Tree-related microhabitats (TreMs) are well-delineated tree structures that provide important resources and refuge to thousands of species. For these reasons, TreMs have been recommended as an indirect indicator of biodiversity in European forests. We conducted a literature review to understand the scale at which different taxonomic groups are associated with TreMs in Europe. Across a pool of 54 papers, we found that TreMs were most frequently studied at the scales of sampling plots (n = 28, 58 %) and individual trees (n = 18, 33 %), and less frequently at the stand scale (n = 5, 9 %). Two studies addressed the landscape scale in addition to the plot and stand scale (n = 1). Birds, saproxylic beetles and bats were the most frequently studied taxonomic groups overall and at the plot scale, whereas amphibians were only studied at the scale of individual trees. Tree cavities were the most frequently studied TreM-form at all scales and were often associated with bats and saproxylic beetles. Crown deadwood and fungal fruiting bodies were associated with saproxylic beetles, and tree injuries were frequently associated with bats. Sample sizes across all TreMs-taxa combinations were low, allowing only descriptive analyses and interpretation. The largest sample size for saproxylic beetles investigated at the plot scale (n = 15) across different TreMs, were found to be significantly associated more than 20 % of the times with tree cavities (33 %), tree injuries (21 %) and fungal fruiting bodies (20 %). We recommend future studies to evaluate species-TreM associations for the poorly studied TreMs such as epiphytic and epixylic structures, excrescences, and exudates, or poorly studied taxa such as meiofauna, and to evaluate multiple scales and taxonomic groups when possible. Our review could also serve as a starting point to consider which taxonomic groups would benefit from those management practices that enhance TreM-associated biodiversity.

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.

Against a background of intensifying climate-induced disturbances, the need to enhance the resilience of forests and forest management is gaining urgency. In forest management, multiple trade-offs exist between different demands as well as across and within temporal and spatial scales. However, methods to assess resilience that consider these trade-offs are presently lacking. Here we propose a hierarchical framework of principles, criteria, and indicators to assess the resilience of a social-ecological system by focusing on the mechanisms behind resilience. This hierarchical framework balances trade-offs between mechanisms, different parts of the social-ecological system, ecosystem services, and spatial as well as temporal scales. The framework was developed to be used in a participatory manner in forest management planning. It accounts for the major parts of the forest-related social-ecological system and considers the multiple trade-offs involved. We demonstrate the utility of the framework by applying it to a landscape dominated by Norway spruce (Picea abies (L.) Karst.) in Central Europe, managed for three different management goals. The framework highlights how forest resilience varies with the pursued management goals and related management strategies. The framework is flexible and can be applied to various forest management contexts as part of a participatory process with stakeholders. It thus is an important step towards operationalizing social-ecological resilience in forest management systems.

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.

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.

The potential of trees and forests to store carbon and provide materials and energy for the green transition is widely recognized, but little guidance is available for the forest manager to make climate friendly decisions in daily forest management practice. Marteloscopes, outdoor learning facilities for field-based training and education of forestry professionals and students, have been established in many European countries for illustrating trade-offs between multiple goals. They offer a unique possibility to facilitate training in understanding the climate implications of forest management actions. We provided the scientific foundation for including climate effects in the complex decision making of foresters at the forest stand and individual tree level. Hereby we intended to provide a framework for assessing climate effects of forest operations at tree and stand level, increase knowledge on forests and their climate effects, and raise awareness of the potential of forests as part of the green transition. We used two Danish marteloscopes combined with two thinning strategies to illustrate the modelling of forest carbon emissions at stand and individual tree level. In all four resulting scenarios, the thinning resulted in loss of forest carbon stock compared to the no-harvest alternative. However, in three of the four scenarios, this loss was more than outweighed by an increase of carbon in harvested wood products and the effect of substituting more carbon intensive materials with wood, resulting in a total net reduction in atmospheric carbon compared to the non-harvested scenario. The results pointed to the carbon storage and substitution effects of harvested wood products as well as the ability of the remaining forest to recapture CO2 as the main drivers determining the climate effect of specific forest operations. Including individual tree carbon storage and substitution potentials alongside existing attributes of biodiversity and economic value enable the consideration of trade-offs between carbon, biodiversity, and economy and facilitates comprehensive learning and discussion about these trade-offs.

