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.

Global warming is driving unprecedented changes in the soil organic carbon (SOC) cycle. Understanding the complex factors controlling SOC stocks and turnover is essential for projecting future SOC storage. In this study, we examined the influence of physico-chemical soil properties and climate on SOC stocks and radiocarbon (14C)-derived turnover times in various SOC fractions across 54 forest soils in Switzerland (0-20 cm depth), spanning diverse forest ecosystems and geo-climatic conditions (e.g., mean annual temperature, MAT, ranging from 1 to 12°C). Soil pH emerged as the dominant predictor for SOC turnover times for all fractions, exhibiting a parabolic relationship with a tipping point at a pH around 5.5 and longer turnover times at both low and high pH values. This is likely related to the high contents of pedogenic oxides observed in acidic soils and exchangeable calcium at high pH, both of which are known to stabilise SOC. Reduced soil biotic activity at low pH may additionally contribute to the observed pH effects. Accordingly, SOC persistence appears lowest in moderately acidic soils (pH 5–6.5). Particulate organic matter (POM) fractions were particularly sensitive to climate, with the organic layer being the most responsive SOC pool, exhibiting increased C stocks and turnover times at lower MAT. In mineral soils, soil water availability played a more pronounced role than MAT in controlling SOC stocks and turnover times, particularly in the occluded light fraction and the fine heavy fraction (fHF). Oxidation of the fHF with peroxide revealed that mineral-associated organic matter consists of two distinct sub-pools: a larger oxidized fraction with turnover times of 170±29 years, and a residual pool exhibiting turnover times on millennial timescales. These sub-pools exhibited distinct relationships with climate and soil properties. While turnover times of the oxidisable pool within the fHF increased significantly with soil water availability, residual organic matter in fHF remained insensitive to climate. Finally, relationships between climate and turnover times were mostly non-linear. Soils at the wetter and colder extremes of Swiss forest ecosystems (MAT<4°C) exhibited particularly large POM stocks and long turnover times – for example, up to 166 years in the organic layer at treeline. These ‘extreme’ soils with large POM stocks are likely particularly sensitive to climate change, as rising temperatures and hydrological changes may disproportionally impact C stored in POM.

Hotter droughts in European forests increasingly combine declining soil moisture with rising atmospheric demand, raising fundamental questions about how trees sustain transpiration while avoiding embolism-induced mortality under drought stress. While stomatal regulation and transpiration responses are well documented, the role of upstream, within-tree water fluxes, particularly the use and replenishment of internal stem water storage, represent an emerging research frontier.Here, we present high-temporal resolution observations of stem water storage use and rehydration dynamics in mature Pinus sylvestris, combining sap-flow and dendrometer measurements from the VPDrought experiment at the Pfynwald research platform in the dry inner-Alpine Rhône valley of Switzerland. By independently manipulating soil moisture and vapour pressure deficit (VPD), this experiment allows us to disentangle atmospheric and soil controls on internal tree water fluxes.We show that under drought, trees increasingly “run on savings”: the contribution of stem water storage to daily transpiration rises sharply from approximately ~5% under well-watered soil conditions to up to ~40% under dry soil conditions, when transpiration declines but storage water use persists. In parallel, the replenishment of stem storage-water reserves through water flow into the stem declines with decreasing soil water potential. Notably, even under mild soil drought, elevated VPD substantially constrains nighttime rehydration of stem storage-water reserves.The findings we present emphasize stem water storage as a dynamic and drought-responsive component of tree-water use. Accounting for both the mobilization and rehydration of internal water reserves is essential for understanding how trees buffer hydraulic stress during drought and enhance model representations of plant-water interactions under increasingly frequent hotter droughts.

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.

The onset of leaf discoloration in deciduous trees is primarily triggered by seasonal declines in temperature and photoperiod. However, severe drought conditions can induce early leaf discoloration and shedding. Early discolored European beech (Fagus sylvatica L.) trees may be more susceptible to crown dieback and mortality following a drought. These persistent legacy effects of drought underlie efforts to understand the environmental drivers of early leaf discoloration but also complicate ongoing monitoring. Here, field observations of early leaf discoloration in Switzerland's European beech forests and intra-annually normalized Sentinel-2 vegetation index time series were used to model and isolate the signal of discoloration from drought legacy effects in both drought and post-drought years. To demonstrate model efficacy and examine empirical inter-and intra-annual trends in early leaf discoloration, the trained model was applied to predict May to August early leaf discoloration across Switzerland's European beech dominated forests at a 10 m resolution from 2017 to 2023. Results reveal an order of magnitude greater proportion of early leaf discoloration at the end of the drought-affected months of August 2018 (5% discolored) and August 2023 (2% discolored) as compared to the non-drought affected month of August 2019 (0.4% discolored). In 2018 and 2023, discoloration hotspots formed as early as June in Switzerland's most severely affected northwestern regions and spread rapidly in the following weeks. Efforts here establish a robust framework for disentangling seasonal drought related discoloration from legacy effects of drought and forest management forming a baseline for further investigations of biotic and abiotic early leaf discoloration drivers.

