Scientific evidence shows that climate and meteorological conditions strongly modulate forest CO₂ fluxes measured using Eddy-Covariance method. However few studies have investigated the impact of synoptic-scale circulation on gas exchange variability.Seasonal estimates of Gross Primary Production (GPP), Net Ecosystem Exchange (NEE) and Ecosystem Respiration (RECO) were derived using linear regression models applied to Eddy Covariance data collected at the Collelongo-Selva Piana LTER-Italy site (3000 ha beech forest) from 1996 to 2014. Circulation Weather Types (CWTs) derived from ERA5 MSLP and HGT500 (used individually and combined) are generated via principal methods available in the used software (Simulated Annealing, Leader, Principal Component Analysis, Threshold methods).We use COST Action 733 software to construct and compare multiple CWT classifications over a fixed Italian domain, selecting the optimal classification for explaining CO₂ flux variability.Classifications are ranked by minimizing intraclass and maximizing interclass variance of GPP, NEE and RECO. This approach identifies the CWT classification that best discriminates flux regimes driven by synoptic patterns (uptake under westerlies vs. emissions under blocking).The optimized Italian CWT enables diagnostic attribution of observed CO₂ anomalies to specific circulation drivers, helping to clarify Collelongo beech forest ecosystem processes.In addition, the CO₂‑optimized CWT classification could be applied in CORDEX climate scenarios to quantify the future impacts of changing circulation frequencies on forest ecosystem activities.Hence, understanding how CWTs influence natural ecosystem fluxes — and how these circulation patterns may evolve under future scenarios — could provide valuable guidance for forest management. This includes strategic species selection for adaptive silvicultural practices, supporting decision makers in climate-resilient forestry planning.

Abstract The development of harmonized, standardized, and integrated environmental observation systems is a key challenge in Earth system science. Such capability is essential for advancing the interdisciplinary research needed to improve understanding of the Earth system and support global sustainability. The Integrated European Long‐Term Ecosystem, Critical Zone and Socio‐ecological Research Infrastructure (eLTER RI) is a recently developed pan‐European network of in situ research sites that facilitates the collection long‐term, comprehensive observation, analysis, and modeling of environmental and ecosystem change. This initiative focuses on Europe's primary ecosystems, encompassing the atmosphere, geosphere, hydrosphere, biosphere, and their socio‐ecological interactions with the anthroposphere. A fundamental prerequisite for effective environmental monitoring and observation is a standardized and harmonized design that facilitates consistent and comparable environmental data across diverse spatial and temporal scales. The objective of this paper is to introduce the eLTER Framework of Standard Observations (eLTER SO) as a harmonized conceptual and operational standard for long‐term, integrated in situ environmental observations, and to demonstrate how it supports consistent cross‐sphere monitoring and international collaboration in environmental research. The eLTER SO delineates essential ecosystem variables, their measurement methods, and protocols. These Standard Observations (SOs) constitute the conceptual foundation of eLTER RI and provide a basis for overcoming existing disciplinary barriers to the international harmonization of environmental research and a foundation for cross‐sphere observation concepts. The eLTER SO combines the scientific‐academic perspective, as known from “classical” Essential Variable concepts, with the operational perspective required for the establishment and long‐term operation of in situ observatories.

This paper analyzes the seasonal and annual patterns of precipitation and temperature in the Castelporziano Nature Reserve from 1980. It also considers an index that combines precipitation and evapotranspiration. The results indicate various patterns within this small region, primarily influenced by the distance from the coast. Additionally, there is a clear upward trend in temperature. The joint analysis of precipitation and temperature shows that the most recent years have been characterized as either the hottest and driest or the hottest and most humid. Furthermore, there has been a significant increase in tropical nights and a longer duration of warm spells for maximum temperatures.

