Abstract Climate change is expected to alter species assemblages by affecting the outcome of competition between species. Investigating processes of competition remains challenging in tree communities, as they unfold over extensive spatio-temporal scales. Here, we used a deep learning-based meta-model trained on 135 million simulated tree responses to climate across Europe to investigate changes in the competitiveness of nine major tree species under future climate. We harnessed projections from local process models to train a Deep Neural Network of forest state transitions to investigate climate-induced changes in competition at continental scale. We found decreasing competitive strength for evergreen conifers across their distribution, while deciduous broadleaved species increased in competitiveness. Most investigated species lost competitive strength at their warm range edges. Consequently, up to 25% of Europe’s forests could experience a change in the dominant tree species until the end of the 21st century, suggesting a profound climate-induced reassembly of Europe’s forests.

Wildfires, insect outbreaks, and storms cause large pulses of tree mortality. Climate change amplifies these forest disturbances, yet their future magnitude and extent remain uncertain. Here, we simulated future forest disturbance regimes at 100-meter resolution across Europe using a deep learning-based simulation framework. Our results show that forest disturbances will continue to increase throughout the 21st century, with disturbed areas more than doubling relative to the recent past under an unabated continuation of climate change. Wildfires are the main agent driving future disturbance change. Changing disturbances result in an increase in young forests, substantially altering Europe's forest demography. Because of their profound implications for forest carbon storage and the habitat value of forest ecosystems, disturbances should be a priority of forest policy and management.

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

When an even-aged forest stand reaches maturity, it can be renewed within a limited period to maintain the even-aged structure or gradually transformed to an uneven-aged stand. However, there is still debate as to which silvicultural approach is more profitable, conducive to carbon storage, favourable to biodiversity or resilient. In this study, we simulated the evolution of fifteen stands representative of the Walloon forest, whose initial structure was even-aged. The stands were managed according to two contrasting silvicultural approaches (continuation of the even-aged system vs transformation to an uneven-aged one), and the simulations were run with the SSP3-7.0 climate projections produced by five global circulation models. Our simulations indicate that even-aged and uneven-aged silviculture yield similar outcomes in terms of carbon storage. Financial indicators were likewise largely unaffected, except in the oak-beech mixture, where uneven-aged silviculture increased profitability through the substitution of oak by beech. This shift reduced tree species diversity in uneven-aged oak-beech stands. Tree microhabitats, except in beech stands, were more abundant under uneven-aged silviculture. While mean values of forest ecosystem functioning indicators are largely comparable between the two approaches, uneven-aged stands exhibit higher temporal stability. Uneven-aged silviculture produces stands that are more wind-resistant and avoid periods of extreme vulnerability. Our study shows that the most appropriate silviculture may vary depending on the aspect considered. Uneven-aged silviculture has a definite advantage in terms of stability, risk management, and tree microhabitats. However, maintaining even-aged patches at the landscape scale remains important to facilitate the regeneration of shade-intolerant species.

Abstract. Droughts in western Central Europe have major impacts on agriculture, ecosystems, and society, yet their long-term variability and drivers remain poorly understood. This study investigates drought variability over the past 180 years and its link to atmospheric circulation to identify their dynamic drivers. Three reanalysis datasets (ERA5, 20CRv3, and ModE-RA) are used to detect meteorological drought events via the 3-month Standardized Precipitation (Evapotranspiration) Indexes (SPI-3 & SPEI-3) and to connect them to atmospheric circulation patterns through k-means clustering. Dataset reliability is assessed over western Central Europe, providing consistent coverage from 70 to 165 years. Results show that recent severe and successive droughts, such as the 2018-drought, have historical precedents and display strong multidecadal variability. Diverging trends between SPI-3 and SPEI-3 over the last decades indicate an increasing role of atmospheric evaporative demand (AED). Summer dryness has intensified over the past 180 years, whereas winter dryness has declined. Regional and seasonal contrasts further emphasize the complexity of drought dynamics. Four distinct circulation patterns associated with droughts are identified: the Baltic High, British Isles High, North–South Dipole, and European High. Over time, droughts have become increasingly linked to the European High, a pattern characterized by strong AED anomalies, intense droughts, and this had a central role in the recent spring drying. The findings highlight the recent emergence of circulation patterns that enhance AED, marking a shift in the dynamic drivers of regional droughts under climate change.

