Dr. Golo Stadelmann

Les forêts suisses et françaises font face à des défis similaires liés aux changements climatiques. Des outils sont nécessaires pour guider les gestionnaires forestiers dans l'adaptation de leurs pratiques. La Suisse utilise une approche stationnelle basée sur les types de stations avec l'outil Tree App, tandis que l'Office National des Forêts, gestionnaire des forêts publiques françaises, se concentre sur les niches climatiques des essences avec l'outil Zoom 50. Malgré leurs différences, les deux approches visent à anticiper les changements climatiques et à adapter la gestion forestière en conséquence. Favoriser davantage d'échanges franco-suisses permettrait d’intéressantes synergies et apprentissages.

European forests are being shaped by active human use and management, and by harvesting of wood in particular. Yet, our understanding of how forests are harvested across Europe is limited, as the real harvest regimes are not well described by currently available data. Here, we analyse recent harvests, as observed in permanent plots of forest inventories in 11 European countries, totaling to 182,649 plots and covering all major forest types. We (a) characterize harvest regimes through the frequency and intensity of harvest events spatially across Europe, and (b) build models for the probability and intensity of harvest events at the plot-level and examine the links to potential drivers of harvest, including the pre-harvest forest structure and composition, climatic, topographic and socio-economic factors, and past natural disturbances. The results revealed notable variation in harvest regimes across Europe, ranging from high-frequency and low-intensity harvests in eastern Central Europe to low-frequency and high-intensity harvests in the north, with different strategies emerging in regions with similar total harvest rates. The harvest regimes were strongly driven by country-level variation, emphasizing the role of national-level factors. Pre-harvest forest properties were important drivers for the intensity of harvest, whereas the probability of harvest was more related to socio-economic factors and natural disturbances. The presented quantification of the forest harvesting regimes provides much needed detail in our understanding of the contemporary forest management practices in Europe, providing a baseline against which to assess future changes in management and strengthening the knowledge-base for decision-making on European level.

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.

A systemic understanding of the use of wood resources is required to defossilize society and promote bio-based developments. This study provides a novel approach to represent a comprehensive material flow analysis (MFA) spanning the entire wood value chain; encompassing wood harvest to products, use in society, collection, reuse, recycle, energy generation, and trade. By recognizing wood as a complex material, with changing properties throughout its lifespan, we developed a method where we employed a color-coded system for processes (27 boxes) and 110 flows to symbolize distinct life stages and the dynamic characteristics, including 23 different trade flows. The wood processes and flows are categorized (11 categories) into different wood types (e.g., softwood and hardwood) and harmonized to include all available data on each single step of the wood life cycle, improving data traceability and visualization, and allowing for replicable analysis with respective data noted for each process and flow. Methodological obstacles due to different units, uncertainty of flows, and discrepancies in data are addressed and adjustments proposed. Switzerland was chosen as a case study as a large number of various types of data were available to perform the analysis. The categorized and harmonized flows of woody biomass mapped and analyzed in the MFA provide a comprehensive basis to identify and recommend avenues to increase cascading use of wood as a carbon sink, by considering relevant aspects like the network of flows and processes, the quality and availability of the woody biomass, and the organization of the industry.

Der Schweizer Wald beherbergt zahlreiche Baumarten mit unterschiedlichen ökologischen Ansprüchen an Licht, Nährstoffen und Klima. In der empirischen Waldmodellierung werden ähnliche Baumarten oft gruppiert, weil zu wenige Beobachtungen vorliegen, um artspezifische Modelle für Einwuchs, Wachstum und Mortalität zu entwickeln. Wir präsentieren eine neue Baumartengruppierung, die ähnliche ökologische Ansprüche und deren Einfluss auf das Wachsen und Absterben berücksichtigt.

