Dr. Stephan Zimmermann

Schweizer Waldböden speichern mit 143 Tonnen pro Hektar die grössten Mengen an Kohlenstoff aller europäischer Wälder. Dieser hohe Vorrat hat sich über Jahrtausende aufgebaut und ist wahrscheinlich auf die feuchten und kühlen Klimaverhältnisse zurückzuführen. Auf den Flächen des Projekts «Langfristige Waldökosystem-Forschung» besteht eine negative Beziehung zwischen Waldbiomasse und Bodenkohlenstoff, weil bei günstigen Wuchsverhältnissen, etwa in einem warmen Klima, mehr Bodenkohlenstoff abgebaut wird. Bodenminerale, die die Stabilisierung von Kohlenstoff steuern, haben die grösste Bedeutung für den Kohlenstoffvorrat im Boden. Schweizer Waldböden weisen unter trockeneren und wärmeren Klimabedingungen niedrigere Kohlenstoffvorräte auf, was auf potenzielle Kohlenstoffverluste in einem zukünftigen Klima hindeutet.

Forests can contribute to climate change mitigation by sequestering carbon when management intensity is reduced. However, there is high uncertainty regarding biomass dynamics in temperate forests after the cessation of management. We used forest inventory data from an extensive network of 224 plots in 37 natural forest reserves (NFR) covering a wide environmental gradient with mean annual temperatures ranging from 1 to 10.4 °C and mean annual precipitation ranging from 901 to 2317 mm. Inventories had been conducted approximately every 10 years during the last 60 years. We used mixed effect models to (i) analyse biomass development, (ii) assess the role of time since the cessation of management (TSCM) and (iii) disentangle the environmental and forest structural drivers of biomass change. After the cessation of management and in the absence of high-severity natural disturbances, biomass accumulated gradually along a saturation curve. There were large differences in biomass among reserves and plots, with values ranging from 101 Mg ha⁻¹ to 851.2 Mg ha⁻¹, with a median of 362.1 Mg ha⁻¹ (SD = 122.5 Mg ha⁻¹). The biomass curve did not yet tend towards an equilibrium, most likely because the majority of the NFRs do not exceed 100 years of TSCM. Compared to higher elevations, forests at lower, warmer sites showed a larger total biomass and higher rates of biomass accumulation. We found a reduction by 148 Mg ha⁻¹ of biomass per 1000 m of elevation gain. The strongest positive rate of change (>8 Mg ha⁻¹ year⁻¹) was found in forests with high basal area (>60 m² ha⁻¹) and medium to high levels of tree density (1500 to 2000 stems ha⁻¹). Overall, most reserves have not reached a biomass equilibrium yet and continue to act as carbon sinks in tree biomass. This highlights the carbon sequestration capacity of forest reserves and their role as carbon pools.

Steigende Temperaturen verschieben die Baumgrenze in höhere Lagen. Dies hat erhebliche Auswirkungen auf den Kohlenstoffkreislauf und die ober- und unterirdischen Kohlenstoffspeicher in alpinen Ökosystemen. Drei Höhengradienten von Bodenkohlenstoff im Raum Davos-Flüelapass illustrieren diese Vorgänge qualitativ und quantitativ.

With this study, our aim was to estimate the nutrient fluxes relevant for assessing nutrient sustainability as accurately as possible and to calculate nutrient balances for alternative forest management scenarios. Furthermore, we tested whether mapping units from existing geologic maps can serve as a basis for forest practitioners to estimate nutrient sustainability or whether more detailed data are needed. Positive fluxes include deposition and weathering, while negative fluxes include losses due to leaching and nutrient removal through timber harvesting in the balance. Weathering and leachate losses were modeled with a geochemical model. The SwissStandSim model was used to simulate the biomass growth under different harvesting and silvicultural strategies, allowing for sustainability to be assessed for each nutrient at a given intensity of use. This assessment was made per rotation period based on two criteria: (i) nutrient supply and (ii) total stocking volume. As a result, it can be noted that the accurate estimation of individual fluxes is essential for assessing the sustainability of forestry practices and that it needs detailed site-specific data. Various influencing factors turned out to be important, particularly the assumed depth of the root zone.