Forests are central to climate-change mitigation but are increasingly threatened by global change components, such as more frequent extreme weather and climate events (particularly drought and heatwaves) and increasing nitrogen deposition, resulting in great uncertainties for the future of the essential ecosystem services they provide. Drought and heatwaves impair physiological mechanisms underpinning tree growth and forest productivity, and they may trigger tree mortality, thus constraining the forest carbon sink. On the one hand, nitrogen deposition stimulates tree growth in nitrogen-limited forests, but when exceeding the empirical nitrogen critical load could cause forest dieback, through soil acidification and nutrient imbalances, but also by making trees more vulnerable to drought. Many questions remain: How do global change components interact and affect forest functioning? Which tree ecophysiological mechanisms are involved? Are those mechanisms synchronized at tree and ecosystem scales (in terms of temporal trends and intra-annual seasonal changes)? Does nitrogen deposition affect tree and forest responses to climate extremes under a CO2 richer world? The NEXTRES project aims at addressing these questions by applying a multi-scale approach combining tree-based measurements (including long-term growth and stable carbon, oxygen and nitrogen isotopes together with intra-annual scale carbon isotope analyses) to ecosystem responses (Gross Primary Production and Evapotranspiration). We studied eleven forest sites along climatic and nitrogen deposition gradients (3–42 kg N ha⁻¹ yr⁻¹) across Europe, within the ICOS and ICP Forests networks, focusing on four widespread tree species (Fagus sylvatica, Quercus spp., Picea abies, Pinus sylvestris). Across sites, basal area increment generally declined during recent climate extremes (e.g. the 2018 drought), with a stronger response in the case of broadleaf vs. conifer species, followed by recovery in subsequent years at most of the sites. Preliminary isotope results for Fagus sylvatica at two sites show contrasting responses: intrinsic water-use efficiency (iWUE) increased during the 2018 drought at Sorø (Denmark), coinciding with reduced growth, whereas a severe late frost at Collelongo (Italy) reduced growth without a clear iWUE response, suggesting different plant strategies in terms of leaf gas exchanges and carbon allocation. Preliminary intra-annual δ¹³C analyses from Picea abies trees in Davos (Switzerland) reveal higher and more variable δ¹³C values during the extreme year in 2018, with elevated values in latewood compared to earlywood, highlighting strong seasonal modulation of drought responses. The coupling between tree-level and ecosystem responses will be assessed at the multidecadal and intra-seasonal scale, as well as the contribution of nitrogen deposition in modulating forest vulnerability and resilience to climate extremes.Acknowledgments. Project funded by the European Union - NextGenerationEU under the National Recovery and Resilience Plan (PNRR) - Mission 4 Education and research - Component 2 From research to business - Investment 1.1 Notice Prin 2022 - DD N. 104 del 2/2/2022, title “Effects of nitrogen deposition and climate extremes on European forests: combining stable isotopes in tree rings and ecosystem fluxes (NEXTRES)”, proposal code 202299J927 - CUP J53D23002640006. We thank all collaborators at the forest sites for assistance in the field.

The atmospheric transport of microplastics plays a critical role in understanding input rates and distribution patterns across ecosystems. Forest ecosystems remain understudied with regard to microplastic contamination. Within this study we aim to evaluate microplastic concentrations in the air of a montane Fagus sylvatica forest and to identify influencing factors on sampling sites of different altitudes during the vegetation and dormant seasons, using a vertical and horizontal spatial approach. The methodology integrated simultaneous, parallel sampling with portable active aerosol samplers at multiple heights beneath the canopy. Sampling was conducted along a transect from outside towards the margins to the interior of three forests of different altitudes. Laboratory analyses featured minimal chemical application, with microplastics identified via fluorescence microscopy and & micro;Raman spectroscopy. We observed average microplastic concentrations of 11.9 +/- 7.1 MP/m3, with fragments dominating and a median size of 12 & micro;m. Results indicated significantly higher microplastic transport into the area during summer (14.9 MP/m3) compared to autumn (8.7 MP/m3), with altitudinal differences influenced by proximity to potential sources related to human activities. Particle concentrations varied by up to 39.8 % between sampling heights, with the lowest mean abundances observed at medium height compared to under canopy and crown height. However, no significant differences were detected in the vertical or horizontal distribution. The variability among parallel samples and across sampling heights highlights the importance of this sampling approach for enhancing statistical robustness.

