Climate change mitigation and biodiversity conservation are key forest functions, but how to pursue them jointly in timber-managed forests is still unclear. We use a Europe-wide dataset of forest multi-taxon diversity and stand structure to (i) evaluate the importance of aboveground carbon stocks in determining species richness of six taxonomic groups; (ii) assess relationships between species richness and carbon stocks; (iii) discuss the potential to jointly enhance carbon and biodiversity and policy implications. Carbon-diversity relationships are positive for several groups, but mostly when deadwood pools are considered. Forest policies should consider the complex relationship between different carbon pools and taxonomic groups. Environmental policies emphasizing carbon sequestration in aboveground living biomass may conflict with biodiversity conservation by promoting homogeneous, fast-growing forests that fail to support species diversity of multiple groups. Sustainable forest management should acknowledge that deadwood carbon instead may translate into positive outcomes for both carbon storage and biodiversity conservation. Forests are essential for both climate change mitigation and biodiversity conservation, yet how to balance these goals in managed forests remains unclear. Here, using a Europe-wide dataset, the authors find that biodiversity increases with carbon stocks, but mostly when deadwood is included.

The transferability of single or joint species distribution models ((j)SDMs) depends on their ability to predict beyond the observed environmental range and to remain consistent despite shifts in biotic interactions. Transfer accuracy may be improved by recent advances in the application of deep learning that provide greater flexibility and potentially superior predictive accuracy than traditional approaches. We implemented jSDMs with deep and machine learning algorithms and measured the transfer accuracy from continental to regional areas in communities with different species composition. We ran jSDMs with deep neural networks (DNN), elastic net (EN), and stacked SDMs (sSDM) with random forests (RF). We used 134 689 occurrence records representing 1776 species of six taxonomic groups (beetles, birds, bryophytes, fungi, lichens and plants) from 2387 forest plots in Europe. We employed an agnostic modelling approach that covered most of the environmental conditions by including more than 100 satellite-derived variables and 98 climatic variables. The predictive power of the models within the training continental area was evaluated using AUC, whereas the transfer accuracy in the regional area was evaluated with the Boyce index calculated with independent presence records. We found that the DNN-jSDMs outperformed other models at continental scale, but model transfer from continental to regional extent was less accurate. We found that the accuracy of regional predictions was higher for taxonomic groups with better representation in the continental data, such as birds, bryophytes and plants. Depending on the algorithm and the taxonomic group, we achieved acceptable (Boyce > 0) to accurate (Boyce > 0.5) transferability for 32-78% of the species. Our findings underscored the need of considering trade-offs among hyperparameter tuning, spatial scales and model complexity. Our findings also suggest that the varying biotic interaction structures and, particularly, the different species compositions of the transfer areas, may affect model transferability more than previously considered.

Abstract Ectomycorrhizal (ECM) fungi are well-known for their crucial roles in forest health and productivity, yet their responses to various forest management practices are understudied, particularly in oak-dominated forests. The purpose of this study was to better understand the effects of silvicultural treatments on the diversity and community composition of ECM fungi in an oak-hornbeam forest in northern Hungary. We analyzed ITS2 rDNA metabarcoding data of soil-borne fungi to compare richness and community composition of ECM fungi among forest treatment types (clear-cutting, gap-cutting, preparation-cutting, tree retention in clear-cut areas, and control) and between sampling years (2020 and 2021). We found 268 ECM fungal genotypes, with the most diverse phylogenetic clades being /russula-lactarius (52), /tomentella-thelephora (47), /inocybe (40), /sebacina (27), and /cortinarius (20). We found significant compositional difference of ECM fungi among silvicultural treatments in both years, with some variations in richness. There were also small, but still significant compositional differences between the two years. Treatment effect was partly explained by altered environmental variables, such as relative humidity and soil temperature. These results highlight the importance of forest structure and the abiotic environment in driving community dynamics of plant-symbiotic fungi, with potential implications for forest health and productivity.

