The summer of 2022 was characterized by compound soil and atmospheric drought (CSAD; low soil water content, SWC, and high vapor pressure deficit, VPD) in Central Europe, threatening forest carbon sequestration and water balance, yet linked ecosystem- and tree-level hydraulic responses remain poorly resolved. We assessed the response and recovery of water relations of a beech-dominated montane mixed deciduous forest in Switzerland (CH-Lae) to the 2022 CSAD through a series of ecosystem-scale (evapotranspiration, ET; canopy conductance, Gs) as well as tree-scale measurements, including leaf-level (leaf water potential, LWP; stomatal conductance, gs), stem-level (nighttime stem rehydration, NSR) and root-level (root water uptake depth, RWU depth) variables. During peak CSAD, ET and Gs declined by up to 50%, driven primarily by low SWC. Midday and predawn LWP declined significantly. Beech and maple accessed deeper water (> 70 cm), maintaining NSR and gs during CSAD and fully recovering thereafter; spruce and fir relied on shallow water (< 30 cm), showing stronger NSR reductions but still full recovery post-CSAD. Our findings demonstrate that species-specific hydraulic traits drive forest-scale water dynamics under CSAD conditions. Integrating these insights into forest adaptation strategies will be critical for sustaining forests under intensifying compound droughts in the future.

Abstract. Agriculture is the largest anthropogenic source of nitrous oxide (N2O), primarily due to nitrogen (N) fertilization. Understanding how the influence of key drivers and the relative contribution of source processes change throughout the cropping season is crucial for developing effective strategies to mitigate N2O emissions. In this study, we combined high-resolution eddy covariance flux measurements and stable isotope analyses over one winter wheat cropping season and the subsequent summer cover crop season. Two phases, crop establishment and early spring, were identified as critical periods for N2O emissions, characterized by a mismatch between N supply and plant demand, resulting in surplus soil mineral N and elevated N2O fluxes under favorable environmental conditions. Gross primary productivity (GPP), used as a proxy for crop N uptake, suppressed N2O emissions, especially under high soil moisture, highlighting the importance of active vegetation in mitigating emissions. Source partitioning, based on stable isotopes, revealed denitrification as the dominant process of N2O production, driven by poor soil drainage and high soil moisture. Over the nine-month winter wheat season, the Tier 1 N2O emission factor was 1.8 %, with cumulative emissions of 5.5 kg N2O-N ha−1, offsetting 70 % of the net CO2 uptake. Our findings emphasize the need to better synchronize N supply with crop demand and to adopt agronomic practices that promote rapid crop establishment to mitigate N2O emissions in cropping systems.

Grasslands serve a unique role in the global carbon (C) cycle and cover about 30 % of the European and about 70 % of the Swiss agricultural area. Carbon dioxide (CO2) fluxes of managed grasslands are substantially influenced by land management practices and meteorological conditions, but the temporal development of drivers and their effects are still uncertain. We used 20 years (2005-2024) of eddy-covariance (EC) fluxes, meteorological data, and detailed management information collected from an intensively managed grassland site (Chamau) in Switzerland, and employed machine learning approaches, i.e., eXtreme Gradient Boosting (XGBoost) models in combination with SHapley Additive exPlanations (SHAP) analyses, to identify drivers and their temporal contributions over two decades. Our study aimed to (1) identify intra- and inter-annual variations in grassland CO2 fluxes, (2) assess magnitude and drivers of gross primary production (GPP) and ecosystem respiration (Reco) during regrowth periods (i.e., the period after mowing, grazing, or reseeding events until the next event), and (3) quantify driver contributions to GPP and Reco over time, with focus on management and extreme events. Our results showed pronounced intra- and inter-annual variations in CO2 fluxes, driven by both management activities as well as meteorological conditions. Despite significant increases in temperature and decreases in soil water content (SWC) during the two decades, GPP and Reco during regrowth periods remained stable, and no significant trend over time was detected, suggesting adapted, climate-smart decision making of the farmer. The most important drivers of GPP in the long-term were light, management, and temperature, while Reco was mainly driven by temperature, GPP, and management. However, during extreme drought periods in the peak growing seasons (June, July, August), SWC increased in importance and limited GPP. In contrast, the impact of nitrogen (N) fertilization was more differentiated, either acting in parallel with SWC, suggesting low N availability during drought periods, or increasing GPP in years after sward renewal despite low SWC. Overall, our study provided novel insights into relevant drivers of grassland CO2 fluxes and their complex temporal contributions in the short- and long-term. Our results suggest that even small climate-smart management adaptations could be promising solutions for stabilizing important grassland processes, such as grassland regrowth, under current and future climate.