Although the hydrogen (δ2H) and oxygen (δ18O) isotopic signature of tree rings is dependent on the environmental water, such as precipitation and soil water that trees have taken up (i.e. “source water”), estimating the spatio-temporal origin of water sources through analysis of water stable isotopes in tree rings is not a straightforward approach. This is because 1) our knowledge on the contribution and the variability of individual isotopic fractionation steps between source water and tree rings is limited, and 2) in situ measurements that consider the seasonality of the isotopic composition of source water and cellulose synthesis are rare. Within the framework of the EU Cost Action WATSON (#CA19120 - WATer isotopeS in the critical zONe), we analyzed (1) δ2H and δ18O in tree-ring cellulose and stem sugar, (2) δ2H and δ18O in soil water at shallower (15 cm) and deeper (80 cm) depths in up to bi-weekly resolution and (3) modelled isotopic variations in precipitation, soil water, stem xylem water, and leaf water using mechanistic and process-based models for three long-term forest monitoring sites in Switzerland over 20 years. We used this data to explain intra-annual (2021-2022) and inter-annual (2003-2022) δ2H and δ18O variations in tree-ring cellulose of beech (Fagus sylvatica) and spruce (Picea abies). At the intra-annual scale, preliminary findings indicate a pronounced isotopic enrichment in the second half of the growing season and marked seasonal variations in the isotopic composition of soil water at shallower depths compared to deeper layers. However, such fluctuations were strongly dampenend in the intra-annual δ2H and δ18O variations observed in the tree-ring cellulose and stem sugars of both tree species, which may indicate the use of deeper soil water sources or scrambling of the source water isotope signal because of isotope fractionation before cellulose synthesis. In further analyses at the inter-annual scale, we will investigate how well δ2H and δ18O in tree rings can function as indicators of source water through time-window correlation analysis between water and tree-ring stable isotopes and comparisons between measured and modelled data. Our study aims to enhance models of hydrogen and oxygen isotope fractionation. This will improve the use of both elements in tree rings as innovative ecohydrological proxies for retrospectively reconstructing environmental water sources.

Climate change, including a reduction in precipitation, increased atmospheric moisture demand, and drying soils, threaten the life-supporting function in trees. In response, trees can exhibit different below-ground drought acclimation strategies, including increased root-water uptake depth and root growth to increase water supply. We initiated a long-term monitoring experiment at the Swiss Canopy Crane II (SCCII) site in Switzerland in 2018, including a rainfall exclusion of 50% during the vegetation period (April-October) since 2023. The SCCII site provides growing conditions representative of a central European mid-mountain range forest and hosts 10 co-occurring European temperate tree species. For six years, we measured the δ 2 H and δ 18 O values of samples collected from tree xylem, soil water in different depths, and precipitation, as well as a multitude of ecophysiological measurements within a great range of environmental conditions (wet and dry), including the exceptionally dry summer in 2023. The extreme conditions in 2023 caused canopy dieback and mortality in individuals of Fagus sylvatica , Picea abies , and Abies alba in the drought treatment indicating critically low soil water supply. We utilized the data to parameterize the hydrological model LWFBrook90.jl with the goal of simulating soil moisture, soil water potential and soil water isotope transport, as well as root water uptake depth under different environmental conditions. By quantifying the temporal origins of root water uptake and running scenarios of further increased drought conditions, we will quantify the access of different tree species to soil water from various depths and the soil water residence time. Moreover, we will quantify how soil water residence time and, thus, the supply of water to tree species at different soil depths varies under different climate change scenarios. First results show that depending on the severity of drought and tree water consumption, the soil water is used up almost entirely within one growing season indicating the vital role of summer precipitation and winter-time refilling. We expect the final results of this study to provide us with valuable insights on soil water retention time and the temporal dynamics of root water uptake under various drought conditions. These findings will increase our understanding critical below-ground drought effects and acclimation of temperate trees.

