Orals: Thu, 7 May, 10:45–12:30 | Room 3.16/17
Trees are powerful mediators within ecosystem water and nutrient cycles. Through their roots, they take up these essential resources from the soil, distributing them in the system and upward into the canopy to maintain transpiration and photosynthesis.
However, understanding and predicting real-world dynamics remains challenging: tree species identity, species mixture, site and soil conditions may shape tree water and nutrient uptake fundamentally, particularly in mature forests. Moreover, in the face of climate change, access to resources in deeper, less drought-prone soil layers is crucial for buffering drought impacts and maintaining forest functioning. Studies targeting tree resource uptake in mature forests are still scarce; but they are emerging, with stable isotopes as a central tool.
We investigated root water uptake depth and subsoil water and nitrogen uptake in mature temperate forests of north-western Germany, using 2H, 18O and 15N as tracers. Native European beech, non-native Douglas fir, and native but drought-sensitive Norway spruce were studied, revealing tree species-specific uptake strategies and influences of species mixture. Furthermore, we found consistent site effects: on well-drained, sandy soils, the trees integrated more resources from deeper layers than on loamy soils. Notably, transit times from soil to canopy were slower for nitrogen than for water, highlighting the biotic and abiotic interactions that decouple nitrogen from water.
We conclude that species-specific traits in interaction with soil characteristics are crucial for understanding and predicting water and nutrient fluxes in forests. Our findings underscore the importance of belowground processes when assessing forest functioning and resilience.
How to cite: Hackmann, C. A., Mrak, K., Paligi, S. S., Magh, R.-K., Marshall, J. D., Mund, M., and Ammer, C.: Tracing water and nitrogen uptake in mature forests using stable isotopes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15061, https://doi.org/10.5194/egusphere-egu26-15061, 2026.
Ecohydrological studies aiming to understand patterns in root water uptake by trees based on plant and soil water isotope data are often confined to one or a few nearby locations. In this study, we took advantage of a recently established pan-European hydrogen (δ2H) and oxygen (δ18O) isotope dataset (10.16904/envidat.542) to assess root water uptake depth for beech and spruce trees across Europe. For a subset of sites, δ17O data were available as well.
Our analysis revealed consistent isotopic enrichment in xylem water of spruce trees compared to beech trees across all mixed-species sites (N=13), suggesting that spruce predominantly used shallower soil water regardless of environmental conditions. Additionally, we observed isotopic enrichment in stem xylem water from spring to summer at most beech and spruce sites (N=32), suggesting both species relied on isotopically enriched summer precipitation. Interestingly, for a subset of sites (N=8), there was an inverse pattern, with isotopic depletion in summer, implying shifts to deeper soil water sources or uptake of shallow soil water that was isotopically depleted in summer compared to spring conditions.
To further explore these findings, we will visually and statistically examine them using isotope data from the soil (10–90 cm depth). We will analyze the role of climate (using gridded data), alongside site-, soil-, and tree-specific metadata to better understand the factors influencing the variation in root water uptake at the continental scale. Additionally, we will explore the potential of oxygen-17 excess to provide further insights into root water uptake dynamics.
Lehmann et al., 2025. Soil and stem xylem water isotope data from two pan-European sampling campaigns. Earth System Science Data, 17, 6129–6147, https://doi.org/10.5194/essd-17-6129-2025
How to cite: Lehmann, M., Geris, J., Penna, D., Rothfuss, Y., van Meerveld, I., and Meusburger, K.: Exploring root water uptake of beech and spruce trees across Europe, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21902, https://doi.org/10.5194/egusphere-egu26-21902, 2026.
Shifts in tree water sources are important for understanding the spatiotemporal dynamics of ecosystem water fluxes. However, our understanding of tree water uptake remains limited, constraining reliable predictions of local and global hydrological processes under ongoing climate change. The isotopic composition of water (δ2H and δ18O) is a powerful tracer of the Earth’s water cycle, as isotopic differences among water reservoirs, together with mixing and fractionation processes, allow water movements to be traced across the hydraulic continuum.
This study aims to characterize the tree water sources of Scots pine (Pinus sylvestris L.) in a Mediterranean forest (Pyrenees, NE Spain) during the 2024 growing season. To do so, the isotopic composition of water in several ecohydrological compartments was measured. Precipitation, soil water pools at multiple depths (10, 20, 30, 40, and 60 cm), and xylem water from four individuals were sampled biweekly. Bulk soil water was extracted using cryogenic vacuum distillation, whereas xylem water was obtained using a flow-rotor centrifuge (cavitron). The cavitron enables access to mobile xylem water (e.g., sap) and is not affected by the well-known methodological artifacts associated with cryogenic extraction. Bayesian isotope mixing models were applied to quantify the relative contributions of distinct water pools to xylem water and their temporal evolution. Dynamics of total water uptake were estimated from transpiration data.
