Abstract Saproxylic insect community assembly is structured by deadwood and forest habitat gradients, as well as biotic interactions such as competition, predation, and parasitism. However, co-variation between abiotic and biotic conditions limits our ability to disentangle their contributions. Furthermore, a focus on beetles in temperate and boreal forests has left important taxonomic and geographic knowledge gaps. Here, we tested how experimentally manipulated tree diversity, deadwood position (lying vs. standing), and biotic interactions with a dominant antagonist (ant exclusion) structure communities of deadwood-cavity-nesting bees, wasps, and their parasitoids in a subtropical forest. Lying deadwood supported less diverse and abundant communities than standing deadwood while retaining approximately twice as much moisture. Moreover, host emergence declined along moisture gradients within each deadwood type. Together, these patterns identify substrate moisture as an important factor limiting bee and wasp communities nesting in deadwood – reflecting reduced brood production, survival, or both – and as a candidate driver for their positive association with standing deadwood in forests. By contrast, neither ant exclusion nor observed ant occurrence was associated with host or parasitoid responses. Because standing deadwood is typically scarce in managed forests, our findings support retaining or creating standing deadwood structures alongside lying deadwood and, more broadly, promoting dry and elevated nesting substrates for cavity-nesting bees and wasps in conservation and restoration measures. Graphical abstract

Many forests have a long history of human land use, which shapes species communities and ecosystem processes, making robust and quantitative measures of land-use intensity in forests desirable. We here introduce the ForMIX (Forest Management IndeX), a compound index combining altered tree species composition, tree removal, deadwood availability and stand maturity, which are each calculated as the deviation from expectations in an unmanaged old-growth forest reference. The index and its components allow for mechanistic inference on the consequences of land use in forests as they are based on biotic resources and niches directly affected by forest land use. Using basic forest inventory data from 150 sites distributed over three regions of Germany, we demonstrate the properties of ForMIX, which differentiates well among forest types and silvicultural systems and is robust to decisions regarding reference values and components. Reference values used in ForMIX are dynamic, could be adapted to ongoing climate change and may require refinement for different geographic regions. ForMIX advances the quantification of land-use intensity in forests by being biologically meaningful, usable and comparable across forest types, derivable from standard forest inventory data, and easy to apply, understand and interpret.

Forest ecosystems play a key role in mitigating both climate change and biodiversity loss, as they store large amounts of carbon and provide habitat for a diverse flora and fauna. However, for temperate European forests, several studies suggest that there are not unequivocally unidirectional correlations between carbon storage (above- and below-ground) and the diversity of biota across taxa. Although the effects of forest management on carbon stocks and biodiversity are well understood, the impact on their interrelationship remains unclear. Furthermore, previous studies have primarily examined the aboveground strata, despite the substantial contribution of the soil to carbon stocks and biodiversity. In this study, we synthesized data on carbon stocks, forest management intensity, and arthropods from the above- and below-ground strata, collected at 150 forest plots representative of Central European forests. In line with previous studies, total and aboveground carbon stocks decreased, whereas belowground carbon stocks increased with increasing forest management intensity. Furthermore, the response of arthropods to increasing management intensity was mixed. While total and aboveground carbon stocks were positively associated with the biomass of belowground arthropods and aboveground myceto-detritivorous insects, relationships between aboveground carbon stocks and aboveground arthropods were mainly positive, albeit negatively affected by increasing forest management intensity. Belowground carbon stocks were negatively associated with increases in belowground arthropods, but higher management intensity positively influenced these correlations. Our findings reveal that adjusting forest management to promote individual arthropod groups and carbon stocks can lead to trade-offs for other arthropod groups and for carbon stocks. Nevertheless, the observed trade-offs between different arthropod groups and carbon stocks suggest that promoting a mosaic of management types while reducing general management intensity might enhance habitat suitability for arthropods and the carbon storage potential. This could help to maximise synergies between storing large amounts of carbon and mitigating biodiversity loss in forests.