The lack of energy balance closure in Eddy-Covariance (EC) measurements is a well-known, still unresolved challenge in micrometeorology, with energy balance closure (EBC) rates typically ranging between 60% and 80%. While numerous hypotheses have been proposed to explain this imbalance, the relative contributions of neglected energy storage terms, data quality and flux processing options remain insufficiently disentangled. Using standardized ICOS and NEON datasets, we show that a significant portion of the observed energy imbalance can be attributed to overlooked or inconsistently handled energy components and turbulent flux quality control. Using data drawn from 84 sites, we show that comprehensive energy accounting-including soil heat flux, storage terms (soil, air, biomass), photosynthetic energy demand, and strict quality filtering of turbulent fluxes-improved EBC by 16% on average, with site-specific gains up to 40%. However, we also identify a persistent residual imbalance that is unlikely to be resolved through methodological refinements or additional measurements alone, pointing to fundamental physical processes that are not accounted for in the standard measurement and processing. We argue that this unresolved imbalance should be explicitly acknowledged and bounded, rather than implicitly absorbed into correction schemes, and we outline practical guidance for diagnosing and interpreting EBC in standardized flux networks. This perspective evaluates methodological advances and residual uncertainties, providing an actionable framework for the appropriate use of EC energy fluxes in carbon, water, and climate research.

Abstract. Agroecosystems regulate carbon, water, and nitrogen cycles, yet robust modeling of water and greenhouse gas (GHG) fluxes remains limited by incomplete or inaccessible information on field management practices. Although high-resolution remote sensing (RS) observations can detect management events such as mowing or harvest, their use for representing management intensity and associated impacts on ecosystem flux dynamics remains limited in existing models. Here, we developed an RS-assisted modeling framework to estimate daily latent heat flux (LE), net ecosystem CO2 exchange (NEE), nitrous oxide (N2O), and methane (CH4) fluxes across six Swiss FluxNet sites (two croplands and four grasslands) between 2016 and 2025. Sentinel-2 time series were used to derive leaf area index and RS-based field management indices (RS-FMIs), detecting mowing events, quantifying defoliation intensity, and identifying crop rotation and bare soil periods. These indicators were combined with meteorological drivers to train XGBoost models for each ecosystem type and target variable separately, and driver contributions were evaluated using SHapley Additive exPlanations (SHAP) analysis. The RS-FMIs effectively captured in situ recorded management events and enabled improved reconstruction of daily flux variability. Model performances were strong for LE (R2 ≈ 0.89–0.90) and NEE (R2 ≈ 0.59–0.71), whereas N2O and CH4 fluxes were reproduced with moderate accuracy (R2 ≈ 0.37–0.55). Models using RS-FMIs performed similarly to those using well-compiled in situ management records, supporting the ability of RS-derived vegetation and management indicators to represent management effects. LE variability was primarily energy-driven and dominated by meteorological conditions, whereas vegetation dynamics and RS-FMIs played stronger roles in shaping NEE, N2O, and CH4 variability. These results demonstrate that RS-FMIs offer new opportunities to reconstruct management information and improve the representation of management effects in agroecosystem flux modeling.

As climate change leads to more frequent droughts, understanding forest ecosystem health becomes increasingly critical. Monitoring forest canopy water content provides valuable insights into their resilience to these stressors. Space-based optical vegetation indices like the normalized difference water index (NDWI) are often used to monitor the water content over more extended time periods in large areas. However, the top-of-canopy view of satellites limits the sensitivity as they neglect the lower canopy and understory vegetation. This study explores the seasonal correspondence between the NDWI retrieved from Sentinel-2 satellite data and in situ measured vegetation optical depth (VOD). We use a unique time series of concurrent and complementary measurements acquired in two contrasting forest ecosystems (i.e., an evergreen coniferous and a deciduous broadleaf forest) spanning four seasons. VOD is calculated from global navigation satellite system (GNSS) data measured with one receiver antenna above and one below the canopy, assessing the full vertical extent of the forest canopy. In addition, we used the Sentinel-2 derived enhanced vegetation index (EVI), eddy flux based evapotranspiration (ET) estimates, and measurements of soil moisture, air temperature, precipitation, and shortwave incoming radiation to facilitate our interpretation. Our results showed a large seasonal variation in VOD and NDWI in the deciduous forest, a pattern that coincided with a large variation in biomass, as expected and indicated by the EVI. We saw indications that a short-term summer drought in 2022 affected VOD and ET but not the NDWI in the deciduous forest. In contrast, in the evergreen forest, we found a pronounced seasonality of canopy water content only for ET, while VOD and NDWI followed different trajectories. We conclude that the satellite-based NDWI tended to saturate at higher levels of canopy water content. VOD showed a sensitivity to changing canopy water content, as indicated by ET and soil moisture dynamics, as well as to changing canopy biomass, as suggested by varying EVI. These initial exploratory insights into the sensitivity of VOD could stimulate discussions within the community and potentially help optimize the sampling design of future VOD networks.