The European Alps are experiencing more than twice the increase in air temperature observed in the rest of the world. Thus, the treeline ecotone, and the unique habitats above it, offer a preview of drastic changes in plant and animal communities. However, our knowledge about climate change impacts on microbial diversity belowground is scarce. Here we investigate how upslope shift of the treeline ecotone, associated with changes in soil nutrient content, temperature and precipitation, will influence alpine ectomycorrhizal (EM) communities of Dryas octopetala, Bistorta vivipara and Salix herbacea across different habitat types in the Alps. We also assessed the degree of EM community taxonomic composition turnover in these habitats across three different climatic projections for 2040 and 2070. Our results indicate that the specialized EM fungal communities from snowbed habitats will be mostly negatively influenced under the current trajectory of environmental shifting predicted for the region. In contrast, fungi from the treeline ecotone, having wider niches, will be positively influenced by future climate and extend upwards. In addition, our predictions of EM community turnover for putative future climatic scenarios revealed high rates of turnover across the entire alpine region. This, together with glacier retreats, will aid colonization of alpine snowbed habitats by new EM plants and associated fungi, bringing additional pressures on local mycorrhizas and likely leading to fungal species extinctions.

Nitrogen (N) deposition has decreased in the last decades in Europe but in many cases remains higher than the critical loads, i.e., higher than what could be considered safe for biodiversity and ecosystem functioning. The main concerns about N deposition are eutrophication and acidification. In a long-term experiment (1994 to present) in a montane (1200 m a.s.l.) coniferous forest in Alptal, central Switzerland, we simulated increased N deposition by adding NH₄NO₃ to rainwater. This treatment consisted of an additional N input of 22 kg ha⁻¹ yr⁻¹ to the 12 kg ha⁻¹ yr⁻¹ ambient bulk deposition rate or 17 kg N ha⁻¹ yr⁻¹ throughfall rate. The treatment was applied simultaneously to a small catchment area and to plots in a replicated block experiment (n=5). The site has a carbonate-rich parent material and is thus not particularly at risk of acidification. Nevertheless, we examined soil acidification as affected both by ambient and experimentally increased N deposition. In the 2.5 decades since the beginning of the study, nitrate (NO₃^-) and especially sulfate (SO₄^2-) concentrations decreased in precipitation, while pH increased by slightly more than 1 unit. In the same time period a reduction in pH of the soil was measured. The exchangeable acidity in the soil increased, especially in the N-addition treatment. This was mainly observed on small mounds because the drier mounds are less well buffered than wet depressions. This trend, however, was limited in time, as exchangeable acidity later declined again to reach values not much higher than 26 years before. This was also the case in the N-addition treatment and can be considered a progressive recovery mainly due to the reduced acid inputs and, at this site with a carbonate-rich subsoil, to the biological cycling of base cations. The pH of the runoff from the experimental catchments decreased by 0.3 units, both in the control and under N addition. Decreasing Ca²⁺ and increasing Al³⁺ and Fe²⁺ concentrations in runoff also show that the recovery observed in the exchangeable soil acidity is not yet able to stop the slow acidification of water leaving the catchments. However, with the runoff water pH remaining above 7, this trend is not alarming for water quality or for the health of water bodies. Future monitoring will be necessary to see if and when a recovery takes place in the soil and runoff pH.

Anthropogenic nitrogen (N) input is known to alter the soil microbiome, but how N enrichment influences the abundance, alpha-diversity and community structure of N-cycling functional microbial communities in grasslands remains poorly understood. Here, we collected soils from plant communities subjected to up to 9 years of annual N-addition (10 g N m^-2 per year using urea as a N-source) and from unfertilized plots (control) in 30 grasslands worldwide spanning a large range of climatic and soil conditions. We focused on three key microbial groups responsible for two essential processes of the global N cycle: N₂ fixation (soil diazotrophs) and nitrification (AOA: ammonia-oxidizing archaea and AOB: ammonia-oxidizing bacteria). We targeted soil diazotrophs, AOA and AOB using Illumina MiSeq sequencing and measured the abundance (gene copy numbers) using quantitative PCR. N-addition shifted the structure of the diazotrophic communities, although their alpha-diversity and abundance were not affected. AOA and AOB responded differently to N-addition. The abundance and alpha-diversity of AOB increased, and their community structure shifted with N-addition. In contrast, AOA were not affected by N-addition. AOA abundance outnumbered AOB in control plots under conditions of low N availability, whereas N-addition favoured copiotrophic AOB. Overall, N-addition showed a low impact on soil diazotrophs and AOA while effects for AOB communities were considerable. These results reveal that long-term N-addition has important ecological implications for key microbial groups involved in two critical soil N-cycling processes. Increased AOB abundance and community shifts following N-addition may change soil N-cycling, as larger population sizes may promote higher rates of ammonia oxidation and subsequently increase N loss via gaseous and soil N-leaching. These findings bring us a step closer to predicting the responses and feedbacks of microbial-mediated N-cycling processes to long-term anthropogenic N-addition in grasslands.