Our results show that Scots pine predominantly relied on shallow soil water (10 cm) during most of the growing season, with xylem water closely reflecting the isotopic signature of recent precipitation. A decoupling between the isotopic signature of precipitation and xylem water emerged as seasonal drying progressed. Under dry conditions, tree water uptake was low, and tree water sources shifted towards deeper soil layers (40-60 cm). Overall, these patterns indicate a strong coupling between rainfall inputs and tree water use during periods of high transpiration demand, suggesting that the contribution of deeper soil water reserves represent only a very small fraction of tree total water use during a growing season. These findings underscore the ecological importance of shallow soil water and recent precipitation in sustaining forest function and highlight the role of vegetation water use in regulating atmospheric water fluxes.
How to cite: Cara-Abad, P., Barbeta, A., Llorens, P., Latron, J., Castro-López, J. A., Fu, H., Gutiérrez, E., and Martínez-Sancho, E.: Spatiotemporal contribution of soil water sources to total tree water uptake in a Mediterranean Scots pine forest, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14267, https://doi.org/10.5194/egusphere-egu26-14267, 2026.
Differentiation in water use strategies is essential for the function and resilience of dryland ecosystems. Under prolonged drought, dominant shrubs self-organize into two spatial configurations: scattered and clumped. However, the mechanisms by which root–leaf trait coordination drives these divergent water use strategies remain poorly understood. In this study, the soil moisture, morphological and root traits, leaf-level physiological traits, and stable isotope (δ2H, δ18O, and δ13C) of scattered and clumped Vitex negundo were observed during the 2022–2024 growing seasons in the semi-arid Loess Plateau, to elucidate the water use strategies and physiological responses of self‑organized shrubs. Our findings indicate that scattered shrubs primarily utilized middle and deep soil water (69.4±7.8%), facilitated by isolated canopies that promote precipitation infiltration and recharge deeper soil layers. In contrast, clumped shrubs predominantly relied on shallow and middle soil water (82.0±6.5%), supported by their aggregated canopies and root systems. Scattered shrubs adopted a conservative strategy, exhibiting higher intrinsic water use efficiency (iWUE) and stable midday water potential during dry seasons, due to lower specific leaf area and moderate stomatal conductance. Conversely, clumped shrubs exhibited an opportunistic strategy, characterized by larger specific leaf area and higher stomatal conductance, enabling rapid photosynthetic accumulation and peak iWUE during rainy seasons. However, under drought, clumped shrubs accelerated the depletion of shallow soil water, leading to depressed midday water potential and constrained photosynthesis. These shrub types illustrate complementary mechanisms for drought adaptation: scattered shrubs enhance resilience, while clumped shrubs improve precipitation capture efficiency, collectively promoting the stability of dryland ecosystems.
How to cite: Wang, L., Gao, G., Ma, Y., and Rietkerk, M.: Self-organization shapes divergent water use strategies among co-occurring shrubs for drought resilience in drylands, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4361, https://doi.org/10.5194/egusphere-egu26-4361, 2026.
Up to date, isotopic techniques in ecohydrology have been rarely applied in viticulture, despite their potential to identify plant water uptake sources and resolve their temporal dynamics, largely because traditional approaches for sampling grapevine xylem sap are destructive and often impractical in productive agroecosystems such as commercial vineyards, particularly when a high number of replicates is required. Moreover, recent evidence indicates that cryogenic vacuum distillation (CVD), commonly used to extract xylem water, can induce a methodological bias, yielding water that is artificially depleted in δ²H relative to the original xylem water.
These limitations have stimulated interest in alternative, non-destructive approaches. Recent studies have shown that transpired water can be collected by enclosing grapevine branches in plastic bags and sampling condensed water. Nevertheless, it remains unclear whether the isotopic composition of transpired water can be reliably used to infer plant water sources when the true isotopic signature of xylem water is unknown.
Here, we tested whether transpired water condensation can be used to retrieve the isotopic composition of plant water sources. We conducted a controlled experiment on potted grapevine plants irrigated with water of known isotopic composition, under contrasting water availability and different atmospheric conditions.
Multiple plant water sampling techniques were applied to detect isotopic changes along the soil–plant–atmosphere hydraulic continuum and to evaluate the validity of using transpired water to infer plant water uptake sources. In particular, we employed a vacuum pump–based sap extraction method designed to retrieve flowing xylem sap water and expected to closely reflect source water isotopic composition. Xylem bulk water, leaf bulk water, and bulk soil water were extracted using CVD.
The isotopic composition of vacuum-extracted sap water and of CVD-extracted waters were compared with transpired water and the original source of water (irrigation). Vacuum-extracted sap water closely reflected the isotopic composition of source water. Interestingly, transpired water collected in plastic bags also showed potential to be used as a proxy to infer the source water; however, its interpretation is less straightforward, requiring many replicates and explicit consideration of atmospheric conditions.