Continuous cover forestry is gaining importance in temperate forests as an alternative to rotation forestry. However, for light-demanding oaks, especially when coexisting with shade-tolerant tree species, the optimal gap size and shape for successful regeneration remain unclear. We therefore examined the effects of four gap types—two sizes (150 and 300 m2) and two shapes (circular and elongated)—on initial sessile oak (Quercus petraea) regeneration, abiotic conditions, and competitive vegetation in an oak–hornbeam forest in Hungary. We found that tended oak saplings showed the best growth in the initially brightest and most moist large circular gaps. Growth in large elongated gaps (initially with similar light level but drier soil) and small circular gaps (with similar soil moisture but lower light) was slightly lower. In small elongated gaps, where resources were limited, oak growth was slower but still better than in the closed stand. Survival of tended oak saplings was the highest in large gaps and small circular gaps, with intermediate survival in small elongated gaps. Hornbeam abundance and growth was greater in initially moister circular gaps. In untended quadrats, where competition was present, oak saplings were the most abundant in small elongated gaps. However, their growth was slow. Our results suggest that oak regeneration can be initiated in all 150–300 m2 gap types, though tending effort requirements and sapling growth vary. Large circular gaps provide the best abiotic conditions, but if competition is also considered, oak regeneration can be initiated most efficiently (requiring less tending) in elongated gaps. Large elongated gaps allow for higher survival rates and greater growth than small ones, but due to the higher levels of competition, more tending is required. Small elongated gaps may need expansion after a few years to avoid high mortality, but they offer the advantage of continued acorn availability, increasing regeneration chances.

The species richness of vascular plants in forests can have contrasting effects on the occurrence of non‐native insects. The establishment of non‐native insect populations may be facilitated by low plant species richness, which reflects the availability of few but easily accessible resources, or hampered by high plant species richness due to spatial dilution of resources or biotic resistance (i.e., resistance against biological invasions). The relationship between the species richness of plants and non‐native insects is likely influenced by disturbance regimes, which, in European forests, mostly consists of timber harvesting. We investigated this relationship considering two major forest attributes: (i) species richness of non‐native vascular plants and (ii) forest management. From 1101 forest plots in Europe, we gathered occurrences of 1212 vascular plant species, including 160 non‐native species, and of 2404 beetle species, including 29 non‐native species. We tested the relationship between the species richness of non‐native beetles and plants using non‐linear quantile regressions. We disentangled the effect of non‐native plant species richness from that of management on the species richness of non‐native beetles, while accounting for forest structural variables, using structural equation models. We found clear evidence of a hump‐shaped relationship between non‐native beetle and plant species richness. The general shape of the relationship persisted when considering only woody or non‐woody plants, as well as only non‐native plants. The relationship was also similar between managed and unmanaged forests. However, the proportion of non‐native beetles in managed forests was higher than in unmanaged forests at the same plant species richness. Management had a direct negative effect on non‐native beetle species richness, whereas non‐native plant species richness had a direct positive effect. When considering all direct and indirect effects, management facilitated the occurrence of non‐native beetles indirectly via non‐native plants rather than directly. Synthesis and applications . Species richness of native and non‐native vascular plants modulates the species richness of non‐native beetles through relationships with opposite signs. The interplay with management regimes and forest structures determines whether non‐native beetles are promoted. Forest management aimed at reducing the intensity of disturbance while encouraging native plant species richness could promote the dominance of dilution effects and biotic resistance and could moderate the establishment of non‐native insects.

Forest biodiversity is threatened by the use of conventional rotation forestry system, while fine-scale interventions of continuous cover forestry, such as gap-cutting, could protect forest habitats and enhance the stand structural heterogeneity. Consequently, their use could maintain biodiversity during the timber production process. It is unclear which gap sizes and shapes can trigger ample natural regeneration while simultaneously maintaining or improving the near-natural character of the understory. The Pilis Gap Experiment examined the five-year effects of four gap types comparing two gap sizes (150 and 300 m2) and two gap shapes (circular and elongated) on the light and soil moisture conditions and understory vegetation in an oak-hornbeam forest. The investigated understory variables included species richness, total cover, height, shrub cover and cover of five functional groups. Our results indicate an initially increased light in all gap types, but later it decreased in large circular gaps, while remaining more stable in other gap types. Soil moisture increased first, transiently in the circular gaps, and later in the elongated gaps. Species richness temporarily increased in large circular gaps, whereas total cover increased in all gap types. Understory height and shrub cover also increased in large circular gaps. Annual and perennial forb cover remained unchanged in all gap types, although graminoid cover showed transient growth in large elongated gaps. Small gaps had the highest cover of woody seedlings, whereas bramble ( Rubus fruticosus agg.) cover increased the most in large circular gaps. Species composition exhibited the most significant changes in large circular gaps. From a conservation aspect, all gap types can be considered favorable, as they increase the heterogeneity of the openness and understory vegetation in homogeneous closed stands. Vegetation changes are the most prominent in large circular gaps w spread of bramble here multiple vegetation layers developed. However, the dense cover of bramble and shrubs hinders the effective regeneration of sessile oak ( Quercus petraea). Smaller gaps slightly increase the heterogeneity of the forest understory and provide ample light and soil moisture to initiate regeneration. In larger gaps, oak regeneration may be supported by applying an elongated shape, mitigating the competition from bramble.