The UN Decade on Ecosystem Restoration aims to stop biodiversity losses1. Approximately 60% of tropical forests have already been lost or severely degraded2, making restoration essential to achieve conservation goals. Recovery trajectories of trees have been studied intensively3,4, but a comprehensive understanding of biodiversity recovery is lacking. Here we analyse recovery trajectories across trophic levels including 16 taxonomic groups from three kingdoms in a lowland tropical forest by investigating resistance to perturbation, recovery times and return rates to old-growth forest conditions. Abundance and diversity regained more than 90% and composition approximately 75% similarity to old-growth forests within 30 years, but full recovery takes several decades. Mobile animal communities acting as seed dispersers or pollinators had high resistance levels and recovered faster than trees or tree seedlings. Return rates contributed 1-2.5 times more than resistance to the recovery times of species composition. Taxon-specific recovery times could not be explained by simple mechanisms (life-history strategies, trophic level or mobility). We show the enormous potential of protecting naturally recovering secondary forests to stop and reverse biodiversity losses.

Wild bees are widely distributed and effective pollinators, yet they face significant threats such as degradation of forests. Forest restoration has been advocated as a strategy to mitigate these threats and stabilize biodiversity. However, there is a lack of understanding of the ecological consequences of forest restoration on bee diversity, particularly regarding interactions with tree diversity and microenvironment. Using data from the world's largest tree diversity experiment (BEF-China), this study examines how tree species richness, canopy cover, understorey vegetation, and microclimatic conditions affect bee diversity in the context of forest restoration. Our analysis of bee diversity data (8341 individuals from 79 species) revealed that these biotic factors had distinct effects on three dimensions of bee diversity. Specifically, canopy cover had a negative effect on bee taxonomic diversity but a positive effect on phylogenetic and functional diversity. However, these patterns were reversed when the cover of understorey vegetation was accounted for. Moreover, tree species richness exerted an indirect influence on bee diversity through understorey microenvironment. Our findings provide nuance into how tree species richness shapes bee communities via vegetation cover and microclimate, which is informative on habitat characteristics in forest restoration and conservation that better enable the safeguarding of pollinators.

1. Although the biodiversity crisis and insect decline have received increasing scientific and public attention, they persist. Measures to stop biodiversity loss do not yet appear to be effective. A key step forward is to bridge science and practice, with digital tools and data-driven environmental education playing a crucial role. 2. To provide an evidence-based tool that shows the negative effects of mowing on insects and spiders, we developed the 'Insect Calculator' (insektentaschenrechner. de/en). Using a large, comprehensive dataset from suction sampling across various grassland types and urban green spaces in Germany and Switzerland, the Insect Calculator models, visualises and communicates the effects of different detailed mowing regimes and other parameters, such as urban isolation surrounding grasslands, on arthropod density and species richness. 3. Two different modes of the Insect Calculator, which require different levels of knowledge, are available to users: the 'Walking mode' for interested laypersons and the 'Expert mode' mainly for grassland management practitioners. For example, the tool shows that 73% more arthropods can be found in unmown meadows than in mown ones. It also highlights that, compared to bar mowers, mulchers cause a greater decline in insect populations, as well as the influence of other factors, including cutting height, width and frequency. One particular benefit of the Insect Calculator is that it shows how arthropods are affected by these various factors applied in conjunction. 4. Solution. The Insect Calculator is an informative and user-friendly tool designed to enhance environmental education by illustrating the severe effects of mowing on insects and spiders. Users themselves can answer questions about how to support arthropods. The proposed tool aims to inspire the development of similar tools that bridge the gap between science and practice in biodiversity conservation while also addressing other biodiversity threats.

Biodiversity loss threatens the multifunctionality of ecosystems on which human well-being ultimately depends. Multitrophic species interactions may be key to explaining the ecological consequences of biodiversity loss, but research explicitly linking species interactions and ecosystem multifunctionality remains rare. To fill this gap, we synthesize data from a large-scale biodiversity experiment established in 2009 in subtropical China that manipulates tree species richness (1-24 species). We integrate 11 types of antagonistic and mutualistic species interaction networks, and 34 ecosystem functions associated with a diverse set of species and trophic levels. Our analysis highlights that characterizing the structure of species interaction networks is invaluable for assessing interaction-mediated biodiversity effects and underlying mechanisms. Positive effects of network size align with expected benefits of multitrophic diversity for ecosystem multifunctionality. Positive effects of niche overlap among interacting species and negative effects of highly connected species (i.e. high linkage density) reveal additional, interaction-mediated drivers. The effects of niche overlap suggest benefits of functionally similar species, and the effects of linkage density underscore the importance of specialized interactions in promoting ecosystem multifunctionality. These findings emphasize that ecosystem service provisioning does not only rely on biodiversity across trophic levels, but to a similar degree on how species interact.

