Jonas Stillhard

Tree-related Microhabitats (TreMs) are of prime concern for biodiversity since they host thousands of taxa. TreMs are discrete habitat patches borne by trees and cover a wide range of lifespans, from days to decades to centuries. The taxa associated with such discrete and sometimes highly ephemeral habitat patches are likely to be sensitive to spatial distribution because they need to search for new habitats after the occupied one disappears. Although many studies have recently been dedicated to TreMs, only very few have investigated their spatial distribution. Focusing on European beech-dominated forests, we used a European TreM database with 12,362 trees and 296 plots (ranging from local (0.1–1 ha) to landscape scale (10,000 ha)) to assess TreM spatial distribution patterns in long unmanaged forests. Then, with a TreM sub-database with 6828 trees and 21 plots, we assessed whether and if so, how harvesting changes spatial patterns at the plot scale. In long unmanaged forests, most TreMs showed a regular pattern at the plot scale and an aggregated pattern at larger scales. Tree diameter was the most influential factor for spatial patterns in TreMs. Spatial patterns at the plot scale in harvested forests differed slightly from those observed in unmanaged forests. To favor TreM-dwelling taxa in harvested stands, our results suggest retaining habitat trees in a regular spatial pattern to mimic the natural pattern. However, some TreMs should be conserved in an aggregate pattern; we specify the spatial scale at which this should be done.

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.

Estimating growing stock is one of the main objectives of forest inventories. It refers to the stem volume of individual trees which is typically derived by models as it cannot be easily measured directly. These models are thus based on measurable tree dimensions and their parameterization depends on the available empirical data. Historically, such data were collected by measurements of tree stem sizes, which is very time- and cost-intensive. Here, we present an exceptionally large dataset with section-wise stem measurements on 40'349 felled individual trees collected on plots of the Experimental Forest Management project. It is a revised and expanded version of previously unpublished data and contains the empirically derived coarse (diameter ≥7 cm) and fine branch volume of 27'297 and 18'980, respectively, individual trees. The data were collected between 1888 and 1974 across Switzerland covering a large topographic gradient and a diverse species range and can thus support estimations and verification of volume functions also outside Switzerland including the derivation of whole tree volume in a consistent manner.

1. Human intervention often alters the availability of habitat for biodiversity. The conservation of biodiversity therefore requires an optimized habitat management. In forests, dead wood represents one of the most important habitats and in boreal and temperate regions around 25% of forest species depend on it (= saproxylic species). Increasing the amount of dead wood in managed forests has thus become a policy objective, but there is no consensus on how to best distribute dead wood in space.
2. In a landscape experiment, we exposed freshly cut beech branches in bundles of different sizes (one, three, six and 12 branches) in the forest, representing newly created habitat patches to be colonized by saproxylic beetles. We investigated how species richness in a 'single large' branch bundle compares to that in 'several small' bundles (SLOSS debate). We further tested the effects of dead wood availability (amount and isolation) in the surrounding landscape (20–200 m) and environmental factors (temperature and light availability) on species richness, abundance and community composition.
3. The species richness of the pooled small bundles (1 + 3 + 6 = 10 branches) was as high as that of the large bundle (12 branches), despite having a smaller total surface, demonstrating the benefit of spatially dispersed habitat patches for total diversity. Also community composition differed and every bundle size yielded some unique species. Dead wood availability in the surrounding landscape had a minor effect in comparison. Our results further highlight the importance of microsite heterogeneity: species richness was related to light availability, and abundance and community composition were related to temperature.
4. Synthesis and applications: Larger amounts of dead wood harbour more saproxylic beetle species and the distribution of dead wood in patches of different sizes within the forest can promote the development of variable species communities. Combined, this results in a higher species diversity. In managed forests, where retained dead wood is often homogeneous in terms of size or tree species, increasing heterogeneity by distributing dead wood in the forest could foster higher diversity of saproxylic species.

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.

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.
 