Climate change stands as a primary stressor, exerting various adverse effects on forests that are particularly susceptible to swift alterations in climatic parameters. At the same time, forests provide a range of ecosystem services beneficial for society. Therefore, a proper management and planning of forests is essential to mitigate the effects of climate change and provide valuable services. Forest management and planning is a complex process due to numerous socio-economic, administrative or environmental aspects that should be considered at different spatial scales. To this end, Decision Support Systems (DSSs) proved to be valuable tools that guide forest managers in enhancing forest resilience and its capacities to mitigate climate change. Engaging stakeholders from the very beginning of the DSSs development process is seen as a prerequisite for the project’s success, adding value and delivering more serviceable outputs. Here, we summarize the most important outputs stemming from a stakeholder engagement process that occurred between July−December 2023, in order to raise awareness about the role of forests in achieving climate ambitions, identify relevant stakeholders and build relationships. These aspects serve as a basis for achieving the following research objectives: provide an improved characterisation of the forest services to mitigate climate change related risks, utilise end-user focused process modelling, empower forest end-users to make informed decisions to enhance forest resilience and forest mitigation, provide a novel decision support tool, bridging different European Union strategic priorities, robust science, and stakeholders in the forest and forest-based sectors. In addition, a novel set of Essential Forest Mitigation Indicators (EFMI) will be proposed to assess the climate change impact and its relation to forest management. Their relevance will be validated through stakeholder consultation.The stakeholder engagement was performed through on-site workshops, and online, phone and email consultations, in eight European countries (Norway, Lithuania, United Kingdom, Germany, Austria, Romania, Spain, and Italy). Common issues that arose through the engagement of stakeholders are related to the challenges of handling different variables (e.g. scale of the study area, public/private forest ownership) between countries and differences in forest management across case studies. The most important lessons learned after the stakeholders workshops are: the importance of trusted relationships with local partners for an effective stakeholder engagement, the significance of including the stakeholders needs and expectations for a successful, long-term partnership, avoiding language barriers by using a non-technical language, as well as long-term policies and funding sources for planning security. A unique feature of the conducted workshops is the interest of stakeholders to be involved and contribute to the development of the Forest DSS, as a user-friendly and tailored tool to their needs.AcknowledgementsThis research received funds from the project “OPTimising FORest management decisions for a low-carbon, climate resilient future in Europe (OptFor-EU)” funded by the European Union Horizon Europe programme, under Grant agreement n°101060554

Climate change undermines forests' health, vitality, and, as a consequence, tree functionality, productivity, and resilience to biotic disturbances. Mountain and sub-alpine forests are particularly susceptible to climate extremes and are showing signs of degradation in Europe. Warmer temperatures, drought, higher frequency and intensity of natural disturbances increasingly alter species distribution and survival, their growing capacity, reproduction, establishment, as well as their potential adaptation to climate change. Real-time monitoring of trees' and stands' responses to such events provides an effective way to better understand and even foresee the adverse side effects of climate change. The use of advanced and innovative monitoring tools and devices is required for ensuring long-term, large-scale, and real-time monitoring of forest dynamics. Here, we present the TreeTalker Italia Network (TTIN), i.e., the first largescale network of tree-proximal sensors (TreeTalkers (c)) at a national scale in Italy. We describe the recent advances, innovations, and potential of such devices for continuous monitoring and research. As a primer, we argue that TTIN will provide effective support to ongoing science and policy efforts for monitoring natural resources' dynamics on a large scale (e.g., forest inventory, climate impacts), including their effects on human well-being.

The ability of forests to continue providing important ecosystem services and mitigating climate change depends on their ability to adapt to global change pressures, such as more frequent climate extremes (specifically drought and heatwaves) and changes in atmospheric pollutants, such as reactive nitrogen compounds. On the one hand, nitrogen deposition could stimulate tree growth in a CO2 richer word, but on the other hand increasing atmospheric nitrogen input, above the critical load, could result in forest dieback, through soil acidification and nutrient imbalances but also by making trees more vulnerable to climate extremes. How do these global change components interact and affect forest carbon, water and nitrogen cycling? What are tree ecolophysiological mechanisms involved? Are those mechanisms synchronized (in terms of magnitude and temporal trends) at tree and ecosystem scales? Does nitrogen deposition affect tree and forest responses to climate extremes? In order to answer these fundamental questions, we considered 12 forests along a climate and nitrogen deposition gradient (from 3 to 42 kg ha-1 yr-1) in Europe, including four of the most widespread tree species in European forests: Fagus sylvatica, Quercus spp., Picea abies, Pinus sylvestris. Forests sites were selected within established networks, namely ICOS and eLTER (for the ecosystem scale measurements of carbon and water fluxes with eddy covariance technique and other ecological parameters) and ICP Forests (for atmospheric nitrogen deposition). We will present preliminary results on the combinination of existing data on ecosystem fluxes with dendroecological data (growth and stable carbon, oxygen and nitrogen isotope ratios) to explore multidecadal changes in forest water-use efficiency and elucidate tree physiological mechanisms underpinning those responses. Moreover, in specific years characterized by climate extremes, an intra-annual isotope approach will be considered to evaluate possible divergences among tree species in the physiological signal and between tree and ecosystem responses, but also to elucidate the contribution of nitrogen deposition in affecting responses to climate extremes. Collaborators at the ICOS and ICP Forests sites selected for the study are greatly acknowledged