The tree species composition of Swiss forests is influenced by both environmental conditions and centuries of forest management, resulting in varying degrees of naturalness. Here, we estimate the naturalness of Swiss forests by comparing the tree species composition (i.e., dominance and presence/absence of tree species) recorded by the national forest inventory with the idealised species composition of the potential natural forests. We aimed to (1) evaluate the naturalness of different forest types, (2) identify the main drivers of low naturalness and (3) investigate the influence of naturalness on the susceptibility to disturbances. Based on our analysis, 45 % of Swiss forests had a tree species composition classified as ‘not natural’ while 42 % were classified as ‘natural’ or ‘close to natural’. The vast majority (65 %) of the forests classified as ‘not natural’ were potential European beech (Fagus sylvatica L.) and other broadleaf forests which are currently dominated by conifers. In addition, at higher elevations, forests often showed a higher proportion of European larch (Larix decidua Mill.) than expected, based on the potential natural forest. Overall, forests classified as ‘natural’ had a significantly lower risk of being affected by stand-level disturbance events. Potential natural European beech and other broadleaf forests were significantly less affected by disturbances, namely insect outbreaks and wind throw, when comparing ‘natural’ to ‘not natural’ (i.e., afforested with conifers). The observed dieback and associated loss of ecosystem function of conifer afforestations (mainly Norway spruce; Picea abies (L.) H. Karst.) in the Swiss plateau is likely a combination of climatic stress and higher pathogen/parasite pressure. These Norway spruce afforestations already experience a significantly elevated temperature of about 2.5 °C compared to their climatic optimum (i.e., primary natural distribution within Switzerland), representing a large-scale, long-term transplantation experiment. We discuss the question of whether similar diebacks and problems of maladaptation are to be expected for many other species, including European beech, due to accelerated climate warming during the 21st century and the slow transformation of long-lived systems such as forests. Consequently, an adaptation of current management practices might be needed to allow a faster transition of forests minimizing the risk of large-scale diebacks.

Die in den nächsten hundert Jahren im Kanton Bern nutzbaren Energieholzpotenziale wurden analysiert. Die Analyse basiert auf dem 4. Landesforstinventar, der Simulation von drei national anerkannten Waldbewirtschaftungsszenarien und zwei lokal definierten Holzmarktpräferenzen bzw. -situationen. Letztere bestimmen, in welchem Umfang das Holz im Zuge der Vermarktung stofflich oder energetisch genutzt wird. [...]

Mit der Entwicklung eines konzeptionellen Rahmens sind wir in der Lage, die Auswirkungen ökologischer und ökonomischer Restriktionen auf die räumlich-zeitliche Verfügbarkeit von Waldenergieholz in der Schweiz zu quantifizieren. Wir haben sie über einen Zeitraum von 40 Jahren für drei Waldbewirtschaftungsszenarien und zwei Holzmarktsituationen mit und ohne Subventionen für die Bewirtschaftung von Schutzwäldern simuliert. Ökologische und ökonomische Restriktionen schränkten die Verfügbarkeit von Waldenergieholz stark ein. Das theoretische Potenzial entsprach der jährlichen Holzproduktion von 9 bis 16 Mio. m³/a oder 67 bis 118 PJ/a. Ökologische Restriktionen reduzierten es auf etwa die Hälfte bis ein Drittel, die zusätzlichen ökonomischen auf ein Drittel bis ein Fünftel (14-41 PJ/a), und nach Abzug der aktuellen Nutzung verblieb ein Zehntel des theoretischen Potenzials oder weniger. Verglichen mit der derzeitigen Waldbewirtschaftung erhöhten die Szenarien zum Vorratsabbau die verfügbaren Energieholzmengen über 40 Jahre um das Drei- bis Vierfache - dies insbesondere kurzfristig. Günstige Energieholzmärkte erhöhten die Verfügbarkeiten um 20 bis 25% und Subventionen um 30%, vor allem in den alpinen Regionen. Unter den aktuellen Marktpreisen sind das Mittelland und der Jura mit den grössten ungenutzten Potenzialen und der geringsten Abhängigkeit von Subventionen vielversprechende Regionen für die Energieholzmobilisierung. Die Reduktion der Vorräte bietet sich an, um in der frühen Phase der Energiewende mehr Energieholz im Wald zu nutzen und auf diese Weise Versorgungslücken anderer erneuerbarer Energien auszugleichen.