Searching for drought tolerant species is one of the adaptative management measures that could be implemented to improve forest resilience in the context of more intense and frequent droughts brought be climate change. Western European forests are already suffering from drought-induced tree mortality, in native as well as in well- established non-native species. Novel non-native species (i.e., non-native species that have not yet undergone thorough operational testing or previously been grown at forestry scale) with high drought resistance in their original geographic range could be an alternative. To this end, the ecology and drought response of these species need to be investigated in their area of introduction. We applied a dendroecological approach on trees of Abies nordmanniana (Steven) Spach (AN), Chamaecyparis lawsoniana (A.Murray bis) (CL), and Thuja plicata Donn ex. D. Don. (TP) from Belgian arboreta (Western Europe). First, we identified the main climatic drivers of species radial growth, using Bootstrapped Correlation Coefficients between tree ring indices and climate indicators related to drought, heat, and cold stresses. Second, we assessed the species growth response to exceptional drought events, using resistance, recovery and resilience indices and an integrated index comparing the actual resilience to a theoretical full resilience. We investigated the effects of species and drought timing on these indices using linear mixed models. The radial growth of the three species was negatively influenced by the water deficit during the previous growing season (especially in fall and summer), lower precipitation in the previous October, colder temperatures in late winter-early spring, and lower minimal temperatures in May. TP is the most sensitive species to previous summer conditions as it was negatively affected by the number of days with a mean temperature above 30 degrees C, while AN was the least sensitive species with no significant Bootstrapped Correlation Coefficients for previous summer precipitation and temperatures. AN and TP differed from CL in being negatively affected in spring by higher maximal temperatures versus lower precipitation respectively. The effects of species and drought timing on resistance and recovery were significant. Overall, early and whole growing season droughts had a stronger negative effect than late droughts. AN and CL were more resistant to early and late droughts than TP, while the opposite was observed for recovery. The species showed less pronounced differences in resilience. CL was the species the closest to the theoretical full resilience, followed by AN: these two species appear to be good candidates for improving drought resistance of Western European forests. However, one must not forget that introducing novel species is associated with ecological risks and a thorough assessment of these risks must be carried out before promoting these species in forestry. Future research could focus on a comparison of novel nonnative species with already well-established species in Western Europe such as Norway spruce and Douglas fir and explore how AN, CL and TP interact with native species in mixed stands.

Droughts in Western Central Europe (WCE) have recently attracted attention due to their detrimental impact on crops, ecosystems, and society, as evidenced by events in 2018 and 2022. In this region, however, their variability and underlying causes remain unclear. This study aims to associate droughts with the atmospheric circulation to gain insight into their drivers. We employed reanalysis datasets (ERA5, 20CRv3, and ModE-RA) to identify meteorological drought events using the Standardized Precipitation Evapotranspiration Index at a 3-month scale and consistently connect them to atmospheric circulation patterns through k-means clustering. The three datasets are evaluated over the WCE regions, showing that they are highly reliable over periods ranging from 70 to 180 years, providing a long perspective on the recent events. Firstly, we demonstrate that droughts in WCE display a strong multidecadal variability with no significant long-term trend. Although precipitation has increased over time, this has been offset by the rising atmospheric evaporative demand due to warming. Secondly, we identify three distinct atmospheric circulation patterns associated with drought events in WCE: a high-anomaly geopotential height centred over Western Central Europe (WCE+); a dipole of high-anomaly geopotential height over the British Isles and low-anomaly geopotential height over the Maghreb (BIM+); and the negative phase of the North Atlantic Oscillation (NAO-), predominantly in winter. Our analysis shows that droughts have become increasingly associated with WCE+ over the last century, while their association with NAO- has decreased over the past 180 years. This research provides a regional historical analysis of meteorological drought and its drivers, offering better insight into long-term regional climate change.