Storms represent a major disturbance factor in forest ecosystems, but the effects of windthrows on soil organic carbon (SOC) stocks are poorly quantified. Here, we assessed the SOC stocks of windthrown forests at 19 sites across Switzerland spanning an elevation gradient from 420 to 1550 m, encompassing a strong climatic gradient. Results show that the effect size of disturbance on SOC stocks increases with the size of the initial SOC stocks. The largest windthrow-induced SOC losses of up to 29 t C ha^-1 occurred in high-elevation forests with a harsh climate developing thick organic layers. In contrast, SOC stocks of low-elevation forests with thin organic layers were hardly affected. A mineralization study further revealed high elevation forests to store higher amounts of easily mineralizable C in thick organic layers that got lost following windthrow. These findings are supported by a meta-analysis of available windthrow studies, showing an increase of storm-induced SOC losses with the size of the initial SOC stocks. Modelling simulations further indicate longer-lasting SOC losses and a slower recovery of SOC stocks after windthrow at high compared to low elevations, due to a slower regeneration of mountain forests and associated lower C inputs into soils in a harsh climate. Upscaling the experimental findings/observed patterns by linking them to a data base of Swiss forest soils shows a total SOC loss of ∼0.4 Mt. C for the whole forested area of Switzerland after two major storm events, counteracting the forest net carbon sink of decades. Our study provides strong evidence that the vulnerability of SOC stocks to windthrow is particularly high in forests featuring thick and slowly forming organic layers, such as mountain soils. Thus, the risk of losing SOC to more frequent windthrows in mountain forests strongly limits their potential to mitigate climate change.

Maps of soil pH are an important tool for making decisions in sustainable forest management. Accurate pH mapping, therefore, is crucial to support decisions by authorities or forest companies. Soil pH values typically exhibit a distinct distribution characterized by two frequently encountered pH ranges, wherein aluminium oxides (AlO) and carbonates (CaCO) act as the primary buffer agents. Soil samples with moderately acid pH values (pH CaCl₂ of 4.5-6) are less commonly observed due to their weaker buffering capacity. The different strength of buffer agents results in a distinct bimodal distribution of soil pH values with peaks at pH of around 4 and 7.5. Commonly used approaches for spatial mapping neglect this often observed characteristic of soil pH and predict unimodal distributions with too many moderately acid pH values. For ecological map applications this might result in misleading interpretations.
This article presents a novel approach to produce pH maps that are able to reproduce pedogenic processes. The procedure is suitable for bimodal responses where the response distribution is naturally inherent and needs to be reproduced for the predictions. It is model-agnostic, namely independent from the used statistical prediction method. Calibration data is optimally split into two parts corresponding each to a data culmination, i.e. for soil pH values belonging to the ranges of the two principal buffer agents (Al₂O₃ and CaCO₃). For each subset a separate model is then built. In addition, a binary model is fitted to assign every new prediction location a probability to belong either to Al₂O₃ or CaCO₃ buffer range. Predictions are combined by weighted mean. Weights are derived from probabilities predicted by the binary model. Degree of smoothness is chosen by sigmoid transform which allows for optimal continuous transition of the pH values between Al₂O₃ and CaCO₃ buffer ranges. For each location uncertainty distributions may be combined by using the same weights.
We illustrated application of the new approach to a medium and strong bimodal distributed response (1) pH in 0-5 cm and (2) pH in 60-100 cm of forest soils in Switzerland (2 530 calibration sites). While model performance measured at 354 validation sites slightly dropped compared to a common modelling approach (drop of R² of 0.02-0.03) distributional properties of the predictions are much more meaningful from a pedogenic point of view. We were able to demonstrate the benefits of considering specific distributional properties of responses within the prediction process and expanded model assessment by comparing observed and predicted distributions.