Climate change is endangering natural and anthropogenic ecosystems, as pointed out by the recent IPCC Reports and the COPs' statements. The impacts of climate change on natural ecosystems can affect their production capacity, particularly in those systems characterized by a high quality of yields, especially in densely populated and industrialized countries. We analyze two recent intense rainfall events that hit the Emilia-Romagna and Tuscany (Italy) regions and the damage caused to the agricultural ecosystems downstream of forests and woodlands. Although the scientific debate on these events' climatic or purely meteorological origin is still open, these occurrences provide a potential direct example of the harm climate change may bring. The topic of forest management for risk reduction is also analyzed on the forest itself and anthropized systems and related economies. The study was conducted within the European OptFor-EU Project.

Costal pine Italian forests, like mostly monospecific plantations, are much more vulnerable than natural and multi-specific stands. This fragility was completely expressed in the Castelporziano Presidential Natural Reserve in Italy (west coast of central Italy), where, in only 6 years, the combined action of the alien pathogens Toumeyella parvicornis with the native one Tomicus destruens led to the disappearance of the stone pines (Pinus pinea L.) that were covering more than 250 hectares of monospecific stands. The subsequent removal of standing dead trees left large open areas where various ecosystem restoration strategies can be applied including reforestation to natural recolonization and different options for grazing control. An ICOS station was already present inside the natural reserve and now, thanks to the fruitful collaboration of three European Research Infrastructures (ICOS, eLTER and LifeWatch), five additional monitoring plots will be established. At moment of the present abstract submission three station plots have been already implemented and started to measure in mid-August 2024, while the other two are under implementation with the start of the measurements planned for spring 2025. The different plots, covering each a different post-pine option with a different ecosystem structure, are all equipped with an eddy covariance system for CO2, water and energy continuous exchange measurement. Beyond the functionality in terms of carbon absorption, other investigation activities will be carried out in the plots with a specific focus on vegetation and soil characteristics, biodiversity evolution and hyperspectral and SIF local measurements among others. In this presentation the first preliminary results will be illustrated, together with the plan and the activities on-going. The data, collected in the context of the European Research Infrastructures, are open access and FAIR and will be fundamental for better evaluating and understanding different restoration options and the consequent vegetation dynamics from a holistic point of view including carbon storage, water balance, plants and animal biodiversity, with a link to the remote sensing for their possible upscaling.

Forest ecosystems are particularly threatened by global change components, i.e., more frequent extreme weather and climate events (particularly drought and heatwaves) and increasing (N) deposition, resulting in great uncertainties for the future of the essential ecological, economic and social benefits that humanity relies on from forests. Drought and heatwaves impair physiological mechanisms underpinning tree growth and forest productivity, and they may trigger tree mortality, thus constraining the forest carbon sink (Adams and ì 2017; Gazol and Camarero 2022; Hartmann et al. 2022; Hubau 2020). On the one hand, N deposition stimulates tree growth in nitrogen-limited forests, under steady increase in atmospheric CO 2 (Etzold 2020; Fernández-Martínez 2017 ; Flechard 2020; Wang et al. 2017). On the other hand, increasing atmospheric N input above the empirical nitrogen critical load (above which harmful effects can occurr in the ecosystem) (Braun et al. 2022) (*) could reduce the positive effect on tree growth and could cause forest dieback, through soil acidification and nutrient imbalances, but also by making trees more vulnerable to climate extremes (Dalton et al. 2024; Etzold 2020; Ferretti et al. 2015; Flechard 2020; Gharun et al. 2021; Thomas et al. 2009). Many questions remain: How do these global change components interact and affect forest carbon, water and N cycling? Which tree ecophysiological mechanisms are involved? Are those mechanisms synchronized (in terms of magnitude and temporal trends) at tree and ecosystem scales? Does N deposition affect tree and forest responses to climate extremes? The NEXTRES project aims at answering these questions by applying a multi-scale approach (from tree to ecosystem responses) to eleven forests along a climate and total N deposition gradient (from 3 to 42 kg ha -1 yr -1 ) across Europe, selected within established monitoring networks, namely ICOS and ICP Forests (Fig. 1). We will present preliminary results combining existing ecosystem CO 2 and water vapor fluxes with dendroecological information on growth as well as stable carbon isotope ratios to explore multidecadal changes in water-use efficiency, and to elucidate underpinning tree physiological mechanisms. Moreover, we will target years characterized by climate extremes to follow the intra-annual carbon isotope fingerprint within individual in tree rings to evaluate possible divergences: among tree species in their recovery strategies, and between tree and ecosystem responses. among tree species in their recovery strategies, and between tree and ecosystem responses. Finally, we will elucidate whether an excess of atmospheric N input can affect tree and forest responses to climate extremes.

