Stable isotopes are essential tools for tracing water and nutrient fluxes in terrestrial ecosystems.  In recent years, studies of the soil-plant-atmosphere continuum have yielded impressive volumes of stable isotope tracer data at previously unattainable precision and spatiotemporal resolution.  These emerging data sets facilitate new methods of analysis that promise new insights into transport, storage and mixing.  For decades, end-member mixing analysis (EMMA) has been the standard workhorse for interpreting tracer data, but new methods can overcome some of its limitations and facilitate new inferences into ecosystem processes. 

At the catchment scale, for example, end-member mixing has been widely used to quantify streamflow as a mixture of isotopically distinct sources, but knowing where streamwater comes from is not the same as knowing where precipitation goes, for which one needs end-member splitting (Kirchner and Allen, 2020) instead.  End-member splitting allows summer and winter precipitation to be partitioned between evapotranspiration, summer streamflow, and winter streamflow, without direct measurements of evapotranspiration water fluxes or their isotopic composition. 

At the hillslope and plot scale, end-member mixing has been widely used to quantify the relative contributions of isotopically distinct sources to soils and xylem waters.  If any end-members are missing or their tracer distributions overlap, however, conventional mixing models become unusable.  These constraints can be overcome by exploiting the information contained in tracer time-series using ensemble end-member mixing analysis (EEMMA; Kirchner, 2023).  EEMMA can potentially quantify many sources using a single tracer, even if their mean concentrations are indistinguishable. EEMMA can also quantify source contributions when some sources are unknown, and even infer the tracer time series of a missing source. 

These new methods will be demonstrated using benchmark data, and proof-of-concept applications will be presented.

How to cite: Kirchner, J.: New methods for studying the soil-plant-atmosphere continuum with stable isotope data, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12264, https://doi.org/10.5194/egusphere-egu24-12264, 2024.

Stable isotopes of hydrogen and oxygen in water are common tools for investigating water uptake apportionment, but many of the existing methods rely on simple linear mixing approaches that do not mechanistically incorporate additional information about site physical properties and conditions. Here, we develop a 'physically based root water uptake isotope mixing estimation' model (PRIME) that combines a continuous and parametric probability density function for root water uptake with site physical data in a process-based linear mixing framework. To demonstrate the application of PRIME, water uptake patterns of boreal forest Pinus banksiana trees were estimated on four dates in 2019. To aid in validation, estimates were compared with that of the Bayesian linear mixing model framework, MixSIAR. The two approaches provided similar results, but due to its continuous and parametric nature, PRIME provided estimates of superior resolution, certainty, and model parsimony. Although both models incorporate additional physical information into their mixing frameworks , PRIME does so in a mechanistic manner, thereby reflecting the relevant hydrological processes more effectively than the purely empirical approach taken by MixSIAR. Furthermore, because PRIME uses a continuous function to describe the predicted uptake pattern, it allows users to quantify water uptake with essentially infinite resolution, through integration over the desired depth ranges. These findings demonstrate the advantages of utilizing a continuous, parametric, and process-based mixing model to estimate root water uptake apportionment, thus providing a relatively simple yet powerful tool with which to approach plant water sourcing. 

How to cite: Si, B., Si, E., and Fu, H.: A process-based water stable isotope mixing model for plant water sourcing, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11966, https://doi.org/10.5194/egusphere-egu24-11966, 2024.

In urbanized areas the natural flow paradigm from river management is being increasingly challenged by hydroclimatic changes and marked anthropogenic influences on flow regulation. Although the impacts of urbanization, increased runoff and reduced baseflow are increasingly well quantified; the characteristics of flow regimes that are sustainable and wanted for urban streams to preserve or restore their hydrologic and ecological integrity in the face of future climate change and rapid urbanization, remain less well understood.