The European biodiversity and forest strategies rely on forest sustainable management (SFM) to conserve forest biodiversity. However, current sustainability assessments hardly account for direct biodiversity indicators. We focused on forest multi-taxon biodiversity to: i) gather and map the existing information; ii) identify knowledge and research gaps; iii) discuss its research potential. We established a research network to fit data on species, standing trees, lying deadwood and sampling unit description from 34 local datasets across 3591 sampling units. A total of 8724 species were represented, with the share of common and rare species varying across taxonomic classes: some included many species with several rare ones (e.g., Insecta); others (e.g., Bryopsida) were represented by few common species. Tree-related structural attributes were sampled in a subset of sampling units (2889; 2356; 2309 and 1388 respectively for diameter, height, deadwood and microhabitats). Overall, multi-taxon studies are biased towards mature forests and may underrepresent the species related to other developmental phases. European forest compositional categories were all represented, but beech forests were over-represented as compared to thermophilous and boreal forests. Most sampling units (94%) were referred to a habitat type of conservation concern. Existing information may support European conservation and SFM strategies in: (i) methodological harmonization and coordinated monitoring; (ii) definition and testing of SFM indicators and thresholds; (iii) data-driven assessment of the effects of environmental and management drivers on multi-taxon forest biological and functional diversity, (iv) multi-scale forest monitoring integrating in-situ and remotely sensed information.

Das Einrichten von Naturwaldreservaten ist eine wichtige Massnahme zur Erhaltung und Förderung der Biodiversität im Wald - das gilt besonders für Arten, die auf späte Waldentwicklungsstadien und Totholz angewiesen sind. Im Rahmen des Projekts «Biodiversität und Habitatstrukturen im Wildnispark Zürich Sihlwald» untersuchten wir zwischen 2016 und 2020 vier Organismengruppen (Moose, Flechten, xylobionte Käfer und totholzbewohnende Pilze) auf 69 Probeflächen der kantonalen Stichprobeninventur. Um zu verstehen, wie ihr Vorkommen mit verschiedenen Waldparametern zusammenhängt, verglichen wir Modelle, die auf Inventurdaten basieren, mit solchen, die nur Fernerkundungsdaten verwenden. Während ein etwas stärkerer Zusammenhang der Artenvielfalt totholzbewohnender Pilze mit Inventurdaten (z.B. Totholzvolumen) gefunden wurde, waren es bei den xylobionten Käfern eher Fernerkundungsdaten (z.B. Vegetationshöhe). Bemerkenswert ist, dass der Sihlwald nach knapp 20 Jahren ohne Bewirtschaftung einige Artenhighlights beherbergt, etwa die Zitronengelbe Tramete (Antrodiella citrinella) - der Pilz gilt als sogenannte Urwaldart. Unsere Modelle liefern wertvolle Hinweise darauf, wie zukünftige Biodiversitätsmonitorings mithilfe von Fernerkundung unterstützt werden können. Zudem bilden die erhobenen Daten die Grundlage für ein Langzeitmonitoring der Biodiversität im grössten Naturwaldreservat der Schweiz in der Buchenwaldstufe und ermöglichen in Zukunft Aussagen über die Entwicklung des Waldes zu einem Refugium für Arten, die in bewirtschafteten Wäldern kaum überleben können.

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.

Most strategic and operational forest management decisions are taken based on stand-level information, and quantitative models of forest dynamics are key for developing sustainable management strategies. However, data on forest stands for the initialisation of such models that are representative at large spatial scales, e.g., countries or ecoregions, are often lacking. National Forest Inventories (NFIs) provide forest data from small sample plots at large spatial scales, yet deriving full stand information based on such data is challenging. Here, we evaluate seven methods of varying complexity for deriving quantitative stand descriptions based on sample data as provided by the Swiss NFI. We selected 271 extensively measured Swiss forests stands with unimodal diameter distributions, classified them as beech- vs. spruce-dominated in five development stages and randomly placed a small sized sample plot in each stand using the Swiss NFI sampling design (i.e., a circular plot of 500 m²). Seven modelling approaches were used to derive diameter distributions and species-specific stem numbers (i.e., tree species composition) from the sample data that are representative for a particular stand (local scale) and for stand types in general (generalised scale). The prediction performance of the modelling approaches was evaluated using 100 random samples per stand to calculate prediction errors. Generalised even-aged diameter distributions were best predicted by the simultaneous parameter prediction method (PPM), i.e. a combined three-step regression approach, with on average 1.3 to 2.5 times lower prediction errors compared to the simple pooling of diameter samples. However, uneven-aged diameter distributions were best predicted by pooling. At the local scale, the simultaneous PPM performed best for data from sample plots with fewer than 17 to 19 trees across all development stages. Prediction performance of the PPMs increased for structurally and spatially diverse local stands with positively skewed diameter distributions. A Random Forest approach was most suitable for predicting species composition at both the generalised and the local scale. Our study evaluates the strengths and weaknesses of methods to model stands based on data from small sample plots. We emphasise terminological pitfalls by consequently distinguishing local accuracy and generalised representativity of the stand descriptions. We demonstrate the feasibility of deriving locally accurate stands using data from small forest sample plots and evaluate the derivation of generalised stands representative at large regions. At both scales, our developments contribute to an improved initialisation of forest models and thus to a more realistic modelling of forest development under future boundary conditions.

