Dr. Katrin Di Bella Meusburger

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.

Capillary suction across the soil-snow interface is a possible mechanism for the formation of wet basal snow layers, which are necessary for snow gliding and glide-snow avalanches. However, little is known about the conditions under which this process occurs. We investigated capillary suction across the soil-snow interface considering realistic snow and soil properties. Snow properties were determined from snow profiles and soil properties were determined from field measurements of liquid water content, matric potential, soil texture, and bulk density for 40 alpine soils in Davos, Switzerland, as well as a field site in the region (Seewer Berg) with glide-snow avalanche activity. For the alpine soils investigated here, the results show that capillary flow from the soil to the snow is possible for realistic snow properties, but requires a soil saturation of approximately 90% or higher at the soil surface. When comparing the 90% saturation threshold to field measurements, the results suggest that capillary suction across the soil-snow interface is unlikely to contribute significantly to the formation of wet basal layers on Seewer Berg. These results are also relevant for soil and snow hydrology, where water transport across the soil-snow interface is important and understudied.

• We investigate the impact of a 20-yr irrigation on root water uptake (RWU) and drought stress release in a naturally dry Scots pine forest.
• We use a combination of electrical resistivity tomography to image RWU, drone flights to image the crown stress and sensors to monitor soil water content.
• Our findings suggest that increased water availability enhances root growth and resource use efficiency, potentially increasing trees' resistance to future drought conditions by enabling water uptake from deeper soil layers.
• This research highlights the significant role of ecological memory and legacy effects in determining tree responses to environmental changes.

In ecohydrology, water isotopologues are used to assess potential sources of root water uptake by comparing xylem water signatures with source water signatures. Such comparisons are affected by the variability and uncertainty of the isotope signatures of plant water and water sources. The tree-scale and stand-scale variabilities of the isotope signatures in stem xylem water are often unknown but are important for sampling design and uncertainty estimation in assessing the sources of tree water uptake. Here, we quantified tree-scale and stand-scale variabilities of xylem water isotope signatures in beech, oak and spruce trees in a mature forest on the Swiss plateau. For stem xylem water, sub-daily replicates and replicates in different cardinal directions showed no systematic differences, but we found systematic differences with sampling height. The observed variability of isotope signatures at different heights along the stem suggests that water residence times within trees need to be considered, along with their effects on the isotope signatures in different compartments (stem, branches, leaves). Further, concerning the hydrogen signatures, we found height- and species-specific offsets (SW-excess δ²H). Stem xylem water's tree-scale variability was similar in magnitude to its stand-scale variability and smaller than the variabilities in branch xylem and bulk soil water around each tree. Xylem water from stem cores close to the ground, therefore, can give a more precise estimate of the isotopic signal of the most recent root water uptake and facilitate more accurate source water attribution.

Forests are under pressure and going through rapid changes. However, current inventorying and monitoring (IM) programs are often either disjointed, too narrow in their scope and/or do not operate at fine enough temporal resolutions, which may hinder scientific understanding, the timely supply of information, fast decision making, and may result in the sub-optimal use of resources. For these reasons, there is an urgent need for Advanced Forest Inventorying and Monitoring (AIM) programs to (i) achieve expanded relevance (by augmenting data/information across ecosystem properties and trophic levels), (ii) have increased temporal resolution (by tailored data collection frequency), and (iii) make use of technological advances (by incorporating novel tools and technologies). The Advanced Inventorying and Monitoring for Swiss Forests (SwissAIM) initiative was launched in 2020 to address these needs. SwissAIM builds upon the foundation offered by the existing programs (e.g., national forest inventory, long-term forest ecosystem research, biodiversity monitoring). It aims to offer a collaborative and adaptive framework to enable integrated data collection, evaluation, interpretation, analysis, and modeling. Ideally, it will result in a more responsive system with respect to current and predicted biotic/abiotic stressors that will challenge Swiss forests. Developing such a system implies identifying the information needs of different stakeholders (e.g., science, policy, practice), related technical requirements, and governance frameworks. Here, we present (i) the main features of the SwissAIM initiative (vision, scientific questions and variables, governance and engagement), (ii) the main outcomes of the participatory design process (measurements, sampling, and plot design), (iii) the potential transferability of AIM initiatives outside Switzerland (timing, relevance, practicability), and (iv) the key messages that emerged (i.e., need for advancement, integration and transdisciplinarity, statistical underpinning). Since similar needs related to forest inventorying and monitoring are emerging throughout Europe and elsewhere, the objective of this opinion paper is to share our experience and promote a dialog with those interested in developing AIM initiatives in other countries and regions.

