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.

A large European forest monitoring dataset reveals a pattern of reduced foliar nitrogen (N) and phosphorus (P) concentrations following drought conditions in spruce and pine, and, in the case of P, beech and oak, often exhibiting N:P imbalances. Gradual nutritional imbalance and nutrient deficiency during droughts raise concern for tree vitality and forest carbon sequestration under climate change. Nitrogen (N) and phosphorus (P) are essential nutrients for tree metabolism, forest growth, and carbon sequestration, yet the drivers of their availability to trees are often complex to untangle. In this study, we investigated environmental controls of foliar N, P, and N:P based on > 4100 N and P measurements in foliage samples of main tree species (beech, oak, spruce, and pine) across 279 European monitoring sites by applying mixed regression models. We found overall nutritional declines over the past three decades that ranged from − 1.8

Forests form a major organic carbon reservoir, both above- and belowground. In the course of global change, predicting possible changes in these carbon reservoirs is essential. To this end, the Horizon Europe PathFinder project aims to develop an innovative forest monitoring system allowing consistent EU greenhouse gas reporting of LULUCF (Land Use, Land Use Change & Forestry) in combination with advanced policy pathway assessments. Greenhouse gas reporting of soil organic carbon (SOC) stock changes in forests commonly relies on simulations by soil carbon cycling models, such as Yasso (Y20), which uses only climate data and soil carbon inputs that can be derived by country-specific approaches from National Forest Inventories. However, the agreement between measured versus simulated carbon stocks and changes at the European scale has not yet been established. Within the framework of this project, this study aims to derive European-wide harmonised soil carbon inputs and stock estimates since the 1990s and further develop the current estimation methodology. After exploration of the available data sets, the ICP Forests Level II forest condition monitoring database was found the most suitable to set the initial modelling conditions. It is the only harmonised data set at the European scale that comprises above- and belowground compartments and contains repeated assessments on a subset of about 200 plots across Europe. The pre-processing of the observed data on soil carbon stock, growth and litterfall from the central ICP Forests database was very labour-intensive. As part of the ICP Forests monitoring programme, carbon concentrations and bulk densities are measured down to a depth of 80 cm. Using mass-preserving splines, soil carbon stocks were estimated down to a depth of 100 m to make them comparable with Y20. Regression models were developed to estimate litterfall inputs based on forest inventory data. We simulated SOC stocks by Y20 in ICP Forests Level II plots with available stand inventory data and soil characterization. Soil carbon inputs were obtained using two approaches: an inventory approach, with litterfall estimated by the above-mentioned regression models, and root and coarse-woody inputs by allometric functions, and a satellite approach, with net primary production (NPP) from MODIS at 500 m resolution. The Y20-simulated SOC stocks were compared with the SOC stocks to 100 cm depth based on the soil inventory data. an inventory approach, with litterfall estimated by the above-mentioned regression models, and root and coarse-woody inputs by allometric functions, and a satellite approach, with net primary production (NPP) from MODIS at 500 m resolution. The Y20-simulated SOC stocks were compared with the SOC stocks to 100 cm depth based on the soil inventory data. On average, the satellite approach estimated higher soil carbon inputs than the inventory approach (+20%). The SOC stocks simulated by Y20 were overall in line with observed SOC stocks. The simulations for broadleaf-dominated stands agreed well with SOC measurements, with average deviations below 1 kg C m -2 using the satellite approach. In coniferous stands, Y20-simulated SOC stocks were lower than observed by 3-5 kg C m -2 . This is likely due to the intrinsic soil properties driving SOC storage and stabilization in highly acidic, coniferous forests (i.e. Podzols and Umbrisols), which are not accounted for in Y20.

Forests are increasingly affected by global change. Building resilient forests requires – amongst others - leveraging the wealth of knowledge from existing ground-based, field inventory and monitoring programs as well as Earth Observation systems to better assess the status, detect changes, understand processes, predict future dynamics, and guide forest management. A proposal from the European Commission for a new forest monitoring framework at the European level aims in this direction but lacks the integration of some crucial and readily available resources and infrastructures. For this reason, the proposal risks to be a missed opportunity rather than a step forward. Here we provide suggestions to help reconciling the proposal with its objectives and a more comprehensive monitoring vision.