Tree to tree interactions are important structuring mechanisms for forest community dynamics. Forest management takes advantage of competition effects on tree growth by removing or retaining trees to achieve management goals. Both competition and silviculture have, thus, a strong effect on density and distribution of tree related microhabitats which are key features for forest taxa at the stand scale. In particular, spatially-explicit data to understand patterns and mechanisms of tree-related microhabitats formation in forest stands are rare. To train and eventually improve decision-making capacities related to the integration of biodiversity aspects into forest management plot of one hectare, so called marteloscopes were established in the frame of the ‘European Integrate Network’. In each plot, a set of data is collected at the individual tree level and stored in a database, the ‘I+ repository’. The 'I+ repository' is a centralised online database which serves for maintaining the data of all marteloscope plots. A subset of this repository was made publicly available via the Global Biodiversity Information Facility, based on a data-sharing policy. Data included are tree location in plot, tree species, forest mensuration data (diameter at breast height [cm], tree height [m]), tree status (living or standing dead) and tree-related microhabitats. Further, a visual assessment of timber quality classes is performed in order to provide an estimate of the economic value (market price) for each tree. This information is not part of the GBIF dataset. Currently 42,078 individual tree observations from 111 plots are made available via the Global Biodiversity Information Facility (GBIF). As the network of plots continues to expand, so does the database of tree-related microhabitats. Therefore, the database will undergo a regular update. The current version has a temporal coverage from March 2014 to December 2020. The innovation of this unique dataset is that it is based on a commonly agreed catalogue of tree microhabitats as a field reference list when assessing assessment protocol. The reference list is available in 17 languages and, thus, helps to guarantee compatibility of tree-related microhabitat assessments across countries and plots.

Closer-to-Nature Forest Management 3. Use adaptive management as a way to tackle uncertainties: We need to regularly monitor forest responses to management interventions, evaluate these responses and adjust management strategies accordingly.A similar adaptive approach is urgently required to evaluate the impact of policy measures and support mechanisms proposed to encourage adoption of Closer-to-Nature Forest Management. Not a quick-fix, long-term measures are needed:The introduction of Closer-to-Nature Forest Management is not a 'quick-fix' and policy makers must provide long-term and consistent support measures to encourage forest managers and other stakeholders to adopt this strategy.Support for forest owners for training and application of the strategy is key. Review existing subsidy and taxation regimes for private owners:Convincing private owners to follow this approach will require the creation of schemes that reward them for providing ecosystem services.Closer-to-Nature Forest Management has the potential to support biodiversity, adapt forests to climate change and provide ecosystem services to a higher level than conventional forest management.There is an urgent need to review existing subsidy and taxation regimes affecting private forestry, and to consider how these might be changed to further the uptake of Closer-to-Nature Forest Management. Develop and use new technologies and tools:There is a need to harmonize monitoring systems and to develop and use new technologies and tools (GIS, GPS and remote sensing) to ease management of these more diverse and structure-rich forests.Finally, there are still some uncertainties about the effect of certain elements of Closer-to-Nature Forest Management on biodiversity conservation and ecosystem health, and how they will affect other ecosystem services including wood production under different management conditions throughout Europe.This calls for more collective learning, experimentation and research.

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. 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. 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. 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.