Forests provide essential ecosystem services that range from the production of timber to the mitigation of natural hazards. Rapid environmental changes, such as climate warming or the intensification of disturbance regimes, threaten forests and endanger forest ecosystem services. In light of these challenges, it is essential to understand forests' demographic processes of regeneration, growth, and mortality and their relationship with environmental conditions. Specifically, understanding the regeneration process in present-day forests is crucial since it lays the foundation for the structure of future forests and their tree species composition. We used Swiss National Forest Inventory (NFI) data covering vast bio-geographic gradients over four decades to achieve this understanding. Trees that reached a diameter at breast height of 12 cm between two consecutive NFI campaigns were used to determine regeneration and were referred to as ingrowth. Employing three independent statistical models, we investigated the number, species, and diameter of these ingrowth trees. The models were subsequently implemented into a forest simulator to project the development of Swiss forests until the mid-21st century. The simulation results showed an ingrowth decrease and a shift in its species composition, marked by a significant reduction in Norway spruce Picea abies and concurrent increases in broadleaves. Nevertheless, the pace of this change towards climatically better adapted species composition is relatively slow and is likely to slow down even further as ingrowth declines in the future, in contrast to the fast-changing climatic conditions. Hence, support through adaptive planting strategies should be tested in case ingrowth does not ensure the resilience of forests in the future. We conclude that since the regeneration of forests is becoming increasingly challenging, the current level at which ecosystem services are provided might not be ensured in the coming decades.

Climate change is predicted to increase atmospheric vapor pressure deficit, exacerbating soil drought, and thus enhancing tree evaporative demand and mortality. Yet, few studies have addressed the longer-term drought acclimation strategy of trees, particularly the importance of morphological versus hydraulic plasticity. Using a long-term (20 years) irrigation experiment in a natural forest, we investigated the acclimation of Scots pine (Pinus sylvestris) morpho-anatomical traits (stomatal anatomy and crown density) and hydraulic traits (leaf water potential, vulnerability to cavitation (ψ₅₀), specific hydraulic conductivity (K_s), and tree water deficit) to prolonged changes in soil moisture. We found that low water availability reduced twig water potential and increased tree water deficit during the growing season. Still, the trees showed limited adjustments in most branch-level hydraulic traits (ψ₅₀ and K_s) and needle anatomy. In contrast, trees acclimated to prolonged irrigation by increasing their crown density and hence the canopy water demand. This study demonstrates that despite substantial canopy adjustments, P. sylvestris may be vulnerable to extreme droughts because of limited adjustment potential in its hydraulic system. While sparser canopies reduce water demand, such shifts take decades to occur under chronic water deficits and might not mitigate short-term extreme drought events.

Most attempts to correlate soil erosion rates with soil physiochemical properties have not fully succeeded. We employed ¹³⁷Cs and ²¹⁰Pb_ex tracers to understand the interactive impacts of medium- to long-term soil erosion rates on soil texture, structure, and composition in different cropping management systems and slope gradients. To do so, cultivated areas of a sub-watershed were subdivided into three iso-sectors based on slope gradient and typical cropping systems in the region: dry farming with slopes of 10–30 and < 10 % and irrigated farming with a slope of < 10 %. Some soil physiochemical properties were assessed, including particle size distribution, organic carbon (OC), aggregate percolation stability (PS), sealing index (SI), and semi-quantitative XRD of clays. The weighted average net erosion rates in cultivated lands estimated by ¹³⁷Cs and ²¹⁰Pb_ex methods were 26.3 and 31.4 Mg ha⁻¹ yr⁻¹, respectively. Among studied iso-sectors, the dry farming site with a slope of < 10 % had the highest net soil erosion rates and the lowest values for PS and SI, highlighting that land management is a more influential factor on soil erosion than the slope gradient. Both Pearson coefficient and spatial cross-correlation index (SCI) showed a negative correlation between clay content, aggregate stability and soil erosion due to the abundance of expandable clays in the region. However, unlike the Pearson coefficient, SCI showed that the soil loss rates had the strongest association with PS in space, but very weak ones with OC and SI, highlighting PS as a promising indicator of soil quality. SCI proved a superior analysis, which provides the spatial causality between the two variables in space. Fundings suggest that to ensure food security in the face of future climate change, careful management practices should be implemented in dry farming lands with high amounts of expandable clays, making them prone to aggregate breakdown and soil erosion.

