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.

Climate change is increasingly impairing forest ecosystems in Europe, reducing tree vitality and increasing mortality. Crown defoliation and stem growth are widely used indicators of early stress responses, yet their drivers remain difficult to disentangle due to complex, non-linear interactions among climatic, edaphic, and biotic factors.Here, we apply explainable AI (XAI) to model annual diameter growth of individual trees from four dominant European tree species (Norway spruce, Scots pine, Common beech, and Oak) using long-term data from the ICP Forests Level II network. Gradient-boosted decision trees trained on tree-level attributes (e.g., defoliation, social class) and plot-level variables (e.g., soil solution chemistry, atmospheric deposition, and topography) outperform linear baselines across all ablations and grouping strategies. Ablation experiments show that plot-level features account for most of the predictive power while defoliation contributes only marginally.XAI analyses reveal strong non-linear, species-specific response regimes, including marked growth reductions at high defoliation levels (∼35–60%) and optimal regimes of nitrogen and sulphate deposition, illustrating the capacity of XAI to identify candidate growth-relevant regimes and interactions.However, temporally explicit validation using tree-wise cross validation results in a 54% reduction in R2 score whereas spatially explicit validation based on plot-wise cross-validation leads to a near-complete collapse in predictive performance, indicating strong reliance on both temporal and spatial autocorrelation and context-specific patterns. Moreover, ablating defoliation features causes attribution to shift toward correlated environmental variables, highlighting the role of proxying and statistical confounding.Overall, our results illustrate both the potential and the limitations of XAI for forest ecology: while effective for screening large observational datasets and generating hypotheses, XAI outputs require cautious interpretation, mechanistic understanding and spatially robust validation.

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 «Lang- fristige Waldökosystem-Forschung» zeigen sich Veränderungen in der Waldstruktur und im Wachstum. Die Wäl- der 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 glei- cher Bestandesdichte auf, was auf eine Abnahme der Standortskapazität hinweist.

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 Forschungspro- gramms «Langfristige Waldökosystem-Forschung» erhoben. In dieser Periode hat die Vitalität des Waldes deut- lich abgenommen. Starke und grossflächige Verschlechterungen zeigten sich insbesondere nach Trockensom- mern. 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 be- sonders 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 gros- ser 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 erken- nen und zu verstehen und daraus entsprechende Empfehlungen an die Politik und Praxis abzuleiten.

Climate models project a further increase in the average global temperature for the following decades, with Alpine regions (and their ecosystems) expected to be over-proportionally more affected. Biogenic volatile organic compounds (BVOCs) comprise the largest, most highly complex, and diverse fraction of the volatile organic compounds (VOCs) emitted into the atmosphere (1). By emitting BVOCs, plants communicate, fight herbivores, and attract pollinators (2). It is well known that biotic stressors (e.g., insects feeding on plants) lead to changes in plants' BVOC emissions: certain compounds can be promoted, and others reduced. Atmospheric oxidation of BVOCs affects the concentration of methane, carbon monoxide, and tropospheric ozone, leading to the formation of Secondary Organic Aerosol (SOA). Atmospheric aerosol load is crucial in defining the radiative balance and negatively impacts air-quality standards (3). Stress-induced changes in plant emissions may thus lead to changes in atmospheric chemistry and SOA properties (e.g., ref. 4). The impact of prolonged changes in abiotic factors and abiotic stress (e.g., heat and drought) on plants' BVOC composition and emissions quantities, and how this may impact atmospheric chemistry and SOA properties, need to be better understood. Within the experimental project "Acclimation and environmental memory” (AccliMemo), we study BVOC composition and quantities at basal conditions and under prolonged heat and drought. To this purpose, Scots pine (Pinus Sylvestris) seedlings were grown from seeds collected from selected mother trees from the long-term irrigation experiment Pfynwald. Those mother trees experienced different long-term water availability. This also allows us to examine the consequence of transgenerational memory on BVOC emissions (5). Our conference contribution will give insight into our findings from plant chamber experiments and address i) gas-phase BVOC samples collected on sorbent tubes and analyzed by Thermal Desorption GC-MS and ii) gas-phase BVOC measurements collected in-situ using a PTR-ToF-MS. These data provide a well-resolved picture of terpene compositions and diurnal trends in emission levels. The BVOC analysis in the gas phase is complemented by a detailed analysis of the secondary metabolites in needle samples. Secondary metabolites are extracted in organic solvents and analyzed by liquid injection GC-FID/MS. Bibliography (1) Sindelarova, K., Granier, C., Bouarar, I., Guenther, A., Tilmes, S., Stavrakou, T., Müller, J.-F., Kuhn, U., Stefani, P., and Knorr, W.: Global data set of biogenic VOC emissions calculated by the MEGAN model over the last 30 years, Atmospheric Chem. Phys., 14, 9317–9341, https://doi.org/10.5194/acp-14-9317-2014, 2014.(2) Niinemets, Ü. and Monson, R. K. (Eds.): Biology, Controls and Models of Tree Volatile Organic Compound Emissions, Springer Netherlands, Dordrecht, https://doi.org/10.1007/978-94-007-6606-8, 2013.(3) Seinfeld, John H. and Pandis, Spyros N.: Atmospheric Chemistry and Physics: From Air Pollution to Climate Change, 3rd Edition., Wiley, 1152 pp., 2016.(4) Smith, N. R., et al.: Viscosity and liquid–liquid phase separation in healthy and stressed plant SOA, Environ. Sci. Atmospheres, 1, 140–153, https://doi.org/10.1039/D0EA00020E, 2021.(5) Bose, A. K., et al.: Memory of environmental conditions across generations affects the acclimation potential of scots pine, Plant Cell Environ., 43, 1288–1299, https://doi.org/10.1111/pce.13729, 2020.Funding: Swiss National Science Foundation, Project Numbers 189109, 199317, and, 194390.

