Dr. Sophia Etzold

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.

• Nitrogen deposition exceeds critical loads in almost 90% of forest areas. Increased nitrogen deposition leads to nutrient imbalances in the soil and vegetation and can affect the biodiversity of plants, lichens and soil organisms.
• Ground-level ozone damages the photosynthetic cells in leaves. The ozone dose taken up through the stomata has decreased in beech trees, but it remains above the critical threshold.
• Although nitrogen and ozone pollution has been reduced, it still exceeds the relevant limits in many places and can inhibit forest’s growth and drought resistance.

• In quasi il 90 per cento della superficie forestale gli apporti di azoto sono superiori ai valori limite di contaminazione. Questo fenomeno comporta squilibri nutritivi nel suolo e nella vegetazione e può influire sulla biodiversità di piante, licheni e organismi del suolo.
• L'ozono troposferico danneggia le cellule fotosintetiche delle foglie. Nei faggi la dose di ozono assorbita
attraverso le aperture degli stomi è diminuita, ma rimane superiore al livello critico.
• Sebbene sia stato possibile contenerle, le emissioni di azoto e ozono spesso superano ancora i valori
limite di contaminazione e possono inibire la crescita dei boschi e la loro resistenza alla siccità.

• Die Stickstoffeinträge liegen für knapp 90 % der Waldfläche über den kritischen Belastungsgrenzen. Erhöhter Stickstoffeintrag führt zu Nährstoffungleichgewichten in Boden und Vegetation und kann die Biodiversität von Pflanzen, Flechten und Bodenorganismen beeinträchtigen.
• Bodennahes Ozon schädigt die fotosynthetischen Zellen der Blätter. Die durch die Spaltöffnungen aufgenommene Ozondosis hat bei den Buchen abgenommen,
verharrt aber über dem kritischen Grenzwert.
• Obwohl die Immissionen von Stickstoff und Ozon reduziert werden konnten, überschreiten sie vielerorts immer noch die Belastungsgrenzen und können
das Wachstum des Waldes und seine Resistenz gegen Trockenheit hemmen.

• Les dépôts azotés dépassent les charges critiques sur près de 90% de la surface forestière. Un apport excessif d'azote provoque des déséquilibres nutritifs
dans le sol et la végétation et peut nuire à la biodiversité des plantes, des lichens et des organismes du sol.
• L'ozone troposphérique endommage les cellules photosynthétiques des feuilles. La dose d'ozone absorbée par les stomates a diminué chez le hêtre, mais reste supérieure à la valeur limite.
• Les immissions d'azote et d'ozone ont pu être réduites. Cependant, elles dépassent encore en bien des endroits les charges critiques, et peuvent affecter la croissance de la forêt et sa résistance à la sécheresse.

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.

La végétation du sous-bois est une composante majeure de la biodiversité des écosystèmes forestiers. Sa composition floristique est recensée depuis le milieu des années 1990 à intervalles de deux à dix ans dans le cadre du programme de recherches à long terme sur les écosystèmes forestiers LWF. Le nombre d’espèces vasculaires et le recouvrement de la strate herbacée ont diminué significativement depuis le début des inventaires, vraisemblablement en raison d’une fermeture des peuplements due à l’absence d’interventions sylvicoles. La variation temporelle des valeurs indicatrices de moyennes suggère une faible diminution de la disponibilité en nutriments (valeur N) lorsque les bryophytes sont incluses dans le calcul des moyennes par site, et une faible augmentation de la température (valeur T) lorsqu’elles ne le sont pas.

The size of a tree and its crown are key drivers of its ability to intercept light. These characteristics are also constrained by biomechanical and hydraulic limitations, leading to tremendous variations in crown size and tree height between species. To date, there is no consensus on how covariation between crown characteristics (i.e. tree height, crown diameter and relative crown depth) control the spectrum of crown sizes, nor on the species characteristics (i.e. species climate niche or functional traits) that govern their variation. Using species-specific allometric relationships accounting for local competition data on individual crown characteristics of tree species across Europe and North America, we showed that the spectrum of tree crown size is mainly driven by two independent axes: one correlating tree height with relative crown depth, and another linking crown diameter to crown volume. While both angiosperms and gymnosperms exhibited similar patterns of covariation for these crown characteristics, distinct ecological strategies emerged for maximum tree height. We also highlighted that shade-tolerant species were characterized by wider and deeper crowns and showed reduced sensitivity to local competition in relative crown depth, indicating enhanced competitive resilience. Finally, simulations of light interception in virtual stands performed using a ray tracing algorithm highlighted the importance of different crown characteristics in various competitive contexts. Tree height was the primary determinant of light interception in low competitive contexts, common in early successional stages, whereas relative crown depth and crown diameter played a more significant role in high-competitive contexts typical of late succession. These findings suggest that shade-tolerant species, with their deeper crowns, are better equipped to intercept light under competition and ultimately emphasize the ability of shade tolerance to improve the realism of forest dynamic models as it can be used as an indicator of crown size and its response to competition.