Present and future climatic trends are expected to markedly alter water fluxes and stores in the hydrologic cycle. In addition, water demand continues to grow due to increased human use and a growing population. Sustainably managing water resources requires a thorough understanding of water storage and flow in natural, agricultural, and urban ecosystems. Measurements of stable isotopes of water (hydrogen and oxygen) in the water cycle (atmosphere, soils, plants, surface water, and groundwater) can provide information on the transport pathways, sourcing, dynamics, ages, and storage pools of water that is difficult to obtain with other techniques. However, the potential of these techniques for practical questions has not been fully exploited yet. Here, we outline the benefits and limitations of potential applications of stable isotope methods useful to water managers, farmers, and other stakeholders. We also describe several case studies demonstrating how stable isotopes of water can support water management decision-making. Finally, we propose a workflow that guides users through a sequence of decisions required to apply stable isotope methods to examples of water management issues. We call for ongoing dialogue and a stronger connection between water management stakeholders and water stable isotope practitioners to identify the most pressing issues and develop best-practice guidelines to apply these techniques.

Scots pine (Pinus sylvestris L.) is a common European tree species, and understanding its acclimation to the rapidly changing climate through physiological, biochemical or structural adjustments is vital for predicting future growth. We investigated a long-term irrigation experiment at a naturally dry forest in Switzerland, comparing Scots pine trees that have been continuously irrigated for 17 years (irrigated) with those for which irrigation was interrupted after 10 years (stop) and non-irrigated trees (control), using tree growth, xylogenesis, wood anatomy, and carbon, oxygen and hydrogen stable isotope measurements in the water, sugars and cellulose of plant tissues. The dendrochronological analyses highlighted three distinct acclimation phases to the treatments: irrigated trees experienced (i) a significant growth increase in the first 4 years of treatment, (ii) high growth rates but with a declining trend in the following 8 years and finally (iii) a regression to pre-irrigation growth rates, suggesting the development of a new growth limitation (i.e. acclimation). The introduction of the stop treatment resulted in further growth reductions to below-control levels during the third phase. Irrigated trees showed longer growth periods and lower tree-ring δ¹³C values, reflecting lower stomatal restrictions than control trees. Their strong tree-ring δ¹⁸O and δ²H (O–H) relationship reflected the hydrological signature similarly to the control. On the contrary, the stop trees had lower growth rates, conservative wood anatomical traits, and a weak O–H relationship, indicating a physiological imbalance. Tree vitality (identified by crown transparency) significantly modulated growth, wood anatomical traits and tree-ring δ¹³C, with low-vitality trees of all treatments performing similarly regardless of water availability. We thus provide quantitative indicators for assessing physiological imbalance and tree acclimation after environmental stresses. We also show that tree vitality is crucial in shaping such responses. These findings are fundamental for the early assessment of ecosystem imbalances and decline under climate change.