Overall, our results provide a methodological framework for evaluating non-destructive approaches to trace plant water sources and contribute to a better understanding of isotopic fractionation processes along the soil–plant–atmosphere continuum, with implications extending beyond viticulture to ecohydrological studies in managed and natural ecosystems.
How to cite: Peschiutta, M., Lehmann, M. M., Bonet Caballol, G., Benettin, P., Penna, D., Masiol, M., Stenni, B., and Barbeta, A.: Rethinking isotope-based plant water uptake tracing in viticulture: alternative sampling approaches, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7698, https://doi.org/10.5194/egusphere-egu26-7698, 2026.
Common hydrogeological methods make use of natural gases as tracers to better understand the spatial and temporal evolution of groundwater flow, to constrain water residence time, and to reconstruct environmental conditions at recharge [1-3]. Noble gases can be used as complement of the stable water isotope tracers for understanding complex hydrological systems [4,5,6].
We adapted these methods to in-situ measurements of gases in tree xylem sap to better understand the plant-mediated water and gas flux between the hydrosphere, the biosphere, and the atmosphere.
Using a “miniRuedi” portable mass-spectrometer [7] and tailored semi-permeable membrane probes, the partial pressures of He, Ar, Kr, N2, O2, CO2, and CH4 were continuously monitored in-situ in the soil, the tree, and the atmosphere. Diurnal variations of CO2 and O2 were observed that reflected the tree physiological activities [8].
Since transpiration by plants is a major component of the hydrological cycle, such measurement techniques offer new opportunities to better understand plant water and CO₂ dynamics, within the soil-plant-atmosphere continuum.
[1] Kipfer et al. (2002), Reviews in Mineralogy and Geochemistry, 47, 615–700; [2] Brennwald et al. (2013), Advances in Isotope Geochemistry – The Noble Gases as Geochemical Tracers, 123-153; [3] Brennwald et al. (2022), Frontiers in Water, 4, 107-115; [4] Althaus et al. (2009), Journal of Hydrology, 370, 64-72. [5] Schilling et al. (2019), Reviews of Geophysics, 57, 146-182. [6] Xu et al. (2017). Hydrogeology Journal, 25(7), 2015–2029; [7] Brennwald et al. (2016), ES&T, 50, 13455-1346; [8] Marion et al. (2024), Tree Physiology, tpae062.
How to cite: Marion, C., Zweifel, R., Buchmann, N., Brennwald, M., and Kipfer, R.: In-situ measurements of dissolved gases in tree xylem sap as tracers for plant physiology, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22892, https://doi.org/10.5194/egusphere-egu26-22892, 2026.
As temperatures rise, heat waves, droughts, and intense rainfall are expected to become more common and severe, posing significant risks to forest ecosystems. More frequent droughts make forests progressively vulnerable, leading to increased tree mortality particularly among drought-sensitive species like European beech (Fagus sylvatica S.). Understanding the interactions between forests and the water cycle is crucial to predict how forest ecosystems will respond to climate change and to develop adapted management strategies accordingly. By analysing the stable isotopes of water (δ2H, δ18O) which act as a natural fingerprint, we elucidate how beech trees cope with climate extremes and quantify water fluxes across the soil-plant-atmosphere continuum. Among these fluxes, groundwater recharge is essential for replenishing groundwater storage and sustaining stream baseflow. Here, we focus on estimating groundwater recharge under natural rainfall conditions, drought, and after extreme rainfall using HYDRUS-1D.
This study is conducted in a mature beech stand in the Rosalia forest located in the alpine forelands of Austria. The elevation at the study site is 650 m with an average slope of 16°. The mean annual precipitation is 790 mm, 60% of which falls between May to October, and the mean annual temperature is 8.2 °C. The soil is predominantly Cambisol, exhibits strong heterogeneity, and consists of 41% sand, 46% silt and 13% clay.
Climate change scenarios are simulated with rain‑out shelters (6x6 m) that induce drought stress in two trees and the surrounding soil. Sprinklers simulate extreme rainfall (75 mm per event) at two‑month intervals during the growing season, while two other trees serve as references under natural rainfall conditions. Soil water isotope profiles are collected via two complementary approaches: first, 100 cm soil cores are subdivided into 10 cm increments and analysed in the laboratory using the direct liquid-vapour equilibration method every three weeks. Second, since July 2025, in-situ soil water vapour is sampled within the rooting zone of one drought-treated and one reference tree at 10, 20, 30 and 60 cm, and analysed with an isotope ratio spectrometer (Picarro L2130-i) for daily measurements. These isotope data are supported by meteorological data including isotopic composition of precipitation, soil moisture, and matric potential.