Das Monitoring-Netzwerk TreeNet untersucht mit automatischen Sensoren an Baumstämmen (Punktdendrometern) den Wasserhaushalt und das Wachstum von Waldbäumen an rund 70 Standorten in der Schweiz (inkl. 13 LWF-Standorten). Alle zehn Minuten werden an über 500 Bäumen die Stammradien in Mikrometergenauigkeit gemessen sowie Daten in Atmosphäre und Boden gesammelt. Hier zeigen wir charakteristische physiologische Durchschnittswerte von neun Baumarten basierend auf Einzelbaummessungen (1998–2023). Unterschiede in den Art- (Wachstumsraten, Baumwasserdefizit) und Standorteigenschaften (Niederschlag und Temperatur während der Wachstumsphase) bestimmen das Gedeihen der Bäume. Das durchschnittliche Jahreswachstum wurde hauptsächlich durch die Anzahl der Wachstumsstunden pro Jahr und die mittlere Wachstumsrate pro Stunde erklärt. Die Douglasie und die Tanne zeigten die grössten Zuwächse. Die Fichte hatte die höchste Wachstumsrate pro Stunde, erreichte aber aufgrund der wenigen Wachstumsstunden einen nur durchschnittlichen Jahreszuwachs. Die Wasserspeicherkapazität eines Baumes – hergeleitet aus der maximalen, täglichen Schrumpfung des Stammes – und das Baumwasserdefizit sind weitere Faktoren, die das Wachstum beeinflussen. Innerhalb derselben Art deutet eine grössere Wasserspeicherkapazität auf mehr Wachstum hin. Über alle neun untersuchten Arten hinweg wachsen jedoch Arten mit einer generell kleinen Wasserspeicherkapazität am besten (Douglasie, Tanne, Buche). Wir zeigen, wo die TreeNet-Bäume am besten wachsen (Schweizer Mittelland) und wo sie unter dem grössten Trockenstress leiden (Wallis und Nordschweiz).

Forests are undergoing increasing risks of drought-induced tree mortality. Species replacement patterns following mortality may have a significant impact on the global carbon cycle. Among major hardwoods, deciduous oaks (Quercus spp.) are increasingly reported as replacing dying conifers across the Northern Hemisphere. Yet, our knowledge on the growth responses of these oaks to drought is incomplete, especially regarding post-drought legacy effects. The objectives of this study were to determine the occurrence, duration, and magnitude of legacy effects of extreme droughts and how that vary across species, sites, and drought characteristics. The legacy effects were quantified by the deviation of observed from expected radial growth indices in the period 1940–2016. We used stand-level chronologies from 458 sites and 21 oak species primarily from Europe, north-eastern America, and eastern Asia. We found that legacy effects of droughts could last from 1 to 5 years after the drought and were more prolonged in dry sites. Negative legacy effects (i.e., lower growth than expected) were more prevalent after repetitive droughts in dry sites. The effect of repetitive drought was stronger in Mediterranean oaks especially in Quercus faginea. Species-specific analyses revealed that Q. petraea and Q. macrocarpa from dry sites were more negatively affected by the droughts while growth of several oak species from mesic sites increased during post-drought years. Sites showing positive correlations to winter temperature showed little to no growth depression after drought, whereas sites with a positive correlation to previous summer water balance showed decreased growth. This may indicate that although winter warming favors tree growth during droughts, previous-year summer precipitation may predispose oak trees to current-year extreme droughts. Our results revealed a massive role of repetitive droughts in determining legacy effects and highlighted how growth sensitivity to climate, drought seasonality and species-specific traits drive the legacy effects in deciduous oak species.