1. Tree species richness is known to enhance biodiversity and ecosystem functioning, but its effects across trophic levels during forest restoration remain insufficiently understood. In reforestation on complex terrain, habitat complexity may moderate the effect of canopy closure on animal community reassembly, a relationship further shaped by the abiotic environment. 2. Ants, as key functional organisms sensitive to vegetation structure and microclimate, provide an ideal system to test how tree species richness, canopy closure and abiotic environmental variables jointly influence animal communities in young plantations. 3. We examined ground ant diversity, community composition, and functional traits in subtropical tree plantations, using nearby secondary forest as a reference. No ant metric, including incidence, species richness, taxonomic or functional diversity, or community-weighted trait means, responded to tree species richness. Instead, canopy closure and soil pH explained variation in most metrics. 4. Canopy closure and topographical exposure shaped community composition, with canopy closure negatively influencing ant incidence and species diversity, while soil pH consistently had a positive effect on both across the observed range (4.1–5.2). More closed canopies favored leaf-litter-nesting ants, a higher proportion of predators, and communities with increased mandible length, indicating convergence toward forest-adapted predatory assemblages. Soil-nesting ants approached secondary forest values only where both canopy closure and tree species richness were high. 5. Early canopy closure appears to be the primary biotic environmental variable influencing ant community recovery in young plantations, which is further attenuated by the abiotic environment such as soil conditions. This highlights canopy development as a key mechanism of faunal recovery, beyond the effects of tree species richness alone, with increasing predator dominance suggesting potential functional restoration.

Abstract Climate change strongly impacts forest ecosystems, exacerbating drought stress, tree mortality, and shifts in species composition. While aboveground changes are well documented, belowground biodiversity and its response to climate‐driven alterations remain less studied. Soil‐dwelling decomposer organisms such as oribatid mites (Oribatida), earthworms (Lumbricidae), and terrestrial isopods (Oniscidae) play a crucial role in maintaining ecosystem functions through the decomposition of organic matter and nutrient cycling. Abiotic soil conditions drive soil animal communities and the effect of climate change on a forest also depends on soil properties such as water‐holding capacity. This study examines the relationship between environmental conditions and decomposer communities in the municipal forest of Darmstadt, Germany, characterized by distinct edaphic and climatic gradients (dry in the West and moist in the East). In the study region, climate change strongly increased tree mortality albeit with high variation across sites. Oribatid mites and earthworms were significantly more abundant in the moister eastern region, while isopods showed no clear abundance differences but higher diversity in the drier west. Decomposer communities were primarily related to soil moisture and tree community composition, with canopy openness playing a crucial role in microclimatic variation. Microclimatic changes in response to tree damage and altered species composition significantly influenced decomposer communities. Drought‐prone sites were favored by drought‐tolerant taxa, suggesting ongoing shifts in belowground biodiversity. The study underscores the importance of integrating belowground organisms and processes in climate impact assessments in forests to better understand ecosystem resilience and functionality in changing forest environments.

1. How changes in habitat conditions influence insect diversity has been intensively studied. However, whether trophic interactions of insects are also influenced by such changes is largely unknown. Higher habitat heterogeneity is often hypothesized to promote niche partitioning and complementarity in resource use among interacting species, yet evidence from animal interaction networks is sparse. 2. We tested in a biodiversity experiment how experimentally-manipulated tree species richness influences ant-nutrient interaction networks using a replicated and standardized set of nutrient baits. The structure of ant-nutrient networks was quantified with selected quantitative network indices representing diversity, redundancy and specialization of interactions. 3. Contrary to expectations, tree species richness had no direct or indirect effects on network complementarity. Instead, diversity of interactions declined slightly with increasing tree species richness, while network redundancy and specialization did not change. This indicates that higher tree diversity may influence the diversity of trophic links, possibly through altered competitive dynamics in more diverse ant assemblages, but not overall network structure. 4. These patterns are consistent with nutrient regulation theory predicting that foragers should concentrate interactions on resources that best satisfy the nutritional targets of colonies, reducing interaction diversity when nutritionally relevant resources are readily available. Our results suggest that for trophic networks of ants, habitat heterogeneity alters interaction frequencies rather than promoting complementarity or structural reorganization, indicating that ant trophic interactions are robust to tree species loss.

