Dr. Peter Waldner

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.

Der Zustand der Baumkrone ist ein wichtiger Indikator für die Vitalität eines Baumes. In Schweizer Wäldern wird der Kronenzustand seit 1985 im Rahmen der Sanasilva-Inventuren und seit 1994 als Teil des Forschungsprogramms «Langfristige Waldökosystem-Forschung» erhoben. In dieser Periode hat die Vitalität des Waldes deutlich abgenommen. Starke und grossflächige Verschlechterungen zeigten sich insbesondere nach Trockensommern. Der Zustand der Baumkronen ist auch ein Indikator für die Absterbewahrscheinlichkeit eines Baumes in den Folgejahren. Hauptursache für den Vitalitätsverlust des Waldes ist der erhöhte Trockenstress, der sich besonders auf Wälder in tieferen Lagen und trockenen Regionen der Schweiz auswirkt. Ein Extrembeispiel dafür ist die Entwicklung der Waldföhre (Pinus sylvestris L.) im Wallis. Regional starb dort in mehreren Wellen ein grosser Teil der Waldföhren ab. Über die Jahre haben sich die Erkenntnisse zu Ursachen und Prozessen immer klarer abgezeichnet. Die immer längeren und wertvolleren Zeitreihen einer Vielzahl beobachteter Parameter erlauben es uns, auch einzelne Ereignisse und Entwicklungen in einen grösseren Kontext zu stellen und zu interpretieren. Kontinuierliche Langzeitbeobachtungen sind deshalb unersetzlich, um Veränderungen im Wald früh zu erkennen und zu verstehen und daraus entsprechende Empfehlungen an die Politik und Praxis abzuleiten.

Good things take time: findings on forest health from long-term observations

The condition of the tree crown is an important indicator of the vitality of a tree. In Swiss forests, the crown condition has been surveyed since 1985 as part of the Sanasilva inventories and since 1994 as part of the research programme "Longterm Forest Ecosystem Research". During this period, the vitality of the forest has declined significantly. Pronounced and widespread deterioration was particularly evident after dry summers. The condition of the tree crowns is also an indicator of the probability of a tree dying within a few years. The main reason for the loss of vitality in the forest is the increased drought stress, which particularly affects forests at lower altitudes and in dry regions of Switzerland. An extreme example of this is the development of the Scots pine (Pinus sylvestris L.) in Valais. In several waves, a large proportion of the Scots pines there died off regionally. Over the years, the findings on causes and processes have become increasingly clear. The increasingly longer and more valuable time series of a large number of observed parameters also allow us to place and interpret individual events and developments in a wider context. Continuous long-term observations are therefore indispensable for recognising and understanding changes in the forest at an early stage and for making appropriate recommendations to policymakers.

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.

Wälder haben eine essenzielle Bedeutung für die Biodiversität, die Klimaregulation und weitere Ökosystemleistungen. Das Programm «Langfristige Waldökosystem-Forschung» der Eidgenössischen Forschungsanstalt WSL hat im Verlauf der vergangenen drei Jahrzehnte wertvolle Erkenntnisse über die komplexen Wechselwirkungen zwischen biotischen, abiotischen und anthropogenen Faktoren gewonnen. Luftverschmutzung und Klimawandel stellen eine erhebliche Belastung für die Wälder dar, was zu Bodenversauerung, Nährstoffungleichgewichten und erhöhter Baumsterblichkeit führt. Technologische Innovationen wie Fernerkundung, Echtzeitüberwachung und Machine Learning machen detaillierte Analysen möglich, die ein verbessertes Verständnis der Waldökosystemprozesse unter verschiedenen Klimaszenarien zulassen. Die Ergebnisse aus drei Jahrzehnten «Langfristige Waldökosystem-Forschung» lieferten wertvolle Leitlinien für Entscheidungsträgerinnen und Waldbesitzer. Angesichts der steigenden Umweltbelastungen ist die Erhaltung resilienter Wälder entscheidend, um ihre Schutz- und Regulierungsfunktionen langfristig zu sichern.