Recent studies indicate an increase in the frequency of extreme compound dryness days (days with both extreme soil AND air dryness) across central Europe in the future, with little information on their impact on the functioning of trees and forests. This study aims to quantify and assess the impact of extreme soil dryness, extreme air dryness, and extreme compound dryness on the functioning of trees and forests. For this, >15 years of ecosystem-level (carbon dioxide and water vapor fluxes) and 6–10 years of tree-level measurements (transpiration and growth) each from a montane mixed deciduous forest (CH-Lae) and a subalpine evergreen coniferous forest (CH-Dav) in Switzerland, is used. The results showed extreme air dryness limitation on CO₂ fluxes and extreme soil dryness limitations on water vapor fluxes. Additionally, CH-Dav was mainly affected by extreme air dryness whereas CH-Lae was affected by both extreme soil dryness and extreme air dryness. The impact of extreme compound dryness on net CO₂ uptake (about 75 % decrease) was more due to higher increased ecosystem respiration (40 % and 70 % increase at CH-Dav and CH-Lae, respectively) than decreased gross primary productivity (10 % and 40 % decrease at CH-Dav and CH-Lae, respectively). A significant negative impact on evapotranspiration and transpiration was only observed at CH-Lae during extreme soil and compound dryness (about 25 % decrease). Furthermore, with some differences, the tree-level impact on tree water deficit, transpiration, and growth were consistent with the ecosystem-level impact on carbon uptake and evapotranspiration. Finally, the impact of extreme dryness showed no significant relationship with tree allometry (diameter and height) but across different tree species. The projected future is likely to expose these forest areas to more extreme and frequent dryness conditions, thus compromising the functioning of trees and forests, thereby calling for management interventions to increase the adaptive capacity and resistance of these forests.

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.

In an increasingly dry environment, it is crucial to understand how tree species use soil water and cope with drought. However, there is still a knowledge gap regarding the relationships between species-specific stomatal behaviour, spatial root distribution, and root water uptake (RWU) dynamics.
Our study aimed to investigate above- and below-ground aspects of water use during soil drying periods in four temperate tree species that differ in stomatal behaviour: two isohydric tracheid-bearing conifers, Scots pine and Norway spruce, and two more anisohydric deciduous species, the diffuse-porous European beech, and the ring-porous Downy oak. From 2015 to 2020, soil-tree-atmosphere-continuum parameters were measured for each species in monospecific forests where trees had no access to groundwater. The hourly time series included data on air temperature, vapor pressure deficit, soil water potential, soil hydraulic conductivity, and RWU to a depth of 2 m. Analysis of drought responses included data on stem radius, leaf water potential, estimated osmotically active compounds, and drought damage.
Our study reveals an inherent coordination between stomatal regulation, fine root distribution and water uptake. Compared to conifers, the more anisohydric water use of oak and beech was associated with less strict stomatal closure, greater investment in deep roots, four times higher maximum RWU, a shift of RWU to deeper soil layers as the topsoil dried, and a more pronounced soil drying below 1 m depth. Soil hydraulic conductivity started to limit RWU when values fell below 10⁻³ to 10⁻⁵ cm/d, depending on the soil. As drought progressed, oak and beech may also have benefited from their leaf osmoregulatory capacity, but at the cost of xylem embolism with around 50 % loss of hydraulic conductivity when soil water potential dropped below −1.25 MPa.
Consideration of species-specific water use is crucial for forest management and vegetation modelling to improve forest resilience to drought.

Radial stem growth is a key ecosystem process resulting in long-term carbon sequestration. Despite recognition of its importance to global carbon cycling, high uncertainties remain regarding how radial growth phenology (e.g., the onset, mid, and cessation of radial growth) will be affected by climate change. In this study, we evaluated to what extent high spatially (3 × 3 m) and temporally (up to daily) resolved satellite imagery from PlanetScope can be used to monitor stem growth phenology. For this, we made use of detailed stem growth phenological observations of six common European tree species measured by automated point dendrometers at 14 distinct sites across Switzerland between 2017 and 2021. These growth phenological observations were then linked through multiple regression modeling with metrics extracted from spectral index time series. Our results show that the remote sensing-based models enable monitoring the onset (root mean squared deviation (RMSD) ranges from 5.96 to 27.04 days) and mid-stages of stem growth (RMSD ranges from 10.20 to 36.34 days) with reasonable accuracy as opposed to the cessation of stem growth that showed low accuracy (RMSD ranges from 16.02 to 153.63 days). The accuracy of the remote sensing-based prediction models and their optimal suite of predictors varied across species. The latter has important implications for the remote sensing of stem growth phenology in mixed forests, suggesting that it is important for satellite sensors to resolve individual tree crowns. Overall, our results suggest the need for novel spectral indices that capture the spectral components of mechanistic linkages between stem growth and canopy properties that go beyond the mere detection of leaf phenology. When employing such spectral indices, remote sensing could make it possible to detect not only shifts in leaf phenology caused by climate change but also those in stem growth on a broad spatial scale.

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.