Several regional initiatives and reporting efforts assess the state of forest biodiversity through broad-scale indicators based on data from national forest inventories. Although valuable, these indicators are essentially indirect and evaluate habitat quantity and quality rather than biodiversity per se. Therefore, their link to biodiversity may be weak, which decreases their usefulness for decision-making. For several decades, Forest Europe indicators assessed the state of European forests, in particular their biodiversity. However, no extensive study has been conducted to date to assess their performance - i.e. the capacity of the indicators to reflect variations in biodiversity - against multitaxonomic data. We hypothesized that no single biodiversity indicator from Forest Europe can represent overall forest biodiversity, but that several indicators would reflect habitat quality for at least some taxa in a comprehensive way. We tested the set of Forest Europe's indicators against the species richness of six taxonomic and functional groups across several hundreds of sampling units over Europe. We showed that, while some indicators perform relatively well across groups (e.g. deadwood volume), no single indicator represented all biodiversity at once, and that a combination of several indicators performed better. Forest Europe indicators were chosen for their availability and ease of understanding for most people. However, we showed that gaps in the monitoring framework persist, and that surveying certain taxa along with stand structure is necessary to support policymaking and tackle forest biodiversity loss at the large scale. Adding context (e.g. forest type) may also contribute to increase the performance of biodiversity indicators.

Managing forests to sustain their diversity and functioning is a major challenge in a changing world. Despite the key role of understory vegetation in driving forest biodiversity, regeneration and functioning, few studies address the functional dimensions of understory vegetation response to silvicultural management. We assessed the influence of the silvicultural regimes on the functional diversity and redundancy of European forest understory. We gathered vascular plant abundance data from more than 2000 plots in European forests, each associated with one out of the five most widespread silvicultural regimes. We used generalized linear mixed models to assess the effect of different silvicultural regimes on understory functional diversity (Rao's quadratic entropy) and functional redundancy, while accounting for climate and soil conditions, and explored the reciprocal relationship between three diversity components (functional diversity, redundancy and dominance) across silvicultural regimes through a ternary diversity diagram. Intensive silvicultural regimes are associated with a decrease in functional diversity and an increase in functional redundancy, compared with unmanaged conditions. This means that although intensive management may buffer communities' functions against species or functional losses, it also limits the range of understory response to environmental changes. Policy implications. Different silvicultural regimes influence different facets of understory functional features. While unmanaged forests can be used as a reference to design silvicultural practices in compliance with biodiversity conservation targets, different silvicultural options should be balanced at landscape scale to sustain the multiple forest functions that human societies are increasingly demanding. Different silvicultural regimes influence different facets of understory functional features. While unmanaged forests can be used as a reference to design silvicultural practices in compliance with biodiversity conservation targets, different silvicultural options should be balanced at landscape scale to sustain the multiple forest functions that human societies are increasingly demanding.image