Climate-adaptive forest management aims to sustain the provision of multiple forest ecosystem services and biodiversity (ESB). However, it remains largely unknown how changes in adaptive silvicultural interventions affect trade-offs and synergies among ESB in the long term. We used a simulation-based sensitivity analysis to evaluate popular adaptive forest management interventions in representative Swiss low- to mid-elevation beech- and spruce-dominated forest stands. We predicted stand development across the twenty-first century using a novel empirical and temperature-sensitive single-tree forest stand simulator in a fully crossed experimental design to analyse the effects of (1) planting mixtures of Douglas-fir, oak and silver fir, (2) thinning intensity, and (3) harvesting intensity on timber production, carbon storage and biodiversity under three climate scenarios. Simulation results were evaluated in terms of multiple ESB provision, trade-offs and synergies, and individual effects of the adaptive interventions. Timber production increased on average by 45% in scenarios that included tree planting. Tree planting led to pronounced synergies among all ESBs towards the end of the twenty-first century. Increasing the thinning and harvesting intensity affected ESB provision negatively. Our simulations indicated a temperature-driven increase in growth in beech- (+ 12.5%) and spruce-dominated stands (+ 3.7%), but could not account for drought effects on forest dynamics. Our study demonstrates the advantages of multi-scenario sensitivity analysis that enables quantifying effect sizes and directions of management impacts. We showed that admixing new tree species is promising to enhance future ESB provision and synergies among them. These results support strategic decision making in forestry.

Der Waldboden und die in ihm gespeicherten Nährstoffe sind neben dem Wasser- und Wärmeangebot des Standorts entscheidend für die Entwicklung der Baumbestände. Im Zusammenhang mit einer intensivierten Holzernte gilt es, dem Nährstoffpotential im Boden Sorge zu tragen. Insbesondere die Nährstoffnachhaltigkeit sollte nicht gefährdet werden. Es gilt, den Nährstoffpool im Boden trotz Nährelementexport durch die Holzernte langfristig zu erhalten, indem dem Standort nur so viele Nährstoffe entzogen werden, wie durch natürliche Prozesse nachgeliefert werden. Um die Nachhaltigkeit der Holz- und Nährstoffversorgung zu gewährleisten, benötigt das Waldmanagement standortsspezifische Entscheidungsgrundlagen.

The forest soil and the nutrients stored in it are crucial for the healthy growth of tree stands, in addition to the site’s water and heat supply. In connection with intensified timber harvesting, the nutrient potential in the soil must be taken care of. In particular, nutrient sustainability should not be endangered. This means at least the maintenance of the nutrient pool in the soil after timber harvesting and balancing of all relevant nutrient fluxes at a given site, including nutrient export through timber harvesting. Timber harvesting should only remove as many nutrients from the site as are replenished by natural processes.

Process-based forest models are important tools for predicting forest growth and their vulnerability to factors such as climate change or responses to management. One of the most widely used stand-level process-based models is the 3-PG model (Physiological Processes Predicting Growth), which is used for applications including estimating wood production, carbon budgets, water balance and susceptibility to climate change. Few 3-PG parameter sets are available for central European species and even fewer are appropriate for mixed-species forests. Here we estimated 3-PG parameters for twelve major central European tree species using 1418 long-term permanent forest monitoring plots from managed forests, 297 from un-managed forest reserves and 784 Swiss National Forest Inventory plots. A literature review of tree physiological characteristics, as well as regression analyses and Bayesian inference, were used to calculate the 3-PG parameters.
The Swiss-wide calibration, based on monospecific plots, showed a robust performance in predicting forest stocks such as stem, foliage and root biomass. The plots used to inform the Bayesian calibration resulted in posterior ranges of the calibrated parameters that were, on average, 69% of the prior range. The bias of stem, foliage and root biomass predictions was generally less than 20%, and less than 10% for several species. The parameter sets also provided reliable predictions of biomass and mean tree sizes in mixed-species forests. Given that the information sources used to develop the parameters included a wide range of climatic, edaphic and management conditions and long time spans (from 1930 to present), these species parameters for 3-PG are likely to be appropriate for most central European forests and conditions.