Aim: The microbial metabolic quotient (MMQ; mg CO₂-C/mg MBC/h), defined as the amount of microbial CO₂ respired (MR; mg CO₂-C/kg soil/h) per unit of microbial biomass C (MBC; mg C/kg soil), is a key parameter for understanding the microbial regulation of the carbon (C) cycle, including soil C sequestration. Here, we experimentally tested hypotheses about the individual and interactive effects of multiple nutrient addition (nitrogen + phosphorus + potassium + micronutrients) and herbivore exclusion on MR, MBC and MMQ across 23 sites (five continents). Our sites encompassed a wide range of edaphoclimatic conditions; thus, we assessed which edaphoclimatic variables affected MMQ the most and how they interacted with our treatments.
Location: Australia, Asia, Europe, North/South America.
Time period: 2015–2016.
Major taxa: Soil microbes.
Methods: Soils were collected from plots with established experimental treatments. MR was assessed in a 5-week laboratory incubation without glucose addition, MBC via substrate-induced respiration. MMQ was calculated as MR/MBC and corrected for soil temperatures (MMQsoil). Using linear mixed effects models (LMMs) and structural equation models (SEMs), we analysed how edaphoclimatic characteristics and treatments interactively affected MMQsoil.
Results: MMQsoil was higher in locations with higher mean annual temperature, lower water holding capacity and lower soil organic C concentration, but did not respond to our treatments across sites as neither MR nor MBC changed. We attributed this relative homeostasis to our treatments to the modulating influence of edaphoclimatic variables. For example, herbivore exclusion, regardless of fertilization, led to greater MMQsoil only at sites with lower soil organic C (< 1.7%).
Main conclusions: Our results pinpoint the main variables related to MMQsoil across grasslands and emphasize the importance of the local edaphoclimatic conditions in controlling the response of the C cycle to anthropogenic stressors. By testing hypotheses about MMQsoil across global edaphoclimatic gradients, this work also helps to align the conflicting results of prior studies.

Covering approximately 40% of land surfaces, grasslands provide critical ecosystem services that rely on soil organisms. However, the global determinants of soil biodiversity and functioning remain underexplored. In this study, we investigate the drivers of soil microbial and detritivore activity in grasslands across a wide range of climatic conditions on five continents. We apply standardized treatments of nutrient addition and herbivore reduction, allowing us to disentangle the regional and local drivers of soil organism activity. We use structural equation modeling to assess the direct and indirect effects of local and regional drivers on soil biological activities. Microbial and detritivore activities are positively correlated across global grasslands. These correlations are shaped more by global climatic factors than by local treatments, with annual precipitation and soil water content explaining the majority of the variation. Nutrient addition tends to reduce microbial activity by enhancing plant growth, while herbivore reduction typically increases microbial and detritivore activity through increased soil moisture. Our findings emphasize soil moisture as a key driver of soil biological activity, highlighting the potential impacts of climate change, altered grazing pressure, and eutrophication on nutrient cycling and decomposition within grassland ecosystems.

Bei der Beurteilung von Standortsbedingungen im Schweizer Wald spielt der Boden neben Klima und Relief eine entscheidende Rolle. Flächige Bodeninformationen für den Schweizer Wald sind jedoch nur in grober Form verfügbar. Basierend auf rund 2000 Waldbodenprofilen haben wir mittels maschinellen Lernens verschiedene Bodeneigenschaften (z.B. pH- und Sandgehalt) von sechs Bodentiefen für die gesamte Waldfläche der Schweiz mit einer Auflösung von 25 m × 25 m vorausgesagt. Während die Güte der Modellierung für die Bodendichte zufriedenstellend war, waren der Gehalt an organischem Kohlenstoff und die Bodengründigkeit schwieriger vorauszusagen. Zu jeder Bodenkarte berechneten wir zudem eine Unsicherheitskarte, sodass die Qualität der Bodenkarten auch räumlich beurteilt werden kann.

