime, dead canopy trees are often replaced by advance regeneration. We studied the abundance of saplings (6-10 cm dbh) and poles (10-25 cm dbh) under canopy trees based on four inventories on a 10-ha permanent plot in a large primary forest dominated by Fagus sylvatica in Ukraine. Saplings and poles of Fagus sylvatica, Acer platanoides, A. pseudoplatanus and Ulmus glabra were spatially linked to canopy trees ('hosts', dbh > 25 cm) based on their crown radius, and the hosts' neighborhood was classified according to the presence of dead trees. The number of saplings and poles under hosts was modelled with a Bayesian approach. There was a higher number of advance regeneration under hosts with increasing host diameter. The abundance of advance regeneration was the lowest under hosts in the canopy (i.e., under shaded conditions) and highest for hosts growing in gaps. 1) Under the canopy, only F. sylvatica poles can replace heterospecific hosts > 80 cm dbh, while other pole species are clearly below the replacement threshold. 2) Near dead lying trees, Acer pseudoplatanus rarely achieved an abundance of one pole at a host dbh > 80 cm. 3) In gaps, Acer spp. poles were able to replace F. sylvatica hosts. The proximity to dead lying trees and gaps is crucial for Acer spp. and U. glabra saplings for recruitment to the pole stage. The higher rate of recruitment under hosts adjacent to dead lying trees suggests that the total length of gap edges may be as important as gap size or disturbance frequency. In gaps > 550 m², advance regeneration without hosts (growing in gaps) of Acer spp. can recruit higher number of saplings than F. sylvatica, but not poles. We conclude that almost all canopy trees of Acer spp. and U. glabra are likely to be replaced by Fagus sylvatica if the current small-scale disturbance regime and single-tree mortality continue to prevail.

Aim: Predicting biodiversity responses to global changes requires good models of species' distributions. Both environmental conditions and human activities determine population density patterns. However, quantifying the relationship between wildlife population densities and their underlying environmental conditions across large geographical scales has remained challenging. Our goal was to explain the abundances of mammal species based on their response to several remotely sensed indices including the Dynamic Habitat Indices (DHIs) and the novel Winter Habitat Indices (WHIs).
Location: Russia, the majority of regions.
Taxon: Eight mammal species.
Methods: We estimated average population densities for each species across Russia from 1981 to 2010 from winter track counts. The DHIs measure vegetative productivity, a proxy for food availability. Our WHIs included the duration of snow-free ground, duration of snow-covered ground and the start, end and length of frozen season. In models, we included elevation, climate conditions, human footprint index. We parameterized multiple linear regression and applied best-subset model selection to determine the main factors influencing population density.
Results: The DHIs were included in some of the top-twelve models of every species, and in the top model for moose, wild boar, red fox and wolf, so they were important for species at all trophic levels. The WHIs were included in top models for all species except roe deer, demonstrating the importance of winter conditions. The duration of frozen ground without snow and the end of frozen season were particularly important. Our top models performed well for all the species (R²_adj 0.43-0.87).
Main Conclusions: The combination of the DHIs and the WHIs with climate and human-related variables resulted in high explanatory power. We show that vegetation productivity and winter conditions are key drivers of variation in population density of eight species across Russia.