A Soil Monitoring Law to improve soil health across all land uses has been proposed by the European Commission. As forests soils have different chemical and physical properties as well as biogeochemical dynamics compared to agricultural land, they also face different challenges in maintaining and restoring soil health. Examples are soil acidification, eutrophication by atmospheric deposition, responses to climate change, and loss of biodiversity. Therefore, we propose forest soil specific health descriptors and thresholds based on experience and knowledge from existing long-term monitoring programs.

Climate change effects on tree reproduction are poorly understood, even though the resilience of populations relies on sufficient regeneration to balance increasing rates of mortality. Forest-forming tree species often mast, i.e. reproduce through synchronised year-to-year variation in seed production, which improves pollination and reduces seed predation. Recent observations in European beech show, however, that current climate change can dampen interannual variation and synchrony of seed production and that this masting breakdown drastically reduces the viability of seed crops. Importantly, it is unclear under which conditions masting breakdown occurs and how widespread breakdown is in this pan-European species. Here, we analysed 50 long-term datasets of population-level seed production, sampled across the distribution of European beech, and identified increasing summer temperatures as the general driver of masting breakdown. Specifically, increases in site-specific mean maximum temperatures during June and July were observed across most of the species range, while the interannual variability of population-level seed production (CVp) decreased. The declines in CVp were greatest, where temperatures increased most rapidly. Additionally, the occurrence of crop failures and low seed years has decreased during the last four decades, signalling altered starvation effects of masting on seed predators. Notably, CVp did not vary among sites according to site mean summer temperature. Instead, masting breakdown occurs in response to warming local temperatures (i.e. increasing relative temperatures), such that the risk is not restricted to populations growing in warm average conditions. As lowered CVp can reduce viable seed production despite the overall increase in seed count, our results warn that a covert mechanism is underway that may hinder the regeneration potential of European beech under climate change, with great potential to alter forest functioning and community dynamics.

Integrated long-term, in-situ observations are needed to document ongoing environmental change, to “ground-truth” remote sensing and model outputs and to predict future Earth system behaviour. The scientific and societal value of in-situ observations increases with site representativeness, temporal duration, number of parameters measured and comparability within and across sites. Research Infrastructures (RIs) can support harmonised, cross-site data collection, curation and publication. Integrating RI networks through site co-location and standardised observation methods can help answers three questions about the terrestrial carbon sink: (i) What are present and future carbon sequestration rates in northern European forests? (ii) How are these rates controlled? (iii) Why do the observed patterns exist? Here, we present a conceptual model for RI co-location and highlight potential insights into the terrestrial carbon sink achievable when long-term in-situ Earth observation sites participate in multiple RI networks (e.g., ICOS and eLTER). Finally, we offer recommendations to promote RI co-location.

Forests and wood products play a major role in climate change mitigation strategies and the transition from a fossil-based economy to a circular bioeconomy. Accurate estimates of future forest productivity are crucial to predict the carbon sequestration and wood provision potential of forests. Since long, forest managers have used empirical yield tables as a cost-effective and reliable way to predict forest growth. However, recent climate change-induced growth shifts raised doubts about the long-term validity of these yield tables. In this study, we propose a methodology to improve available yield tables of 11 tree species in the Netherlands and Flanders, Belgium. The methodology uses scaling functions derived from climate-sensitive process-based modelling (PBM) that reflect state-of-the-art projections of future growth trends. Combining PBM and stand information from the empirical yield tables for the region of Flanders, we found that for the period 1987-2016 stand productivity has on average increased by 13% compared to 1961-1990. Furthermore, simulations indicate that this positive growth trend is most likely to persist in the coming decades, for all considered species, climate or site conditions. Nonetheless, results showed that local site variability is equally important to consider as the in- or exclusion of the CO2 fertilization effect or different climate projections, when assessing the magnitude of forests' response to climate change. Our projections suggest that incorporating these climate change-related productivity changes lead to a 7% increase in standing stock and a 22% increase in sustainably potentially harvestable woody biomass by 2050. The proposed methodology and resulting estimates of climate-sensitive projections of future woody biomass stocks will facilitate the further incorporation of forests and their products in global and regional strategies for the transition to a climate-smart circular bioeconomy.