The contribution from foliage, roots, and fungi to soil organic matter (SOM) represents a key unknown in SOM dynamics. Since stable hydrogen isotope ratios (δ²H_n) can greatly differ among these sources, we explored δ²H_n to elucidate sources contribution to SOM. Moreover, we assessed whether addition of ²H-depleted water - providing ²H-signal that gets incorporated into organic matter - allows tracing of new organic H and thus assessing SOM turnover.
In a 17-year-long irrigation experiment in a dry pine forest, we measured δ²H_n and stable carbon and nitrogen isotope ratios (δ¹³C, δ¹⁵N) in SOM sources, bulk SOM and particle-size fractions. Relative source contribution to SOM was estimated by Bayesian mixing models including all three isotopes.
The δ²H_n increased from foliar litter (-153‰) to fine roots (-118‰) and fungal mycelia (-81‰). Mixing models indicated that foliar litter dominated organic layers (69 ± 15%) and coarse particulate organic matter (POM) at 0-5 cm (65 ± 12%). In contrast, roots dominated fine POM at 2-5 cm (58 ± 12%). Fungal mycelia contributed only to 1-5% of coarse and fine POM, but to 5-25% of mineral associated organic matter (MOM) at 0-5 cm.
The soil water ²H-depletion with irrigation (-76‰ vs -68‰ at 0-10 cm) was paralleled by lower δ²H_n in coarse POM of irrigated vs dry plots (-122‰ vs -111‰ at 0-5 cm), indicating organic H turnover of less than 17 years. In contrast, δ²H_n in fine POM and MOM decreased only by 3‰ with irrigation, implying mean residence times of approximately 30 years.
While δ¹³C and δ¹⁵N differed by a maximum of 1‰ between plant sources, δ²H_n were 30-39‰ higher in roots than foliar litter, thus improving SOM sources differentiation. Long-term ²H-labelling by irrigation indicated higher organic H turnover in coarse vs fine soil fractions, offering a novel tool to identify SOM cycling and microbial H incorporation into SOM pools.

To investigate the effects of converting forests into vineyards typical to Zarivar Lake watershed, Iran, which occurred mainly in the 1970s and 80s, on soil erosion,¹³⁷Cs and ²¹⁰Pb_ex, being mid-and-long-term soil loss tracers, were applied. In Chernobyl-contaminated areas like those found in some parts of Europe and Asia, the proportion of ¹³⁷Cs Chernobyl fallout needs to be determined to convert ¹³⁷Cs inventories into soil erosion rates. To do so, Pu radioisotopes were applied for the first time in Iran. The soil samples were gathered from two adjacent, almost similar hillslopes under natural forest (slope length: 250 m; slope gradient: 20%) and rainfed vineyard (slope length: 200 m; slope gradient: 17%). ¹³⁷Cs/^239+240Pu ratios indicated that 49.8 ± 10.0% of ¹³⁷Cs originated from Chernobyl. The net soil erosion rates derived by ¹³⁷Cs, and ²¹⁰Pb_ex approaches were 5.0 ± 1.1 and 5.9 ± 2.9 Mg ha^-1 yr^-1 in the forested hillslope, and 25.9 ± 5.7 and 32.5 ± 14.5 Mg ha^-1 yr^-1 in the vineyard hillslope, respectively. Both ¹³⁷Cs and ²¹⁰Pb_ex highlighted that deforestation increased soil erosion by around five times. Moreover, the impacts of deforestation on soil physicochemical properties were investigated in surface and subsurface soils. Compared to forested hillslope, soil organic carbon stock in the upper 40 cm of the vineyard reduced by 14 Mg C ha^-1 (29%), 8 Mg C ha^-1 of which was removed by erosion within 35 years, and the remaining have likely been lost via emissions (6 Mg C ha^-1). The vineyard topsoil experienced the most dramatic drops in percolation stability (PS), sealing index, and organic matter by about 55, 51, and 49%, respectively. Among all measured physicochemical properties, PS showed the greatest sensitivity to land-use change. Overall, the present study's findings confirmed that deforestation for agricultural purposes triggered soil loss, deteriorated soil quality and possibly contributed to the reduction of the lake's water quality and climate change.