30 years of Long-Term Forest Ecosystem Research: synthesis and outlook
Forests are of vital importance for biodiversity, climate regulation and other ecosystem services. The Long-Term Forest Ecosystem Research (LWF) conducted by the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) over the past three decades has provided valuable insights into the complex interactions between biotic, abiotic and anthropogenic factors. Air pollution and climate change are a significant burden on forests, leading to soil acidification, nutrient imbalances and increased tree mortality. Technological innovations such as remote sensing, real-time monitoring and machine learning allow for detailed analyses that provide an improved understanding of forest ecosystem processes under different climate scenarios. The results from three decades of LWF research provided valuable guidelines for decision-makers and forest owners. In the face of increasing environmental pressures, maintaining resilient forests is crucial to ensure their protective and regulatory functions in the long term.

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.

Forest soils harbor hyper-diverse microbial communities which fundamentally regulate carbon and nutrient cycling across the globe. Directly testing hypotheses on how microbiome diversity is linked to forest carbon storage has been difficult, due to a lack of paired data on microbiome diversity and in situ observations of forest carbon accumulation and storage. Here, we investigated the relationship between soil microbiomes and forest carbon across 238 forest inventory plots spanning 15 European countries. We show that the composition and diversity of fungal, but not bacterial, species is tightly coupled to both forest biotic conditions and a seven-fold variation in tree growth rates and biomass carbon stocks when controlling for the effects of dominant tree type, climate, and other environmental factors. This linkage is particularly strong for symbiotic endophytic and ectomycorrhizal fungi known to directly facilitate tree growth. Since tree growth rates in this system are closely and positively correlated with belowground soil carbon stocks, we conclude that fungal composition is a strong predictor of overall forest carbon storage across the European continent.

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 are increasingly affected by global change. Building resilient forests requires – amongst others - leveraging the wealth of knowledge from existing ground-based, field inventory and monitoring programs as well as Earth Observation systems to better assess the status, detect changes, understand processes, predict future dynamics, and guide forest management. A proposal from the European Commission for a new forest monitoring framework at the European level aims in this direction but lacks the integration of some crucial and readily available resources and infrastructures. For this reason, the proposal risks to be a missed opportunity rather than a step forward. Here we provide suggestions to help reconciling the proposal with its objectives and a more comprehensive monitoring vision.

Human activities have greatly increased the reactive nitrogen in the biosphere, thus profoundly altering global nitrogen cycling. The large increase in nitrogen deposition over the past few decades has led to eutrophication in natural ecosystems, with negative effects on forest health and biodiversity. Recent studies, however, have reported oligotrophication in forest ecosystems, constraining their capacity as carbon sinks. Here we demonstrate the widespread biological transformation of atmospheric reactive nitrogen in the canopies of European forests by combining nitrogen deposition quantification with measurements of the stable isotopes in nitrate and molecular analyses across ten forests through August–October 2016. We estimate that up to 80% of the nitrate reaching the soil via throughfall was derived from canopy nitrification, equivalent to a flux of up to 5.76 kg N ha⁻¹ yr⁻¹. We also document the presence of autotrophic nitrifiers on foliar surfaces throughout European forests. Canopy nitrification thus consumes deposited ammonium and increases nitrate inputs to the soil. The results of this study highlight widespread canopy nitrification in European forests and its important contribution to forest nitrogen cycling.