Animals select habitat at multiple spatial scales, suggesting that biodiversity modeling, for example of species richness, should be based on environmental data gathered at multiple spatial scales, and especially multiple grain sizes. Different satellite sensors collect data at different spatial resolutions and therefore provide opportunities for multi-grain habitat measures. The dynamic habitat indices (DHIs), which are derived from satellite data, capture patterns of vegetative productivity and predict bird species richness well. However, the DHIs have only been analyzed at single resolutions (e.g., 1-km), and have not yet been derived from high-resolution satellite data (< 10 -m). Our goal was to predict bird species richness based on measures of vegetation productivity (DHIs, NDVI median and NDVI percentile 90th) across a range of spatial resolutions both from different sensors, and from resampled high-resolution imagery. We analyzed bird species richness within 215 forest, grassland and shrubland plots (56.25 ha) located at 26 terrestrial field sites of the National Ecology Observatory Network (NEON), in the continental US. To obtain our multi-resolution measures of vegetation productivity, we acquired data from Planetscope (3-m), RapidEye (5-m), Sentinel-2 (10-m), Landsat-8 (30-m) and MODIS (250-m) from 2017 to 2020, generated time series of NDVI, calculated the three DHIs (cumulative, minimum and variation), NDVI median and the 90th percentile NDVI and calculated 1st and 2nd order texture measures. We evaluated the performance of the derived measures to predict bird species richness of habitat specialist guilds based on (i) univariate models (ii) multivariate models with single-resolution measures and (iii) multivariate models with multi-resolution measures. Single-spatial resolution measures predicted bird species richness moderately well (R² up to 0.51) and the best performing spatial resolution and measure differed among bird species guilds. High-spatial resolution (3-5 m) measures outperformed medium-resolution measures (10–250 m). Models for all guilds performed best when incorporating multiple resolutions, including for all species richness (R² = 0.63) and for forest (R² = 0.72), grassland (R² = 0.53) and shrubland specialists (R² = 0.46). In addition, models based on multi-resolution data from different sensors performed better than models based on resampled high-resolution data for any of the guilds. Our results highlight, first, the value of the DHIs derived from high-resolution satellite data to predict bird species richness and, second, that remotely-sensed vegetation productivity measures from multiple spatial resolutions offer great promise for quantifying biodiversity.

The Jorat is one of the largest continously forested areas in the Swiss Plateau. On a forested area of 778 ha, the Parc Naturel du Jorat (PNJ), a periurban parc, has been established in 2022. To document the initial state of the forest within the perimeter of the PNJ, a sample plot inventory (SPI) was carried out on 132 sample plots (SP) in winter 2021/22. The aim was to describe the condition and development of the forest structure and to compare it with the results of previous sample inventories in the same area carried out in 1994 and 2004. This report presents the results and interprets them in the context of research results from natural and virgin forests. The analysis is based on the two zones (core and buffer) of the PNJ. [...]

Global biodiversity loss is most pronounced in the tropics. Monitoring of broad-scale patterns of habitat is essential for biodiversity conservation. Image texture measures derived from satellite data are proxies for habitat heterogeneity, but have not been tested in tropical forests. Our goal was to evaluate image texture to predict tropical forest bird distributions across Thailand for different guilds. We calculated a suite of texture measures from cumulative productivity (1-km fPAR-MODIS data) for Thailand's forests, and assessed how well texture measures predicted distributions of 86 tropical forest bird species in relation to body size, and nesting guild. Finally, we compared the predictive performance of combining (a) satellite image texture measures, (b) habitat composition, and (c) habitat fragmentation. We found that texture measures predicted occurrences of tropical forest birds well (AUC = 0.801 ± 0.063). Second-order homogeneity was the most predictive texture measure. Our models based on texture were significantly better for birds with larger body size (p < 0.05), but did not differ among nesting guilds (p > 0.05). Models that combined texture with habitat composition measures (AUC = 0.928 ± 0.038) outperformed models that combined fragmentation with habitat composition measures (AUC = 0.905 ± 0.047) (p < 0.05). The incorporation of texture, composition, and fragmentation variables significantly improved model accuracy over texture-only models (AUC = 0.801 ± 0.063 to AUC = 0.938 ± 0.034; p < 0.05). We suggest that texture measures are a valuable tool to predict bird distributions at broad scales in tropical forests.