Quantifying soil redistribution rates is a global challenge addressed with direct sediment measurements (e.g. traps), models, and isotopic, geochemical, and radionuclide tracers. The isotope of plutonium, namely ^239+240Pu, is a relatively new soil redistribution tracer in this challenge. Direct validation of ^239+240Pu as a soil redistribution tracer is, however, still missing. We used a unique sediment yield time series in southern Italy, reaching back to the initial fallout of ^239+240Pu to verify ^239+240Pu as a soil redistribution tracer. Distributed soil samples (n=55) were collected in the catchment, and at undisturbed reference sites (n=22), ^239+240Pu was extracted, measured with inductively coupled plasma mass spectrometry (ICP-MS), and converted to soil redistribution rates. Finally, we used a generalized additive model (GAM) to regionalize soil redistribution estimates for the catchment. For the catchment sites, mean ^239+240Pu inventories were significantly reduced (16.8 ± 10.2 Bq m^-2) compared to the reference inventory (40.5 ± 3.5 Bq m^-2), indicating the dominance of erosion. Converting these inventory losses into soil erosion rates resulted in an average soil loss of 22.2 ± SD 7.2 t ha^-1 yr^-1. The uncertainties of the approach stemmed mainly from the high measurement uncertainties of some low-activity samples where samples have been bulked over depth. Therefore, we recommend taking incremental soil samples and extracting larger soil volumes (∼ 20 g). The geographic coordinates and the flow accumulation best described the spatial pattern of erosion rates in the GAM model. Using those predictors to upscale Pu-derived soil redistribution rates for the entire catchment resulted in an average on-site loss of 20.7 t ha^-1 yr^-1, which corresponds very well to the long-term average sediment yield of 18.7 t ha^-1 yr^-1 measured at the catchment outlet and to ¹³⁷Cs-derived soil redistribution rates. Overall, this comparison of Pu-derived soil redistribution rates with measured sediment yield data validates ^239+240Pu as a suitable retrospective soil redistribution tracer.

The effects of tree pollen on precipitation chemistry are not fully understood and this can lead to misinterpretations of element deposition in European forests. We investigated the relationship between forest throughfall (TF) element fluxes and the Seasonal Pollen Integral (SPIn) using linear mixed-effects modelling (LME). TF was measured in 1990-2018 during the main pollen season (MPS, arbitrary two months) in 61 managed, mostly pure, even-aged Fagus, Quercus, Pinus, and Picea stands which are part of the ICP Forests Level II network. The SPIn for the dominant tree genus was observed at 56 aerobiological monitoring stations in nearby cities. The net contribution of pollen was estimated as the TF flux in the MPS minus the fluxes in the preceding and succeeding months. In stands of Fagus and Picea, two genera that do not form large amounts of flowers every year, TF fluxes of potassium (K⁺), ammonium-nitrogen (NH₄⁺-N), dissolved organic carbon (DOC), and dissolved organic nitrogen (DON) showed a positive relationship with SPIn. However- for Fagus- a negative relationship was found between TF nitrate-nitrogen (NO₃^--N) fluxes and SPIn. For Quercus and Pinus, two genera producing many flowers each year, SPIn displayed limited variability and no clear association with TF element fluxes. Overall, pollen contributed on average 4.1-10.6% of the annual TF fluxes of K⁺ > DOC > DON > NH₄⁺-N with the highest contribution in Quercus > Fagus > Pinus > Picea stands. Tree pollen appears to affect TF inorganic nitrogen fluxes both qualitatively and quantitatively, acting as a source of NH₄⁺-N and a sink of NO₃^--N. Pollen appears to play a more complex role in nutrient cycling than previously thought.

Bewirtschaftung und Umweltbedingungen beeinflussen die Gestalt von Wäldern. Mit der Einrichtung eines Naturwaldreservats wurden am Bettlachstock im Kanton Solothurn die Weichen zur Entwickung in Richtung eines Naturwaldes gestellt. Im Waldreservat werden im Rahmen des «Monitoring Naturwaldreservate Schweiz» mit Stichprobeninventuren die natürlichen Prozesse der Waldentwicklung und in der «Langfristigen Waldökosystemforschung (LWF)» mit kontinuierlichen Messungen der Einfluss von Luftverschmutzung und Klimawandel auf den Wald erforscht. Beides sind langfristige Untersuchungen, die Methoden der Waldinventur und Erfassung des Waldzustandes verwenden. Das Reservat ist aufgrund der Standortsvielfalt reich an verschiedenen Baumarten. Diese widerspiegeln das natürliche Artenspektrum. Der hohe Anteil an Fichten zeigt jedoch, dass die frühere Bewirtschaftung diese bewusst gefördert hat. Die Luftbelastung und das Klima hatten und haben nachweislich einen Einfluss auf den Wald am Bettlachstock. Die Die Luftbelastung ist eher rückläufig und die Klimaveränderung eher zunehmend. Beide stehen aus heutiger Sicht grundsätzlich der Entwicklung zu einem naturnahen Wald an diesem Standort nicht im Wege. Eine Zunahme von trockenen Sommern ist zu erwarten und könnte die Standorteigenschaften verändern. In Teilen des Reservats könnte dies mittelfristig zu vermehrtem Absterben von Tannen und Fichten und in der Folge zu einer Zunahme des Totholzvolumens führen. Langfristig könnte dies Verschiebungen der Höhenstufen und in der Baumartenzusammensetzung hin zu mehr Laubbaumarten auslösen.