Abstract. Agriculture is the dominant source of anthropogenic nitrous oxide (N2O), a potent greenhouse gas with a high global warming potential. In Switzerland, substantial changes in fertiliser use alongside climatic conditions have occurred over the past four decades, yet the relative contributions of management practices versus environmental change to long-term N2O emission trends remain incompletely understood. Here, we applied the biogeochemical model DayCent at 1 km resolution across Switzerland, integrating spatially explicit datasets on climate, soil properties and agricultural management, to quantify N2O emissions for the period 1981–2020 from croplands and grasslands and attribute emission changes to management versus climate drivers. Simulated national N2O emissions from agricultural soils declined by roughly 5 % from 4.0 to 3.8 kt N yr⁻¹ between the 1980s and 2010s, primarily due to a 25 % reduction in N fertiliser use. Our attribution simulations suggested that under real climate conditions, such a decrease in fertiliser N inputs (–25 %) over the period studied lowered emissions by 15.2 % in croplands and by 12.0 % in grasslands (including permanent meadows and pastures, and high alpine summer pastures). However, compared to a control scenario (in the absence of climate change), rising temperatures over the past 40 years offset these gains, increasing emissions by 5.7 % in croplands and 13.6 % in grasslands. These results show that warming-induced N2O emissions partially negate mitigation from improved fertiliser management, highlighting the need for integrated agricultural N2O mitigation strategies that are resilient to future warming.

Our understanding of forests as methane (CH4) and nitrous oxide (N2O) sinks or sources remains limited, contributing to large uncertainties in terrestrial greenhouse gas (GHG) budgets. We investigated the CH4 and N2O exchange in a subalpine spruce forest in Davos (Switzerland) at multiple scales over seven years. Measurements included forest-floor fluxes with automatic chambers, GHG concentrations along a vertical canopy profile, and eddy covariance (EC) fluxes below and above the canopy. The forest floor was a small net CH4 sink (-0.5 + 0.05 g CH4-C m-2 yr-1; mean + standard deviation), while the forest was a small net CH4 source (0.54+0.22 g CH4-C m-2 yr-1). CH4 concentrations near the forest floor were low, increased with height, and stayed relatively constant within and above the forest canopy (10 to 35 m). While supporting the observed forest-floor sink, the profiles could not explain the discrepancy to the ecosystem CH4 budget. Forest-floor CH4 fluxes measured by chambers and below-canopy EC showed comparable magnitudes and seasonal dynamics, driven by snow depth, soil temperature, soil moisture, and below-canopy photosynthetic photon flux density. These findings suggest a coupling between above-ground CO2 assimilation and below-ground CH4 dynamics, possibly mediated by plant-soil interactions. Above-canopy N2O fluxes were negligible, with annual net ecosystem N2O budgets close to zero (-0.012 to 0.035 g N2O-N m-2 yr-1). Considering the global warming potentials of CO2, CH4 and N2O over a 100-year period, the subalpine spruce forest remained a net GHG sink, though CH4 and N2O emissions offset its sink strength by 3-12%.

Grasslands are worldwide spread ecosystems involved in the provision of multiple functional services, including biomass production and carbon storage. However, the increasingly adverse climate and non-optimised farm management are threatening these ecosystems. In this study, the original semi-mechanistic remotely senseddriven VISTOCK model, which simulates grass growth as limited by thermal and water stress, was modified and integrated with the RothC model to simulate the ecosystem fluxes. The new model (GRASSVISTOCK) showed satisfactory performance in simulating above-ground biomass (AGB) in dry matter (d.m.) and fractional transpirable soil water (FTSW) along Alps (AGB, RMSE = 85.39 g d.m. m- 2; FTSW, RMSE = 0.21) and Mediterranean (AGB, RMSE = 136.84 g d.m. m- 2; FTSW, RMSE = 0.13) grasslands. Also, GRASSVISTOCK was able to simulate the net ecosystem exchange (NEE - RMSE = 0.03 Mg C ha- 1), the gross primary production (RMSE = 0.04 Mg C ha- 1), the ecosystem respiration (RMSE = 0.04 Mg C ha- 1) and the evapotranspiration (RMSE = 1.44 mm), where these observations were available (Alps). The model was applied under present and two climate datasets characterised by temperature increase and precipitation decrease (+2 degrees C temperature, -10 % precipitation) and reference or enriched CO2 concentration (394 vs. 540.5 ppm) scenarios. The results showed that, while changes in temperature and precipitation alone had a negative impact by increasing NEE (+0.69 Mg C ha- 1) and decreasing total biomass (-0.20 Mg d.m. ha- 1) in the reference CO2 scenario, the enriched atmospheric CO2 concentration partially smoothed the NEE trend (+0.27 Mg C ha- 1) and increased total biomass (+0.60 Mg d.m. ha- 1) compared to the present period. It is concluded that the GRASSVISTOCK model represents a first step towards an integrated tool for estimating the performance of the agro-pastoral systems in terms of biomass production, water and carbon fluxes, in the face of ongoing climate change.