Results showed that the soil exhibits strong heterogeneity in both isotopic composition and physical properties, with three to four soil horizons identified within the top 100 cm. Following irrigation, the isotope profile was largely replaced by the irrigation water isotope ratio within 100 cm, indicating preferential flow and rapid infiltration. We found strong temporal heterogeneity in soil water isotope profiles, and the isotopic profiles from in-situ vapour sampling and soil cores were only partly comparable, likely reflecting differences in isotopic composition of bulk water (core samples) and mobile water fractions (in-situ analysis), soil heterogeneity, and possibly method-specific biases. These discrepancies currently prevent robust estimates of groundwater recharge estimation with different approaches, underscoring the difficulty in applying these methods in strongly heterogeneous environments. Ongoing work includes system refinements and experimental redesign, alongside evaluation of more suitable methods to enable groundwater recharge quantification.
How to cite: Asanza-Grabenbauer, M., Stumpp, C., and Stockinger, M.: Constraints of using in-situ water vapour stable isotope analysis for estimating groundwater recharge in a beech forest , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10134, https://doi.org/10.5194/egusphere-egu26-10134, 2026.
Saltwater intrusion (SWI) alters water movement and redox-sensitive biogeochemical processes in tidal marshes. It remains challenging to distinguish conservative freshwater–seawater mixing from process-driven effects such as evaporation, residence time, and redox reactions. Electrical conductivity (EC) is commonly used to trace mixing but provides limited insight into how these processes modify porewater chemistry across space and time. Here, we evaluate whether stable water isotopes and an Evaporative Enrichment Index (EEI) improve the interpretation of porewater mixing and redox-sensitive responses along a marsh transect experiencing SWI.
Porewater was sampled along a forest–marsh transect at the St. Jones Reserve (Delaware, USA) across seasons, depths, and tidal settings. Mixing was quantified using stable water isotopes (δ²H, δ¹⁸O, δ¹⁷O), EC, and end-member mixing analysis (EMMA). Results from isotope-only and isotope+EC EMMA were compared, and EEI was applied to isolate non-conservative isotopic modification associated with evaporation, transpiration, and prolonged residence time. Mixing metrics were related to redox-sensitive variables, including redox potential, nitrate, iron, and manganese.
Mixing fractions calculated from isotopes and EC both captured the freshwater–seawater gradient but diverged most strongly in the marsh transition zone. Isotope-only EMMA preserved seasonal and tidal variability that was dampened when EC was included. EEI exhibited strong seasonal structure and was negatively correlated with redox potential, indicating that isotopic enrichment coincides with more reducing conditions. Near-channel sites showed conservative mixing and consistent nitrate decline with increasing seawater fraction, whereas the transition zone exhibited enhanced nitrate loss and
depth-dependent, nonlinear iron and manganese responses associated with extended inundation and residence time.
These results demonstrate that isotope tracers, when combined with EEI, provide process-level insight beyond EC by resolving evaporative modification and hydrologic isolation. EEI helps identify when and under what hydrologic conditions redox-sensitive nutrient and metal transformations occur during saltwater intrusion.
How to cite: Bradach, S., Lui, Y., and Jin, Y.: Stable Water Isotopes Reveal Non-Conservative Mixing and Redox Dynamics During Saltwater Intrusion in Tidal Marshes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14383, https://doi.org/10.5194/egusphere-egu26-14383, 2026.
Identifying and quantifying sources and cycling of nitrogen is important for understanding not only aquatic ecosystems but also planning water resource management, mitigating urban and agricultural pollution, and optimizing government policy. Stable isotopes of dissolved nitrate and nitrite (δ15N, δ18O and δ17O) have been useful in distinguishing between the diverse nitrogen sources and sinks and help understand large scale global ocean processes as well as revealing major changes in agricultural land use and urbanization.
Despite the strength of dissolved nitrate and nitrite stable isotope analysis, the strong barrier for uptake using the favored contemporary methods (bacterial denitrifier and Cd-azide reaction) due to the laborious multi-step methods, maintenance of anerobic bacterial cultures and use of highly toxic chemicals has limited the analysis to highly specialized laboratories. We evaluate the performance of the Elementar EnvirovisION using the new Titanium (III) reduction method (Altabet et al., 2019) for one step conversion of nitrate into N2O for IRMS analysis.
The EnvirovisION has been developed for high performance analysis of CO2, N2O and CH4 and dissolved nitrate. The system has the capacity to be rapidly customized for specific needs with options for dual GC columns supporting the Weigand ‘heart-cut’ N2O method (Weigand et al., 2016) and sequential N2 and N2O analysis from a single atmospheric sample.
How to cite: Barker, S., Preece, C., Seed, M., and Berstan, R.: Analysis of dissolved nitrate stable isotopes using the one-step Ti (III) reduction method and Elementar EnvirovisION System, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11255, https://doi.org/10.5194/egusphere-egu26-11255, 2026.