Stand-level process-based models have rarely been applied to uneven-aged forests that contain many size classes and negative exponential shaped size distributions. However, the relative simplicity of such models, in terms of parameterisation, use and interpretation, could make them valuable tools for studying and managing such forests. In particular, the effects of climate change on the stand-level growth of forests with negative exponential shaped size distributions has received very little attention compared with even-aged forests. The first objective of this study was to validate 3-PG, a stand-level process-based model, for five types of uneven-aged forests in Switzerland; (1) Fagus sylvatica dominated, (2) Picea abies dominated, (3) mixtures of Picea abies and Abies alba, (4) mixtures of Picea abies, Abies alba and Fagus sylvatica, and (5) mixtures of Larix decidua, Pinus cembra and P. abies. The second objective was to use 3-PG to examine how climate change has influenced the growth of these forests since the 1930s.
3-PG predictions of biomass, biomass partitioning in above- and belowground components, and light absorption were validated using inventory data from 23 plots, which had been monitored for an average of 81 years (15 to 112 years). For all species and size classes (2–3 per species), 3-PG produced accurate predictions of root biomass, stem biomass and outputs derived from it such as mean diameter, basal area and height, which were all highly correlated with the observed values (R² > 0.86). The slope of predicted versus observed values was often not significantly different to 1 (averaged 1.13) and the bias averaged −1.2%.
3-PG simulations to examine the effects of climate change without the confounding effects of stand structure and management, showed that the growth of the five forests types has, on average, increased by 17% since the 1930s. The growth was mainly influenced by temperature, while in the case of A. alba, growth was largely influenced by vapour pressure deficit. The accelerated growth rates imply that thinning intensities also need to increase to prevent high stand densities from inhibiting regeneration in these uneven-aged forests. This study shows that 3-PG can be used to predict the growth dynamics of uneven-aged mixed-species forests, and to our knowledge, this is the first time a stand-level process-based forest growth model has been used and validated for such forests.

1. Tree regeneration is a key process for long-term forest dynamics, determining changes in species composition and shaping successional trajectories. While tree regeneration is a highly stochastic process, tree regeneration studies often cover narrow environmental gradients only, focusing on specific forest types or species in distinct regions. Thus, the larger-scale effects of temperature, water availability, and stand structure on tree regeneration are poorly understood.
2. We investigated these effects in respect of tree recruitment (in-growth) along wide environmental gradients using forest inventory data from Flanders (Belgium), northwestern Germany, and Switzerland covering more than 40 tree species. We employed generalized linear mixed models to capture the abundance of tree recruitment in response to basal area, stem density, shade casting ability of a forest stand as well as site-specific degree-day sum (temperature), water balance, and plant-available water holding capacity. We grouped tree species to facilitate comparisons between species with different levels of tolerance to shade and drought.
3. Basal area and shade casting ability of the overstory had generally a negative impact on tree recruitment, but the effects differed between levels of shade tolerance of tree recruitment in all study regions. Recruitment rates of very shade-tolerant species were positively affected by shade casting ability. Stem density and summer warmth (degree-day sum) had similar effects on all tree species and successional strategies. Water-related variables revealed a high degree of uncertainty and did not allow for general conclusions. All variables had similar effects independent of the varying diameter thresholds for tree recruitment in the different data sets.
4. Synthesis: Shade tolerance and stand structure are the main drivers of tree recruitment along wide environmental gradients in temperate forests. Higher temperature generally increases tree recruitment rates, but the role of water relations and drought tolerance remains uncertain for tree recruitment on cross-regional scales.

Climate-smart forestry (CSF) has gained increasing attention in recent years. Yet, a framework to assess the three pillars of CSF, in terms of mitigation and adaptation to climate change and ecosystem service (ES) provision based on national forest inventory (NFI) data is currently lacking. In this study, we present an assessment framework of CSF based on forest indicators derived from NFI. To quantify the three pillars of CSF we defined 1) mitigation based on the forest carbon stock, 2) adaptation using biodiversity and susceptibility to disturbances caused by storm and bark beetles and 3) ES provision with timber production and protection against avalanches and rockfall. We demonstrate the application of this framework using Swiss NFI data and projections of future forest development under four different management scenarios. Our results show that, in general, the carbon stock is increasing at the national scale when current management practices are maintained. Increases are especially pronounced in the Jura and Southern Alps regions. Trade-offs occur between enhancing mitigation and adaption, and the sustainable provision of ES such as timber production. Scenarios with increased harvesting intensities lead to a higher timber supply in the short-term, while decreasing the carbon stock, biodiversity-related old-growth indicators and the protective function of forests. The indicator framework presented in this study provides a first methodological approach that can support policy making to balance the different forest services in the context of CSF.