Droughts have garnered global attention due to their adverse effects on crops, ecosystems, and society. Despite their frequent occurrence in north-western Europe, the causes of these droughts remain poorly understood. This study investigates the historical climate drivers of meteorological droughts in the region. The identification of drought events since 1836 is conducted using the Standardized Precipitation Evapotranspiration Index at a 3-month scale, based on reanalysis datasets (ERA5 and 20CRv3). Subsequently, by employing clustering methods, we categorize the diverse atmospheric conditions leading to droughts into discernible patterns. Our next objective is to assess the long-term variability and trends within these patterns. This research provides a long-term regional analysis of meteorological drought drivers, contributing to a deeper understanding of regional climate changes over the past two centuries.

The temporal change of soil chemistry in the forest floor and mineral soil down to a depth of 40 cm was assessed for the 102 permanent plots of the French Network for the Monitoring of Forest Ecosystems (RENECOFOR), over a 15-year period (from 1993–1995 to 2007–2012). In examining the separate and joint evolutions of a large set of parameters, many significant changes were detected reflecting the fact that French forest soils were not in a steady state. A significant increase in soil organic carbon (SOC) stocks was found, mainly in the surface soil (13.0% increase over the forest floor and the 0–10 cm layer). Conversely, the relative increase of the total nitrogen (Ntot) stocks was lower in the surface soil (4.8% increase), and a general and sharp decline of Ntot was detected between 10 and 40 cm depth (12.0% decrease). These results led to a substantial raise of C/N ratio over the whole soil profile. Another major finding is the difference in soil acidification recovery depending on the initial trophic level. In highly acidified contexts (top soil pH H2O < 4.5), increased soil acidification (pH and base saturation decrease, exchangeable Al increase) over the profile was observed while exchangeable base cation (Ca, Mg, K) pools increased. On the other hand, less acidic soils saw their global buffer capacity enhanced. These observations contrast with what is measured in other European inventories. While a previous study carried out on the same plots and over the same period highlighted SOC as a major driver of soil evolution in the top mineral soil, the possible mechanisms behind the large N decrease in the lower mineral soil remain to be confirmed.

Process-based forest models combine biological, physical, and chemical process understanding to simulate forest dynamics as an emergent property of the system. As such, they are valuable tools to investigate the effects of climate change on forest ecosystems. Specifically, they allow testing of hypotheses regarding long-term ecosystem dynamics and provide means to assess the impacts of climate scenarios on future forest development. As a consequence, numerous local-scale simulation studies have been conducted over the past decades to assess the impacts of climate change on forests. These studies apply the best available models tailored to local conditions, parameterized and evaluated by local experts. However, this treasure trove of knowledge on climate change responses remains underexplored to date, as a consistent and harmonized dataset of local model simulations is missing.Here, our objectives were (i) to compile existing local simulations on forest development under climate change in Europe in a common database, (ii) to harmonize them to a common suite of output variables, and (iii) to provide a standardized vector of auxiliary environmental variables for each simulated location to aid subsequent investigations. Our dataset of European stand- and landscape-level forest simulations contains over 1.1 million simulation runs representing 135 million simulation years for more than 13,000 unique locations spread across Europe. The data were harmonized to consistently describe forest development in terms of stand structure (dominant height), composition (dominant species, admixed species), and functioning (leaf area index). Auxiliary variables provided include consistent daily climate information (temperature, precipitation, radiation, vapor pressure deficit) as well as information on local site conditions (soil depth, soil physical properties, soil water holding capacity, plant-available nitrogen). The present dataset facilitates analyses across models and locations, with the aim to better harness the valuable information contained in local simulations for large-scale policy support, and for fostering a deeper understanding of the effects of climate change on forest ecosystems in Europe.

