Climate change, particularly the associated increase in extreme events and disturbances, threatens the numerous environmental, social, and economic benefits that forests provide, both locally and globally. Heat and drought pose significant risks to forest ecosystems; the anticipated future climate is expected to exacerbate this trend. Management interventions should aim to maximise the provision of ecological functions amid the uncertain conditions ahead. A better understanding of the mechanisms regulating forest responses to drought, heat, pests, and diseases - and how management interventions interact with these - is necessary for evidence-based, climate-smart forest management. We first provide an overview of the ecophysiological mechanisms that drive the loss of ecosystem functioning induced by heat and drought. We then explore how various commonly adopted management interventions at the stand level - such as tree species selection and mixture, stand density regulation, measures to optimise stand structure, tree height, and age distribution, as well as nutrient management - may positively or negatively influence forest ecophysiological responses to heat and drought. In this work, we present a mechanism-based critical assessment of forest management practices to support climate-smart forestry/forest management in response to shifting environmental and climatic conditions.

Hotter droughts in European forests increasingly combine declining soil moisture with rising atmospheric demand, raising fundamental questions about how trees sustain transpiration while avoiding embolism-induced mortality under drought stress. While stomatal regulation and transpiration responses are well documented, the role of upstream, within-tree water fluxes, particularly the use and replenishment of internal stem water storage, represent an emerging research frontier.Here, we present high-temporal resolution observations of stem water storage use and rehydration dynamics in mature Pinus sylvestris, combining sap-flow and dendrometer measurements from the VPDrought experiment at the Pfynwald research platform in the dry inner-Alpine Rhône valley of Switzerland. By independently manipulating soil moisture and vapour pressure deficit (VPD), this experiment allows us to disentangle atmospheric and soil controls on internal tree water fluxes.We show that under drought, trees increasingly “run on savings”: the contribution of stem water storage to daily transpiration rises sharply from approximately ~5% under well-watered soil conditions to up to ~40% under dry soil conditions, when transpiration declines but storage water use persists. In parallel, the replenishment of stem storage-water reserves through water flow into the stem declines with decreasing soil water potential. Notably, even under mild soil drought, elevated VPD substantially constrains nighttime rehydration of stem storage-water reserves.The findings we present emphasize stem water storage as a dynamic and drought-responsive component of tree-water use. Accounting for both the mobilization and rehydration of internal water reserves is essential for understanding how trees buffer hydraulic stress during drought and enhance model representations of plant-water interactions under increasingly frequent hotter droughts.

Forests play a major role in wood production and other ecosystem services, such as carbon (C) sequestration and filling reservoirs in drinking water quality. However, it is still under discussion to what extent environmental changes, such as elevated nitrogen (N) deposition and related eutrophication, may affect such services.Our study aimed to assess long-term changes in N and C storage in Swiss forest soils along a gradient in N deposition (about 10 to 30 kg N/ha/y). At five long-term forest ecosystem research plots in Switzerland, which are part of the ICP Forests Level II network, nutrient fluxes (atmospheric deposition, litterfall, soil solution) have continuously been measured since the 1990s. Soil samples were taken from fixed depth layers in the course of soil inventories in the 1990s and 2022.The observed flux patterns indicated that the forests had reached nitrogen saturation on some sites, resulting in nitrogen leaching. At sites with a higher carbon-to-nitrogen ratio (C/N), we found comparatively lower levels of N leaching. The comparison of the two soil inventories showed that the N concentration in soils has actually increased (and the C/N ratio decreased) on the sites with high C/N ratio and high N deposition. We will discuss the observed accumulation and transformations of organic C and N in these soils and the potential impacts on selected ecosystem services.