With ongoing climate change, effective science communication has become increasingly important. Anthropogenic climate change, driven by excessive greenhouse gas emissions ‒ primarily CO₂ ‒ requires innovative solutions for mitigation. Among those, nature-based solutions have gained significant attention to offset some of the anthropogenic CO₂ emissions. One of the best available methods to study land-atmosphere CO₂ exchange, i.e., CO 2 fluxes, is the eddy covariance (EC) technique, which results in continuous long-term time series of half-hourly CO 2 fluxes. While such data are extensively used in scientific research, for instance to evaluate the impacts of climate change and land management on ecosystems, effective communication of these findings to the general public remains a challenge. Global and regional EC networks, such as FLUXNET and ICOS, hold a great potential to engage with lay persons to increase public understanding of climate change effects on ecosystems as well as of feedbacks ecosystems have on the atmosphere. Within the Swiss FluxNet, the Swiss national EC network, CO₂ fluxes have been measured across different land use types ‒ grasslands, forests, and croplands for many years and decades. Since measurements began at one site in 1997, five permanent measuring sites have been added since then. Now, the Swiss FluxNet has collected a total of 129 years of CO₂ flux data from these six long-term sites, and it continues to grow steadily. Such a research database also faces the challenges of an effective communication to the public. Therefore, we initiated an interdisciplinary collaboration between science and visual art. The goal was to create a visually engaging and meaningful representation of the flux data, using accessible and intuitive colours and an attractive design, thus simplifying scientific data without compromising content. The collaboration resulted in a figure that showcases annual CO₂ budgets across multiple sites with a uniform colour scale, highlighting year-to-year differences in ecosystem performance as well as typical characteristics of the respective ecosystem (Fig. 1). While the scientists provide flux data and information about the sites, the artist engages with the historical, artistic, and cultural significance of colours, and both partners address accessibility for colour-blind individuals. This approach involves a critical examination of how natural scenes are represented in artworks across diverse cultural contexts, how their corresponding colour scales look like and can be used, with insights from this research applied to the field of data visualization. First outputs of the project were presented at the Sustainable University Day at the University of Zurich and ETH Zurich in November 2024. The presented work exhibited the background research and the rationale behind the choice of colours (Fig. 2), and the scientific concepts of ecosystem CO₂ exchange as well as the final visual representation of the CO 2 fluxes (Fig. 1). The public, including individuals from diverse fields such as marketing, waste management, and sustainability, responded very positively, acknowledging the value of the collaboration and the clarity of the communication. This experience stresses the importance of interdisciplinary collaboration between science and art, showcasing how we can bridge the gap between complex scientific data and public understanding.

Methane (CH4) and nitrous oxide (N2O) substantially contribute to global greenhouse gas (GHG) emissions together with carbon dioxide (CO2). To understand their impact on future climate change, prioritizing the study of CH4 and N2O fluxes becomes critical. Forest ecosystems, primarily investigated for CO2 exchange, are less explored concerning their exchange of CH4 and N2O. Forests are known to be sinks for CH4, while their role in N2O fluxes varies, acting as either sources or sinks. However, comprehensive studies that concurrently examine CH4 and N2O fluxes in forests, particularly over extended periods and at high elevation, remain scarce. At high altitudes, measuring GHG fluxes with chambers during snowy periods is challenging, leading to a lack of winter flux data which are crucial for understanding flux dynamics related to freeze-thaw cycles and snow patterns. This study addresses this gap by investigating long-term CH4 and N2O fluxes in a subalpine Norway spruce forest (Davos, CH-Dav, ICOS Class 1 Ecosystem station, Switzerland), encompassing both soil and canopy interactions with the atmosphere.Over five years (2017, 2020-2023 for CH4; 2017, 2020 for N2O), we employed automatic chambers to measure forest-floor fluxes, complemented by below-canopy eddy covariance CH4 flux measurements starting from May 2023, as well as static chamber measurements in 2023. Our research objectives were to 1) characterize the magnitude and seasonal dynamics of CH4 and N2O forest-floor fluxes, and 2) compare CH4 fluxes using chamber and eddy covariance techniques to better understand the interaction of soil and vegetation with the atmosphere.We hypothesized that the forest floor primarily acts as a net sink for CH4, with soil temperature and snow dynamics being important drivers due to their impact on microbial activity and diffusion rates between soil and atmosphere. Given the low nitrogen availability at the study site, we anticipated very low N2O emissions. Additionally, we hypothesized that comparing CH4 fluxes from chambers and eddy covariance would reveal small differences in their magnitudes, attributable to the distinct measurement scales and scopes of these two techniques. Our results confirmed the forest floor as a consistent CH4 sink, exhibiting substantial short-term fluctuations driven predominantly by air temperature and snow cover. N2O fluxes were negligible over the two-year observation period. Our study contributes to a deeper understanding of how environmental drivers and seasonal dynamics influence CH4 and N2O fluxes in high-elevation forests.