Increasingly frequent droughts in Central Europe underscore the need for regular forest health monitoring. For decades, vegetation indices like Normalized Difference Vegetation Index (NDVI) and Normalized Difference Water Index (NDWI) have been used as an indicator of forest condition and these indices are the focus of next-generation near real-time forest water stress monitoring which aims to update stress information coincident with satellite revisit times. However, few studies have robustly correlated vegetation indices with continuous ground-based monitoring tracking the water stress of individual trees. This study examines the relationship between 7 years of Sentinel-2 derived vegetation index time series (NDWI, NDVI, Chlorophyll Red Edge: CIRE, Chlorophyll Carotenoid Index: CCI, and Enhanced Vegetation Index: EVI) and continuous stem-level tree water deficit measurements from 41 European Beech (Fagus sylvatica L.) and 61 Norway Spruce (Picea abies (L.) H. Karst.) trees in Switzerland's temperate forests. Results show both an inter-and intra-seasonal correlation between tree water deficit and vegetation indices at a monthly time step (linear fit R2 values reaching 0.62 for Norway Spruce and 0.58 for European Beech), but it is more challenging to resolve recoverable tree water deficit at a finer temporal resolution. Common satellite-derived vegetation indices are unlikely to fully replace the sensitivity of in-situ tree water stress measurements but some indicators like EVI for Norway Spruce or NDWI for European Beech show potential for estimating broad tree water stress trends during the vegetated period. Monthly, or ongoing rolling average estimates could offer a scalable approach for regular drought stress assessment.

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 stable isotope ratios of hydrogen (δ2H) and oxygen (δ18O) are useful for studying ecohydrological dynamics in forests. However, most isotope-based eco-hydrological studies are limited to single sites, resulting in a lack of large-scale isotope data for understanding tree water uptake. Here, we provide a first systematic isotope dataset for soil and stem xylem water collected during two pan-European sampling campaigns at 40 beech (Fagus sylvatica), spruce (Picea abies), or mixed beech-spruce forest sites in spring and summer 2023 (https://doi.org/10.16904/envidat.542, Lehmann et al., 2024). The dataset is complemented by additional site-, soil-, and tree-specific metadata. The samples and metadata were collected by different researchers across Europe following a standardized protocol. Soil samples were taken at up to 5 depths (ranging from 0 to 90 cm) and stem xylem samples from the trunks of three beech and/or spruce trees per site. All samples were sent to a single laboratory, where all analytical work was conducted. Water was extracted using cryogenic vacuum distillation and analyzed with an isotope laser spectrometer. Additionally, a subset of the samples was analyzed with an isotope ratio mass spectrometer. Data quality checks revealed a high mean total extraction efficiency, mean water amount (>1 mL), accuracy, and precision. The isotopic signature of soil and stem xylem water varied as a function of the geographic origin and changed from spring to summer across all sites. While δ2H and δ18O were strongly correlated, the soil water data plotted closer to the Global Meteoric Water Line (GMWL) than the stem xylem water. Specifically, the δ2H values of the xylem water were more enriched than those of the soil water, leading to a systematic deviation from the GMWL. Isotopic enrichment of the stem xylem water at mixed forest sites was larger for spruce trees than for beech trees. This dataset is particularly useful for large-scale studies on plant water use, ecohydrological model testing, and isotope mapping across Europe.

Soil-vegetation systems partition incoming precipitation into either the freshwater system ("blue water") or back to the atmosphere as evapotranspiration ("green water"). The isotope signatures of these fluxes are observed to be distinct. We investigate the partitioning of precipitation and illustrate one potential mechanism for this "apparent" isotopic fractionation of root water uptake by means of an isotope-enabled mechanistic water balance model [1].Stable water isotope signatures were collected at a Swiss forest site dominated by beech trees (>60 cm DBH, 37m stand height) with a mean annual precipitation of ~1050 mm/year, at 800 m.a.s.l throughout two vegetation seasons. Up to bi-weekly samples of xylem and mobile soil water and six bulk soil water campaigns (down to 150 cm) were combined with continuous hydrometric measurements (down to 200 cm) to constrain modelled water fluxes such as preferential infiltration patterns and seasonal patterns of root water uptake.During the model validation period in 2022, the model faithfully reproduced seasonal and vertical patterns in isotope signatures. The goodness of fits of time series showed δ18O RMSE smaller than 0.4‰ for mobile soil water at 50cm or 80cm or smaller than 1.0‰ for stem xylem water at breast height, while the goodness of fits of vertical profiles had δ18O RMSE smaller than 1.7‰ for bulk soil water profiles down to 150cm. Reduced soil water availability in the topsoil during the summer of 2021 led to a downward shift of the flux-weighted average water uptake depths of beech trees. However, while the relative contribution to water uptake of soil layers below 80 cm increased during the summer of 2021, their absolute contribution did not increase sufficiently to compensate the water missing in the topsoil layers where most roots are located. Modelled infiltration pathways and root water uptake illustrate how seasonal and vertical selectivity of root water uptake leads to distinct isotope signatures in the modelled green and blue water fluxes. This behaviour is obtained at this site without a two-domain representation of the soil domain nor preferential flow to deeper layers. Further, simulations with synthetic seasonal isotope patterns in precipitation demonstrate how this "apparent" fractionation factor depends on the timing of transpiration together with the seasonality of the precipitation isotope signature. In conclusion, this study highlights that the soil-vegetation system may fractionate heavier precipitation for the green water fluxes mainly because of the seasonal patterns in precipitation isotope signatures and transpiration rates. [1] Fabian Bernhard. (2024). fabern/LWFBrook90.jl: v0.9.8 (v0.9.8). Zenodo. https://doi.org/10.5281/zenodo.10463109