Posters on site: Thu, 7 May, 16:15–18:00 | Hall A
Understanding how streamflow is generated is essential for managing water quantity, quality, aquatic ecosystems and drinking water resources. Stable water isotopes are widely used to separate streamflow into event and pre-event water, offering valuable insights into catchment storage dynamics, water ages and transit times of water. Typically, precipitation or stream water for isotope analysis are sampled using autosamplers to reduce effort and ensure an adequate temporal resolution.
However, hydrograph separation using stable isotopes is often limited by reliance on single-point rainfall sampling, which therefore assumes that precipitation inputs are spatially uniform across the catchment. This can introduce substantial errors, as spatial variability in the isotopic composition of precipitation—even within small catchments—may lead to misestimations of event water endmember contributions. Also, the various transit time models may experience biases due to an incorrect precipitation input time series of stable isotopes. Furthermore, typical autosamplers are susceptible to evaporative losses from the stored water samples, resulting in isotopic fractionation and compromised data integrity.
To solve these difficulties, we have developed and deployed the low-cost, evaporation-proof Portable Autosampler for Liquids (PAUL). Nine PAUL units were distributed across the 1.5 km² Krummenbach sub-catchment of the Brugga watershed, a mountainous headwater catchment located in the Black Forest, Germany. Eight units measured precipitation and one sampled streamflow, with biweekly collection over the course of one month.
Our findings show that spatially distributed, evaporation-secure sampling significantly improves the characterization of event water inputs and reduces uncertainty in hydrograph separation. The PAUL system provides a robust and accessible solution for high-resolution, catchment-scale isotope monitoring, providing spatial and temporal coverage that was previously unfeasible with standard autosamplers. This approach advances process-based hydrology by increasing the accuracy and reliability of isotope-based precipitation and streamflow analyses.
How to cite: Pyschik, J. and Weiler, M.: PAUL: A Novel Autosampler for High-Resolution Isotopic Monitoring in Catchment Hydrology, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5434, https://doi.org/10.5194/egusphere-egu26-5434, 2026.
Soil water management in Mediterranean vineyards is increasingly critical under increasing aridity. Soil management practices such as cover crops are promoted for improving soil structure, reducing erosion, and enhancing ecosystem services. However, grapevine production is often reduced under cover crops, and their effects on vine water use are still not fully understood. Here, we investigated the impact of soil management, i.e. conventional tillage vs. spontaneous cover crop, on the soil–plant–atmosphere continuum of a rainfed vineyard in Rioja Alavesa during veraison, with the aim of contributing to the ongoing discussion on soil management effects on vine water use.
Determining root water uptake depth using water isotopes (δ¹⁸O and δ²H) revealed contrasting uptake strategies between conventional tillage and spontaneous cover crop. Building on that, we focused on aboveground responses by combining measurements of vine water status (midday leaf water potential, Ψₘ) with stable isotopes of carbon, oxygen, and nitrogen (δ¹³C, δ¹⁸O, and δ¹⁵N) in leaves and berries. The Ψₘ values showed a clear management effect, with vines under cover crop exhibiting improved water status compared to vines under tillage (Ψₘ= -0.62 and -0.83 MPa respectively, p < 0.01). Leaf δ¹⁵N also differed between treatments, indicating changes in nitrogen availability or uptake associated with soil management (mean leaf δ¹⁵N under cover crop = 2.14‰ and tillage = 0.15‰, p < 0.01). In contrast, leaf δ¹⁸O and berry δ¹³C showed substantial plant-to-plant variability with no consistent treatment effect (p = 0.22 and 0.51, respectively).
Taken together, our results show that cover crops can enhance vine hydraulic status (Ψₘ) and modify nitrogen dynamics (δ¹⁵N), without altering long-term carbon assimilation efficiency (δ¹³C and δ¹⁸O). They also demonstrate that soil management effects are strongly dependent on the temporal scale of observation, as instantaneous indicators (Ψₘ) revealed treatment differences that were not captured by seasonally integrated isotopic signals (δ¹³C and δ¹⁸O). Overall, our study highlights the value of combining hydraulic measurements with multiple stable isotopes to improve the assessment of sustainable soil and water management strategies in vineyards.
How to cite: Ruiz, I., Schwendenmann, L., Barbeta, A., Lehmann, M. M., Pérez-Parmo, R., and Aizpurua, A.: Stable isotopes as tracers of the effects of vineyard management practices, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6149, https://doi.org/10.5194/egusphere-egu26-6149, 2026.