A Tree-related Microhabitat (TreM) is a distinct, well-delineated morphological singularity occurring on living or standing dead trees, which constitutes a crucial substrate or life site for various species. TreMs are widely recognized as key features for biodiversity. Current TreM typology identifies 47 TreM types according to their morphology and their associated taxa. In order to provide a range of resolutions and make the typology more user-friendly, these 47 TreM types have been pooled into 15 groups and seven forms. Depending on the accuracy required and the time available, a user can now choose to describe TreMs at resolution levels corresponding to type, group or form. Another way to more easily record TreMs during routine management work would be to use co-occurrence patterns to reduce the number of observed TreMs required. Based on a large international TreM database (2052 plots; 70,958 individual trees; 78 tree species), we evaluated both the significance and the magnitude of TreM co-occurrence on living trees for 11 TreM groups. We highlighted 33 significant co-occurrences for broadleaves and nine for conifers. Bark loss, rot hole, crack and polypore had the highest number of positive co-occurrences (N = 8) with other TreMs on broadleaves; bark loss (N = 4) had the highest number for conifers. We found mutually exclusive occurrences only for conifers: Exposed Heartwood excluded both dendrotelm and sap run. Among the four variables we tested for their positive contribution to significant co-occurrences, tree diameter at breast height was the most consistent. Based on our results and practical considerations, we selected three TreM groups for broadleaves, and nine for conifers, and formed useful short lists to reduce the number of TreM groups to assess during routine forest management work in the field. In addition, detecting potential similarities or associations between TreMs has potential theoretical value, e.g. it may help researchers identify common factors favouring TreM formation or help managers select trees with multiple TreMs as candidates for retention.

Integrating nature conservation effectively in forests managed for timber production implies reconciling a trade-off between ecological and economic objectives. In continuous cover forest management, this culminates in decisions about tree harvesting (or retention) determining both the prevalence of tree-related microhabitats in the forest and the economic viability of timber management. Applying an innovative mixed methods approach, we compare conservationists and foresters performing a tree selection exercise. We assess the outcomes of their forest management decisions quantitatively and explore their strategies and the underlying reasoning based on qualitative data. Our findings show that particularly the habitat trees differ greatly between the two groups: while conservationists retained almost exclusively large oaks at often high opportunity costs, foresters retained a notable number of smaller-diameter hornbeams. These differences are related to a different perception of opportunity costs of retention by both groups, as well as because they do not agree about how to value current tree-related microhabitats and their projection into the future. Such diverging patterns of reasoning imply incompatible interpretations of what constitutes a habitat tree. Our results indicate that it is important to apply benchmarks for evaluating ecological goals as well as to increase foresters’ and conservationists’ understanding about the motivations and restrictions of the respective counterpart. Our study points out a significant potential for (mutual) learning, and illustrates the complementarity of quantitative and qualitative research methods to examine tree selection behaviour.

Marteloscopes as training tools for the retention and conservation of habitat trees in forests Recently several initiatives on the political and practical level have aimed at promoting forest biodiversity and at halting the loss of species in forest ecosystems. Nevertheless numerous species are still threatened. The main reason for this phenomenon is most likely the fact that habitat trees – i.e. trees providing microhabitats for various different species – are rare in managed forests. In the course of the projects Integrate and Integrate+ (carried out at the European Forest Institute between 2011 and 2017) we looked for methods on how to retain habitat trees within the scope of integrative forest management. It became more and more apparent that most foresters very successfully recognize economically valuable trees, but not habitat trees. For that reason we categorized habitat structures occurring on trees and developed a microhabitat catalogue to determine habitat values. Additionally we used marteloscopes as training tools for the retention of habitat trees within integrative forest management. Altogether we established 40 such marteloscopes with a biodiversity focus across Europe. Three of these are located in Switzerland. Using a mobile app on a tablet computer we analyzed effects of different tree selections with respect to habitat and economic value of a stand. Furthermore the marteloscopes are ideal tools to convince decision makers or conservationists that harvesting trees and promoting biodiversity can be achieved in the very same stand.

The most significant European forest-related strategies highlight the importance of multifunctional forests for human wellbeing, due to the provision of a wide range of goods and services. However, managing competing aims, such as timber production, economic drivers and biodiversity conservation is often difficult for practitioners. In order to assess the loss and gain of ecosystem services caused by forestry, trade-off evaluation has been increasingly used to aid decision-making. In this study, four silvicultural scenarios are simulated using the Marteloscope approach to evaluate the trade-offs between biodiversity conservation and timber production. Tree-related Microhabitats (TreMs) are used as a proxy to evaluate forest habitat value, while timber production is assessed by the number of harvested trees, biomass removal and economic income. This study takes an innovative approach by investigating TreMs using the Marteloscope in mixed Mediterranean forest. The main findings from this paper confirm that tree-related microhabitats can be considered ecological indicators effective in identifying important habitat trees, to assess forest habitat value and support tree marking for thinning operations and management.