Denitrification, the reduction of nitrogen oxides (NO3-and NO2-) to NO, N2O and, ultimately, to N2 gas in soils, is classified as a microbiologically ‘broad process’ which can be conducted by a wide array of microbes belonging to remote phylogenetic groups. Further, understanding how environmental and management factors drive denitrification is challenging because they are scale-dependent, with large scale drivers affecting denitrification fluxes both directly and through drivers working at detailed small scales. Despite of this, we hypothesized that denitrification processes, although highly complexes due to the multiple processes and environmental conditions involved, they could present a functional convergence at the microbial community level explained by a short list of microbial groups or functions. On the other hand, different methodological approaches to assess soil microbial diversity are currently used; among them are multiple substrate-induced respiration by MicroResp™, enzymes activities and functional genes abundance and structure by GeoChip 5S microarray. We applied all those methods to study functional diversity in 5 different soils from 5 countries: Finland, Belgium; France, Switzerland and Italy. Studied soils have a wide range of soil pH, organic Carbon and Nitrogen content, and texture. Soils were sampled at Hot Moment and Low flux emission of N2O. The main objective of this study was to explore possible convergences in terms of functional microbial diversity in contrasting N2O emission events (low emission versus hot moments). Result showed that MicroResp™ , enzyme activities and GeoChip 5S microarray were reliable ecological indicator to evaluate soil microbial functionality diversity. Results stressed the importance to study soil microbiome at different magnitude of N2O emission with the aim to gain a deeper knowledge of nitrifiers community. Reciprocal relationship of those methodologies, soil proprieties and magnitude of flux emission of N2O are discussed.

Grasslands serve a unique role in the global carbon (C) cycle and cover about 30% of the European and about 70% of the Swiss area used for agriculture. The CO2 fluxes of managed grasslands are substantially influenced by climate conditions and land management practices. The eddy covariance (EC) technique is the only approach to directly measure the net ecosystem exchange (NEE) of CO2. NEE represents the balance between two large ecosystem processes: gross primary production (GPP; amount of CO2 fixation through photosynthesis), and ecosystem respiration (Reco; amount of CO2 released via plant and soil respiration). Our study aimed to (1) investigate intra- and inter-annual changes in grassland NEE as well as regrowth after mowing/grazing events, (2) understand key drivers of GPP regrowth rates, and (3) examine grassland responses to sward renewal. In our study, we measured EC fluxes and meteorological variables at the temperate grassland site Chamau (CH-Cha as part of FLUXNET) in Switzerland. This grassland is intensively managed, with 4-6 mowing/grazing events per year, accompanied by organic fertilization (on average 271 kg N ha-1 yr-1) and sward renewal every 7-10 years. We applied machine learning approaches such as Extreme Gradient Boosting (XGBoost) and Shapley Additive exPlenations (SHAP) analysis to address our aims, using 20 years (2005-2024) of EC flux, meteorological, and detailed management data. Over the 20 years, a pronounced intra-seasonal course of NEE was found due to mowing and grazing, with the maximum CO2 uptake in early spring (March-April) and the largest CO2 loss in early winter (December-January). During the main growing season (April-September), the average GPP regrowth rate was 10 g C m-2 day-1. We did not find a significant trend for GPP regrowth rates over the 20 years. The most important drivers of GPP regrowth rates were air temperature and light, while water-related drivers dominated regrowth rates during summer droughts (e.g., 2015 and 2018). Nitrogen fertilization did not play a key role in GPP regrowth rates. Moreover, sward renewal years resulted in either very large CO2 losses (in 2012) or in reduced CO2 uptake rates (in 2021), most likely caused by the different timing of the renewal, i.e., February vs. August, respectively. Thus, our study provides novel insights into climate-smart management options and helps to develop mitigation strategies for current and future climate risks.

Nitrous oxide (N2O) is a potent greenhouse gas and a significant contributor to global warming and ozone layer depletion. It is primarily emitted from soils through microbial processes such as nitrification and denitrification and shows spatial and temporal variations driven by environmental factors such as the availability of nitrogen (e.g., in the form of fertilizers), organic carbon, soil moisture, temperature and oxygen levels. However, estimates on the relative contribution of different N2O producing pathways are frequently uncertain and knowledge on how environmental factors influence N2O emissions dynamics is still limited. Therefore, closing these knowledge gaps is crucial for improving mitigation strategies.This study aims to analyze the patterns of N₂O emissions across diverse forest and agricultural soils, taking geographic variations into account, and to determine the relative contributions of the primary N2O producing and consuming pathways specific to each soil type.Batch experiments were conducted using four agricultural soils and four forest soils from sites of the ICOS (https://www.icos-cp.eu) and FLUXNET (https://fluxnet.org/about/) networks. Agricultural soils were obtained in France, Belgium, Italy and Switzerland, while forest soils were obtained in Finland, Sweden, Belgium and Italy. These soils exhibited a range of intrinsic characteristics, such as texture, organic matter content and type, and nitrogen sources. The incubations took place in complete darkness at a constant temperature of 22 ºC for approximately 30 hours after rewetting dry soil. Each soil type was tested with five replicates across five time points (i.e., 25 reactors for soil type). For each reactor we measured the production of N2O and its isotopic composition including the δ15N-N2Obulk, δ18O-N2Obulk, and site preference δ15N-N2OSP (i.e., the intramolecular distribution of N isotopes, since the N2O molecule has an asymmetric linear structure [N-N-O]). Additionally, the isotopic compositions of nitrate and ammonium from soil KCl extracts are being analyzed (δ15N-NO3-, δ18O-NO3-, δ15N-NH4+) and microbiological characterization is also being performed.Preliminary results revealed significantly higher N2O production in agricultural soils compared to forest soils during the 30-hour incubation period, with rates reaching up to 130 μg N-N2O/kg/h in agricultural soils and only 0.3 μg N-N2O/kg/h in forest soils. Notable differences were also observed among the four tested soils within each category (agricultural or forest). These differences might be mainly attributed to differences in the nitrogen and organic carbon content as well as the texture. The isotopic analysis of N2O suggests that denitrification is the primary process driving N₂O emissions in the studied soils, with nitrification also contributing to varying extents depending on the soil type.Ongoing isotopic analyses of nitrate and ammonium in soil KCl extracts alongside microbial characterization, will provide deeper insights into the dominant processes driving N2O emissions in each soil type and the key environmental factors influencing them.

