The oxygen (δ18O) and hydrogen (δ2H) isotope compositions of leaf and xylem water shape tree-ring isotope baselines, while the fraction of sugars undergoing isotopic modification downstream of leaves (fO, fH) determines the dominant hydrologic signal. However, limited information on the seasonal dynamics of these isotope sources and on the drivers of f variation constrains tree-ring isotope interpretation. We measured intra-annual δ18O and δ2H in stem water, sugar, starch, and tree-ring α-cellulose of beech and spruce over two growing seasons. Using modelled leaf water δ18O and δ2H, we estimated seasonal f values and examined their relationships with nonstructural carbohydrate concentrations and climate. Tree rings primarily recorded δ18O and δ2H signatures of leaf water, despite seasonal changes in fO and fH. We found no clear transfer of intra-annual xylem water isotopic signals into sugars, starch, or cellulose. Seasonal fO and fH can be negatively correlated. Both were related to climate variables, but only fO was correlated with nonstructural carbohydrate concentrations. Thus, isotopic fractionation downstream of leaves does not always override the seasonal imprint of leaf water in tree rings. These findings provide insight into the controls on the fO-fH covariation, supporting more robust interpretations of climate variability from tree-ring isotope records.

The phase equilibria of the Cu-Fe-S system are calculated from liquidus to medium temperatures based on Gibbs energy modeling of the intermediate solid solution (ISS). The compound energy formalism is used for a suitable thermodynamic model to account for the pronounced solid solution character of ISS. The obtained Gibbs energy as a function of composition and temperature allows the computation of complex phase relations with high-temperature modifications of bornite and pyrrhotite, pyrite and chalcopyrite, of which a Gibbs energy expression as a function of temperature is also provided. For the first time, a direct comparison is made public between the well-known experimental isoplethal phase diagram along the CuFe-S join, which was provided more than fifty years ago by Barton (1973), and a thermodynamically computed phase diagram section. A detailed comparison with experimental phase equilibria data on the homogeneity range of ISS at 800 degrees C, 760 degrees C, 700 degrees C and 600 degrees C with calculated results is carried out. In total three isoplethal and four isothermal sections of the Cu-Fe-S phase diagram are computed, and the agreement with experimental data available in the literature ranges from fair to very satisfactory.

Forests are one of the most important terrestrial carbon sinks, but are increasingly under pressure due to drought, heat and the occurrence of extreme events. There are opposing longer term trends for European forest growth reported, and severe drought and disturbance events additionally impact forest ecosystems, so that the overall trend of forest productivity is uncertain. Thirty years of harmonized forest monitoring at 18 forest sites along an altitudinal gradient in Switzerland provides a good basis for assessing the effects of climate change on forest conditions. We found a decreasing trend of forest productivity (basal area index and net carbon uptake by growth), particularly pronounced since 2015 across all altitudinal ranges, age classes and species, which could not solely be attributed to stand density and ageing of the forest, but also to soil water availability and nitrogen deposition. The growth rate of trees, as well as the ingrowth rate, were hereby the most important factors explaining the overall forest productivity. At a given stand density, forest productivity was lower in recent years compared to earlier decades. Overall, our results indicate a decreasing stand-level growth trend irrespective of site conditions and stand structure. This 30-year declining trend can be partly attributed to water and nitrogen availability, and points to a decreasing growth capacity of the forest sites that is the long-term potential of a site to sustain tree growth. The pivotal role of water availability for sustainable forest production and the long-term effect of drought years on forest vitality urges us to rethink the adaptability of forests in view of the increasing frequency of drought and heat periods predicted for the future.Read the free for this article on the Journal blog.