Elevational gradients along mountain slopes offer opportunities to study key factors shaping species diversity patterns. Several environmental factors change over short distances along the elevational gradient in predictable ways. However, different taxa respond to these factors differently resulting in various proposed models for biodiversity patterns along elevational transects. Using a multi-taxa approach, we investigated the effects of elevation, area, habitat and soil characteristics on species richness, individual abundance and species composition of six groups of ground-dwelling arthropods along four transect lines in the Swiss National Park and its surroundings (Eastern Alps). Spiders, millipedes, centipedes, ants, ground beetles and rove beetles were sampled using standardized methods (pitfall traps, cardboard traps, visual search) in 65 sites spanning an elevational range from 1800 to 2750 m a.s.l.. A total of 14,782 individuals comprising 248 species were collected (86 spider, 74 rove beetle, 34 ground beetle, 21 millipede, 19 centipede and 14 ant species). Linear mixed model-analysis revealed that rarefied species richness in five out of the six arthropod groups was affected by elevation (the quadratic term of elevation provided the best fit in most cases). We found three different patterns (linear decrease in centipedes, low elevation plateau followed by a decrease in ants and rove beetles, and midpoint peak in spiders and millipedes). These patterns were only partially mirrored when considering individual abundance. Elevation influenced species composition in all groups examined. Overall, elevation was the most important factor explaining the diversity patterns, while most local habitat and soil characteristics have little influence on these patterns. Our study supports the importance of using multi-taxa approaches when examining effects of elevational gradients. Considering only a single group may result in misleading findings for overall biodiversity.

Der Boden ist eine zentrale Schnittstelle in Waldökosystemen. Er erfüllt wichtige Funktionen als Lebensraum, als Speicher für Kohlenstoff und Nährstoffe, und er reguliert die Wasser- und Stoffkreisläufe. Für alle Bodenfunktionen spielt die organische Bodensubstanz eine Schlüsselrolle. Aufgrund des feuchten und kühlen Klimas sowie der relativ nachhaltigen Waldnutzung speichern Schweizer Waldböden pro Flächeneinheit die grössten Vorräte organischer Bodensubstanz Europas. Dieser Vorrat hat sich über Tausende Jahre hinweg aufgebaut. Der Klimawandel - höhere Temperaturen und eine intensivere Trockenheit - sowie grossflächige Störungen, beispielsweise durch Windwürfe und Kahlschläge, gefährden die organische Bodensubstanz. Eine bodenschonende Waldbewirtschaftung sowie eine permanente Bestockung mit standortsangepassten Baumarten tragen zum Erhalt dieser wichtigen Ressource bei.

Soil represents an essential component in forest ecosystems. It fulfills key functions as living space, storage for carbon and nutrients, regulation of water and element cycles. Soil organic matter plays a key role for all soil functions. Swiss forest soils contain the greatest organic matter stocks per unit area of all European countries due to the relatively cool and humid climate and the overall sustainable forest management. The stocks have built up over thousands of years, but are put at risk by climate change - higher temperatures and more intense drought - as well as disturbances by windthrows or clear cuts. A soil-friendly forest harvest, a continuous forest cover, as well as the promotion of a structure- and species-rich forest are all contributing to preserve soil organic matter as an essential resource in the soil.

Wälder sind zunehmend von Sturmschäden betroffen. In einem Projekt der Eidgenössischen Forschungsanstalt WSL wurde untersucht, wie sich Windwürfe auf die Kohlenstoffspeicherung im Waldboden auswirken. Hoch gelegene Nadelwälder sind besonders anfällig für Kohlenstoffverluste aus dem Boden. Im Unterschied zu Wäldern tieferer Lagen werden dort grössere Mengen an Kohlenstoff in rein organischen, mächtigen Humusauflagen gespeichert. Eine Stabilisierung durch mineralische Interaktionen fehlt weitgehend, was die Humusauflagen empfindlich macht für Kohlenstoffverluste durch störungsbedingte Veränderungen des Bestandesklimas. Die langsame Baumverjüngung und die geringen Streueinträge in den Boden bremsen den Humusaufbau nach einer Störung zusätzlich. Eine weitere Zunahme von Windwürfen könnte die Bodenkohlenstoffvorräte in Bergwäldern drastisch reduzieren, was negative Auswirkungen auf das Klima hätte. Darüber hinaus könnten störungsbedingte Humusverluste die Bodenqualität nachhaltig verschlechtern.