Plants’ non-structural carbohydrates (NSCs) serve as their capital for growth, reproduction, defense, and survival. To increase the NSC availability of carbon-limited trees, a recent study revealed the possibility of adding exogenous soluble sugars to carbon-starved trees. This provides an opportunity to investigate carbon allocation between source and sink, as well as the growth and physiological responses to external sugars. Using this method, we infused 13C-labeled glucose solution into the stem xylem of sycamore maple (Acer pseudoplatanus L.) trees (Experiment 1) and immersed branch cuttings of various tree species in a 13C-labeled glucose solution (Experiment 2). Our aim was to study whether infused sugars contribute to structural growth and how they influence photosynthesis. Specifically, we focused on whether trees can transport and utilize exogenous sugars for growth, and if sugar addition might trigger negative feedback mechanisms on carbon gain. We then traced the 13C label in bulk tissue and cellulose, and measured tissue NSC concentrations and leaf photosynthesis. Glucose addition consistently increased leaf NSC concentrations (Experiments 1 and 2), and exogenous sugars added were transported and incorporated into biomass formation in Experiment 1. However, excessive sugar addition triggered a negative feedback response, leading to leaf senescence (Experiments 1 and 2) and reduced photosynthesis (Experiment 2). Our findings validate the recently developed sugar addition method but emphasize the importance of carefully controlling the amount and rate of sugar addition to avoid negative feedback responses. This study will contribute to carbon physiological research, particularly in understanding carbon balance and source-sink relationships at the whole-plant level.

In most tree species, roots serve as major carbon (C) sinks, where C is depleted first when C assimilation is limited. Recent methodological advancements in sugar infusion allow for a better understanding of physiological processes alleviating root C limitation. We conducted a glasshouse experiment with maple (Acer pseudoplatanus L.) and pine (Pinus sylvestris L.) saplings that underwent defoliation followed by either slow, fast, or no 13C-labeled glucose infusion. We measured photosynthetic parameters, nonstructural carbohydrate (NSC) concentrations, and δ13C in cellulose of leaves, twigs, and fine roots, as well as the isotopic composition of dark-respired CO2. Sugar infusion induced photosynthetic downregulation and leaf senescence in maple but not in pine. Leaf photosynthesis was negatively correlated with leaf NSC concentration in maple. These responses exacerbated root C limitation in maple. Conversely, pine maintained stable photosynthetic rates and needle NSC concentrations across treatments, showing the potential of sugar infusion to mitigate root C limitation. Our study suggests that exogenous sugar supply reduces the root C availability when it impairs a plant's photosynthetic performance. Species-specific differences influence infused sugar transport and overall source-sink responses. Alleviating C limitation in roots via exogenous sugar addition is feasible only if photosynthesis is not impeded.

Forest soils have significant potential to mitigate climate change through their ability to store large amounts of organic carbon. However, forests are increasingly subject to natural disturbances such as windthrow, wildfire or disease outbreaks, which threaten the permanence of this large carbon stock. In response to increasing disturbances and ongoing climate change, forests are expected to lose their ability to return to pre-disturbance conditions involving a reorganization of tree species composition and stand structure. If tipping points are crossed, even a complete vegetation shift and conversion to non-forest ecosystems is possible. Here we aimed to assess the sensitivity of forest soil carbon to disturbance and its recovery with contrasting successional trajectories by combining two field studies on soil carbon stocks in windthrown forest stands and a global meta-analysis on the effects of different disturbance agents. Our results along an altitudinal gradient in Switzerland show that mountain forests with high carbon stocks in thick organic layers were particularly sensitive to disturbance by windthrow, losing up to 90% of their carbon stored belowground. In contrast, low-elevation forest soils with thin organic layers and smaller carbon stocks were barely affected. These results are consistent with our meta-analysis, which shows that disturbance-induced carbon losses increase with the size of initial carbon stocks. Boreal and high-elevation forests with large soil carbon stocks are highly sensitive to severe and long-lasting carbon losses due to damage from storms, wildfire, insects, and harvesting, while in most temperate and tropical forests soil carbon stocks recover more rapidly and losses are smaller. Results from a disturbance chronosequence in Austria also suggest that vegetation shifts following forest damage can strongly influence the recovery of soil carbon stocks after disturbance. Disturbed sites that remained in a non-forest, grass-dominated state for three decades accumulated about a third more soil carbon than sites that regenerated with trees. In addition to high litter inputs from herbaceous fine roots at grass-dominated sites, we relate this difference to changes in microbial community structure and function. In conclusion, our results underline that the magnitude and duration of soil carbon losses after disturbance depend on the forest type and site specific soil properties. Moreover, vegetation shifts during succession significantly modify the re-accumulation of soil carbon after disturbance.