Carbon sustains life, whereas mercury is a global toxin, yet their cycling in forests appears to be intimately linked. Here we show, using elemental stoichiometry, carbon and mercury isotopes and a global forest synthesis, that forests simultaneously couple and decouple mercury from carbon along contrasting ecosystem continua. Mercury/carbon ratios remain tightly conserved (0.5-0.9 × 10-6) along the aqueous-phase continuum, indicating proportional mercury transport with dissolved organic carbon. In comparison, mercury/carbon ratios increase by nearly three orders of magnitude from the atmosphere to soils (0.007-3.6 × 10-6) along the solid-phase continuum, reflecting progressive mercury enrichment during litter and soil organic matter decomposition. Standing litter acts concurrently as a net carbon source and mercury sink, whereas biomass regulates coupled carbon and mercury storage and litterfall deposition. These contrasting carbon-mercury trajectories reveal how forests both retain and redistribute atmospheric mercury and provide a conceptual framework for understanding terrestrial mercury cycling under environmental change.

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

Wälder sind eine wichtige Kohlenstoffsenke, stehen jedoch durch sich häufende Klimaextreme wie Hitze und Trockenheit zunehmend unter Druck. Nach 25 Jahren Monitoring an den 19 Standorten des Programms «Lang- fristige Waldökosystem-Forschung» zeigen sich Veränderungen in der Waldstruktur und im Wachstum. Die Wäl- der wurden generell dichter und älter. Das Wachstum nahm sowohl in den Tief- als auch in den Hochlagen ab, vor allem bei Fichte, Tanne und Buche. Das Wachstum korreliert auf Einzelbaum- und auf Bestandesebene mit Bestandesstruktur (Bestandesdichte und Alter) und negativ mit Temperatur und Trockenheit. Verglichen mit den 1990er-Jahren weisen die Wälder an den untersuchten Standorten heute eine geringere Produktivität bei glei- cher Bestandesdichte auf, was auf eine Abnahme der Standortskapazität hinweist.

The input of nitrogen (N) into forests through atmospheric deposition has been determined for the main forest types within the ICP Forests Level II monitoring network and the Swedish Throughfall Monitoring Network (SWETHRO) since the 1990s from measured concentrations in continuously collected precipitation (bulk deposition) and throughfall (below tree canopy) samples. Recently, aggregated data sets have been created, containing gap-filled monthly and annual bulk and throughfall depositions (including stemflow in beech stands) for more than 500 forest stands. Total deposition was calculated from throughfall deposition accounting for canopy exchange. Here, we present trends for throughfall deposition of inorganic N, including ammonium (NH4+-N) and nitrate (NO3--N), for plots with a complete time series, during the period 2000-2020 and in the first and last decade separately. Furthermore, we highlight and discuss spatial trends of total inorganic N deposition across Europe.

Forests play a major role in wood production and other ecosystem services, such as carbon (C) sequestration and filling reservoirs in drinking water quality. However, it is still under discussion to what extent environmental changes, such as elevated nitrogen (N) deposition and related eutrophication, may affect such services.Our study aimed to assess long-term changes in N and C storage in Swiss forest soils along a gradient in N deposition (about 10 to 30 kg N/ha/y). At five long-term forest ecosystem research plots in Switzerland, which are part of the ICP Forests Level II network, nutrient fluxes (atmospheric deposition, litterfall, soil solution) have continuously been measured since the 1990s. Soil samples were taken from fixed depth layers in the course of soil inventories in the 1990s and 2022.The observed flux patterns indicated that the forests had reached nitrogen saturation on some sites, resulting in nitrogen leaching. At sites with a higher carbon-to-nitrogen ratio (C/N), we found comparatively lower levels of N leaching. The comparison of the two soil inventories showed that the N concentration in soils has actually increased (and the C/N ratio decreased) on the sites with high C/N ratio and high N deposition. We will discuss the observed accumulation and transformations of organic C and N in these soils and the potential impacts on selected ecosystem services.