Most broad-scale forest biodiversity indicators are based on data from national forest inventories and are used to assess the state of biodiversity through several regional initiatives and reporting. Although valuable, these indicators are essentially indirect and evaluate habitat quantity and quality rather than biodiversity per se . Besides, most of these indicators are applicable at regional or national scales, while their use at a more local level is difficult. Therefore, their link to biodiversity may be weak, which decreases their usefulness for decision-making.For several decades, Forest Europe indicators assessed the state of European forests, in particular its biodiversity. However, no extensive study has been conducted to date to assess the performance of these indicators against multitaxonomic data. We hypothesized that – as implied by the reporting process – no single biodiversity indicator from Forest Europe can represent overall forest biodiversity, but that several – eventually combined – indicators would reflect habitat quality for at least some taxa in a comprehensive way. We tested the set of indicators proposed by Forest Europe against the species richness of six taxonomic and functional groups (tracheophytes, epixylic and epiphytic bryophytes, birds, saproxylic beetles, saproxylic non-lichenized fungi and epixylic and epiphytic lichenized fungi) across several hundreds of plots over Europe. We showed that, while some indicators perform relatively well across groups (e.g. deadwood volume), no single indicator represented all biodiversity at once, and that a combination of several indicators performed better. Surprisingly, some indicators showed weak links with the biodiversity of the six taxonomic and functional groups.Forest Europe indicators were chosen for their availability and ease of understanding for most people. However, our analyses showed that there are still gaps in the monitoring framework, and that surveying certain taxa along with stand structure is necessary to support policymaking and tackle forest biodiversity loss at the large scale.Impact statement Biodiversity indicators used to assess the state of Europe’s forests perform unequally; a combination of indicators is more informative### Competing Interest StatementThe authors have declared no competing interest.

Forest biodiversity studies conducted across Europe use a multitude of forestry terms,often inconsistently.This hinders the comparability across studies and makes the assessment of the impacts of forest management on biodiversity highly context-dependent.Recent attempts to standardize forestry and stand description terminology mostly used a top-down approach that did not account for the perspectives and approaches of forest biodiversity experts.This work aims to establish common standards for silvicultural and vegetation definitions,creating a shared conceptual framework for a consistent study on the effects of forest management on biodiversity.We have identified both strengths and weaknesses of the silvicultural and vegetation information provided in forest biodiversity studies.While quantitative data on forest biomass and dominant tree species are frequently included,information on silvicultural activities and vegetation composition is often lacking,shallow,or based on broad and heterogeneous classifications.We discuss the existing classifications and their use in European forest biodiversity studies through a novel bottom-up and top-driven review process,and ultimately propose a common framework.This will enhance the comparability of forest biodiversity studies in Europe,and puts the basis for effective implementation and monitoring of sustainable forest management policies.The standards here proposed are potentially adaptable and applicable to other geographical areas and could be extended to other forest interventions.

This research was started in 2014, led by the Pilis Forestry Systems Experiment (PFSE), a long-term ecological study established in the Pilis Mountains that investigaties the effects of the forestry treatments on forest site, regeneration and multi-taxon biodiversity. We compared the effects of different treatments of rotation and selection

Forest management has a major impact on the understorey vegetation, with the intensity and type of the applied silvicultural treatments driving variable vegetation responses. We compared understorey variables across different experimental silvicultural treatments in a temperate oak-hornbeam forest in Central Hungary. Five treatment types were used in six replicates representing rotation and selection silvicultural systems: control (C), clear-cutting (CC), gap-cutting (G), preparation cutting (P), and retention tree group (R). The response of several understorey variables was investigated to the treatments in the first six years after their implementation in 2014.We assessed how understorey variables change in response to different forestry treatments, how these responses vary with time, and how game exclusion affects them. We then evaluated how well the treatments can preserve the forest character of the vegetation.We found a large temporal variability in the understorey variables over the study period. In all cases, the interventions led to an initial increase in species richness, followed by a decline later, where the regeneration layer started to close. The regeneration layer grew most intensively in G and CC. At the end of the study, R had the highest average species number, comprising a heterogeneous group of perennial forb species. The interventions all resulted in a rapid increase in total herb layer cover, mainly in favour of graminoid and perennial species. The extent of cover increase depended primarily on the amount of additional light received (CC > G > P > R > C). Turnover and beta diversity values also decreased in a similar order. The effect of game exclusion was especially pronounced in the case of the CC and G, where game browsing significantly slowed the regeneration outside the fences. The most significant changes in almost all variables were in the CC. It had the highest number of indicator species, many of them annual, disturbance-related, and invasive. G preserved the forest character of the vegetation better and proved to be less susceptible to the mass appearance of disturbance-related and invasive herbaceous species.Increasing the share of continuous cover forestry methods is crucial to preserve the forest herb layer. Rotation forestry with large cutting areas is not recommended or should be kept at low landscape rates, as these areas are highly exposed to disturbance-related and invasive species. Leaving retention tree groups can be key to the survival of numerous forest plant species.