Biodiversity-ecosystem functioning (BEF) research has shown that ecosystem functioning and stability are closely linked to biodiversity. A cornerstone of this field is the BEF-China research platform, i.e. the world’s largest forest biodiversity experiment in subtropical China. It has demonstrated that tree diversity enhances productivity, carbon sequestration and ecosystem stability. However, the strength of these positive tree diversity effects varies widely across forests, possibly because higher trophic levels (such as herbivores and predators) mediate how biodiversity influences ecosystem functioning.To better understand how tree diversity influences higher trophic levels and their contributions to forest functioning, the German Research Foundation (DFG) is funding the project MultiTroph. MultiTroph quantifies species interactions and integrates them into food webs to understand when and why ecosystem functions change or destabilise with species loss. We expect that trophic interaction networks reveal how species share or separate their ecological roles, with more niche overlap in species-rich forests and more niche specialisation in species-poor forests.Here, we outline our conceptual framework and research goals. We are convinced that MultiTroph will expand existing BEF research and provide a more holistic understanding of the role of multi-trophic food webs in forest ecosystems.

Positive effects of plant diversity on productivity increase over time through stronger complementarity among species 1 . However, whether these temporal dynamics propagate across trophic levels towards higher-order ecosystem functions remains largely unexplored. Here we tested how tree diversity drives attack rates and trophic interactions of host-parasitoid communities over the course of a decade in a young large-scale forest experiment. We found that the effect of tree species richness on parasitism increased over time, driven both by accelerating stand productivity and changes in food web structure. Parasitism increasingly reflected an attenuation of consumer responses to accumulating resources and a shift towards tree and parasitoid diversity-dependent regulation. The influence of parasitoid species richness on parasitism rose over time as generalist parasitoids concentrated interactions on shared hosts, increasing overall resource use and network robustness to species extinctions. Our results reveal that the functional benefits of biodiversity strengthen over time through shifts in the mechanisms linking trophic levels, underscoring the long-term value of diversity-focused restoration.

Understanding soil organic matter dynamics is essential for evaluation of the carbon (C) sequestration potential of soils, a critical factor in mitigating climate change. However, the dynamics of soil C processes under the canopy of extrafloral nectary (EFN) trees that are widely distributed in subtropical forests remain poorly explored, particularly in the context of declining tree diversity. In this study, we investigated EFN tree effects on the soil C and nitrogen (N) fractions in forest communities characterized by five levels of tree species richness (TSR, i.e. one‐, two‐, four‐, eight‐ and 16‐tree species). Emphasis was placed on the roles of the phyllosphere and soil functional fungal guilds associated with the target and neighbouring trees. The results revealed that the proportion of EFN trees negatively affected the C content of particulate organic matter (POM), the C/N ratios of the POM and mineral‐associated organic matter (MAOM), as well as the POM‐to‐MAOM ratio. The peak soil C/N ratios across all fractions, as well as the POM‐to‐MAOM ratio, shifted from four‐tree species to eight‐tree species under the canopy of EFN trees and their neighbouring non‐EFN trees. The POM C/N ratio was directly associated with the alpha diversity of soil functional fungi and indirectly associated with the alpha diversity of functional fungi colonizing damaged leaves. In addition, the C/N ratios of the POM and MAOM, and the POM‐to‐MAOM ratio were potentially mediated by the complexity, stability and potential keystone taxa of fungal co‐occurrence networks colonizing leaves and in the soil. The changes in microbial communities are likely driven by the interaction between EFN trees and herbivorous insects. Synthesis . These findings demonstrate a positive response of soil C sequestration under extrafloral nectary (EFN) tree canopies. By highlighting the significance of EFN tree–phyllosphere/soil fungi associations and their role in shaping the effect of tree species diversity, this study contributes to a comprehensive understanding of the mechanisms by which above‐ground–below‐ground synergies govern soil C sequestration in a subtropical forest.

