Dr. Brigitte Rohner

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.

Tree cores are a highly valuable scientific resource. Annual growth ring data can, for example, improve our understanding of climate change impacts and effects of environmental pollution, allow for better annual estimations of tree growth patterns and carbon storage, and help quantify dynamics and changes in forest ecosystems. The value of coring trees for research has been weighted against concerns around the potential harm coring might cause to trees. To date, there is indeed limited research accurately quantifying the potential effects of coring on tree growth and only a handful of studies assessing its influence on mortality. Consequently, many European long-term forest inventorying and monitoring programs are concerned that tree coring might bias the repeated tree measurements in permanent plots, which they rely on for assessments of states and changes of forests. In this study, we assessed the effects of tree coring on the growth and mortality of three widespread European tree species approximately 10 years after they were cored. We used repeated tree measurements from permanent research sites in Switzerland and Ukraine. In Switzerland, we assessed 35 cored and 159 uncored Norway spruce (Picea abies) trees as well as 147 cored and 332 uncored silver fir (Abies alba) trees. In Ukraine, we assessed 348 cored and 6'611 uncored European beech (Fagus sylvatica) trees. We found no statistical evidence that coring negatively affected the growth or mortality of the three tree species assessed. Although we cannot rule out subtle effects on tree health and wood quality, our findings do not provide any evidence that coring affects or biases repeated measurements (such as DBH measures and recording of mortality) performed on the investigated tree species. Tree coring could therefore be considered more often for routine incorporation, particularly in long-term forest inventorying and monitoring programs and initiatives.

Dass die Artenvielfalt von der Flächengrösse abhängt, ist ein bekanntes Phänomen. Mit zunehmender Fläche steigt im Allgemeinen die Anzahl festgestellter Arten. Dieser Zusammenhang ist jedoch nicht linear. Die Baumartenvielfalt von Wäldern unterschiedlicher Grösse zu vergleichen, stellt deshalb oftmals eine Herausforderung dar. Auf der Grundlage von Daten des Schweizerischen Landesforstinventars wurde nun eine Methodik entwickelt, mit der Schätzungen zur erwarteten Baumartenvielfalt auf beliebige Flächengrössen skaliert werden können. Die Methodik wurde in erster Linie zur Verbesserung der internationalen Berichterstattung entwickelt. Nun soll sie auch der Schweizer Forstpraxis zugänglich gemacht werden – mittels einer öffentlichen Web-App.

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.

While enhanced tree growth over the last decades has been reported in forests across the globe, it remains unclear whether it drives persistent biomass increases of forest stands, particularly in mature forests. Enhanced tree growth and stand-level biomass are often linked with a simultaneous increase in density-driven mortality and a reduction in tree longevity. Identifying empirical evidence regarding the balance between these processes is challenging due to the confounding effects of stand history, management, and environmental changes. Here, we investigate the link between growth and biomass via the negative relationship between average tree size and stand density (tree number per area). We find increasing stand density for a given mean tree size in unmanaged closed-canopy forests in Switzerland over the past six decades and a positive relationship between tree growth and stand density across forest plots - qualitatively consistent with our simulations using a mechanistic, cohort-resolving ecosystem model (BiomeE). Model simulations show that, in the absence of other disturbances, enhanced tree growth persistently increases biomass stocks despite simultaneous decreases in carbon residence time and tree longevity. However, the magnitude of simulated biomass changes for a given growth enhancement critically depends on the shape of the mortality functions. Our analyses reconcile reports of growth-induced reductions of tree longevity with model predictions of persistent biomass increases, and with our finding of trends toward denser forests in response to growth - also in mature stands.

Eine diverse Gruppe von Waldfachleuten versammelte sich am 29. Juni an der ETH Zürich, als das SwissForestLab gemeinsam mit der Konferenz der Kantonsförster (KOK) und Wald-Schweiz zum Dialog über die "Baumarten im Schweizer Wald der Zukunft" einlud.

De nombreux spécialistes du secteur forestier ont participé le 29 juin dernier, à l'ETH Zurich au Dialogue sur les essences de la forêt suisse de demain, organisé conjointement par SwissForestLab, la Conférence des inspecteurs et inspectrices cantonaux des forêts (CIC) et ForêtSuisse.

Forests account for nearly 90 % of the world's terrestrial biomass in the form of carbon and they support 80 % of the global biodiversity. To understand the underlying forest dynamics, we need a long-term but also relatively high-frequency, networked monitoring system, as traditionally used in meteorology or hydrology. While there are numerous existing forest monitoring sites, particularly in temperate regions, the resulting data streams are rarely connected and do not provide information promptly, which hampers real-time assessments of forest responses to extreme climate events.
The technology to build a better global forest monitoring network now exists. This white paper addresses the key structural components needed to achieve a novel meta-network.
We propose to complement - rather than replace or unify - the existing heterogeneous infrastructure with standardized, quality-assured linking methods and interacting data processing centers to create an integrated forest monitoring network.
These automated (research topic-dependent) linking methods in atmosphere, biosphere, and pedosphere play a key role in scaling site-specific results and processing them in a timely manner. To ensure broad participation from existing monitoring sites and to establish new sites, these linking methods must be as informative, reliable, affordable, and maintainable as possible, and should be supplemented by near real-time remote sensing data.
The proposed novel meta-network will enable the detection of emergent patterns that would not be visible from isolated analyses of individual sites. In addition, the near real-time availability of data will facilitate predictions of current forest conditions (nowcasts), which are urgently needed for research and decision making in the face of rapid climate change. We call for international and interdisciplinary efforts in this direction.