Forests are expected to be strongly affected by modifications in climate and disturbance regimes, threatening their ability to sustain the provision of essential services. Promoting drought-tolerant species or functionally diverse stands have recently emerged as management options to cope with global change. Our study aimed at evaluating the impact of contrasting stand-level management scenarios on the resilience of temperate forests in eastern North America and central-western Europe using the individual process-based model HETEROFOR. We simulated the evolution of eight stands over 100 years under a future extreme climate according to four management scenarios (business as usual- BAU; climate change adaptation- CC; functional diversity approach- FD; no management- NM) while facing multiple disturbances, resulting in a total of 160 simulations. We found that FD demonstrated the greatest resilience regarding transpiration and tree biomass, followed by CC and then BAU, while these three scenarios were equivalent concerning the net primary production. These results were however dependent on forest type: increasing functional diversity was a powerful option to increase the resilience of coniferous plantations whereas no clear differences between BAU and adaptive management scenarios were detected in broadleaved and mixed stands. The FD promoted a higher level of tree species diversity than any other scenario, and all scenarios of management were similar regarding the amount of harvested wood. The NM always showed the lowest resilience, demonstrating that forest management could be an important tool to mitigate adverse effects of global change. Our study highlighted that tree-level process-based models are a relevant tool to identify suitable management options for adapting forests to global change provided that model limitations are considered, and that alternative management options, particularly those based on functional diversity, are promising and should be promoted from now on.

This dataset contains forest floor and mineral soil chemical properties down to 40 cm depth for two soil sampling campaigns carried out at around 15-year time interval (campaign 1 from 1993 to 1995, campaign 2 from 2007 to 2012) within the 102 permanent plots of the French Network for the Monitoring of Forest Ecosystems (RENECOFOR). Data are reported for soil organic carbon (Corg), total nitrogen (Ntot), C/N ratio (CNratio), exchangeable calcium (Caexch), exchangeable magnesium (Mgexch), exchangeable potassium (Kexch), exchangeable aluminium (Alexch), effective cation exchange capacity (ECEC), extractable phosphorus (P), pH CaCl2 and pH H2O.

The increased frequency of climate change-induced droughts poses a survival challenge for forest trees, particularly for the common beech (Fagus sylvatica L.). Drought conditions adversely affect water supply and nutrient uptake, yet there is limited understanding of the intricate interplay between nutrient availability and drought stress on the physiology, growth, and biomass accumulation in young trees. We aimed to address this knowledge gap by examining the effects of irrigation and fertilisation and their interaction with various parameters in common beech saplings, including foliar and root N, P, and K concentrations; height and diameter increments; and aboveground and belowground biomass production. Our findings revealed that a higher fertilisation dose increased nutrient availability, also partially mitigating immediate drought impacts on foliar N concentrations. Also, higher fertilisation supported the post-drought recovery of foliar phosphorus levels in saplings. Prolonged drought affected nitrogen and potassium foliar concentrations, illustrating the lasting physiological impact of drought on beech trees. While drought-stressed beech saplings exhibited reduced height increment and biomass production, increased nutrient availability positively impacted root collar diameters. These insights have potential implications for forest management practices, afforestation strategies, and our broader understanding of the ecological consequences of climate change on forests.

Despite being adapted to a wide range of environmental conditions, the vitality of European beech is expected to be significantly affected by the projected effects of climate change, which we attempted to assess with foliar nutrition and crown defoliation, as two different, yet interlinked vitality indicators. Based on 28 beech plots of the ICP Forests Level I network, we set out to investigate the nutritional status of beech in Croatia, the relation of its defoliation and nutrient status, and the effects of environmental factors on this relation. The results indicate a generally satisfactory nutrition of common beech in Croatia. Links between defoliation and nutrition of beech are not very direct or very prominent; differences were observed only in some years and on limited number of plots. However, the applied multinomial logistic regression models show that environmental factors affect the relationship between defoliation and nutrition, as climate and altitude influence the occurrence of differences in foliar nutrition between defoliation categories.