In recent years, an increasing number of studies have reported on forest declines and shifts in species composition in response to changing climatic conditions (Rigling et al. 2012). The intensification of droughts through rising evaporative demand (i.e., vapor pressure deficit or VPD) is a considerable concern because of their disastrous impacts on natural systems (Grossiord et al. 2020; Trotsiuk et al. 2021). For forests, ecosystem services such as wood provisioning and carbon sequestration are severely jeopardized by these changes, leading to significant uncertainties regarding climate regulation. Climate-vegetation models are not only in need of data on atmospheric and soil drought sensing mechanisms but are also critically challenged by insufficient understanding of the processes driving forest vulnerability to climate change. Only by deciphering the single vs . combined VPD and soil moisture effects will we be able to improve global predictions. We apply a scale spanning approach to disentangle the processes affected by atmospheric (i.e., VPD) and soil droughts from the tissue to the tree and the ecosystem level. We set up the first atmospheric humidity and soil moisture manipulative experiment in a mature natural forest. We combine air humidity (and thus VPD) manipulation using a humidification system in the canopy of adult Scots pine trees exposed naturally to high summer VPD and a below canopy through-fall exclusion system (Schaub 2023). The system is installed at the long-term Pfynwald irrigation experiment, which is since 2003 a pivotal WSL long-term experimental monitoring site anticipated to be near its tipping point with respect to climate change (Bose et al. 2022). This experiment helps us understand how the soil moisture responses of trees, shrubs, and microbial communities are altered by atmospheric dryness from the tissue- to the ecosystem-level. This novel manipulative VPD and soil moisture experiment provides an empirical research platform to address the most critical questions in the context of climate impacts in temperate forests. The data will ultimately allow the development of novel predictive methods to assess climate change impacts on forests. Preliminary data will show the effects of altered atmospheric and soil drought on adult Scots pine trees.

Das Monitoring-Netzwerk TreeNet untersucht mit automatischen Sensoren an Baumstämmen (Punktdendro- metern) 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 Mikrometergenauig- keit gemessen sowie Daten in Atmosphäre und Boden gesammelt. Hier zeigen wir charakteristische physiologi- sche 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 Wachs- tumsrate pro Stunde, erreichte aber aufgrund der wenigen Wachstumsstunden einen nur durchschnittlichen Jahreszuwachs. Die Wasserspeicherkapazität eines Baumes – hergeleitet aus der maximalen, täglichen Schrump- fung des Stammes – und das Baumwasserdefizit sind weitere Faktoren, die das Wachstum beeinflussen. Inner- halb derselben Art deutet eine grössere Wasserspeicherkapazität auf mehr Wachstum hin. Über alle neun unter- suchten 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).

Integrated forest management is a fundamental concept for the sustainable provision of demanded ecosystem services and the simultaneous promotion of biodiversity in our forests. Prof, Dr Andreas Rigling from Forest Ecology, Department of Environmental Systems Science, USYS – ETH Zurich, explains. ‘Growth of trees and forests – from germination to tree death’ is the title of one of my lectures at ETH Zurich. The lecture explores the stages from seed to tree, tree to forest, the long time span, and the many transitions and bottlenecks throughout development. It covers the various components of forest ecosystems and their intricate interactions. This complexity varies across space and time, from one forest type to another, shaped by local site conditions and land-use history. Understanding this complexity is already a challenge, but forest dynamics are now shifting as the boundary conditions change. What applies today may no longer hold true tomorrow, with profound consequences for forests and their sustainable management.