Quantifying and partitioning the evapotranspiration (ET) of agricultural ecosystems in various environmental settings allows the study of the determinants of field site-specific plant water use and water stress conditions. ET flux, as determined from eddy covariance measurements, was partitioned into its component fluxes, soil evaporation (E) and plant transpiration (T), with a range of independent methods, i.e. with water stable isotope analysis (δ2H and δ18O), using lysimeter data, by applying the water use efficiency concept, and from process-based numerical modelling with the Community Land Model (CLM) 5.0. Data was collected with a mobile water stable isotope laboratory (IsoMobile) in the vicinity of the ICOS DE-RuR climate station (Rollesbroich, Germany) in an intensively managed temperate grassland ecosystem between May 5 and September 24, 2025. Isotopic partitioning was calculated at sub-daily resolution from mass balance on basis of ET, E, and T isotopic compositions (δET, δE, and δT, respectively). δET was determined statistically with the Keeling-plot approach and non-destructive measurements of atmospheric water vapor inside and above the plant canopy. δE was calculated from the isotopic composition of the atmospheric water vapor and that of soil water, which was either determined destructively and a posteriori in the laboratory or non-destructively and in situ using gas-permeable tubing placed in the soil. Finally, δT was estimated destructively from stem water extracted from composite grass samples (Alopecurus pratensis, Lolium perenne, Poa trivialis, Rumex acetosa) and under the assumption of isotopic steady state transpiration. The collected standardized ICOS data was used additionally to set up both the water use efficiency partitioning approach and the CLM. All partitioning results were confronted with time series of environmental variables measured by the local weather station. Sub-daily T/ET responded to daily and seasonal changes of environmental conditions, as well as farming practices applied to the grassland. T/ET decreased significantly after the plants were cut, followed by an increase during the subsequent period of plant regrowth. T/ET estimates range between 21 to 98 % for δ18O over the course of the seasons, δ2H-based partitioning shows similar temporal developments as δ18O, while overestimating T/ET by ~13 %. Water stress was not detected during the campaign period, as ET did not decrease while T/ET remained high, even during the dryest and hottest period in summer. From a technical view, non-destructive soil water vapor sampling was found to be a good alternative to destructive soil water sampling for the purpose of ET partitioning. It provides similar δE estimations while reducing the need for fieldwork, laboratory time and resources. In conclusion the high-resolution partitioning results presented in this study provide an opportunity to investigate field scale water fluxes in a variety of environments and can aid in improving water flux estimations embedded in large-scale environmental models.
How to cite: Schulz, D., Claß, M., Brüggemann, N., and Rothfuss, Y.: Partitioning of Evapotranspiration at hourly resolution in a temperate grassland using a mobile water isotope laboratory, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11615, https://doi.org/10.5194/egusphere-egu26-11615, 2026.
Transpiration dominates terrestrial water fluxes and plays a critical role in ecosystem productivity and climate regulation, yet the travel time of water within vegetation and the contribution of internal plant water storage to transpiration remain poorly constrained. While decades of tracer studies have revealed that streamflow is sustained by older subsurface storage and responds dynamically to wetness conditions, comparable insights for transpiration are lacking. Here we present the first integrated field-based assessment of transpiration water age in boreal forests, continuously tracking transit times from the onset to the end of the growing season. Using isotope sampling of xylem, soil water, and precipitation, combined with hydrometric measurements at boreal sites dominated by Picea mariana and Pinus banksiana, we quantified mean travel times and the contribution of new versus old water to transpiration. Our results reveal that transpiration is sustained primarily by water older than one week, with newer precipitation contributing only 20–40% to transpiration fluxes. Growing-season travel times were faster than spring-only estimates but consistent with peak-summer sap-flow measurements. These findings demonstrate a "double-buffering" effect: soil water storage dampens and delays isotopic signals from new precipitation, while stem water storage further attenuates the response, particularly in drier periods. This dual buffering mechanism regulates transpiration age dynamics in response to changing wetness conditions, with storage contributions varying throughout the growing season. Our study provides critical empirical constraints on vegetation water use and transit times, essential for improving ecohydrological models and predicting ecosystem responses to water availability under changing climates.
How to cite: Nehemy, M. F. and Knapp, J. L. A.: Double buffering of transpiration: Soil and stem water storage regulate transpiration water ages in boreal forests, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13738, https://doi.org/10.5194/egusphere-egu26-13738, 2026.
Understanding the spatial and temporal origins of water used by plants for transpiration is crucial for improving forest and water resource management under future drought conditions. However, the impact of local factors such as wetness conditions and topography on the temporal origin of soil and plant waters remains largely unexplored.
In this study, we utilized a 6-year isotopic dataset to investigate i) the seasonal origin of water sources in a small headwater catchment in the Italian pre-Alps, ii) the seasonal origin of soil and plant water under different wetness conditions (based on soil moisture data), iii) the influence of topography (riparian zone vs. hillslope) and wetness conditions on water uptake by beech and chestnut trees.