To understand the state and trends in biodiversity beyond the scope of monitoring programs, biodiversity indicators must be comparable across inventories. Species richness (SR) is one of the most widely used biodiversity indicators. However, as SR increases with the size of the area sampled, inventories using different plot sizes are hardly comparable. This study aims at producing a methodological framework that enables SR comparisons across plot-based inventories with differing plot sizes. We used National Forest Inventory (NFI) data from Norway, Slovakia, Spain, and Switzerland to build sample-based rarefaction curves by randomly incrementally aggregating plots, representing the relationship between SR and sampled area. As aggregated plots can be far apart and subject to different environmental conditions, we estimated the amount of environmental heterogeneity (EH) introduced in the aggregation process. By correcting for this EH, we produced adjusted rarefaction curves mimicking the sampling of environmentally homogeneous forest stands, thus reducing the effect of plot size and enabling reliable SR comparisons between inventories. Models were built using the Conway–Maxell–Poisson distribution to account for the underdispersed SR data. Our method successfully corrected for the EH introduced during the aggregation process in all countries, with better performances in Norway and Switzerland. We further found that SR comparisons across countries based on the country-specific NFI plot sizes are misleading, and that our approach offers an opportunity to harmonize pan-European SR monitoring. Our method provides reliable and comparable SR estimates for inventories that use different plot sizes. Our approach can be applied to any plot-based inventory and count data other than SR, thus allowing a more comprehensive assessment of biodiversity across various scales and ecosystems.

• By killing or weakening trees, drought could change the partitioning of growth between tree sizes or species, thereby altering stand structure. Growth partitioning, often quantified using the growth dominance coefficient (DC) or the shape of tree size versus growth relationships (SGR), indicates the relative contribution of differently sized trees to the total stand growth. Changes in growth partitioning due to droughts are rarely examined but provide valuable information that links tree- and stand-level responses to droughts.
• The objective of this study was to test whether the 2018 European megadrought altered the growth partitioning among tree sizes and species. For this purpose, we first evaluated whether DC or SGR can be calculated from small sample sizes of trees typical of individual forest inventory plots.
• DC, and particularly SGR, were sensitive to sample size, forest type (even-aged and uneven-aged), target variable (tree diameter, basal area or stem mass) and range of tree sizes within the sample. SGR could therefore not be used for our analyses. We found no differences in DC prior to and during the 2018 drought. However, when considering only beech (Fagus sylvatica)-dominated stands, DC was lower during post-drought years than during the 2018 drought.
• The growth of larger trees, especially beech, was more negatively affected during post-drought years. Therefore, an extreme drought event can indeed alter the growth partitioning within forest stands. The DC indicates such changes in partitioning and, hence, which trees can be selected for commercial thinning, or released from competition, to minimize potential impacts of droughts.

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.

Changes in regional and global hydrological cycles and temperature regimes will affect forest health, functioning and growth, and therefore their role in carbon (C) sequestration. A severe spring-summer drought hit large parts of central Europe in 2018 leading locally to premature leaf senescence in deciduous tree species. In late summer 2018 and 2019, we carried out a special survey on selected plots (n = 75) of the Swiss National Forest Inventory to detect whether beech (Fagus sylvatica L.) trees (n = 271) responded to the drought, and to evaluate possible lasting effects in the subsequent year. We concentrated on three vitality indicators: severe crown transparency (SCT), severe leaf browning (SLB), and growth in terms of basal area increment (BAI). In late summer 2018, we observed a frequency of beech trees affected by SLB and SCT significantly higher than in long-term baseline data. The most significant predictors of the occurrence of SLB and SCT were previous growth, species composition, tree size and precipitation deficit. Overall, BAI was significantly lower in 2018 compared to the 2010–2017 average. In particular, trees with both SLB and SCT grew significantly less than unaffected trees, with 40–100% lower median C increment. SCT, SLB and BAI largely recovered after more favorable weather conditions in 2019: almost no instances of SLB were recorded, and SCT was comparable to long-term baseline data. BAI in 2019 was significantly higher than in 2018 and 2010–2017, and weakly related to the crown condition recorded in late summer 2018. We conclude that SCT and SLB recorded in late summer 2018 indicated rapid, drought-induced canopy deterioration of beech, associated with an immediate reduction in BAI and tree-level carbon sequestration. These effects were, however, mainly temporary for most of the investigated beech trees. Since the standard long-term forest health assessment was not able to capture the above signals, we discuss opportunities for adapting its design without jeopardizing data comparability over time.