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. [...]

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.

1. The retention of trees bearing tree-related microhabitats (TreMs) has become an important means of conserving biodiversity in production forests. However, we lack estimates of TreM formation rates and evidence on factors driving TreM formation.
2. Based on the observation of 80,099 living trees from 19 species groups in Europe and Iran, we estimated the probability of TreM occurrence on trees and the associated rate of first TreM formation as a function of tree DBH, management, tree species group and random site effects. We built a separate model for each of 11 TreM groups.
3. The hazard rate of first TreM formation (defined as the probability of formation of a first TreM forming on a tree that is known to have none, during an infinitesimal DBH increment) increased with DBH for some TreM groups like breeding-woodpecker-hole, rot-hole or root-concavity, indicating an acceleration in TreM formation during tree growth. However, it decreased with DBH for TreM groups like bark-loss or dendrotelm, indicating slower formation on very large trees. Most TreM groups had reduced formation rates in managed forests (last logging less than 100 years ago) compared to unmanaged forests (no logging for at least 100 years), with the exception of dendrotelms. No general difference appeared between broadleaves and conifers, but early-successional species tended to have different TreMs than mid- and late-successional species. Abies, Alnus, Betula, Fagus, Prunus, Quercus, Sorbus, Tilia and Ulmus displayed high formation rates for six TreM groups or more. Variability among sites was considerable.
4. Synthesis and applications. The rate of formation of tree-related microhabitats (TreMs) varies greatly among TreM groups, tree species, locations, tree diameters at breast height and forest management. The high rate of formation of some TeM groups on small trees implies that tree retention for biodiversity should concern trees of all sizes and start as soon as thinning operations have occurred. Biodiversity conservation should value not only forest stands and trees that already have many TreMs but also those where the likelihood of future TreM formation is high due to species, maturity or local environmental conditions. The addition of quantitative models of TreM formation to forest stand dynamics simulators is necessary to better take into account biodiversity conservation in forest management.

Tree-related microhabitats (TreMs) have been identified as key features for forest-dwelling taxa and are often employed as measures for biodiversity conservation in integrative forest management. However, managing forests to ensure an uninterrupted resource supply for TreM-dwelling taxa is challenging since TreMs are structures with a limited availability, some of which are triggered by stochastic events or require a long time to develop. At the tree scale, the role of tree species, diameter at breast height (dbh) and status (i.e. living vs standing dead) for favouring TreM occurrence has been quantified and modelled in several studies, since these tree features are routinely recorded in the field. However, TreM occurrence remains difficult to predict, hampering the elaboration of applicable management strategies that consider TreMs. Using an international database encompassing 110,000 trees, we quantified the explanatory power of tree species, dbh, status, time since last harvest and plot context for predicting TreM occurrence at the tree level. Plot context is so far a “black box” that combines local environmental conditions, past and current management legacies, with local biotic features that have high explanatory power for predicting TreM occurrence. Then, based on the literature, we established a set of 21 factors related to site, stand and tree features for which there is a strong assumption that they play a key role in TreM formation. Finally, we identified a sub-set of nine features that should be recorded in the future to provide additional information to enable better prediction of the occurrence of particular TreMs: (i) at plot level: slope, exposure, altitude and presence of cliffs; and (ii) at tree level: bark features, phyllotaxis and compartmentalization capacity of the tree species, plus ontogenic stage and physiological state of the individual tree sampled.