It was time to take stock. We modified the humipedon classification key published in 2018 to make it easier and more practical. This morpho-functional taxonomy of the topsoil (humipedon) was only available in English; we also translated it into French and Italian. A standardized morpho-functional classification of humipedons (roughly the top 30–40 cm of soil: organic and organomineral surface horizons) would allow for a better understanding of the functioning of the soil ecosystem. This paper provides the founding principles of the classification of humipedon into humus systems and forms. With the recognition of a few diagnostic horizons, all humus systems can be determined. The humus forms that make up these humus systems are revealed by measuring the thicknesses of the diagnostic horizons. In the final part of the article, several figures represent the screenshots of a mobile phone or tablet application that allows for a fast recall of the diagnostic elements of the classification in the field. The article attempts to promote a standardized classification of humipedons for a global and shared management of soil at planet level.

ObjectivesThe aim of this exploratory study is to investigate total heavy metal (HM) concentrations in European forest soils.The objectives are to: (1) explore the spatial variation (patterns and hotspots) of heavy metal concentrations and stocks in forest floors (FF) and topsoils throughout Europe; (2) investigate if there is a significant temporal change between the data observed during the first (S1) and second (S2) soil survey; (3) evaluate whether the HM concentrations and stocks exceed contamination or pollution levels and (4) compare the observed forest soil concentration levels with reference databases and maps of HM in soils or in mosses at the European scale. MethodsThe study is based on data from the Combined Forest Soil Condition Database (FSCDB.LI) of ICP Forests holding descriptive and analytical information of soil samples obtained from two soil surveys on the ICP Forests' Level I systematic grid, a 16 x 16 km grid covering over 5000 forested sampling locations within Europe.The soil samples were analyzed for their Cd, Cr, Cu, Hg, Ni, Pb and Zn aqua-regia extractable concentrations in FFs and mineral topsoils (0-10 cm).The left-censored data are explored with appropriate statistical techniques in order to take concentrations below quantification limits (LOQ) into account.Sample geometric means are used as distribution metric and the bootstrapping technique to estimate 95% confidence intervals for evaluation of factor differences (e.g soil group) and temporal changes.For each HM, maps are produced and the average HM concentration and stocks by country, biogeographical region, soil group and humus form are calculated and presented. Results and conclusionsHeavy metal specific variation patterns in forest floors and topsoils are found within countries, biogeographical regions and Europe.Regional hotspots where elevated metal concentrations compared to baseline levels occurred are clearly visible on maps, and could be linked to local pollution sources and well-known contaminated areas.Geochemically related metals (e.g.Ni and Cr) show similar spatial distribution patterns.Soil group and humus form help explaining large-scale differences in HM concentrations.The HM concentrations of Cd, Cu, Pb, Zn and Hg in FFs are generally higher than in the underlying mineral topsoil indicating that FF concentrations are interesting indicators for HM contamination.Substantial enrichment of Cd, Pb and Hg in FF compared to mineral soils was found.Generally the HM concentrations in forest soils have declined from 1990 onwards, although rates of change differ by heavy metal and between countries.Undoubtedly a methodological country effect can be seen.The decline between surveys could be evidenced better for FFs than in mineral soils because more temporally paired data is available for FFs.Except for Cr a significant decrease is found in more than half of all paired Level I plots with 62% of plots showing a significant decrease in Pb concentration.About a quarter of plots still show increasing HM concentrations in FFs.In this study two approaches were tested for evaluation of contamination levels in forest soils.Commonly used indicators as the Geo-accumulation Index and the Nemorow Pollution Index were applied.They indicated polluted areas especially for Pb, Hg and Cd, but almost no pollution for Cr and Ni and only regional hotspots for Cu and Zn.The Nemorow index indicated more than 55% of the LI sites as slightly polluted and 7% as heavily polluted, but could only be computed for 10 countries.Another approach was to apply national screening values, for which we calculated median baseline and critical levels and compared these with estimated baselines and critical levels.The estimated baselines, computed as geometric means of the distribution including values below LOQ, are generally lower than the median of national baselines.Significant differences were found among estimated baseline values of biogeographical regions indicating that an evaluation scheme should be developed for each biogeographical region separately.This approach demonstrated that only few percent of the level I plots exceeded the critical levels and is classified as polluted, 5-10% is classified as enriched and for all metals more than 50% of the level I plots is well below the baseline concentration level.An evaluation scheme for HM concentrations in FFs was tested and a FF contamination index (FFMCI) calculated.Pb, Cd and Zn exceeded more the baseline levels than Ni, Cr and Cu.The FFMCI decreased from S1 to S2, also when considering paired plots only.However, 56% (S1) and 70% (S2) of the observed plots show background concentrations for all HM metals in their FFs.When comparing the observed forest soil HM concentration levels with the LUCAS HM topsoil database and maps, no significant differences for Ni and Cu concentrations were found, but higher levels for Cd, Cr, Pb and Hg in the Level I forest topsoils compared to the interpolated LUCAS topsoil maps.Cd and Hg concentrations are a factor 3.5 higher than the predicted LUCAS concentrations at LI plots, Pb about double as high and Cr a factor 1.23.These results support the hypothesis that forest soils accumulate more metals than agricultural land, especially for Cd, Hg and Pb.When qualitatively comparing both maps, regional hotspots of all metals from LUCAS maps are clearly correlated with increased levels at the Level I sites, as expected.Similarly, increased levels indicated by the maps of HM concentrations in mosses, produced by ICP Vegetation, are also related to the concentration in forest floors and topsoil, albeit less strongly than with LUCAS data.The European-wide significant decline of HM concentrations in mosses between 1990 and 2015 was also found in the forest floor for all metals but less pronounced.These temporal changes seem to suggest that Cd and Pb concentrations are indeed decreasing but much slower than observed in mosses or by deposition time-series.Comparison with other datasets learns that heavy metals clearly accumulate and reside in forest soils and that their concentration levels are slightly higher than in mosses and agricultural soils.Finally this study provides suggestions for future surveys and more profound heavy metal data explorations in forest ecosystems.