Severe droughts have the potential to reduce forest productivity and trigger tree mortality. Most trees face several drought events during their life and therefore resilience to dry conditions may be crucial to long-term survival. We assessed how growth resilience to severe droughts, including its components resistance and recovery, is related to the ability to survive future droughts by using a tree-ring database of surviving and now-dead trees from 118 sites (22 species, >3,500 trees). We found that, across the variety of regions and species sampled, trees that died during water shortages were less resilient to previous non-lethal droughts, relative to coexisting surviving trees of the same species. In angiosperms, drought-related mortality risk is associated with lower resistance (low capacity to reduce impact of the initial drought), while it is related to reduced recovery (low capacity to attain pre-drought growth rates) in gymnosperms. The different resilience strategies in these two taxonomic groups open new avenues to improve our understanding and prediction of drought-induced mortality.

Im Wald wird einer der wenigen in der Schweiz natürlich vorkommenden Rohstoffe produziert - das Holz. Forstliche Planungswerke unterstützen eine nachhaltige Waldbewirtschaftung. Für drei Viertel der Waldfläche sind kantonale Waldpläne, und auf gut der Hälfte sind lokale forstliche Planungswerke wie Betriebspläne vorhanden. Fast zwei Drittel der Waldfläche werden regelmässig bewirtschaftet. Die Holzernte hat einen deutlichen Mechanisierungsschub erfahren, die Kosten bleiben aber hoch. So ist auf drei Vierteln der Waldfläche mit einem potenziellen Holzernteaufwand von über 50 Fr./m³ zu rechnen. Nutzung und Mortalität beliefen sich zwischen LFI3 und LFI4 auf 9,3 Mio. m³/Jahr. Der Bruttozuwachs betrug 10,8 Mio. m³/Jahr. Zum ersten Mal war die Zuwachsabschöpfung im Privatwald nicht geringer, sondern mindestens gleich hoch wie im öffentlichen Wald.

 

La forêt produit une des rares ressources naturelles en Suisse : le bois. Les documents de planification permettent une gestion durable de la forêt. Trois quarts de la surface forestière disposent d'une planification cantonale et une bonne moitié est dotée de documents de planification locale, par exemple sous forme de plans de gestion. Près des deux tiers de la surface forestière sont exploités régulièrement. La récolte de bois a connu une forte progression de la mécanisation, les coûts restent toutefois élevés. Sur près des trois quarts de la surface forestière, il faut compter avec des coûts potentiels de récolte supérieurs à 50 francs par m³. Depuis l'IFN3, les exploitations et la mortalité représentent 9,3 millions de m³/an alors que l'accroissement (brut) est de 10,8 millions de m³/an. Pour la première fois, l'accroissement n'est pas épuisé plus faiblement en forêt privée et l'est même au moins aussi fortement qu'en forêt publique.

Während der Trockenheit 2018 wurden in einigen Regionen der Schweiz bei der Buche frühe Kronenverfärbungen und vorzeitige Blattverluste beobachtet. Die tatsächliche Verbreitung und das Ausmass dieser Kronenreaktionen sowie deren Einfluss auf das Wachstum der Bäume blieben aber unklar. Im Spätsommer 2018 wurden deshalb 75 Probeflächen des Schweizerischen Landesforstinventars (LFI), auf denen mindestens eine Buche steht und die bereits im Frühjahr 2018 im Rahmen der regulären LFI-Datenerhebung vermessen wurden, erneut aufgesucht. Dabei wurde bei der Buche der Kronenzustand beurteilt und bei allen Baumarten der seit dem Frühjahr geleistete Grundflächenzuwachs ermittelt. Der vorliegende Artikel fasst die ersten Erkenntnisse dieser Ad-hoc-Erhebung zusammen.

The response of forest productivity to climate extremes strongly depends on ambient environmental and site conditions. To better understand these relationships at a regional scale, we used nearly 800 observation years from 271 permanent long‐term forest monitoring plots across Switzerland, obtained between 1980 and 2017. We assimilated these data into the 3‐PG forest ecosystem model using Bayesian inference, reducing the bias of model predictions from 14% to 5% for forest stem carbon stocks and from 45% to 9% for stem carbon stock changes. We then estimated the productivity of forests dominated by Picea abies and Fagus sylvatica for the period of 1960-2018, and tested for productivity shifts in response to climate along elevational gradient and in extreme years. Simulated net primary productivity (NPP) decreased with elevation (2.86 ± 0.006 Mg C ha^-1 year^-1 km^-1 for P. abies and 0.93 ± 0.010 Mg C ha^-1 year^-1 km^-1 for F. sylvatica). During warm-dry extremes, simulated NPP for both species increased at higher and decreased at lower elevations, with reductions in NPP of more than 25% for up to 21% of the potential species distribution range in Switzerland. Reduced plant water availability had a stronger effect on NPP than temperature during warm‐dry extremes. Importantly, cold-dry extremes had negative impacts on regional forest NPP comparable to warm-dry extremes. Overall, our calibrated model suggests that the response of forest productivity to climate extremes is more complex than simple shift toward higher elevation. Such robust estimates of NPP are key for increasing our understanding of forests ecosystems carbon dynamics under climate extremes.