The Swiss FluxNet provides ecosystem scale flux data for the major land use types in Switzerland. While the current station network includes long-term eddy covariance flux measurements from two forest sites (mixed deciduous forest Lägeren and evergreen spruce forest Davos), three permanent grassland sites (Chamau, Früebüel and Alp Weissenstein) as well as three cropland sites (Oensingen, Tänikon and Forel) complement the network. In addition, the measurements cover an altitude gradient ranging from 393 to 1978 m.a.s.l. Carbon dioxide (CO 2 ) and water vapor (H 2 O) fluxes are measured continuously at all sites, while nitrous oxide (N 2 O) and methane (CH 4 ) fluxes are also quantified at some sites. Currently, 123 site-years of data are openly shared with FLUXNET. Ancillary meteorological and soil microclimate data are collected continuously as well; plant growth is routinely monitored at all agricultural sites, i.e., grasslands and croplands. Together with the management data, such continuous measurements allow integrated multi-year (Feigenwinter et al. 2023b) and multi-site (Zeeman et al. 2010) comparisons, identification of drivers for greenhouse gas (GHG) fluxes (Maier et al. 2022, Feigenwinter et al. 2023a), quantification of C sequestration (Emmel et al. 2018), as well as assessments of management practices towards sustainable agriculture (Fuchs et al. 2018). Here, we will present long-term CO 2 fluxes (since 2004) as well as CH 4 and N 2 O fluxes measured at the six agricultural Swiss FluxNet sites, i.e., three permanent grasslands and three croplands with their typical Swiss crop rotation. Moreover, the contribution of abiotic and biotic drivers to intra- and interseasonal variations in GHG fluxes will be discussed, potential trade-offs among climate mitigation goals identified, and the importance of management information emphasized. We encourage other research teams to use the open-access dataset, growing annually, and seek collaboration in integrated flux measurements worldwide.