BACKGROUND:Soil microbial communities can affect plant nutrient uptake, productivity, and may even confer resistance to global change. Elevated atmospheric CO2 is widely expected to stimulate plant productivity; however, this will depend on the availability of growth limiting nutrients such as nitrogen. Soil microbial communities are the main mediators of soil nitrogen cycling and should therefore play a key role in influencing plant responses to elevated CO2. RESULTS:To test this, we conducted a controlled, growth chamber experiment with Pinus sylvestris to evaluate how soil microbiome variation influences plant physiology, productivity, and responses to elevated CO₂ (eCO₂; 800 ppm versus 400 ppm in the ambient treatment). Field soils were collected from six forests with varying tree growth rates and were used as an inoculant source, either sterilized or living, into a common growth medium seeded with P. sylvestris. After seven months of growth, we measured plant carbon assimilation, photosynthetic nitrogen use efficiency, above- and belowground productivity, and we measured soil microbial biodiversity using DNA metabarcoding. Our findings demonstrate that seedling productivity was stimulated under eCO2 conditions and that this was supported by improved plant photosynthetic nitrogen use efficiency, but only in the presence of living versus sterilized soil inoculant. The magnitude of this response was also dependent on the forest soil microbial inoculant source and was linked to a 70% increase in bacterial species richness, increased relative abundances of bacteria known to have positive effects on plant growth (e.g., Lactobacillus, Bacillus, Flavobacterium), and with a concomitant shift in saprotrophic fungal community composition and root growth. Variation in inorganic nitrogen cycling which favored the accumulation of nitrate under eCO2 was also correlated with a twofold reduction in photosynthetic nitrogen use efficiency, suggesting a decoupling of nitrogen availability and assimilation efficiency with distinct implications for plant growth responses to elevated CO2. CONCLUSIONS:Our results show that soil microbial community variation directly affects P. sylvestris physiology, productivity, and responses to eCO2, and may enhance plant growth through improved nitrogen use efficiency. Surprisingly, growth with different microbial communities even more strongly impacted plant productivity than a doubling of atmospheric CO2 concentrations. The soil microbiome therefore plays a key role in supporting plant nutrition and growth under ambient and eCO2 conditions, and in turn, may confer increased forest resistance to climate change.

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.

Das Programm «Langfristige Waldökosystem-Forschung» in der Schweiz liefert detaillierte Ergebnisse zu den Auswirkungen von Luftverschmutzung und Klimaänderung auf die Waldböden und Baumernährung. Seit den 1980er-Jahren sind die Emissionen von Schwefeldioxid und Stickoxiden in Mitteleuropa signifikant zurückge- gangen. Diese Trends können auch für die Immissionen in der Schweiz bestätigt werden. Die Monitoringdaten zeigen, dass die Auswaschung von Sulfat aus dem Bodenprofil zurückgegangen ist. Auch die Stickstoffauswa- schung nahm grösstenteils ab, jedoch nicht an Standorten mit weiterhin hohen Stickstoffeinträgen, wo sogar ansteigende Trends beobachtet wurden. Daher bleibt die Stickstoffbelastung trotz der insgesamt positiven Ent- wicklungen ein Problem. Ein weiterer kritischer Befund ist die fortschreitende Bodenversauerung, die sich in ab- nehmenden pH-Werten und niedrigen Verhältnissen von basischen Kationen zu Aluminium zeigt. Die Böden puffern die sauren Depositionen durch die Auswaschung von Nährstoffen und die Freisetzung von Aluminium. Diese Veränderungen in der Bodenlösungschemie können lange anhalten und die Nährstoffverfügbarkeit für die Bäume beeinträchtigen. Dies ist einer der Faktoren, die zu einem signifikanten Rückgang wichtiger Nähr- stoffe wie Stickstoff, Phosphor und Schwefel in den Blättern geführt haben. Dies weist auf eine Verschlechte- rung der Baumernährung hin. Zusammengefasst zeigen die Ergebnisse, dass trotz des Rückgangs saurer Depo- sitionen die langfristigen Auswirkungen auf die Waldböden und die Baumernährung bestehen bleiben, was sich auf die Gesundheit und Vitalität der Schweizer Wälder auswirken kann.