Storms represent a major disturbance factor in forest ecosystems, but the effects of windthrows on soil organic carbon stocks are poorly known. The Swiss Federal Institute WSL assessed the soil organic carbon stocks of windthrown forests and neighbouring unaffected stands across Switzerland. The largest soil carbon losses occurred in high-elevation forests with thick organic layers. In contrast, soil organic carbon stocks of low-elevation forests with thin organic layers were hardly affected. The likely reason for this pattern is the high amount of easily mineralizable carbon in thick organic layers which got lost following windthrow. At low elevations, on the other hand, a greater carbon fraction may be stabilized by mineral interactions preventing fast decomposition. Modelling simulations further show longer-lasting soil carbon losses and a slower recovery of carbon stocks after windthrow at high elevations compared to low elevations, due to a slower regeneration of mountain forests and associated lower carbon inputs into soils. The study provides strong empirical evidence that the vulnerability of soil organic carbon stocks to windthrow is particularly high in mountain forests featuring thick and slowly forming organic layers.

Central Europe has been experiencing unprecedented droughts during the last decades, stressing the decrease in tree water availability. However, the assessment of physiological drought stress is challenging, and feedback between soil and vegetation is often omitted because of scarce belowground data. Here we aimed to model Swiss forests' water availability during the 2015 and 2018 droughts by implementing the mechanistic soil-vegetation-atmosphere-transport (SVAT) model LWF-Brook90 taking advantage of regionalized depth-resolved soil information. We calibrated the model against soil matric potential data measured from 2014 to 2018 at 44 sites along a Swiss climatic and edaphic drought gradient. Swiss forest soils' storage capacity of plant-available water ranged from 53 mm to 341 mm, with a median of 137 ± 42 mm down to the mean potential rooting depth of 1.2 m. Topsoil was the primary water source. However, trees switched to deeper soil water sources during drought. This effect was less pronounced for coniferous trees with a shallower rooting system than for deciduous trees, which resulted in a higher reduction of actual transpiration (transpiration deficit) in coniferous trees. Across Switzerland, forest trees reduced the transpiration by 23% (compared to potential transpiration) in 2015 and 2018, maintaining annual actual transpiration comparable to other years. Together with lower evaporative fluxes, the Swiss forests did not amplify the blue water deficit. The 2018 drought, characterized by a higher and more persistent transpiration deficit than in 2015, triggered widespread early wilting across Swiss forests that was better predicted by the SVAT-derived mean soil matric potential in the rooting zone than by climatic predictors. Such feedback-driven quantification of ecosystem water fluxes in the soil–plant-atmosphere continuum will be crucial to predicting physiological drought stress under future climate extremes.

1. Evaluation of restoration activities is indispensable to assess the extent to which targets have been reached. Usually, the main goal of ecological restoration is to restore biodiversity and ecosystem functioning, but validation is often based on a single indicator, which may or may not cope with whole-ecosystem dynamics. Network analyses are, however, powerful tools, allowing to examine both the recovery of various biotic and abiotic properties and the integrated response at community and ecosystem level.
2. We used restoration sites where topsoil was removed from former intensively managed grassland and seeds were added. These sites were between 3 and 32 years old. We assessed how plants, soil biota, soil properties and correlation-based interactions between biotic communities and their abiotic environment developed over time and compared the results with (i) intensively managed (not restored), and (ii) well-preserved targeted semi-natural grasslands.
3. Plant, nematode, fungal and prokaryotic diversity and community structures of the restored grasslands revealed clear successional patterns and followed similar trajectories towards targeted semi-natural grasslands. All biotic communities reached targeted diversity levels no later than 18 years post-restoration.
4. Ecological networks of intensively managed and short-term (~4 years) restored grasslands were less tightly connected compared to those found in mid- and long-term (~18–30 years) restored and target grasslands. Restoration specifically enhanced interactions among biotic communities, but reduced interactions between biotic communities and their abiotic environment as well as interactions among abiotic properties in the short- and mid-term.
5. Synthesis and applications: Overall, our study demonstrated that topsoil removal and seed addition were successful in restoring diverse, tightly coupled and well-connected biotic communities above- and below-ground similar to those found in the semi-natural grasslands that were restoration targets. Network analyses proved to be powerful in examining the long-term re-establishment of functionally connected biotic communities in restored ecosystems. Thus, we provide an approach to holistically assess restoration activities by notably considering the complexity of ecosystems, much in contrast to most traditional approaches.