Climate change exposes ecosystems to strong and rapid changes in their environmental boundary conditions mainly due to the altered temperature and precipitation patterns. It is still poorly understood how fast interlinked ecosystem processes respond to altered environmental conditions, if these responses occur gradually or suddenly when thresholds are exceeded, and if the patterns of the responses will reach a stable state. We conducted an irrigation experiment in the Pfynwald, Switzerland from 2003–2018. A naturally dry Scots pine (Pinus sylvestris L.) forest was irrigated with amounts that doubled natural precipitation, thus releasing the forest stand from water limitation. The aim of this study was to provide a quantitative understanding on how different traits and functions of individual trees and the whole ecosystem responded to increased water availability, and how the patterns and magnitudes of these responses developed over time. We found that the response magnitude, the temporal trajectory of responses, and the length of initial lag period prior to significant response largely varied across traits. We detected rapid and stronger responses from aboveground tree traits (e.g., tree-ring width, needle length, and crown transparency) compared to belowground tree traits (e.g., fine-root biomass). The altered aboveground traits during the initial years of irrigation increased the water demand and trees adjusted by increasing root biomass during the later years of irrigation, resulting in an increased survival rate of Scots pine trees in irrigated plots. The irrigation also stimulated ecosystem-level foliar decomposition rate, fungal fruit body biomass, and regeneration abundances of broadleaved tree species. However, irrigation did not promote the regeneration of Scots pine trees, which are reported to be vulnerable to extreme droughts. Our results provide extensive evidence that tree- and ecosystem-level responses were pervasive across a number of traits on long-term temporal scales. However, after reaching a peak, the magnitude of these responses either decreased or reached a new stable state, providing important insights into how resource alterations could change the system functioning and its boundary conditions.

Radial stem growth dynamics at seasonal resolution are essential to understand how forests respond to climate change. We studied daily radial growth of 160 individuals of seven temperate tree species at 47 sites across Switzerland over 8 years. Growth of all species peaked in the early part of the growth season and commenced shortly before the summer solstice, but with species-specific seasonal patterns. Day length set a window of opportunity for radial growth. Within this window, the probability of daily growth was constrained particularly by air and soil moisture, resulting in intermittent growth to occur only on 29 to 77 days (30% to 80%) within the growth period. The number of days with growth largely determined annual growth, whereas the growth period length contributed less. We call for accounting for these non-linear intra-annual and species-specific growth dynamics in tree and forest models to reduce uncertainties in predictions under climate change.

Leaf morphological traits (LMTs) of forest trees have been observed to vary across space and species. However, long-term records of LMTs are scarce, due to a lack of measurements and systematic leaf archives. This leaves a large gap in our understanding of the temporal dynamics and drivers of LMT variations, which may help us understand tree acclimation strategies. In our study, we used long-term LMT measurements from foliar material collections of European beech (Fagus sylvatica) and Norway spruce (Picea abies), performed every second year from 1995 to 2019 on the same trees within the Swiss Long-term Forest Ecosystem Research Program LWF. The 11 study plots (6 beech, 4 spruce, and 1 mixed) are distributed along gradients of elevation (485-1,650 m a.s.l.), mean annual precipitation (935-2142 mm), and mean annual temperature (3.2-9.8°C). The investigated LMTs were (i) leaf or needle mass, (ii) leaf area or needle length, and (iii) leaf mass per area or needle mass per length. We combined this unique data set with plot variables and long-term data on potential temporal drivers of LMT variations, including meteorological and tree trait data. We used univariate linear regressions and linear mixed-effects models to identify the main spatial and temporal drivers of LMT variations, respectively. For beech LMTs, our temporal analysis revealed effects of mast year and crown defoliation, and legacy effects of vapor pressure deficit and temperature in summer and autumn of the preceding year, but no clear long-term trend was observed. In contrast, spruce LMTs were mainly driven by current-year spring conditions, and only needle mass per length showed a decreasing long-term trend over the study period. In temporal models, we observed that LMTs of both species were influenced by elevation and foliar nutrient concentrations, and this finding was partly confirmed by our spatial analyses. Our results demonstrate the importance of temporal analysis for determining less recognized drivers and legacy effects that influence LMTs, which are difficult to determine across space and species. The observed differences in the temporal drivers of beech and spruce LMTs suggest differences in the adaptation and acclimation potential of the two species.

Das Forschungsnetzwerk TreeNet untersucht seit 10 Jahren mit automatischen Sensoren den Wasserhaushalt und das Wachstum von Waldbäumen in der ganzen Schweiz. Alle 10 Minuten werden an rund 400 Bäumen Stammradien mikrometergenau gemessen. Die Daten zeigen, dass unsere Bäume nur an wenigen Tagen pro Jahr und hauptsächlich nachts wachsen.