Over the past decade, extreme temperature and drought have resulted in widespread early leaf discoloration in European Beech (Fagus sylvatica) forests across central Europe. Discoloration during the particularly hot and dry summer of 2018 was ultimately associated with increased rates of crown dieback and tree mortality. Given the trend towards hotter and drier growing seasons under a changing climate, there is an increasing demand for site-specific recommendations on drought-resilient forest management practices in Switzerland. Making these recommendations requires a robust understanding of empirical forest disturbance and estimates of future forest health under a range of climatic and management conditions. To that end, using 2018 field observations, manual mapping of forest discoloration in aerial imagery, and multispectral Sentinel-2 imagery, we are developing 10 m/pixel estimates of European Beech discoloration across Switzerland during the 2018 to 2023 foliated periods. To date, we have 1) developed a robust interpolated Sentinel-2 time series from 2018 to 2023 for all of Switzerland, 2) trained a random forest model using 2018 ground control data and several vegetation indices from the Sentinel-2 time series to predict 2018 early leaf discoloration across Switzerland’s European Beech forests with c. 90% accuracy and, 3) used the Chlorophyll Red-Edge Index derived from the Sentinel-2 time series to approximate tree phenology and the length of the foliated period. We estimate that the 2018 foliated period was, on average, 45±19 days shorter for discolored sites as compared to sites without discoloration. Our results generally align well with previous studies of the 2018 drought in Switzerland and additional observational data is being compiled to validate the application of 2018 ground truth data across the foliated periods from 2018 to 2023. In combination with high-resolution soil maps, meteorological data, topographic derivatives, and information on Swiss forest structure, we will use empirical discoloration estimates to train ensemble models of site-specific susceptibility to drought. By artificially varying the meteorological and forest structure variables in these models we will have the unique opportunity to better understand European Beech susceptibility to drought and test the influence of a range of future climate scenarios and forest management strategies on Swiss forest health at a high spatial resolution.

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.

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.

Transpiration fluxes from land to the atmosphere hinge significantly on the degree to which trees opt to open their stomata to trade off water for CO2. Yet, the terrestrial ecosystem response to the changes in the atmosphere (CO2, VPD, etc.) and the redistribution of water on land in an era of change are largely unknown. In addition, the effects of such long-term changes in trees’ adaptation strategies and resilience under short-term dry conditions are not yet fully understood. To address this issue we determined stable water isotopologues within a long-term (20-year) irrigation experiment in a drought-prone Scots pine-dominated forest in one of the driest areas of Switzerland, Pfynwald. Our sampling included plots with trees growing under naturally dry conditions (control), irrigated (from 2003 to present), and previously irrigated (irrigation stop; irrigated from 2003–2013; control condition since 2014). We have installed an in-situ high-frequency isotope measurement system in the field to sample stable water isotopologues (2H and 18O) in different soil depths, tree xylem, and in the atmosphere and to track tree water uptake dynamics at the control, irrigated, and irrigation stop plots. The sampling was complimented with manual extraction of soil and xylem water samples at the three treatment plots for isotope analysis in the lab. Our preliminary findings support the hypothesis that pine forests adjust their carbon allocation strategies during long-term wet periods, establishing a deeper rooting system to access deeper water sources. This adaptive mechanism enhances their resilience during short-term dry periods.