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 repre-sented 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 develop-mental 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 stra-tegies in: (i) methodological harmonization and coordinated monitoring; (ii) definition and testing of SFM in-dicators 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.

Ectomycorrhizal (EM) fungi are one of the most ecologically and culturally important fungi in temperate forest ecosystems, as root contacts with EM fungi are essential for the survival of most forest tree species. The Pilis Forestry Systems Experiment, ongoing since 2016, compares five different forest management methods (four treatments and control) in terms of their effects on abiotic environmental variables, vegetation and mesofauna. The project presented here provides the first insight in our country on the impact of forest management on the composition of EM fungal communities based on DNA data from soil. A total of 4480 fungal genotype DNA sequences were determined in 30 plots sampled in October 2020, more than half of which could be identified at the genus level or higher. Of these, EM fungi were represented by 534 genotypes from 38 genera. Silvicultural practices affected both diversity and composition of fungal communities. Diversity of EM fungi was reduced in the clear-cut and 20 m gap

The Pilis Forestry Systems Experiment,

Most European forests are used for timber production. Given the limited extent of unmanaged (and especially primary) forests, it is essential to include commercial forests in the conservation of forest biodiversity. In order to develop ecologically sustainable forest management practices, it is important to understand the management impacts on forest-dwelling organisms. Experiments allow testing the effects of alternative management strategies, and monitoring of multiple taxa informs us on the response range across forest-dwelling organisms. To provide a representative picture of the currently available information, metadata on 28 multi-taxa forest management experiments were collected from 14 European countries. We demonstrate the potential of compiling these experiments in a single network to upscale results from the local to continental level and indicate directions for future research. Among the different forest types, temperate deciduous beech and oak-dominated forests are the best represented in the multi-taxa management experiments. Of all the experimental treatments, innovative ways of traditional management techniques (e.g., gap cutting and thinning) and conservation-oriented interventions (e.g., microhabitat enrichment) provide the best opportunity for large-scale analyses. Regarding the organism groups, woody regeneration, herbs, fungi, beetles, bryophytes, birds and lichens offer the largest potential for addressing management–biodiversity relationships at the European level. We identified knowledge gaps regarding boreal, hemiboreal and broadleaved evergreen forests, the treatments of large herbivore exclusion, prescribed burning and forest floor or water manipulations, and the monitoring of soil-dwelling organisms and some vertebrate classes, e.g., amphibians, reptiles and mammals. To improve multi-site comparisons, design of future experiments should be fitted to the set-up of the ongoing projects and standardised biodiversity sampling is suggested. However, the network described here opens the way to learn lessons on the impact on forest biodiversity of different management techniques at the continental level, and thus, supports biodiversity conservation in managed forests.

Forest management integrating nature conservation aspects into timber production focuses increasingly on small-scale interventions. However, the ecological consequences of gap cuttings remain ambiguous in oak-dominated forests. In the Pilis Gap Experiment, we analyze how combinations of different gap shapes (circular and elongated), and gap sizes (150 m2 and 300 m2) affect the microclimate and biota of a mature sessile oak-hornbeam forest in Hungary. We first report the changes in direct and diffuse light, soil moisture, daily air and soil temperatures, and relative air humidity in the experimental cuttings in the vegetation season directly following their implementation. Diffuse light had a central maximum and a concentric pattern. Direct light was distributed along a north-south gradient, with maxima in northern gap parts. Soil moisture was determined by gap shape: it increased significantly in the center of circular gaps, with multiple local maxima in the southern-central parts of large circular gaps. Its pattern was negatively related to direct light, and larger spatial variability was present in circular than in elongated gaps. The daily mean air temperatures at 1.3 m increased in all, especially in large gaps. Soil and ground-level temperatures remained largely unchanged, reflecting on light and soil moisture conditions affecting evaporative cooling. Relative humidity remained unaltered. Even though the opening of experimental gaps changed microclimatic conditions immediately, effect sizes remained moderate. Gap size and gap shape were both important determinants of microclimate responses: gap size markedly affected irradiation increase, gap shape determined soil moisture surplus, while soil and air temperatures, and air humidity depended on both components of the gap design. We conclude that 150-300 m2 sized management-created gaps can essentially maintain forest microclimate while theoretically providing enough light for oak regeneration; and that the manipulation of gap shape and gap size within this range are effective tools of adaptive management.