1. Forests provide multiple services, and in the face of global change adaptive management strategies are needed, which inevitably must be based on models. However, most locally accurate forest models are tied to the stand scale and cannot readily be applied across large areas. Empirical data for model initialisation are often not available at large spatial scales. National Forest Inventories (NFIs) provide spatially representative tree and stand samples, but their samples are typically small, i.e. only a few trees are measured per plot, and they are truncated, i.e. not each tree has the same probability of being observed. To overcome these issues, we develop and apply a methodology to derive stand descriptions from small sample data, taking the Swiss NFI as a case study.
2. We extended the traditional Weibull function to (multi‐)truncated unimodal and bimodal forms that are suitable for the representation of samples from survey designs with multiple callipering thresholds. Subsequently, we applied these functions in an extended parameter prediction method to derive stand diameter distributions from representative samples. Additionally, we predicted species compositions using a multinomial logistic regression model and assigned them to the diameter distributions of the stands.
3. The diameter distribution of 9.1% of the Swiss NFI samples was better described by a bimodal than a unimodal Weibull function. The uni‐ and bimodal diameter model in combination with the model to determine species composition can be used to predict stand descriptions from single small samples or entire forest types in the target area. Thereby, the bimodal form is suitable for capturing stand structures with distinct under‐ and overstorey. In Switzerland, the diameter distributions of stands are typically positively skewed.
4. Our method can be applied to any large‐scale dataset (e.g. NFI) and allows to generate initial conditions in terms of spatially representative stands. These in turn are suitable for forest stand simulators, which allows for developing adaptive forest management strategies at large scales, by simulating realistic and site‐specific stand development while still reflecting detailed management measures. Further, stand descriptions can be used to assess tree species diversity, regeneration and harvest potentials.

1. In a Europe shaped by centuries of forest management, the task of today's scientists in characterising, understanding and modelling natural forests is highly challenging. Although numerous forest reserves exist, most remain hardly comparable case studies. Contrarily, National Forest Inventories (NFIs) consist of systematically distributed sample plots with varying time since last intervention and provide representative data. These characteristics make NFIs a unique opportunity to investigate hidden natural forests.
2. Here we propose using NFI plots free of human influence for >40 to >70 years (‘latent reserves’) to conduct large-scale studies on near-natural forests. We tested this original concept in Swiss forests. We characterised compositional and structural attributes of ‘latent reserves’ and compared them with those of managed forests to assess whether the former demonstrated more signs of naturalness than the latter. As an example of an application, we analysed the tree- and stand-level factors affecting natural tree mortality in ‘latent reserves’.
3. Up to 15.3% of Swiss NFI plots fulfilled the criteria of ‘latent reserves’, and most of these plots were distributed at mid- to high elevations where accessibility and management opportunities are limited. ‘Latent reserves’ showed more signs of naturalness than managed forests—a higher proportion of broadleaves, higher mortality rates, higher stand density and more deadwood. However, their size structure and basal area did not differ from those of managed forests, most likely because of a lower site productivity. Although ‘latent reserves’ were transitioning towards a natural state, more time without management might be required for these forests to become fully detached from the effects of past management, especially at high elevations. Mortality analyses in ‘latent reserves’ showed that species-specific tree mortality had a U-shaped response to tree size, was negatively related to tree growth and was higher when competition was stronger.
4. Synthesis. Our findings demonstrate the potential of ‘latent reserves’ to study near-natural forests at the country level, and point to further opportunities for larger-scale collaborations. Investigating ‘latent reserves’ represents a first step towards a deeper understanding of such forests using existing long-term data and shows promise for further research in Europe.