Trees can differ enormously in their crown architectural traits, such as the scaling relationships between tree height, crown width and stem diameter. Yet despite the importance of crown architecture in shaping the structure and function of terrestrial ecosystems, we lack a complete picture of what drives this incredible diversity in crown shapes. Using data from 374,888 globally distributed trees, we explore how climate, disturbance, competition, functional traits, and evolutionary history constrain the height and crown width scaling relationships of 1914 tree species. We find that variation in height-diameter scaling relationships is primarily controlled by water availability and light competition. Conversely, crown width is predominantly shaped by exposure to wind and fire, while also covarying with functional traits related to mechanical stability and photosynthesis. Additionally, we identify several plant lineages with highly distinctive stem and crown forms, such as the exceedingly slender dipterocarps of Southeast Asia, or the extremely wide crowns of legume trees in African savannas. Our study charts the global spectrum of tree crown architecture and pinpoints the processes that shape the 3D structure of woody ecosystems.

Gibbs energy modeling of high temperature bornite is carried out from liquidus to mediate temperatures at a total pressure of one bar. A three sublattice approach using the compound energy formalism is developed which is consistent with a recently reported critical assessment and optimization of the Cu-S sulfide digenite. The first comprehensive comparison with experimental phase diagram data can be carried out on the basis of an adequate reproduction of the homogeneity range of high-temperature bornite which emanates from digenite into the Cu-Fe-S phase space with a substantial iron solubility. Ternary heat capacity data at the composition of Cu5FeS4, considered for the first time for Gibbs energy modeling, provides the basis for a reliable extrapolation to lower temperatures. A recently presented two-sublattice model for high-temperature pyrrhotite is adapted for accordance with its limited but relevant copper solubility. Eleven phase diagram sections of the Cu-Fe-S system – five isopleth and six isothermal sections – are calculated over the total ternary composition range for comparison with experimental data available in the literature. Together with further development of the Cu-Fe-S liquid phase model agreement between calculation and experimental data is achieved in a fair to a very satisfactory manner.

Der Zustand der Baumkrone ist ein wichtiger Indikator für die Vitalität eines Baumes. In Schweizer Wäldern wird der Kronenzustand seit 1985 im Rahmen der Sanasilva-Inventuren und seit 1994 als Teil des Forschungspro- gramms «Langfristige Waldökosystem-Forschung» erhoben. In dieser Periode hat die Vitalität des Waldes deut- lich abgenommen. Starke und grossflächige Verschlechterungen zeigten sich insbesondere nach Trockensom- mern. Der Zustand der Baumkronen ist auch ein Indikator für die Absterbewahrscheinlichkeit eines Baumes in den Folgejahren. Hauptursache für den Vitalitätsverlust des Waldes ist der erhöhte Trockenstress, der sich be- sonders auf Wälder in tieferen Lagen und trockenen Regionen der Schweiz auswirkt. Ein Extrembeispiel dafür ist die Entwicklung der Waldföhre (Pinus sylvestris L.) im Wallis. Regional starb dort in mehreren Wellen ein gros- ser Teil der Waldföhren ab. Über die Jahre haben sich die Erkenntnisse zu Ursachen und Prozessen immer klarer abgezeichnet. Die immer längeren und wertvolleren Zeitreihen einer Vielzahl beobachteter Parameter erlauben es uns, auch einzelne Ereignisse und Entwicklungen in einen grösseren Kontext zu stellen und zu interpretieren. Kontinuierliche Langzeitbeobachtungen sind deshalb unersetzlich, um Veränderungen im Wald früh zu erken- nen und zu verstehen und daraus entsprechende Empfehlungen an die Politik und Praxis abzuleiten.