Europe’s semi-natural grasslands support notably high levels of temperate biodiversity across multiple taxonomic groups. However, these ecosystems face unique conservation challenges. Contemporary agricultural practices have replaced historical traditional low-intensity agriculture in many regions, resulting in a spectrum of management intensities within these ecosystems, ranging from highly intensive methods to complete abandonment. Paradoxically, both extremes along this spectrum of management intensity can be detrimental to biodiversity of semi-natural grasslands. Moreover, while anthropogenic climate change is an overarching threat to these ecosystems, rapid changes in land use and its intensity often present more immediate pressures. Often occurring at a faster rate than climate change itself, these land-use changes have the potential to rapidly impact the biodiversity of these grasslands. Here, we divide the ecological processes, threats, and developments to semi-natural grasslands into three sections. First, we examine the different impacts of agricultural intensification and abandonment on these ecosystems, considering their different consequences for biodiversity. Second, we review seminal works on various evidence-based management practices and offer a concise summary that provides support for various conservation and management strategies. However, the socio-economic factors that drive both abandonment and intensification in semi-natural grasslands can also be used to develop solutions through strategic governmental and non-governmental interventions. Accordingly, we conclude with a way forward by providing several key policy recommendations. By synthesizing existing knowledge and identifying research gaps, this essay aims to provide valuable insights for advancing the sustainable management of semi-natural grasslands.

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.

A recent increase in the intensity and frequency of climate extremes under global warming is likely to continue to cause unprecedented rates of forest dieback in different habitats around the world. As one of the most widely distributed tree species, Scots pine (Pinus sylvestris L.) has experienced widespread mortality over the past two decades and many of those forests transitioned to broadleaved dominance inducing massive changes in ecosystem functioning and services. Here, we synthesize the factors and processes underlying drought-induced Scots pine mortality. Our review identifies agreement across studies on the impact of drought, prevalence of crown defoliation prior to mortality, influence of stand density and ecological growth memory, as well as the presence of biotic agents, such as insects and mistletoes. Studies generally agree that drought triggered self-thinning plays an important role. The post-mortality stand density seems far below the comparable pre-drought numbers of trees per hectare, which indicates a significant reduction in the productivity of forest stands. Most surprisingly, we show while Scots pine mortality in the early-2000 s occurred at the species' arid distribution limits, high mortality rates are now also reported from the species' climatic optimum where growth conditions used to be more beneficial. Extreme droughts such as 2003, 2015 and 2018 are causing this observed pattern, which may indicate that an increase in frequency of extreme drought could challenge Scots pine trees growing in climatically favorable areas. Our review indicates that tree level acclimation strategies such as lowering leaf area and enhancing water-use efficiency as well as stand-level adjustments including self-thinning are apparently not sufficient to prevent Scots pine mortality induced by frequent extreme droughts and associated impact of biotic agents (insects and mistletoes). However, we still lack clear understanding in linking functional strategies of the species to local climatic variation to fully understand the capabilities of the species to grow and survive in the future climate.

Summary The link between above‐ and belowground communities is a key uncertainty in drought and rewetting effects on forest carbon (C) cycle. In young beech model ecosystems and mature naturally dry pine forest exposed to 15‐yr‐long irrigation, we performed 13 C pulse labeling experiments, one during drought and one 2 wk after rewetting, tracing tree assimilates into rhizosphere communities. The 13 C pulses applied in tree crowns reached soil microbial communities of the young and mature forests one and 4 d later, respectively. Drought decreased the transfer of labeled assimilates relative to the irrigation treatment. The 13 C label in phospholipid fatty acids (PLFAs) indicated greater drought reduction of assimilate incorporation by fungi (−85%) than by gram‐positive (−43%) and gram‐negative bacteria (−58%). 13 C label incorporation was more strongly reduced for PLFAs (cell membrane) than for microbial cytoplasm extracted by chloroform. This suggests that fresh rhizodeposits are predominantly used for osmoregulation or storage under drought, at the expense of new cell formation. Two weeks after rewetting, 13 C enrichment in PLFAs was greater in previously dry than in continuously moist soils. Drought and rewetting effects were greater in beech systems than in pine forest. Belowground C allocation and rhizosphere communities are highly resilient to drought.