Tree-microbe interactions are essential for forest ecosystem functioning. Most plant-microbe research has focused on the rhizosphere, while composition of microbial communities in the phyllosphere remains underexplored. Here, we use 16S rRNA gene sequencing to explore differences between beech and Scots pine phyllospheric microbiomes at the European continental scale, map their functional profiles, and elucidate the role of host trees, forest features, and environmental factors such as climate and atmospheric deposition in phyllosphere microbiota assembly. We identified tree species and the associated foliar trait (specifically carbon:nitrogen ratio) as primary drivers of the bacterial communities. We characterized taxonomical and functional composition of epiphytic bacteria in the phyllosphere of beech and Scots pine across an environmental gradient from Fennoscandia to the Mediterranean area, with major changes in temperature and nitrogen deposition. We also showed that temperature and nitrogen deposition played a crucial role in affecting their assembly for both tree species. This study contributes to advancing our understanding on factors shaping phyllosphere microbial communities in beech and Scots pine at the European continental scale, highlighting the need of broad-scale comparative studies (covering a wide range of foliar traits and environmental conditions) to elucidate how phyllosphere microbiota mediates ecosystem responses to global change. Phyllosphere microbiota of beech and Scots pine at European continental scale is influenced by the host species and associated foliar traits, as well as by temperature and nitrogen deposition, according to 16S rRNA gene sequencing analyses on leaf epiphytic microbes.

Non-structural carbohydrates (NSCs) represent the primary carbon (C) reserves and play a crucial role in plant functioning and resilience. Indeed, these compounds are involved in the regulation between C supply and demand, and in the maintenance of hydraulic efficiency. Non-structural carbohydrates are stored in parenchyma of woody organs, which is recognized as a proxy for reserve storage capacity of tree. Notwithstanding the importance of NSCs for tree physiology, their long-term regulation and trade-offs against growth were not deeply investigated. This work evaluated the long-term dynamics of mature tree reserves in stem and root, proxied by parenchyma features and focusing on the trade-off and interplay between the resources allocation in radial growth and reserves in stem and coarse root. In a Mediterranean beech forest, NSCs content, stem and root wood anatomy analysis and eddy covariance data were combined. The parenchyma fraction (RAP) of beech root and stem was different, due to differences in axial parenchyma (AP) and narrow ray parenchyma (nRP) fractions. However, these parenchyma components and radial growth showed synchronous inter-annual dynamics between the two organs. In beech stem, positive correlations were found among soluble sugars content and nRP and among starch content and the AP. Positive correlations were found among Net Ecosystem Exchange (NEE) and AP of both organs. In contrast, NEE was negatively correlated to radial growth of root and stem. Our results suggest a different contribution of stem and roots to reserves storage and a putative partitioning in the functional roles of parenchyma components. Moreover, a long-term trade-off of C allocation between growth and reserve pool was evidenced. Indeed, in case of C source reduction, trees preferentially allocate C toward reserves pool. Conversely, in high productivity years, growth represents the major C sink.