Snow gliding affects soil erosion patterns and depends on various factors such as slope angle, precipitation amount, and vegetation roughness. Snow gliding distance can be assessed through measurement or modeling. However, the comparison of measured with modelled data remains limited due to the scarcity of measured data. We present a long-term dataset (2010-2021) of measured snow gliding distances for two Swiss alpine Valleys (Urseren and Val Piora) using glide shoes to address this gap. We also predicted snow gliding between 2010 and 2021 using the spatial snow gliding distance model developed by Leitinger et al. (2018). Our analysis of the measured data indicates that sites with a north aspect generally exhibit shorter snow gliding distances than those facing south. Moreover, we observed an increase in snow gliding distance with steeper slope angles and a decrease with a higher roughness coefficient. Comparing measured and modelled values, the R2 and Concordance Correlation Coefficient (CCC) values are 0.23 and 0.12 for the Urseren Valley and 0.24 and 0.35 for the Val Piora. Generally, the model tended to predict higher values than the measured data for both Valleys, potentially due to the large small-scale variability observed in the replicates of the measured data that cannot be caught with large-scale models. This variability highlights the dynamic nature of snow gliding distance, making it challenging to model or measure accurately. Furthermore, a covariate importance analysis revealed precipitation and slope angle as the dominant drivers of modelled snow gliding distances versus vegetation roughness (a rather local feature) for measured values.

The COST Action WATer isotopeS in the critical zONe (WATSON; https://watson-cost.eu/) aims to elucidate the interactions between groundwater recharge, soil water storage, and vegetation transpiration across various climatic settings. Within this framework, root water uptake by trees is crucial for understanding water partitioning and forest resilience to drought. While isotopic approaches have successfully revealed root water uptake strategies for different species, a comprehensive assessment across different climates and vegetation types is still missing (Beyer and Penna, 2021). To address this research gap, we executed a synchronized, participatory sampling campaign by WATSON Action members in late spring and summer of 2023. Soil and vegetation samples were taken across 39 well-distributed forest sites encompassing 17 European countries. The samples were analyzed for the stable isotopes of oxygen and hydrogen at the WSL laboratory in Switzerland. The data enable us to investigate the spatial and temporal (spring vs summer) variability of root water uptake of the shallower-rooted spruce (Picea abies) and deeper-rooted beech (Fagus sylvatica) trees. We expect beech to exhibit a more pronounced shift to deeper water sources during summer than spruce due to its deeper rooting system. We also expect that the dominant root water uptake depth is influenced by site-specific factors (climate, elevation, latitude, soil type, level of understory cover) and tree characteristics (tree height, stem diameter). This presentation will describe the sampling campaign and the preliminary results on root water uptake for both species across Europe.

In recent years, an increasing number of studies have reported on forest declines and shifts in species composition in response to changing climatic conditions (Rigling et al. 2012). The intensification of droughts through rising evaporative demand (i.e., vapor pressure deficit or VPD) is a considerable concern because of their disastrous impacts on natural systems (Grossiord et al. 2020; Trotsiuk et al. 2021). For forests, ecosystem services such as wood provisioning and carbon sequestration are severely jeopardized by these changes, leading to significant uncertainties regarding climate regulation. Climate-vegetation models are not only in need of data on atmospheric and soil drought sensing mechanisms but are also critically challenged by insufficient understanding of the processes driving forest vulnerability to climate change. Only by deciphering the single vs . combined VPD and soil moisture effects will we be able to improve global predictions. We apply a scale spanning approach to disentangle the processes affected by atmospheric (i.e., VPD) and soil droughts from the tissue to the tree and the ecosystem level. We set up the first atmospheric humidity and soil moisture manipulative experiment in a mature natural forest. We combine air humidity (and thus VPD) manipulation using a humidification system in the canopy of adult Scots pine trees exposed naturally to high summer VPD and a below canopy through-fall exclusion system (Schaub 2023). The system is installed at the long-term Pfynwald irrigation experiment, which is since 2003 a pivotal WSL long-term experimental monitoring site anticipated to be near its tipping point with respect to climate change (Bose et al. 2022). This experiment helps us understand how the soil moisture responses of trees, shrubs, and microbial communities are altered by atmospheric dryness from the tissue- to the ecosystem-level. This novel manipulative VPD and soil moisture experiment provides an empirical research platform to address the most critical questions in the context of climate impacts in temperate forests. The data will ultimately allow the development of novel predictive methods to assess climate change impacts on forests. Preliminary data will show the effects of altered atmospheric and soil drought on adult Scots pine trees.