Climate change exposes ecosystems to strong and rapid changes in their environmental boundary conditions mainly due to the altered temperature and precipitation patterns. It is still poorly understood how fast interlinked ecosystem processes respond to altered environmental conditions, if these responses occur gradually or suddenly when thresholds are exceeded, and if the patterns of the responses will reach a stable state. We conducted an irrigation experiment in the Pfynwald, Switzerland from 2003–2018. A naturally dry Scots pine (Pinus sylvestris L.) forest was irrigated with amounts that doubled natural precipitation, thus releasing the forest stand from water limitation. The aim of this study was to provide a quantitative understanding on how different traits and functions of individual trees and the whole ecosystem responded to increased water availability, and how the patterns and magnitudes of these responses developed over time. We found that the response magnitude, the temporal trajectory of responses, and the length of initial lag period prior to significant response largely varied across traits. We detected rapid and stronger responses from aboveground tree traits (e.g., tree-ring width, needle length, and crown transparency) compared to belowground tree traits (e.g., fine-root biomass). The altered aboveground traits during the initial years of irrigation increased the water demand and trees adjusted by increasing root biomass during the later years of irrigation, resulting in an increased survival rate of Scots pine trees in irrigated plots. The irrigation also stimulated ecosystem-level foliar decomposition rate, fungal fruit body biomass, and regeneration abundances of broadleaved tree species. However, irrigation did not promote the regeneration of Scots pine trees, which are reported to be vulnerable to extreme droughts. Our results provide extensive evidence that tree- and ecosystem-level responses were pervasive across a number of traits on long-term temporal scales. However, after reaching a peak, the magnitude of these responses either decreased or reached a new stable state, providing important insights into how resource alterations could change the system functioning and its boundary conditions.

Taking advantage of the different histories of Hg deposition in Davos Seehornwald in E-Switzerland and Changbai Mountain in NE-China, the influence of atmospheric deposition on Hg soil dynamics in forest soil profiles was investigated. Today, Hg fluxes in bulk precipitation were similar, and soil profiles were generally sinks for atmospherically deposited Hg at both sites. Noticeably, a net release of 2.07 μg Hg m^-2 yr^-1 from the Bs horizon (Podzol) in Seehornwald was highlighted, where Hg concentration (up to 73.9 μg kg^-1) and soil storage (100 mg m^-3) peaked. Sequential extraction revealed that organic matter and crystalline Fe and Al hydr (oxide)-associated Hg decreased in the E horizon but increased in the Bs horizon as compared to the Ah horizon, demonstrating the coupling of Hg dynamics with the podzolisation process and accumulation of legacy Hg deposited last century in the Bs horizon. The mor humus in Seehornwald allowed Hg enrichment in the forest floor (182-269 μg kg^-1). In Changbai Mountain, the Hg concentrations in the Cambisol surface layer with mull humus were markedly lower (<148 μg kg^-1), but with much higher Hg soil storage (54-120 mg m^-3) than in the Seehornwald forest floor (18–27 mg m^-3). Thus, the vertical distribution pattern of Hg was influenced by humus form and soil type. The concentrations of Hg in soil porewater in Seehornwald (3.4-101 ng L^-1) and in runoff of Changbai Mountain (1.26-5.62 ng L^-1) were all low. Moreover, the pools of readily extractable Hg in the soils at both sites were all <2% of total Hg. Therefore, the potential of Hg release from the forest soil profile to the adjacent aquatic environment is currently low at both sites.