Quantitative estimates of change of primeval European beech (Fagus sylvatica L.) forests at the landscape scale over time are scarce due to both the few remnants of such forests and the absence of repeated sample plot inventories. This forest ecosystem is thought to be relatively stable over time, but it remains unclear what drivers contribute to this stability. Here, we studied temporal change in one of the largest primeval beech forests. Our analysis is based on two consecutive inventories on 238 permanent sample plots in the Uholka-Shyrokyi Luh forest in Transcarpathia, Ukraine, covering 102.8 km². The inventories were carried out in 2010 and 2019.
This data allowed us to derive quantitative estimates for the main structural characteristics tree density, basal area and volume on a landscape scale and to characterize the demographic processes that shape this forest ecosystem. The structural characteristics tree density (2010: 441 N ha¹, 2019: 458 N ha⁻¹), basal area (2010: 35.9 m² ha⁻¹, 2019: 35.4 m² ha⁻¹), and standing volume (2010: 578 m³ ha⁻¹, 2019: 584 m³ ha⁻¹) of the living trees remained stable between the two inventories. The species composition, characterized by a pronounced dominance of beech, remained virtually unchanged as well, with 97.9% of the stems being beech trees in the first and 97.1% in the second inventory. In contrast, we observed a relatively high dynamic when looking at the demographic processes more closely. About 11% of the trees found alive in 2010 died until 2019, resulting in an annual mortality rate of 1.3%. Ingrowing trees compensated for trees that died or decayed regarding tree density, but only to a small extent regarding volume. The volume lost was largely compensated by the growth (8.0 m³ ha⁻¹ year⁻¹) of the surviving trees.
This work characterizes the change of forest attributes in a primeval beech forest at the landscape scale over a time of period of nine years and provides baseline indicators on the development and dynamics of primeval beech forests. It broadens our understanding about the contribution of the main demographic processes to the pronounced structural and compositional continuity of primeval beech forests.

Context: Dead wood is a key habitat for saproxylic species, which are often used as indicators of habitat quality in forests. Understanding how the amount and spatial distribution of dead wood in the landscape affects saproxylic communities is therefore important for maintaining high forest biodiversity.
Objectives: We investigated effects of the amount and isolation of dead wood on the alpha and beta diversity of four saproxylic species groups, with a focus on how the spatial scale influences results.
Methods: We inventoried saproxylic beetles, wood-inhabiting fungi, and epixylic bryophytes and lichens on 62 plots in the Sihlwald forest reserve in Switzerland. We used GLMs to relate plot-level species richness to dead wood amount and isolation on spatial scales of 20–200 m radius. Further, we used GDMs to determine how dead wood amount and isolation affected beta diversity.
Results: A larger amount of dead wood increased beetle richness on all spatial scales, while isolation had no effect. For fungi, bryophytes and lichens this was only true on small spatial scales. On larger scales of our study, dead wood amount had no effect, while greater isolation decreased species richness. Further, we found no strong consistent patterns explaining beta diversity.
Conclusions: Our multi-taxon study shows that habitat amount and isolation can strongly differ in the spatial scale on which they influence local species richness. To generally support the species richness of different saproxylic groups, dead wood must primarily be available in large amounts but should also be evenly distributed because negative effects of isolation already showed at scales under 100 m.

1. Tree regeneration is a key process for long-term forest dynamics, determining changes in species composition and shaping successional trajectories. While tree regeneration is a highly stochastic process, tree regeneration studies often cover narrow environmental gradients only, focusing on specific forest types or species in distinct regions. Thus, the larger-scale effects of temperature, water availability, and stand structure on tree regeneration are poorly understood.
2. We investigated these effects in respect of tree recruitment (in-growth) along wide environmental gradients using forest inventory data from Flanders (Belgium), northwestern Germany, and Switzerland covering more than 40 tree species. We employed generalized linear mixed models to capture the abundance of tree recruitment in response to basal area, stem density, shade casting ability of a forest stand as well as site-specific degree-day sum (temperature), water balance, and plant-available water holding capacity. We grouped tree species to facilitate comparisons between species with different levels of tolerance to shade and drought.
3. Basal area and shade casting ability of the overstory had generally a negative impact on tree recruitment, but the effects differed between levels of shade tolerance of tree recruitment in all study regions. Recruitment rates of very shade-tolerant species were positively affected by shade casting ability. Stem density and summer warmth (degree-day sum) had similar effects on all tree species and successional strategies. Water-related variables revealed a high degree of uncertainty and did not allow for general conclusions. All variables had similar effects independent of the varying diameter thresholds for tree recruitment in the different data sets.
4. Synthesis: Shade tolerance and stand structure are the main drivers of tree recruitment along wide environmental gradients in temperate forests. Higher temperature generally increases tree recruitment rates, but the role of water relations and drought tolerance remains uncertain for tree recruitment on cross-regional scales.