Introduction: Among terrestrial ecosystems, forests represent large carbon stocks threatened by changing climatic conditions, deforestation, overexploitation, and forest degradation. Close to nature forestry may help forests to continue to acting as carbon sinks by promoting their resilience against disturbances. The EU decided to carry out carbon accounting of emissions and removals from managed forests under the Paris Agreement (PA) by using a projected Forest Reference Level (FRL) based on the continuation of recent management practices.
Methods: We developed four conceptual scenarios that could build the Swiss Forest Reference Level and performed simulations over 50 years using Swiss National Forest Inventory (NFI) data and the empirical forest model MASSIMO. To improve MASSIMO, we further developed a new tree species-specific model for small scale mortality that accounts for the Swiss NFI design. Then, using projected biomass and mortality from MASSIMO, carbon budgets of mineral soil, litter, and dead wood were estimated using the Yasso07 model.
Results: The U-shaped mortality model performed well (AUC 0.7). Small as well as large trees had the highest mortality probabilities, reflecting both young trees dying due to self-thinning and old trees from age, pests or abiotic influences. All scenarios matched their given harvesting and growing stock targets, whereby the share of broadleaves increased in all regions of Switzerland. This resulted in decreasing biomass growth, possibly due to a species shift from typically fast growing and more shade tolerant conifers to broadleaves. The CO₂-balance of the conceptual scenarios ranged from 1.06 to -3.3 Mt CO₂ a^-1 under Increased Harvesting and Recent Management Practices (RMP), respectively. Rotation periods are shortened under Increased Harvesting, which is an important climate adaptive management strategy, but forests were predicted to become a net carbon source. In contrast, RMP resulted in similar harvesting amounts and forests as carbon sinks, as reported in the past. Further, the RMP scenario does not involve political assumptions and reflects the idea of the CMP approach used by the EU member states, which makes it comparable to other countries. Therefore, we propose the scenario RMP as a suitable and ideal candidate for the Swiss FRL.

The transition to a climate-neutral economy is expected to increase future timber demands and endanger the multifunctionality of forests. National scenario analyses are needed to determine long-term forest management impacts and support forest policy making in defining guidelines for the sustainable provision of forests' ecosystem services and biodiversity (ESB). Using national forestry inventory data, the forest management model MASSIMO and a model to estimate harvesting costs, we simulated forest development in Switzerland under five politically relevant timber harvesting scenarios until 2106 (business as usual and four increased timber mobilisation scenarios). Model results were analysed using a utility-based multi-criteria approach regarding timber production, protection against gravitational hazards, carbon sequestration and biodiversity conservation for the whole of Switzerland and for five sub-regions. The development of ESB benefits over time and existing trade-offs were analysed. Apart from the Plateau region, the business-as-usual scenario resulted in the highest overall ESB benefits. However, this scenario did not mobilise possible timber potentials, which is not in line with current forest policies. In the Plateau region, ESB benefited most under a constant growing stock scenario that guaranteed long-term sustainable timber usage. Nevertheless, both scenarios showed strong trade-offs between biodiversity conservation and the service carbon sequestration. The latter was achieved best under a scenario with conifer promotion and increased harvested timber volumes that can be used for long-living timber products and substitution of energy intensive materials and fossil fuels. Even though weighting the ESB according to regional management priorities further increased the trade-off situation, it also increased the overall benefits of harvesting scenarios, except for in mountainous regions. We conclude that no single scenario can maximize all ESB benefits simultaneously. A combination of locally adapted scenarios with targeted priorities can guarantee a higher degree of multifunctionality and long-term timber supply, but at the cost of locally more accentuated trade-offs. Overall, our study provides new insights into ESB interactions, and the presented multi-criteria framework and results provide a valuable basis to support forest policy decision making in Switzerland and beyond.

Wood fuel has become central in environmental policy and decision-making processes in cross-sectoral areas. Proper consideration of different types of woody biomass is fundamental in forming energy transition and decarbonization strategies. We quantified the development of theoretical (TPs) and sustainable (SPs) potentials of wood fuel from forests, trees outside forests, wood residues and waste wood in Switzerland for 2020, 2035 and 2050. Ecological and economic restrictions, timber market situations and drivers of future developments (area size, tree growth, wood characteristics, population growth, exporting/importing (waste wood)) were considered. We estimated a SP of wood fuel between 26.5 and 77.8 PJ/a during the three time points. Results demonstrate that the SP of wood fuel could be significantly increased already in the short term. This, as a moderate stock reduction (MSR) strategy in forests, can lead to large surpluses in SPs compared to the wood fuel already used today (~36 PJ/a), with values higher by 51% (+18.2 PJ) in 2020 and by 59% (+21.3 PJ) in 2035. To implement these surpluses (e.g., with a cascade approach), a more circular economy with sufficient processing capacities of the subsequent timber industries and the energy plants to convert the resources is required.