Bewirtschaftung und Umweltbedingungen beeinflussen die Gestalt von Wäldern. Mit der Einrichtung eines Naturwaldreservats wurden am Bettlachstock im Kanton Solothurn die Weichen zur Entwickung in Richtung eines Naturwaldes gestellt. Im Waldreservat werden im Rahmen des «Monitoring Naturwaldreservate Schweiz» mit Stichprobeninventuren die natürlichen Prozesse der Waldentwicklung und in der «Langfristigen Waldökosystemforschung (LWF)» mit kontinuierlichen Messungen der Einfluss von Luftverschmutzung und Klimawandel auf den Wald erforscht. Beides sind langfristige Untersuchungen, die Methoden der Waldinventur und Erfassung des Waldzustandes verwenden. Das Reservat ist aufgrund der Standortsvielfalt reich an verschiedenen Baumarten. Diese widerspiegeln das natürliche Artenspektrum. Der hohe Anteil an Fichten zeigt jedoch, dass die frühere Bewirtschaftung diese bewusst gefördert hat. Die Luftbelastung und das Klima hatten und haben nachweislich einen Einfluss auf den Wald am Bettlachstock. Die Die Luftbelastung ist eher rückläufig und die Klimaveränderung eher zunehmend. Beide stehen aus heutiger Sicht grundsätzlich der Entwicklung zu einem naturnahen Wald an diesem Standort nicht im Wege. Eine Zunahme von trockenen Sommern ist zu erwarten und könnte die Standorteigenschaften verändern. In Teilen des Reservats könnte dies mittelfristig zu vermehrtem Absterben von Tannen und Fichten und in der Folge zu einer Zunahme des Totholzvolumens führen. Langfristig könnte dies Verschiebungen der Höhenstufen und in der Baumartenzusammensetzung hin zu mehr Laubbaumarten auslösen.

Obwohl aquatische (blaue) und terrestrische (grüne) Ökosysteme eng miteinander verwoben sind, werden sie oft getrennt voneinander betrachtet und verwaltet. Um Biodiversität Ökosystem-übergreifend besser zu schützen, braucht es integrative Ansätze in Forschung, Praxis und Gesetzgebung.

Bien qu’ils soient interdépendants, les écosystèmes aquatiques (bleus) et terrestres (verts) sont trop souvent appréhendés isolément les uns des autres. Pour que la conservation de la biodiversité soit plus efficace, la recherche, la pratique et la législation doivent dépasser cette approche cloisonnée au profit d’une vision inter-écosystémique.

La forêt abrite une multitude d'habitats et d'espèces qui dépendent des interactions entre l'eau - milieu bleu - et la terre - milieu vert. Une meilleure prise de conscience de cette interdépendance ouvre la porte à une protection plus efficace de la biodiversité.

Im Wald gibt es eine Vielfalt von Lebensräumen und Arten, die von Wechselwirkungen zwischen Wasser und Land beeinflusst sind. Ein grösseres Bewusstsein für diese gegenseitige Abhängigkeit ermöglicht effektiveren Biodiversitätsschutz.

Low mortality rates and slow growth differentiate shade-tolerant from shade-intolerant species and define the survival strategy of juvenile trees growing in deep shade. While radial stem growth has been widely used to explain mortality in juvenile trees, the leaf area ratio (LAR), known to be a key component of shade tolerance, has been neglected so far. We assessed the effects of LAR, radial stem growth and tree height on survival time and the age-specific mortality rate of juvenile Fagus sylvatica L. (European beech), Acer pseudoplatanus L. (sycamore maple) and Acer platanoides L. (Norway maple) in a primeval beech forest (Ukraine). Aboveground and belowground biomass and radial stem growth were analysed for 289 living and 179 dead seedlings and saplings. Compared with the other species, F. sylvatica featured higher LAR, slower growth and a lower mortality rate. The average survival time of F. sylvatica juveniles (72 years) allows it to reach the canopy more often than its competitors in forests with low canopy turnover rate. In contrast, a combination of lower LAR, higher growth rate and higher age-specific mortality rate of the two Acer species resulted in their shorter survival times and thus render their presence in the canopy a rare event. Overall, this study suggests that shade tolerance, commonly defined as a relationship between sapling mortality and growth, can alternatively be formulated as a relationship between survival time and the interplay of growth and LAR.