Understanding how land use affects temporal stability is crucial to preserve biodiversity and ecosystem functions. Yet, the mechanistic links between land-use intensity and stability-driving mechanisms remain unclear, with functional traits likely playing a key role. Using 13 years of data from 300 sites in Germany, we tested whether and how trait-based community features mediate the effect of land-use intensity on acknowledged stability drivers (compensatory dynamics, portfolio effect, and dominant species variability), within and across plant and arthropod communities. Trait-based plant features, especially the prevalence of acquisitive strategies along the leaf-economics spectrum, were the main land-use intensity mediators within and across taxonomic and trophic levels, consistently influencing dominant species variability. Functional diversity also mediated land-use intensity effects but played a lesser role. Our analysis discloses trait-based community features as key mediators of land-use effects on stability drivers, emphasizing the need to consider multi-trophic functional interactions to better understand complex ecosystem dynamics.

1. Understanding how biodiversity varies under different environmental conditions is one of the central aims of ecology. Mean environmental conditions and heterogeneity have an effect on biodiversity. Increased heterogeneity is generally associated with increased diversity, but mean conditions tend to have a stronger influence. Conditions on site are embedded into a landscape context, which adds another layer of complexity to be considered. Due to the rarity of multi-taxon data it remains unclear if resulting patterns are similar across taxa. Most European forests are managed, and management strongly influences both mean conditions and heterogeneity. How different species groups respond to variation in these forest characteristics is therefore crucial for testing ecological theories and designing effective conservation measures for management. 2. We assessed the effects of environmental conditions on biodiversity of seven taxonomic groups in a temperate mountain forest area in Central Europe. We analysed the responses of biodiversity (species richness, Shannon diversity, and $\beta$-diversity) to three groups of environmental variables: local mean conditions, local heterogeneity, and landscape. Our objectives were to determine which group of variables is most important in explaining biodiversity variation, and whether certain environmental conditions have consistent effects on the diversity of multiple taxonomic groups. 3. We found that the effects of environmental conditions varied substantially between taxa and aspects of biodiversity. The proportion of conifers had the largest number of significant effects overall, but the direction varied between taxa. None of the three groups of variables was more relevant in explaining the variation in biodiversity. While an increase in local heterogeneity and higher values in the landscape context were associated with increased diversity, an increase in mean conditions was mainly negatively associated with diversity. 4. Synthesis and applications Our results show that no single factor or group of factors affects biodiversity across different species groups in the same way. For management, this means that stand-level interventions, such as retention of old-growth elements, are likely not sufficient to promote the many aspects of biodiversity. Instead, different types of management are likely needed for objective-specific biodiversity conservation at the landscape scale.

Grasslands are diverse ecosystems that are increasingly threatened by intensive land use. Restoring grasslands by reducing land-use intensity may support insect abundance and diversity, helping to halt insect declines. To test for the effect of reduced land use on invertebrates, we studied an experiment (established 2020) at 45 sites across three regions of Germany. We hypothesized that reduced land use increases invertebrate abundance, with larger effects in less intensively used grasslands. Using suction sampling, invertebrates were quantitatively sampled in May 2021 and May 2023, with 2021 samples identified by DNA meta-barcoding. Reducing land use to a single late mowing increased invertebrate abundance by 41 % after one year and 99 % after three years. However, species diversity did not differ between treatments and controls. The effect of land-use reduction on abundance was consistently influenced by land use in the surrounding matrix, with larger positive effect sizes at grasslands with lower mowing frequency but higher fertilization. In spite of these local differences in the magnitude of restoration effects, the consistent increase in invertebrate abundance suggests that reducing land-use intensity can enhance invertebrate populations with potential benefits for ecosystem functions. It will be important to study how outcomes of land-use reduction develop over time, as land-use reduction is likely more successful when implemented permanently.