• During the particularly severe hot summer drought in 2018, widespread premature leaf senescence was observed in several broadleaved tree species in Central Europe, particularly in European beech (Fagus sylvatica L.). For beech, it is yet unknown whether the drought evoked a decline towards tree mortality or whether trees can recover in the longer term.
• In this study, we monitored crown dieback, tree mortality and secondary drought damage symptoms in 963 initially live beech trees that exhibited either premature or normal leaf senescence in 2018 in three regions in northern Switzerland from 2018 to 2021. We related the observed damage to multiple climate- and stand-related parameters.
• Cumulative tree mortality continuously increased up to 7.2% and 1.3% in 2021 for trees with premature and normal leaf senescence in 2018, respectively. Mean crown dieback in surviving trees peaked at 29.2% in 2020 and 8.1% in 2019 for trees with premature and normal leaf senescence, respectively. Thereafter, trees showed first signs of recovery. Crown damage was more pronounced and recovery was slower for trees that showed premature leaf senescence in 2018, for trees growing on drier sites, and for larger trees. The presence of bleeding cankers peaked at 24.6% in 2019 and 10.7% in 2020 for trees with premature and normal leaf senescence, respectively. The presence of bark beetle holes peaked at 22.8% and 14.8% in 2021 for trees with premature and normal leaf senescence, respectively. Both secondary damage symptoms occurred more frequently in trees that had higher proportions of crown dieback and/or showed premature senescence in 2018.
• Our findings demonstrate context-specific differences in beech mortality and recovery reflecting the importance of regional and local climate and soil conditions. Adapting management to increase forest resilience is gaining importance, given the expected further beech decline on dry sites in northern Switzerland.

• The intensity and frequency of droughts events are projected to increase in future with expected adverse effects for forests. Thus, information on the dynamics of tree water uptake from different soil layers during and after drought is crucial.
• We applied an in situ water isotopologue monitoring system to determine the oxygen isotope composition in soil and xylem water of European beech with a 2-hour resolution together with measurements of soil water content, transpiration and tree water deficit. Using a Bayesian isotope mixing model, we inferred the relative and absolute contribution of water from four different soil layers to tree water use.
• Beech took up more than 50% of its water from the uppermost 5 cm soil layer at the beginning of the 2018 drought, but then reduced absolute water uptake from the drying topsoil by 84%. The trees were not able to quantitatively compensate for restricted topsoil water availability by additional uptake from deeper soil layers, which is related to the fine root depth distribution. Absolute water uptake from the topsoil was restored to pre-drought levels within 3 wk after rewetting.
• These uptake patterns help to explain both the drought sensitivity of beech and its high recovery potential after drought release.

Summer droughts strongly affect soil organic carbon (SOC) cycling, but net effects on SOC storage are unclear as drought affects both C inputs and outputs from soils. Here, we explored the overlooked role of soil fauna on SOC storage in forests, hypothesizing that soil faunal activity is particularly drought-sensitive, thereby reducing litter incorporation into the mineral soil and, eventually, long-term SOC storage.
In a drought-prone pine forest (Switzerland), we performed a large-scale irrigation experiment for 17 years and assessed its impact on vertical SOC distribution and composition. We also examined litter mass loss of dominant tree species using different mesh-size litterbags and determined soil fauna abundance and community composition.
The 17-year-long irrigation resulted in a C loss in the organic layers (-1.0 kg C m^-2) and a comparable C gain in the mineral soil (+0.8 kg C m^-2) and thus did not affect total SOC stocks. Irrigation increased the mass loss of Quercus pubescens and Viburnum lantana leaf litter, with greater effect sizes when meso- and macrofauna were included (+215%) than when excluded (+44%). The enhanced faunal-mediated litter mass loss was paralleled by a many-fold increase in the abundance of meso- and macrofauna during irrigation. Moreover, Acari and Collembola community composition shifted, with higher presence of drought-sensitive species in irrigated soils. In comparison, microbial SOC mineralization was less sensitive to soil moisture. Our results suggest that the vertical redistribution of SOC with irrigation was mainly driven by faunal-mediated litter incorporation, together with increased root C inputs.
Our study shows that soil fauna is highly sensitive to natural drought, which leads to a reduced C transfer from organic layers to the mineral soil. In the longer term, this potentially affects SOC storage and, therefore, soil fauna plays a key but so far largely overlooked role in shaping SOC responses to drought.