Dr. Katrin Di Bella Meusburger

• The groundwater from forest areas is usually of such good quality that it can be fed into the drinking water network without further treatment.
• Levels of nitrate leaching from forest soils have hardly changed in the past decade. However, they can be high in some localities, indicating an excessive nitrogen load.
• In order to lower nitrate concentrations in seepage water and thus in drinking water from forest areas, nitrogen deposition from the air must be reduced. Silvicultural measures that strengthen forests’ resilience can also reduce the risk of nitrate leaching after disturbance events.

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 «Langfristige Waldökosystem-Forschung» zeigen sich Veränderungen in der Waldstruktur und im Wachstum. Die Wälder 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 gleicher Bestandesdichte auf, was auf eine Abnahme der Standortskapazität hinweist.

25 years of growth development in 19 Swiss forests under increasing drought
Forests are an important carbon sink but are under increasing pressure due to more frequent climate extremes such as heat and drought. After 25 years of monitoring at the 19 sites of the Long-Term Forest Ecosystem Research programme (LWF), changes in forest structure and growth are evident. The forests have generally become denser and older. Growth decreased in both low- and high-altitude sites, especially for spruce, fir and beech. Growth was correlated with stand structure (stand density and age) at individual tree and stand level, as well as negatively with temperature and drought. Compared to the 1990s, the LWF forests today show lower productivity at the same stand density, which indicates a decrease in site capacity.

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.

• Die zunehmende Häufung von Trockenjahren mit begrenzter Bodenwasserverfügbarkeit ist eine Belastung für den Wald.
• Die Bodenversauerung aufgrund übermässiger Stickstoffeinträge ist weiter fortgeschritten. Dadurch sinkt die Verfügbarkeit wichtiger basischer Nährstoffe wie Kalzium, Magnesium und Kalium. Gleichzeitig wird toxisches Aluminium freigesetzt.
• Eine weitere Bedrohung sind physikalische Belastungen bei der Waldbewirtschaftung wie zum Beispiel die Bodenverdichtung durch schwere
Forstmaschinen. Sie könnten mit angepassten Bewirtschaftungsmethoden vermieden werden.

• The increasing frequency of drought years with limited soil water availability is a stress factor for forests.
• Soil acidification due to excessive nitrogen deposition has progressed further. This reduces the availability of important nutrients such as calcium, magnesium and potassium, while also releasing toxic aluminium.
• Another threat is physical stress during forest management, such as soil compaction caused by heavy forestry machinery. This could be avoided with adapted management methods.

• La fréquence accrue d'années sèches avec une disponibilité limitée en eau du sol affecte la forêt.
• L'acidification des sols consécutive à des dépôts excessifs d'azote s'est poursuivie. Elle entraîne une baisse de la disponibilité en nutriments basiques importants comme le calcium, le magnésium et le potassium, accompagnée d'une libération d'aluminium toxique.
• Une autre menace réside dans les atteintes physiques aux sols lors de la gestion des forêts, comme la compaction par des engins lourds. Des méthodes de gestion adéquates permettraient de les éviter.

• La maggiore frequenza di annate siccitose con limitata disponibilità idrica nel suolo ha un impatto negativo sul bosco.
• L'acidificazione causata da un eccessivo apportodi azoto è progredita ulteriormente, riducendo la disponibilità di importanti nutrienti alcalini come calcio, magnesio e potassio. Contemporaneamente viene rilasciato alluminio tossico.
• Un'altra minaccia è rappresentata dallo stress fisico causato dalla gestione forestale, come ad esempio il costipamento del suolo provocato dai pesanti macchinari forestali. Questi fenomeni potrebbero essere evitati con metodi di gestione adeguati.

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 zurückgegangen. 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 Stickstoffauswaschung 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 Entwicklungen ein Problem. Ein weiterer kritischer Befund ist die fortschreitende Bodenversauerung, die sich in abnehmenden 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 Nährstoffe wie Stickstoff, Phosphor und Schwefel in den Blättern geführt haben. Dies weist auf eine Verschlechterung der Baumernährung hin. Zusammengefasst zeigen die Ergebnisse, dass trotz des Rückgangs saurer Depositionen die langfristigen Auswirkungen auf die Waldböden und die Baumernährung bestehen bleiben, was sich auf die Gesundheit und Vitalität der Schweizer Wälder auswirken kann.

Trends in atmospheric deposition and effects on soil water quality and tree nutrition
Long-term forest ecosystem research (LWF) in Switzerland provides detailed results on the impacts of air pollution and climate change on forest soils and tree nutrition. Since the 1980s, sulphur dioxide and nitrogen oxides emissions have significantly decreased in Central Europe. These trends can also be confirmed for emissions in Switzerland. Monitoring data show that sulphate leaching from the soil profile has decreased. Nitrogen leaching has also mostly declined, but not at sites with persistently high nitrogen inputs, where even increasing trends have been observed. Therefore, nitrogen deposition remains a problem despite the overall positive developments. Another critical finding is the ongoing soil acidification, reflected in decreasing pH values and low ratios of base cations to aluminium. The soils buffer the acidic deposition through nutrient leaching and aluminium release. These changes in soil solution chemistry can persist for a long time and impair nutrient availability for trees. This is one of the factors contributing to a significant decline in key nutrients such as nitrogen, phosphorus, and sulphur in the leaves. In summary, the results show that despite the reduction in acidic deposition, the long-term impacts on forest soils and tree nutrition persist, potentially affecting the health and vitality of Swiss forests.

• 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.

Wälder tragen zur Erneuerung des Grundwassers in Trinkwasserqualität bei. Wiederholte Untersuchungen zeigen, 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 Umwelt beeinträchtigen. Die Bodeninventur auf fünf ausgewählten Flächen des Programms «Langfristige Waldökosystem-Forschung» zeigt, dass die Stickstoffsättigung auf einem Teil der Flächen, trotz Rückgang der Einträge, eher zugenommen hat. Dadurch sinkt die Fähigkeit von Böden, Stickstoff aufzunehmen, womit das Risiko erhöhter Nitratauswaschung tendenziell steigt.

In vielen Waldgebieten der Schweiz und Europas führt der Klimawandel zu erhöhter Verdunstung und Bodenaustrocknung in der warmen Jahreszeit, was die Waldgesundheit beeinträchtigt. Aber auch die kalte Jahreszeit könnte für den Wasserhaushalt der Wälder entscheidend sein. Wenn Waldböden im Winter nicht mehr vollständig mit Wasser aufgefüllt werden, kann dies die Wasserversorgung und die Vitalität der Bäume in der folgenden Vegetationsperiode beeinträchtigen. Basierend auf Messwerten zeigen wir in diesem Beitrag, in welchen Regionen der Schweiz die Böden im Winterhalbjahr nur unzureichend befeuchtet werden, was eine Hypothek für die Vegetationsperiode darstellt.

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.