Recent declines in arthropod diversity, abundance and biomass are central to the global biodiversity crisis. Yet, we lack a mechanistic understanding of the respective contributions of species richness, species identity and abundance to overall biomass change, and how the environment filters these processes. Synthesizing 11 years of data from a biodiversity experiment and from farmed grasslands in central Europe across a gradient of plant species richness and land-use intensity, we show that local arthropod biomass declines were predominantly (>90%) linked to species richness losses. Abundance declines among persisting species accounted for only 5-8% of lost biomass. The role of species identity depended on the environment and diminished over time: especially under high plant diversity and low land-use intensity, arthropod species with both below-average total biomass and above-average individual biomass (large, rare species) contributed disproportionately to species turnover-but this was only detectable in early years when the communities were still relatively abundant. We conclude that arthropod communities are currently homogenizing towards few common species of similar biomass, probably reducing their adaptability to future environmental change. Increasing the diversity and reducing the land-use intensity of grasslands may mitigate ongoing community simplification and loss of arthropod diversity and functioning.

Tree species richness promotes the diversity of higher trophic levels and ecosystem functioning. Tree species richness may thus also affect communities of insect decomposers, and through this, accelerate the decomposition of animal carrion. However, these effects might be masked by other factors driving decomposition, such as forest structure, topography, and competition between different decomposer groups. We placed 1728 dead mice and observed their decomposition for up to seven days, and captured carrion decomposers with mouse-baited traps across 96 plots in the worldwide largest forest biodiversity experiment (BEF-China) in subtropical China in May 2023 and July 2024. We sampled 30,975 decomposer invertebrates from at least 65 species of nine orders. The abundance, species richness, and composition of decomposer groups (flies, ants, other arthropods) was related to sampling year, canopy cover and slope steepness, but not to tree species richness. Flies (Calliphoridae, Sarcophagidae and Muscidae) were nine times more abundant and were more often the primary decomposer of carrion in 2023 than in 2024, especially in closed forests. In contrast, when flies were rare in 2024, ants primarily decomposed carrion, especially in areas with more ants or fewer flies, independently of environmental factors or tree species richness. Carrion decomposition was accelerated in 2023 compared to 2024, in closed forests and partially on steeper slopes, but was not influenced by tree species richness. Carrion decomposition was faster when flies instead of ants were the primary decomposers. When both insect groups co-occurred on carrion, ants typically outcompeted flies, resulting in slower decomposition. This study shows that carrion decomposers were largely unaffected by tree species richness, and consequently, carrion decomposition also appeared to be insensitive to bottom-up effects of tree species richness. Instead, our results highlight the interactive effects of temporal and environmental factors on decomposer communities of small vertebrate carrion, the competitive interactions between decomposer groups, and decomposition rates. Our findings thus challenge the general expectation that tree diversity promotes higher-trophic diversity and ecosystem functioning.

Land-use change and intensification are major drivers of biodiversity loss, yet their effects on diversity have usually been studied within a single habitat type or land-use category, limiting our understanding of cross-habitat patterns. Moths, a species-rich taxon worldwide, represent a significant portion of the biodiversity in both temperate forests and grasslands, functioning as pollinators and herbivores. While increasing land-use intensity (LUI) in both habitats is expected to negatively impact moth assemblages, the strength of this effect remains uncertain. Moreover, land-use intensification interacts with broader environmental factors, such as weather conditions and the spread of artificial light at night (ALAN), but their combined effects on moth community diversity and turnover across habitats remain poorly understood. We sampled moth communities across 150 grassland and 150 forest plots along land-use gradients in Germany. We quantified plot- and landscape-scale LUI and tested the role of plant diversity, temperature and precipitation during the night of sampling and the preceding season, and ALAN in shaping moth diversity (standardized by coverage) along Hill numbers. Forests supported significantly higher moth abundance, biomass and diversity than grasslands, with habitat type being the main driver of moth community composition. LUI at the plot scale had contrasting effects on moth abundance, increasing it in forests but reducing it in grasslands. Impacts of LUI were more pronounced at the landscape level, reducing moth diversity particularly in areas dominated by grasslands. Plant diversity and temperature were key determinants for moth communities, increasing alpha diversity across diversity metrics, that is Hill numbers. ALAN had no significant influence on moth abundance or biomass but significantly decreased Simpson diversity. Beta diversity increased with geographic distance, habitat change and LUI but decreased with weather differences among plots. Our results highlight the interplay between LUI, habitat type and abiotic factors in shaping moth communities across large spatial scales. Effective conservation strategies should consider maintaining habitat heterogeneity and promoting plant diversity, particularly in temperate habitats exposed to high land-use intensification.