Evapotranspiration (ET) from forested ecosystems is a major component of the water cycle, influencing soil moisture, groundwater recharge, and streamflow. ET also modulates local and regional climate through latent heat exchange, affecting temperature and humidity in forest ecosystems. Because ET is tightly linked to photosynthesis, it also indicates forest stress from drought conditions, making it a key metric for assessing forest ecosystem health. However, long-term measurements of ET are rare, thus the effects of a warming climate on forest ET fluxes remain poorly understood. ET fluxes can be limited by a lack of water availability in the subsurface, forcing trees to close stomata and reduce transpiration. In periods of high atmospheric demand, vapor pressure deficit (VPD) can also induce stomatal closure. Furthermore, limited energy availability, i.e., reduced net radiation on cloudy days, constrains ET by limiting latent heat flux. Thus, more detailed analyses of the dominant drivers of ET fluxes based on long-term datasets may help to identify under which environmental conditions ET is limited in forest ecosystems. Here we present an comprehensive analysis of long-term evapotranspiration data from eddy covariance measurements from two different forest sites in Switzerland: a subalpine evergreen coniferous forest (CH-Dav) and a montane mixed deciduous forest (CH-Lae), both part of Swiss FluxNet (www.swissfluxnet.ethz.ch). H 2 O vapor fluxes have been measured using eddy covariance (EC) since 1997 and 2004 at the Davos and Lägeren sites, respectively. The Davos site is located in the eastern Swiss Alps at an altitude of 1639 m asl., has an average annual precipitation of 876 mm and a mean annual temperature of 4.3 °C. The EC system is installed at 35 m, above the canopy of Norway spruce ( Picea abies (L.) Karst) trees of a mean age of approximately 100 years and an average tree height of 18 m. The Lägeren site is located in the eastern part of the Jura Mountains in Switzerland at an altitude of 682 m asl., has an average annual precipitation of 831 mm and a mean annual temperature of 8.8 °C. The EC system is installed at 47 m, above the canopy of a mixed forest, dominated by European beech ( Fagus sylvatica L.; 40 % cover), ash ( Fraxinus excelsior L.; 19 % cover) and sycamore maple ( Acer pseudoplatanus L.; 13 % cover). In addition, at both sites, climate variables (i.e., precipitation, temperature, vapor pressure deficit, solar radiation) and soil water content at different depths (i.e., 5, 10, 25 and 50 cm) have been measured continuously at 30-min resolution. Using this long-term data collection, we analyzed changes in annual and seasonal ET fluxes and assessed the major drivers of ET fluxes across the two forest sites. analyzed changes in annual and seasonal ET fluxes and assessed the major drivers of ET fluxes across the two forest sites. To analyze the main drivers of ET fluxes, we used the SHapley Additive exPlanations (SHAP) framework, allowing to estimate the contribution from each potential driver variable to the response variable (i.e., daily ET fluxes). Analyzing long-term ET fluxes at Davos and Lägeren, we found considerable differences between the two forest sites. The subalpine Davos site received slightly higher precipitation and had a lower temperature compared to the Lägeren site, thus soil moisture and vapor pressure deficit were less important drivers of ET at Davos compared to Lägeren. Instead, ET limitations in Davos were mainly related to lower net radiation. At the montane Lägeren site, compound dry events combining soil water deficits and higher VPD triggered decreasing forest ecosystem ET fluxes during the growing season. Analyses of especially dry years (i.e., 2003 – only Davos, 2015, 2018 and 2022) indicated that the spruce dominated Davos site showed higher ET compared to average years, due to favorable growing conditions at this typically energy-limited subalpine site. However, we found that even at the Davos site, extended dry periods (i.e., as observed during the year 2018) lead to higher atmospheric demand and thus lower ET. Overall, the Lägeren site was more vulnerable to dry periods, leading to depletion of soil moisture storage and an increase of VPD, resulting in a significant reduction of ET in the especially dry years. Overall, our results indicate that i ) forest ecosystems in low-elevation sites are already experiencing frequent periods of ET reduction due to water limitations in the soil and high atmospheric demand while ii ) high elevation forest ecosystems might become more vulnerable when the durations of high atmospheric demand are extending over longer time periods as projected in a future climate.

Biogeochemical processes within and across ecosystems are core to understand the functioning of terrestrial ecosystems, i.e., forests and agroecosystems, in particular under changing environmental conditions. Measurements are necessary at multiple scales, e.g., for forests at soil, forest floor, tree, canopy, and forest ecosystem scales, using methodology from many different disciplines. Data should be available in high temporal resolution, preferentially for long time periods, to quantify and understand short-term responses to environmental drivers and management, but also to detect and identify long-term responses to climate change. The SwissFluxNet is a network of six long-term research sites in Switzerland with ecosystem-scale eddy-covariance (EC) measurements of biosphere-atmosphere greenhouse gas (GHG) exchange (i.e., CO 2 , H 2 O vapor, CH 4 , N 2 O; Fig. 1). The Swiss FluxNet offers exactly these opportunities, namely long-term, high-temporal resolution GHG flux data and serves as a research platform for many other studies and research programs. It covers the major land-use types in Switzerland: forest (mixed deciduous: Lägeren, CH-Lae; evergreen: Davos, CH-Dav), grassland (Chamau, CH-Cha; Früebüel, CH-Fru; Alp Weissenstein, CH-Aws), and cropland (Oensingen, Ch-Oe2), and is complemented by project-based flux stations which run for 2-4 years (currently, two below-canopy stations in the two forest sites, one young forest plantation, and two cropland sites). Thus, including data of 2024, we provide 129 site-years of continuous GHG flux measurements to the scientific community (19-28 years per site, and continuously growing…), since all data are open access and have been downloaded from FLUXNET and ICOS over 35'250 times between November 2016 and December 2024. In the talk, we will focus on the two forest sites, Davos and Lägeren. At Davos, above-canopy EC flux measurements of CO 2 and H 2 O vapor started in 1997; measurements of above-canopy CH 4 and N 2 O fluxes were carried out between 2016 and 2023 and complemented by below-canopy flux measurements of CO 2 and H 2 O vapor (since 2021) as well as of CH 4 (since 2023). Data on forest floor CO 2 , N 2 O and CH 4 fluxes, measured automatically by chambers, tree phenology, sap flow and stem diameter changes are available for many years as well. Since 2019, Davos is an ICOS RI Class 1 Ecosystem station, where highest standards apply. At Lägeren, EC flux measurements of CO 2 and H 2 O vapor are available since 2004, complemented by below-canopy flux measurements of CO 2 and H 2 O vapor since 2014 as well as by phenology, soil respiration and tree ecophysiology measurements. At both sites, meteorological (above and within the canopy) as well as soil climate variables (soil profiles, 0 to 60/80 cm soil depth) are recorded continuously as well. Thus, measurements from multiple scales for long time periods allow studying short- and long-term responses to changing environments at both forest sites. We will provide selected highlights from almost 50 site-years of measurements, about the short-term responses of forests to weather extremes such as drought and heatwaves as well as about the long-term carbon sink behaviour of both forests and their vulnerability. Results based on machine learning approaches about the environmental and biological drivers of GHG fluxes at multiple scales will be presented along with their temporal contributions within and across years. Disentangling the role of climate vs. nitrogen (N) deposition for water-use efficiency of both tree species beech and spruce as well as linking tree to forest responses across scales will be discussed. Our experiences clearly demonstrate that using long-term, highly equipped EC sites as research platforms for additional research projects, nesting research programs within large-scale research infrastructure networks, and sharing data openly following FAIR principles, pays out, for one’s own curiosity, for career development of the next generation scientists, for policy advice and science at large!