Process-based forest growth models with spatially explicit representation are relevant tools to investigate innovative silviculture practices and/or climate change effects because they are based on key ecophysiological processes and account for the effects of local competition for resources on tree growth. Such models are rare and are often calibrated for a very limited number of species and rarely for mixed and/or uneven-aged stands, and none are suitable for the temperate forests of Québec. The aim of this study was to calibrate and evaluate HETEROFOR (HETEROgeneous FORest), a process-based and spatially explicit model based on resource sharing, for 23 functionally diverse tree species in forest stands with contrasting species compositions and environmental conditions in southern Quebec. Using data from the forest inventory of Quebec, we evaluated the ability of HETEROFOR to predict the short-term growth (5–16 years) of these species at the tree and stand levels and the long-term dynamics (120 years) of red and sugar maple stands. The comparison between the prediction quality of the calibration and evaluation datasets showed the robustness of the model performance in predicting individual-tree growth. The model reproduced correctly the individual basal area increment (BAI) of the validation dataset, with a mean Pearson's correlation coefficient of 0.56 and a mean bias of 18 %. Our results also highlighted that considering tree position is of importance for predicting individual-tree growth most accurately in complex stands with both vertically and horizontally heterogeneous structures. The model also showed a good ability to reproduce BAI at the stand level, both for monospecific (bias of −3.7 %; Pearson's r=0.55) and multi-species stands (bias of −9.1 %; Pearson's r=0.62). Long-term simulations of red maple and sugar maple showed that HETEROFOR was able to accurately predict the growth (basal area and height) and mortality processes from the seedling stage to the mature stand. Our results suggest that HETEROFOR is a reliable option to simulate forest growth in southern Quebec and to test new forestry practices under future climate scenarios.

Ungulate impacts on forest understory alter tree species composition, with cascading effects on forest functions and resilience against future climate conditions. Indeed, the ungulate browsing pressure on tree seedlings is species-specific and causes contrasted growth reductions that alter tree recruitment rates. Untangling the effects of browsing from the effects of the other factors driving regeneration success is required to guide the forest and ungulate management. In particular, Fagus sylvatica L. strongly dominates temperate Quercus-Fagus forests close to their climax, and it remains unclear if controlling ungulate populations can maintain tree species diversity in naturally regenerated forests. We addressed this question by monitoring 734 pairs of fenced and unfenced 6-m(2) plots across a broad gradient of Cervus elaphus L. abundance in Belgian Quercus-Fagus forests managed by continuous cover forestry. Seedling height, density, and vegetation cover were monitored from 2016 to 2021. Species diversity and ecological affinity for light, temperature, and atmospheric humidity conditions were computed from these measures. With ungulates, the mean growth of Betula pendula Roth. and Sorbus aucuparia L. was negligible, whereas, without ungulates, their growth was higher than the growth of other species. With ungulates, the growth of Fagus sylvatica L. and Picea abies (L.) H. Karst was higher than other species. Quercus (Quercus petreae (Matt.) Liebl and Quercus robur L.) growth was the lowest in all conditions. Finally, Carpinus betulus L. was heavily browsed but still grew higher than its competitors with ungulates. Ungulate browsing can then severely affect seedling growth and likely reduce the diversity of future recruited trees. In the study area, browsing unfavored the regeneration of the species that are less shade tolerant, more-drought tolerant, and more-heat tolerant. It thus accelerates the natural succession and reduces forest resilience to heat and drought events. Such an observation was found valid over a wide study area encompassing contrasting levels of Cervus elaphus L. abundance. Combining further reductions of ungulate populations with foodscape improvement is likely required to maintain species diversity in these forests.