Natural disturbances play an important role in shaping the dynamics of mountain forests, yet their effects on essential ecosystem services, such as protection against natural hazards, can be significant. With the challenges posed by climate change and increasing disturbances, as well as the complexities of salvage logging, there is a growing interest in understanding post-disturbance development in unsalvaged mountain forests, alongside the advancement of decision support systems aimed at ensuring sustained provision of ecosystem services. In this study, we combined a space-for-time substitution approach with long-term monitoring data to evaluate regeneration processes and development of deadwood decay following three distinct windthrow events in Central European mountain forests that were locally unsalvaged (specifically Vaia in 2018, Kyrill in 2007, and Vivian in 1990). Our unique dataset additionally provided insights into the long-term effects of disturbance legacies and tree regeneration on protection against natural hazards. Deadwood cover gradually decreased with time since disturbance, from an average of 50% two years after Vaia to 25% twelve years after Kyrill and 15% thirty years after Vivian. Similarly, deadwood height above ground significantly decreased over time, with median values dropping from 1 to 2 m immediately after the disturbance to 25-30 cm three decades later. The decay stage and diameter of deadwood significantly influenced tree regeneration, with larger diameters of logs and deadwood in more advanced decay stage (especially less solid/soft to very loose stage), facilitating seedling establishment, thus a second wave of tree regeneration. About a quarter of saplings grew on deadwood thirty years after disturbance. The analysis of post-windthrow stand development showed an increase in tree cover and height with time since disturbance, with distinct patterns observed across different windthrow events and sites. Three decades post-disturbance, the number of trees had notably increased, with tree cover reaching 50%. Although Norway spruce remained the dominant species, the forest composition had shifted towards a predominance of broadleaves, particularly evident at lower elevations and areas with moderate browsing pressure. Our findings underscore the critical role of post-disturbance forest recovery and deadwood dynamics in promoting unevenaged mixed forest structures, thereby enhancing forest regeneration, structural diversity, and protection against natural hazards. Emphasizing the vital importance of retaining deadwood, our study suggests that its role as a valuable substrate for enhancing resilience and ecosystem services is likely to grow in the future.

Evolving societal demands and accelerated ecological dynamics due to global change are rapidly altering forest ecosystems and their services. This has prompted the need for advancing forest inventorying and monitoring initatives to expand their scope, improve data collection, foster scientific understanding, and better inform policy responses. Here, we discuss the collaborative processes followed to develop an Advanced Inventorying and Monitoring (AIM) system for Swiss forests. Further, we provide the key messages that emerged from this process which can be of interest to those involved in similar processes at the national/international level. 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.

The loss of leaves and needles in tree crowns and tree mortality are increasing worldwide, mostly as a result of more frequent and severe drought stress. Scots pine (Pinus sylvestris L.) is a tree species that is strongly affected by these developments in many regions of Europe and Asia. So far, changes in metabolic pathways and metabolite profiles in needles and roots on the trajectory toward mortality are unknown, although they could contribute to a better understanding of the mortality mechanisms. Therefore, we linked long-term observations of canopy defoliation and tree mortality with the characterization of the primary metabolite profile in needles and fine roots of Scots pines from a forest site in the Swiss Rhone valley. Our results show that Scots pines are able to maintain metabolic homeostasis in needles over a wide range of canopy defoliation levels. However, there is a metabolic tipping point at around 80-85% needle loss. Above this threshold, many stress-related metabolites (particularly osmoprotectants, defense compounds and antioxidants) increase in the needles, whereas they decrease in the fine roots. If this defoliation tipping point is exceeded, the trees are very likely to die within a few years. The different patterns between needles and roots indicate that mainly belowground carbon starvation impairs key functions for tree survival and suggest that this is an important factor explaining the increasing mortality of Scots pines.