This study aimed to simulate oak and beech forest growth under various scenarios of climate change and to evaluate how the forest response depends on site properties and particularly on stand characteristics using the individual process-based model HETEROFOR. First, this model was evaluated on a wide range of site conditions. We used data from 36 long-term forest monitoring plots to initialize, calibrate, and evaluate HETEROFOR. This evaluation showed that HETEROFOR predicts individual tree radial growth and height increment reasonably well under different growing conditions when evaluated on independent sites. In our simulations under constant CO2 concentration ([CO2]cst) for the 2071-2100 period, climate change induced a moderate net primary production (NPP) gain in continental and mountainous zones and no change in the oceanic zone. The NPP changes were negatively affected by air temperature during the vegetation period and by the annual rainfall decrease. To a lower extent, they were influenced by soil extractable water reserve and stand characteristics. These NPP changes were positively affected by longer vegetation periods and negatively by drought for beech and larger autotrophic respiration costs for oak. For both species, the NPP gain was much larger with rising CO2 concentration ([CO2]var) mainly due to the CO2 fertilisation effect. Even if the species composition and structure had a limited influence on the forest response to climate change, they explained a large part of the NPP variability (44% and 34% for [CO2]cst and [CO2]var, respectively) compared to the climate change scenario (5% and 29%) and the inter-annual climate variability (20% and 16%). This gives the forester the possibility to act on the productivity of broadleaved forests and prepare them for possible adverse effects of climate change by reinforcing their resilience.

Monitoring volumetric soil water content (θv) is the key for assessing water availability and nutrient fluxes. This study evaluated the empirical accuracy of θv measurements using standard and in situ calibrated frequency domain reflectometers (FDR) with gravimetric water content and bulk density measurements of 1512 samples gathered from 15 profiles across 5 ICP Forests level II intensive monitoring plots. The predicted θv, calibrated with standard functions, predominantly underestimated the real water content. The measurement error exceeded the threshold of 0.03 m3 m−3 in 93% of all soil layers. Layer specific calibration removed bias and reduced the overall prediction error with a factor up to 2.8. A simple linear regression often provided the best calibration model; temperature correction was helpful in specific cases. To adequately remove bias in our study plots, a calibration dataset of up to 24 monthly observations was required for topsoils (whereas 12 observations sufficed for subsoils). Based on estimated precision errors, 3 sensors per soil layer proved to be sufficient, while up to 16 sensors are needed to meet the required accuracy in organic topsoils. Validating FDR sensor outputs using in situ gravimetric measurements is essential for quality control and assurance of long term θv monitoring and for improving site specific instrumentalization.