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.

Il monitoraggio e lo studio dei boschi al Sud delle Alpi ci hanno consentito di approfondire le conoscenze sugli effetti di cambiamenti climatici e inquinamento atmosferico: frequenza e intensità di eventi siccitosi sono au- mentate; la qualità dell’aria negli ultimi 30 anni è sensibilmente migliorata in seguito all’adozione di misure per la riduzione di emissioni, ma, in certe zone, deposizioni azotate e ozono destano ancora preoccupazione.

Crown condition is considered as one of the most important indicators of a tree’s vitality. As part of the monitoring program Long-term Forest Ecosystem Research, the defoliation of tree crowns in Swiss forests has been monitored on an annual time scale since 1985. This long-term data set makes it possible to track the progress of defoliation until the trees die and to take into account a variety of stress factors that may have played a role in this process.In Swiss forests, the average defoliation of trees and tree mortality has increased in the past decades. However, this only occurred in areas at lower altitude, where climate change has particularly intensified the atmospheric water demand. The importance of water stress as a driver for this development is also confirmed by some of the highest annual increases of defoliation that directly followed exceptionally dry and hot summers.The probability that individual trees die within a few years starts to increase when the crown defoliation exceeds about 30%. Around 75-85%, most trees seem to reach a point of no return, from which they cannot recover, and which leads to death within a few years, even if no further stress occurs. In the needles of such strongly defoliated Scots pines (Pinus sylvestris L.), we found elevated levels of many stress-related metabolites (particularly osmoprotectants, defense compounds and antioxidants), whereas the levels of these metabolites were homeostatic in the needles of trees in lower defoliation classes. In contrast to the needles, these metabolites were reduced in fine roots of the strongly defoliated trees, suggesting that mainly belowground carbon starvation may impair key functions for tree survival, consequently leading to early death.

As a consequence of ongoing climate change, trees are increasingly exposed to atmospheric and soil drought. The project “ VPDrought - a novel approach to disentangle atmospheric and soil drought ” investigates how trees respond to distinct and combined changes in below- and above-ground conditions by manipulating precipitation and vapor pressure deficit (VPD) in a drought-prone natural pine ecosystem in the Swiss Rhone valley. In particular, the effects of changing VPD on mature trees have not been well studied yet, and the VPDrought project, extending a 20-year long-term irrigation project, provides worldwide unique conditions for integrated studies from the cell to the ecosystem level on an eLTER research platform. The manipulation of the environmental conditions began in late spring 2024 and will continue until 2028. We will demonstrate the effectiveness of our approach and present initial results on the short- and long-term responses of mature trees to altered soil- and atmospheric drought conditions, focusing on the temporal dynamics of key indicators for tree functioning, such as stem sap-flow, tree water deficit, stem shrinkage and expansion, and photosynthetic activity. To the extent possible at this early stage of the project, we will explore thresholds and tipping points under extreme conditions and discuss potential implications for forest health and tree mortality under future climate conditions. Our presentation highlights novel opportunities for studying tree responses to changing water availability and atmospheric demand. Thanks to comprehensive and often continuous high-temporal-resolution observations, this project allows us to investigate tree functioning in ways that have not been possible before, paving the way for new insights into forest ecology under climate change.