We examined the seasonality of photosynthesis in 46 evergreen needleleaf (evergreen needleleaf forests (ENF)) and deciduous broadleaf (deciduous broadleaf forests (DBF)) forests across North America and Eurasia. We quantified the onset and end (StartGPP and EndGPP) of photosynthesis in spring and autumn based on the response of net ecosystem exchange of CO2 to sunlight. To test the hypothesis that snowmelt is required for photosynthesis to begin, these were compared with end of snowmelt derived from soil temperature. ENF forests achieved 10% of summer photosynthetic capacity similar to 3 weeks before end of snowmelt, while DBF forests achieved that capacity similar to 4 weeks afterward. DBF forests increased photosynthetic capacity in spring faster (1.95% d-1) than ENF (1.10% d-1), and their active season length (EndGPP-StartGPP) was similar to 50 days shorter. We hypothesized that warming has influenced timing of the photosynthesis season. We found minimal evidence for long-term change in StartGPP, EndGPP, or air temperature, but their interannual anomalies were significantly correlated. Warmer weather was associated with earlier StartGPP (1.3-2.5 days degrees C-1) or later EndGPP (1.5-1.8 days degrees C-1, depending on forest type and month). Finally, we tested whether existing phenological models could predict StartGPP and EndGPP. For ENF forests, air temperature- and daylength-based models provided best predictions for StartGPP, while a chilling-degree-day model was best for EndGPP. The root mean square errors (RMSE) between predicted and observed StartGPP and EndGPP were 11.7 and 11.3 days, respectively. For DBF forests, temperature- and daylength-based models yielded the best results (RMSE 6.3 and 10.5 days). We used records of forest-atmosphere carbon dioxide exchange and weather to determine when photosynthesis begins and ends each year in 46 northern hemisphere forests. We used observations of soil temperature to determine the timing of the end of the snowmelt period. We found that evergreen needleleaf forests began photosynthesis similar to 3 weeks before snowmelt ended, while deciduous broadleaf forests (DBF) waited until similar to 4 weeks after snowmelt ended. The DBF type ramped up photosynthesis in spring, and ramped down in autumn, faster than the ENF, and the length of the photosynthesis (or "growing") season was similar to 50 days shorter for DBF forests. Abundant evidence suggests that spring is occurring earlier in recent decades. We checked whether these forests are starting photosynthesis earlier by looking at forests with long-term records. We found minimal support for changes in photosynthetic phenology over time, but very strong connections between temperature and the timing of spring and autumn transitions. We tested 19 models that use weather data to predict plant phenological events. We used gridded weather data to drive the models, and the best models were able to predict the spring and autumn photosynthetic transitions to within similar to 10 days. Evergreen forests began photosynthesis in spring similar to 3 weeks before end of snowmelt, deciduous forests similar to 4 weeks after end of snowmelt There is little evidence for lengthening of the photosynthetic season in the northern hemisphere forest flux tower record Interannual variation in onset and end of photosynthesis was related to air temperature

Coastal areas are biodiversity hotspots, providing essential ecosystem services, yet they are among the most threatened systems, particularly by alien species invasion. The European regulation on invasive alien species (IAS) highlights early detection as a key prerequisite for effective containment or eradication strategies. Traditional monitoring methods are costly and time-consuming, and Citizen Science (CS) may be a promising alternative. We assessed the contribution of the generalist CS project “Wild Coast Adriatic” (WCA) developed on the iNaturalist platform to the detection of alien species (AS) along the Central Adriatic coast. Using WCA, we extracted alien occurrences and explored AS seasonal patterns, geographic origins, dangers (EU regulation), and distributions inside protected areas (Natura 2000 and LTER sites). Between 2020 and 2023, WCA gathered 2194 research-grade observations of 687 species, including 139 records of 50 AS, five of which are of European concern. Asteraceae and Fabaceae (plants) as well as insects and mollusks (fauna) were the most abundant aliens. The observations increased over time, with more records concentrated in autumn and summer. Most AS come from the Americas and occurred outside the protected areas. Our results underline the contribution of CS data for detecting AS in coastal ecosystems, offering a valid support for early warning, monitoring, and management strategies.

Human activities have greatly increased the reactive nitrogen in the biosphere, thus profoundly altering global nitrogen cycling. The large increase in nitrogen deposition over the past few decades has led to eutrophication in natural ecosystems, with negative effects on forest health and biodiversity. Recent studies, however, have reported oligotrophication in forest ecosystems, constraining their capacity as carbon sinks. Here we demonstrate the widespread biological transformation of atmospheric reactive nitrogen in the canopies of European forests by combining nitrogen deposition quantification with measurements of the stable isotopes in nitrate and molecular analyses across ten forests through August–October 2016. We estimate that up to 80% of the nitrate reaching the soil via throughfall was derived from canopy nitrification, equivalent to a flux of up to 5.76 kg N ha −1 yr −1 . We also document the presence of autotrophic nitrifiers on foliar surfaces throughout European forests. Canopy nitrification thus consumes deposited ammonium and increases nitrate inputs to the soil. The results of this study highlight widespread canopy nitrification in European forests and its important contribution to forest nitrogen cycling.

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.