Strong density differences were observed between stem wood at 1.30 m and other tree components (stem wood, stem bark, knots, branch stumps and branches). The difference, up to 40% depending on the component, should be taken into account when estimating the biomass available for industrial uses, mainly fuelwood and wood for chemistry. Basic density is a major variable in the calculation of tree biomass. However, it is usually measured on stem wood only and at breast height. The objectives of this study were to compare basic density of stem wood at 1.30 m with other tree components and assess the impact of differences on biomass. Three softwood species were studied: Abies alba Mill., Picea abies (L.) H. Karst., Pseudotsuga menziesii (Mirb.) Franco. X-Ray computed tomography was used to measure density. Large differences were observed between components. Basic density of components was little influenced by tree size and stand density. Overall, bark, knot and branch biomasses were highly underestimated by using basic density measured at 1.30 m. Using available wood density databases mainly based on breast height measurements would lead to important biases (up to more than 40%) on biomass estimates for some tree components. Further work is necessary to complete available databases.

Wild ungulate populations have increased throughout the northern hemisphere in the last decades. Their rising pressure on understory vegetation may be a decisive driver of shifts in forest diversity, with cascading effects on numerous forest functions. In particular, ungulates often thwart climate change adaptation strategies in forests. Ungulate impact has been measured locally and browsing indicators have been developed to monitor changes in ungulate pressure. However, the relationships linking browsing pressure, impact on tree recruitment, and ungulate abundance remain poorly understood. To fill this gap, we conducted a large experiment in 734 pairs of fenced and unfenced 6 m² plots installed across a broad gradient of red deer abundance in oak-beech forests managed with a continuous cover forestry system in Belgium. Height of the dominant seedlings, seedling density, and vegetation cover were monitored yearly in each plot from 2016 to 2021. Species diversity and ecological affinity for light, temperature, and atmospheric humidity conditions were derived from these measures.Ungulates strongly reduced seedling growth, density and cover of understory vegetation. Among the species studied, the early successional species Betula pendula and Sorbus aucuparia were the most affected. These species failed to grow in height when unprotected from ungulates but grew faster than the other species in fenced plots. In contrast, the height growth of late successional species was little or not affected by ungulate browsing. Without protection against browsing, late successional species (Fagus sylvatica, Picea abies) then rapidly dominated the other admixed species. Like the other early successional species, oak seedlings failed to grow significantly in height without protection from browsing. Even when protected, they grew more slowly than all the other species. Oak regeneration will thus rarely succeed without sylvicultural intervention.These observations are clear evidence that ungulates are a key driver of oak-beech forest succession as they cause a bottleneck for the regeneration of early successional species. Ungulates can affect forest succession, reduce species richness, and reduce forest resilience if recruitment of heat- and drought-tolerant species is reduced.Early successional species regeneration can be improved by strongly reducing ungulate abundance, but also probably by temporarily modifying the availability of food resources at landscape level, which would alleviate ungulate pressure and create windows of opportunity for early successional species recruitment. The current cutting regime produces local resource hotspots where ungulate pressure is much higher.

European beech (Fagus sylvatica L.) forests provide multiple essential ecosystem goods and services. The projected climatic conditions for the current century will significantly affect the vitality of European beech. The expected impact of climate change on forest ecosystems will be potentially stronger in southeast Europe than on the rest of the continent. Therefore, our aim was to use the long-term monitoring data of crown vitality indicators in Croatia to identify long-term trends, and to investigate the influence of current and previous year climate conditions and available site factors using defoliation (DEF) and defoliation change (ΔDEF) as response variables. The results reveal an increasing trend of DEF during the study period from 1996 to 2017. In contrast, no significant trend in annual ΔDEF was observed. The applied linear mixed effects models indicate a very strong influence of previous year drought on ΔDEF, while climate conditions have a weak or insignificant effect on DEF. The results suggest that site factors explain 25 to 30% DEF variance, while similar values of conditional and marginal R2 show a uniform influence of drought on ΔDEF. These results suggest that DEF represents the accumulated impact of location-specific stressful environmental conditions on tree vitality, while ΔDEF reflects intense stress and represents the current or recent status of tree vitality that could be more appropriate for analysing the effect of climate conditions on forest trees.