Pollen is known to affect forest throughfall biochemistry, but underlying mechanisms are not fully understood. We used generalized additive mixed modelling to study the relationship between long-term series of measured throughfall fluxes in spring (April–June) at forest plots and corresponding airborne pollen concentrations (Seasonal Pollen Integral, SPIn) from nearby aerobiological monitoring stations. The forest plots were part of the intensive long term monitoring (Level II) network of the UNECE International Co-operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests (ICP Forests) with dominant tree genera Fagus, Quercus, Pinus and Picea, and were distributed all across Europe. We also conducted a 7-day laboratory dissolution experiment with bud scales and flower stalks of European beech (Fagus sylvatica L.), pollen of beech, common oak (Quercus robur L.), silver birch (Betula pendula L.), Scots pine (Pinus sylvestris L.), Corsican pine (Pinus nigra Arnold ssp. laricio (Poiret) Maire), Norway spruce (Picea abies (L.) Karst.) and sterilized pollen of silver birch in a nitrate (NO3--N) solution (11.3 mg N L-1). Throughfall fluxes of potassium (K+), ammonium (NH4+-N), dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) showed a positive relationship with SPIn whereas NO3--N fluxes showed a negative relationship with SPIn. In years with massive seed production of beech and oak SPIn and throughfall fluxes of K+ and DOC were higher, but fluxes of NO3--N were lower. The experiment broadly confirmed the findings based on field data. Within two hours, pollen released large quantities of K+, phosphate, DOC and DON, and lesser amounts of sulphate, sodium and calcium. After 24-48 hours, NO3--N started to disappear, predominantly in the treatments with broadleaved pollen, while concentrations of nitrite and NH4+-N increased. At the end of the experiment, the inorganic nitrogen (DIN) was reduced, presumably because it was lost as gaseous nitric oxide (NO). There was no difference for sterilized pollen, indicating that the involvement of microbial activity was limited in above N transformations. Our results show that pollen dispersal might be an overlooked factor in forest nutrient cycling and might induce complex canopy N transformations, although the net-impact on N throughfall fluxes is rather low.

Nutrient availability influences virtually every aspect of an ecosystem, and is a critical modifier of ecosystem responses to global change. Although this crucial role of nutrient availability in regulating ecosystem structure and functioning has been widely acknowledged, nutrients are still often neglected in observational and experimental synthesis studies due to difficulties in comparing the nutrient status across sites. In the current study, we explain different nutrient-related concepts and discuss the potential of soil-, plant- and remote sensing-based metrics to compare the nutrient status across space. Based on our review and additional analyses on a dataset of European, managed temperate and boreal forests (ICP [International Co-operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests] Forests dataset), we conclude that the use of plant- and remote sensing-based metrics that rely on tissue stoichiometry is limited due to their strong dependence on species identity. The potential use of other plant-based metrics such as Ellenberg indicator values and plant-functional traits is also discussed. We conclude from our analyses and review that soil-based metrics have the highest potential for successful intersite comparison of the nutrient status. As an example, we used and adjusted a soil-based metric, previously developed for conifer forests across Sweden, against the same ICP Forests data. We suggest that this adjusted and further adaptable metric, which included the organic carbon concentration in the upper 20 cm of the soil (including the organic fermentation-humus [FH] layer), the C:N ratio and pHCaCl2 of the FH layer, can be used as a complementary tool along with other indicators of nutrient availability, to compare the background nutrient status across temperate and boreal forests dominated by spruce, pine or beech. Future collection and provision of harmonized soil data from observational and experimental sites is crucial for further testing and adjusting the metric.