Tree-microbe interactions are essential for forest ecosystem functioning. Most plant–microbe research has focused on the rhizosphere, while composition of microbial communities in the phyllosphere remains underexplored. Here, we use 16S rRNA gene sequencing to explore differences between beech and Scots pine phyllospheric microbiomes at the European continental scale, map their functional profiles, and elucidate the role of host trees, forest features, and environmental factors such as climate and atmospheric deposition in phyllosphere microbiota assembly. We identified tree species and the associated foliar trait (specifically carbon:nitrogen ratio) as primary drivers of the bacterial communities. We characterized taxonomical and functional composition of epiphytic bacteria in the phyllosphere of beech and Scots pine across an environmental gradient from Fennoscandia to the Mediterranean area, with major changes in temperature and nitrogen deposition. We also showed that temperature and nitrogen deposition played a crucial role in affecting their assembly for both tree species. This study contributes to advancing our understanding on factors shaping phyllosphere microbial communities in beech and Scots pine at the European continental scale, highlighting the need of broad-scale comparative studies (covering a wide range of foliar traits and environmental conditions) to elucidate how phyllosphere microbiota mediates ecosystem responses to global change.

Raised emissions of biologically reactive nitrogen (N) have intensified N deposition, enhancing tree productivity globally. Nonetheless, the drivers of forest sensitivity to N deposition remain unknown. We used stem growth data from 62,000 trees across Europe combined with N deposition data to track the effects of air temperature and precipitation on tree growth's sensitivity to N deposition and how it varied depending on leaf form over the past 30 years. Overall, N deposition enhanced conifer growth (until 30 kg N ha⁻¹ yr⁻¹) while decreasing growth for broadleaved angiosperms. Lower temperatures led to higher growth sensitivity to N deposition in conifers potentially exacerbated by N limitation. In contrast, higher temperatures stimulated growth sensitivity to N deposition for broadleaves. Higher precipitation equally increased N deposition sensitivity in all leaf forms. We conclude that air temperature and leaf form are decisive in disentangling the effect of N deposition in European forests, which provides crucial information to better predict the contribution of N deposition to land carbon sink enhancement.

To understand the role of vegetation and soil in regulating atmospheric Hg⁰, exchange fluxes and isotope signatures of Hg were characterized using a dynamic flux bag/chamber at the atmosphere–foliage/soil interfaces at the Davos-Seehornwald forest, Switzerland. The foliage was a net Hg⁰ sink and took up preferentially the light Hg isotopes, consequently resulting in large shifts (-3.27‰) in δ²⁰²Hg values. The soil served mostly as net sources of atmospheric Hg⁰ with higher Hg⁰ emission from the moss-covered soils than from bare soils. The negative shift of δ²⁰²Hg and Δ¹⁹⁹Hg values of the efflux air relative to ambient air and the Δ¹⁹⁹Hg/Δ²⁰¹Hg ratio among ambient air, efflux air, and soil pore gas highlight that Hg⁰ re-emission was strongly constrained by soil pore gas evasion together with microbial reduction. The isotopic mass balance model indicates 8.4 times higher Hg⁰ emission caused by pore gas evasion than surface soil photoreduction. Deposition of atmospheric Hg⁰ to soil was noticeably 3.2 times higher than that to foliage, reflecting the high significance of the soil to influence atmospheric Hg⁰ isotope signatures. This study improves our understanding of Hg atmosphere–foliage/soil exchange in subalpine coniferous forests, which is indispensable in the model assessment of forest Hg biogeochemical cycling.

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

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

Most trees form symbioses with ectomycorrhizal fungi (EMF) which influence access to growth-limiting soil resources. Mesocosm experiments repeatedly show that EMF species differentially affect plant development, yet whether these effects ripple up to influence the growth of entire forests remains unknown. Here we tested the effects of EMF composition and functional genes relative to variation in well-known drivers of tree growth by combining paired molecular EMF surveys with high-resolution forest inventory data across 15 European countries. We show that EMF composition was linked to a three-fold difference in tree growth rate even when controlling for the primary abiotic drivers of tree growth. Fast tree growth was associated with EMF communities harboring high inorganic but low organic nitrogen acquisition gene proportions and EMF which form contact versus medium-distance fringe exploration types. These findings suggest that EMF composition is a strong bio-indicator of underlying drivers of tree growth and/or that variation of forest EMF communities causes differences in tree growth. While it may be too early to assign causality or directionality, our study is one of the first to link fine-scale variation within a key component of the forest microbiome to ecosystem functioning at a continental scale.