The sampling campaigns were carried out in the Ressi catchment, which has a 2.4-ha area, steep hillslopes and a narrow riparian zone. The climate is humid and temperate, and the catchment is mostly covered by a forest mainly composed of beech, chestnut, maple and hazel trees. Water samples for isotopic analysis (δ2H and δ18O) were taken from precipitation, stream water, shallow groundwater, soil, and twigs from beech and chestnut trees. Samples were taken approximately bi-weekly during the growing season, whereas precipitation, stream water and shallow groundwater were collected monthly from October to May. Bulk soil water and plant water were extracted by cryogenic vacuum distillation before the isotopic analysis.
Our results, based on the estimation of the seasonal origin index (SOI), showed distinct temporal variability for all water sources, except groundwater. The rapid turnover of water in the catchment indicates that precipitation quickly replenishes the soil, becomes available for plant water uptake, and contributes to stream runoff. Interestingly, we found that both beech and chestnut trees primarily use water derived from summer precipitation, with minimal differences in water uptake between riparian and hillslope trees. The seasonality of water fluxes (i.e., precipitation and evapotranspiration) and isotopes in precipitation have a more significant impact on SOI values of soil water and plant water compared to soil moisture.
These findings suggest that in the Ressi catchment, during the growing season, trees and the stream primarily utilize young waters, even during dry years. This research contributes to our understanding of plant water use strategies and their implications for forest and water resource management under changing climate conditions.
How to cite: Zuecco, G., Todini-Zicavo, D., Marchina, C., Brighenti, S., Penna, D., and Borga, M.: Role of summer precipitation in plant water uptake in a pre-Alpine catchment, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17355, https://doi.org/10.5194/egusphere-egu26-17355, 2026.
Hydrological partitioning, the separation of precipitation into different hydrological fluxes, remains poorly constrained in forested prealpine catchments. Here, we apply EcH2O-iso, a distributed process-based tracer-aided ecohydrological model, to investigate hydrological partitioning in the WaldLab forest research site in Zurich, Switzerland. EcH2O-iso simulates water and energy fluxes while tracking stable water isotopes across all compartments of the critical zone. EcH2O-iso was calibrated and validated with five years of hydrometric measurements, along with high-frequency observations of stable water isotope ratios in precipitation, streams, groundwater, xylem, and bulk and mobile soil water. Our results highlight the importance of explicitly representing dual-porosity soil water storage dynamics in models, providing insights into how mobile and immobile soil water storages are partitioned differently. These results were compared with previous findings at the WaldLab, particularly the seasonal dynamics of interception, infiltration, and plant water uptake. Future work will use these results alongside simulations in other snow-dominated alpine and boreal catchments to contrast ecohydrological processes between snow- vs rain-dominated ecosystems.
How to cite: Mungle, D., Floriancic, M., Rouvenaz, C., Molnar, P., and Beria, H.: Tracer-aided ecohydrological modelling to quantify hydrologic partitioning and investigate partitioning processes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19865, https://doi.org/10.5194/egusphere-egu26-19865, 2026.
Global forests are increasingly impacted by repeated and long-term drought events. The survival of trees under such conditions critically depends on their ability to regulate water use and maintain transpiration even when soil water availability is limited. Internal water storage and distribution within tree stems may hereby play a key role in supporting these processes. However, the mechanisms governing water transport and distribution within mature tree stems remain poorly understood.
To investigate internal water transport, we injected deuterated and dyed water as tracers into three mature Picea abies (Norway spruce) trees located in Tharandt (Germany) on one side of the stem at 50 cm height. Deuterated water movement was monitored by repeated daily sampling of xylem water vapor at 1 m and 3 m above the injection point, from the sapwood on the injection side, the opposite side, and the central heartwood. Water vapor samples were hereby collected by drilling a 10 cm deep, 1 cm diameter hole, which was fitted with inlet and outlet tubes. Dry air was pumped into the hole through the inlet, and the air equilibrated with xylem water was collected from the outlet into glass vials. Water vapor samples were subsequentially analyzed in the lab for their water isotopic composition (2H, 18O) via cavity ring-down spectroscopy (Picarro 2130-i). Two weeks after injection, the trees were harvested, and stem discs were collected every 2–4 m along the stem to visualize dyed water distribution using image analysis. Additional xylem water samples were extracted from increment cores taken from each disc in the four cardinal directions for isotope analysis. This experimental setup enabled the examination of water transport dynamics along axial, radial, and tangential pathways within the stem.
We found that injected water remained on the side of the injection within the lower 5 m of the stem (detected via water isotope tracing) but started circulating around the stem higher up, completing approximately 1-1.5 helical turns along the trunk (detected via dye-tracing), likely reflecting the spiral growth pattern of spruce wood. Below the crown base, water movement was predominantly axial, whereas above the crown base, tangential distribution became more pronounced, allowing all upper sun crown branches across the four cardinal directions to receive the tracer water.