Forest ecosystems are particularly threatened by global change components, i.e., more frequent extreme weather and climate events (particularly drought and heatwaves) and increasing (N) deposition, resulting in great uncertainties for the future of the essential ecological, economic and social benefits that humanity relies on from forests. Drought and heatwaves impair physiological mechanisms underpinning tree growth and forest productivity, and they may trigger tree mortality, thus constraining the forest carbon sink (Adams and ì 2017; Gazol and Camarero 2022; Hartmann et al. 2022; Hubau 2020). On the one hand, N deposition stimulates tree growth in nitrogen-limited forests, under steady increase in atmospheric CO 2 (Etzold 2020; Fernández-Martínez 2017 ; Flechard 2020; Wang et al. 2017). On the other hand, increasing atmospheric N input above the empirical nitrogen critical load (above which harmful effects can occurr in the ecosystem) (Braun et al. 2022) (*) could reduce the positive effect on tree growth and could cause forest dieback, through soil acidification and nutrient imbalances, but also by making trees more vulnerable to climate extremes (Dalton et al. 2024; Etzold 2020; Ferretti et al. 2015; Flechard 2020; Gharun et al. 2021; Thomas et al. 2009). Many questions remain: How do these global change components interact and affect forest carbon, water and N cycling? Which tree ecophysiological mechanisms are involved? Are those mechanisms synchronized (in terms of magnitude and temporal trends) at tree and ecosystem scales? Does N deposition affect tree and forest responses to climate extremes? The NEXTRES project aims at answering these questions by applying a multi-scale approach (from tree to ecosystem responses) to eleven forests along a climate and total N deposition gradient (from 3 to 42 kg ha -1 yr -1 ) across Europe, selected within established monitoring networks, namely ICOS and ICP Forests (Fig. 1). We will present preliminary results combining existing ecosystem CO 2 and water vapor fluxes with dendroecological information on growth as well as stable carbon isotope ratios to explore multidecadal changes in water-use efficiency, and to elucidate underpinning tree physiological mechanisms. Moreover, we will target years characterized by climate extremes to follow the intra-annual carbon isotope fingerprint within individual in tree rings to evaluate possible divergences: among tree species in their recovery strategies, and between tree and ecosystem responses. among tree species in their recovery strategies, and between tree and ecosystem responses. Finally, we will elucidate whether an excess of atmospheric N input can affect tree and forest responses to climate extremes.

Climate change affects carbon sequestration dynamics and phenology in forests, especially in alpine and subalpine regions. Here, long-term trends in climate, net ecosystem CO2 exchange (NEE), net carbon uptake period (CUPnet) and their drivers were investigated, using 26 years of flux measurements in a subalpine spruce forest (CH-Dav, Switzerland; 1997 to 2022). CUPnet length, start (SOS) and end of season (EOS) were extracted from smoothed daily NEE time series. We used machine learning to determine the importance of environmental drivers on daily NEE and CUPnet. Annual mean and maximum air temperatures (Tair) increased, while soil water content (SWC) decreased significantly between 1997 and 2022. Annual C sinks increased from 1997 to 2012, leveled off between 2012 and 2015, followed by a decline. Annual NEE was strongly related to CUPnet length, SOS, and EOS. No significant trends in CUPnet, SOS, or EOS were detected, most likely indicating ecophysiological acclimation, that is, physiological adjustments to changing environmental conditions over the past 26 years. We identified 48 days with significant negative trends in mean daily NEE over the 26 years, that is, stronger net C uptake or weaker net C loss, particularly in spring and autumn, but no significant positive trends. Daylength, incoming shortwave radiation (Rg), SWC, and minimum Tair were the main drivers of daily NEE. SOS was mainly driven by daylength and Tair, EOS by daylength and Rg. Thus, the spruce forest benefited from higher temperature between autumn and spring, with higher net C uptake during favorable conditions and reduced C loss when winter photosynthesis compensated respiration. However, high summer temperatures increasingly limited NEE, suggesting adverse effects for subalpine Picea abies forests in the future. Our study demonstrated that identifying driver contributions to NEE dynamics at daily time scales allows better understanding of the complexity of climate change impacts on forest C dynamics.