Stable oxygen (δ18O) and hydrogen (δ2H) isotope compositions of tree-ring compounds preserve information about environmental waters; however, our understanding of their isotopic relationships is hampered by the lack of long-term data sets. We investigated correlations using unique 17-year (2006-2022) δ18O and δ2H time series of bi-weekly measured soil solution, modelled precipitation and xylem water, along with those of tree-ring α-cellulose and lignin methoxy groups from Norway spruce (Picea abies) across three Swiss forest sites. We show that tree-ring cellulose δ18O preserves water source information more effectively than δ2H, making it better suited for ecohydrological reconstructions. We propose δ2H of tree-ring lignin methoxy groups as an alternative proxy for soil water sources, supported by strong correlations where cellulose failed to track soil water isotopes. Significant linear isotopic relationships within and across sites enable the development of transfer functions that link tree-ring to water sources, particularly precipitation and xylem water. We exemplify how these transfer functions can be used to estimate the seasonal origin of water sourced by trees during the growth period. Our findings enhance the interpretation of environmental water isotope signals in tree rings and promote the use of tree-ring isotope-based tools for retrospective retrieval of forest water dynamics.

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.

The hydrogen (δ²H) and oxygen (δ¹⁸O) isotopic signatures of tree rings depend on that of the environmental water sources, such as precipitation and soil water, taken up by trees (i.e., "source water"). Analyzing δ²H and δ¹⁸O of tree rings is thus a promising approach for reconstructing the spatio-temporal origins of tree water sources. However, such reconstructions remain rare, likely due to methodological challenges, including the analysis of hydrogen isotopes in tree rings and the availability of historical source water isotope data.In this study, we present a first attempt to reconstruct the temporal origins of water used by trees during the 20th century (1901–1995) with annually resolved tree-ring δ¹⁸O time series. The reconstruction is based on a δ¹⁸O chronology of whole wood, sampled from the latewood of spruce (Picea abies) at Bettlachstock, Switzerland. Our choice of site and species reflects a conservative approach, as a transfer function linking δ¹⁸O of tree-ring cellulose to the δ¹⁸O of source waters (e.g., stem xylem water and soil solutions) was recently established over a 17-year period (2006–2022) at the same site. After accounting for the isotopic offset between whole wood and cellulose, we estimated δ¹⁸O values of soil solution (80 cm depth) and stem xylem water during the growing season (May–September) using a linear transfer function. Further, using modeled precipitation δ¹⁸O data and the estimated δ¹⁸O of soil solution and xylem water, we deduced interannual variations in the seasonal origin index (SOI) of soil solution and xylem water during the 20th century. Our results show that the reconstructed δ¹⁸O values and SOI of xylem water were higher than those of soil solutions, suggesting a greater contribution of summer water to xylem water than to soil solutions. Interestingly, while conditions from 1900 to 1970 remained relatively stable, we observed abrupt increases in SOI for both soil solutions and stem xylem water between 1970 and 1995. These recent changes were not due to an increase in summer precipitation amount but may be linked to shifts in seasonal precipitation patterns, causing a relative increase in the contribution of summer precipitation in tree water sources. Despite these findings, uncertainties in precipitation isotope data and transfer functions need further investigation to draw more definitive conclusions. We hope this study will stimulate discussion on the advances and limitations of using tree-ring isotopes to reconstruct historical water sources.

Wälder tragen zur Erneuerung des Grundwassers in Trinkwasserqualität bei. Wiederholte Untersuchungen zei- gen, dass die hohe Stickstoffdeposition immer noch zu erhöhten Konzentrationen von Nitrat im Sickerwasser aus Wäldern führen kann. Zu hohe Nitratwerte im Trinkwasser können die Gesundheit für Mensch und Um- welt beeinträchtigen. Die Bodeninventur auf fünf ausgewählten Flächen des Programms «Langfristige Wald- ökosystem-Forschung» zeigt, dass die Stickstoffsättigung auf einem Teil der Flächen, trotz Rückgang der Ein- träge, eher zugenommen hat. Dadurch sinkt die Fähigkeit von Böden, Stickstoff aufzunehmen, womit das Risiko erhöhter Nitratauswaschung tendenziell steigt.

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.