These findings highlight that tangential water mixing within the stem plays a critical role in supplying water to the entire crown of mature spruce trees. This may become even more important under drought conditions.
How to cite: Hikino, K., Kreher, M., Hartwig, B., Renner, F., and Orlowski, N.: Tangential water transport facilitates crown water supply in mature Norway spruce, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1019, https://doi.org/10.5194/egusphere-egu26-1019, 2026.
Large-scale afforestation in China has resulted in widespread soil water deficits. Yet the effects of root water uptake on soil water decline, and how it differs between dry and wet seasons and across different stand ages remain largely unstudied. Using stable water isotopes (δ18O and δ2H), we investigated water uptake patterns across five Robinia pseudoacacia stand ages (~6, 16, 20, 35, and 45 years) and explored the interactions between soil drying and water-use strategies during the pre-rain and rainy season across two years. R. pseudoacacia exhibited clear seasonal and age-related differences in water uptake, with contrasting water-use strategies under dry versus normal years. Overall, R. pseudoacacia predominantly relied on shallow soil water (0.67 ± 0.15) during the pre-rain season and shifted to deep soil water uptake (0.73 ± 0.14) in the rainy season. In the drier year of the 2-year observation period, all stands showed similar seasonal water uptake patterns, with a predominant reliance on deeper soil water, whereas in the typical year, water-use strategies differed markedly among stand ages. While middle-aged and old stands (16 to 45 years) accessed water from all soil layers, the younger individuals (6 years) primarily utilized soil water from intermediate and deep layers. Combining information from stable water isotopes and actual evapotranspiration we calculated soil water decline rates for all stands and found that soil water was declining between 14.0% to 24.7% in the 0–60 cm soil layer, 5.2% to 6.9% in the 60–200 cm soil layer, and 3.3% to 4.8% in the 200–500 cm soil layer. During the pre-rain season the deeper soil layers were substantially depleted, especially for young stands and in the drier year, and soil water decline rates were related to age-related differences in soil water content and soil drying patterns. This study presents the first isotope-based quantification of soil water decline across different R. pseudoacacia stand ages, highlighting the starkly different soil drying dynamics.
How to cite: Zhao, Y., Wang, Y., Zimin, L., and Floriancic, M. G.: Regulation of soil water consumption of Robinia pseudoacacia in different stand ages, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-23111, https://doi.org/10.5194/egusphere-egu26-23111, 2026.
Posters virtual: Tue, 5 May, 14:00–18:00 | vPoster spot A
EGU26-8525 | ECS | Posters virtual | VPS8
Seasonal water uptake pattern in Agathis australis (kauri), a large and long-lived Southern Hemisphere conifer, using water stable isotopesTue, 05 May, 15:12–15:15 (CEST) vPoster spot A
Understanding seasonal water uptake depth (WUD) in trees is critical for assessing trees’ physiological responses to seasonal variability in microclimatic conditions and soil water availability. While broadleaf and conifer species in the Northern Hemisphere have been widely studied, studies of seasonal changes in WUD of large and long-lived evergreen conifers in the Southern Hemisphere are rare.
We investigated the effect of season and tree size on the water uptake pattern of Agathis australis (kauri), a large and long-lived endemic conifer, over an 18-month period. Our study site, a remnant kauri-dominated forest, is located in West Auckland, northern New Zealand. We collected stem cores from seven kauri trees (n = 3 < 50 cm diameter, n = 4 > 100 cm diameter) and soil samples underneath each of the seven trees (organic layer (OL), 0-10 cm, 10-20 cm, 20-30 cm, 30-50 cm, 50-70 cm, 70+ cm) across six seasons (austral spring 23, austral summer 23-24, austral autumn 24, austral winter 24, austral spring 24, and austral summer 24-25). Water from soil and stem cores was extracted using cryogenic vacuum extraction. We measured δ2H and δ18O in all samples and used a Bayesian mixing model (MixSIAR) to determine WUD.
Our preliminary results show that across season and tree size, kauri obtained a larger proportion of water from the shallow layer (OL to 30 cm depth; ~ 60%) compared to ~ 40% sourced from layers below 30 cm. There was greater reliance on water from the shallow layer (up to 75%) during austral summer 23-24 and 24-25. We did not observe strong differences in WUD between small and large trees across our study seasons. These insights advance ecohydrological research on Southern Hemisphere evergreen conifers and highlight the importance of understanding species-specific response to microclimatic conditions and changing water availability.
How to cite: Boseren, M., Schwendenmann, L., and Boswijk, G.: Seasonal water uptake pattern in Agathis australis (kauri), a large and long-lived Southern Hemisphere conifer, using water stable isotopes , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8525, https://doi.org/10.5194/egusphere-egu26-8525, 2026.