To that end, we conducted a paired catchment study of two streams, both sub-catchments of the Spree catchment – a more natural intermittent rural agricultural stream in Brandenburg (Demnitzer Mill Creek) and an anthropogenically impacted urban stream in Berlin (Panke).  We characterized contrasts in inter- and intra-annual streamflow variability, storm period responses, water ages and mixing processes. Through tracer-based analyses, using stable water isotopes, we identified the physical processes (sources, flow paths and age) sustaining stream flow over multiple years (2018-2023), and broadly linking them to biological dynamics obtained through environmental DNA, in order to estimate resilience to future hydroclimate interactions and land use changes. The higher specific discharge of the urban stream emerges as a clear artefact of artificially increased baseflow due to discharge of wastewater, while reacting primarily to convective summer storms with strong runoff reactions and short discharge peaks. In contrast, the rural stream shows a characteristically intermittent behavior with longer periods without baseflow and only limited runoff reactions with only temporary superficial accumulation of water after heavy rainfall. Water ages reflected the respective runoff contributions and mixing processes, with a low contribution of young water observed in the urban stream and a higher, more variable contribution of young water in the rural stream. The strong dichotomy of runoff responses and unmistakable influence of baseflow manipulation on streamflow dynamics and biological processes point to major uncertainties in the suitability of different approaches for the restoration of urban and management of naturally intermittent rivers. Effective stakeholder engagement will be necessary in seeking to manage flow regimes to maintain ecohydrological connectivity and future resilience in the face of urban growth and climate change.

How to cite: Warter, M. M., Tetzlaff, D., Marx, C., and Soulsby, C.: Characterizing changing stream flow components and hydroclimate interactions in cities – implications for future management and restoration of urban ecosystems, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-383, https://doi.org/10.5194/egusphere-egu24-383, 2024.

Agricultural co-cropping, which is the cultivation of two or more crops simultaneously on the same field, is gaining rapid attention in temperate agroecosystems as a viable nature-based solution to improve agricultural productivity. However, relatively little is known about plant water use patterns in temperate agricultural co-cropping systems. Specifically, the functioning and resilience of these systems compared to their equivalent monocultures is likely to depend on whether water use is complementary for the different crops and how this might change during the growing season and under different hydro-climatological conditions.

This study focused on addressing these knowledge gaps by using water stable isotopes to trace the sources of vegetation water uptake (shallow or deep soil water) in 5 different cereal-legume co-cropping systems and 4 of their respective cereal monocultures under field conditions in North-East Scotland. For each treatment, we extracted vegetation water, and soil water from 5 different depths for analysis of isotopic composition. We then performed MixSIAR end-member mixing modelling to explore proportional water uptake patterns for cereal vegetation throughout the growing season and under wet and dry conditions.

The results showed that cereals in all the monocultures and co-cropping systems predominantly used shallow soil water (upper 5 cm), regardless of growth stage and hydro-climatological conditions. Cereal water uptake patterns in monocultures and co-cropping systems were comparable during wet hydrological conditions. However, the analyses revealed that cereals in co-cropping systems exhibited plasticity and increased their water uptake up from deeper soil water (5 – 30 cm) compared to cereals in monocultures during dry conditions. Furthermore, during dry conditions, we found different seasonal responses in the co-cropping systems between cereal genotypes traits. Understanding of plant water use patterns for different cropping systems could inform the design of resilient and sustainable water management practices and agricultural policies. The plasticity observed in co-cropping systems could potentially contribute to optimised water use under climate change.

How to cite: Durodola, O., Rothfuss, Y., Hawes, C., Smith, J., Valentine, T., and Geris, J.: Using stable water isotopes to trace cereal water use in agricultural co-cropping systems under contrasting hydro-climatological conditions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6181, https://doi.org/10.5194/egusphere-egu24-6181, 2024.

Farmers are increasingly adopting practices that add more biodiversity to agro-ecosystems for improving crop yield level and stability. These practices include co-croping, that is the cultivation of different phenotypes (or cultivars) in the same field. However, we are missing clear criteria for the selection of the cultivars as well as a quantified assessment of the effect of the combination on water use and partitioning.

In our study, we proposed to focus on the association of two wheat cultivars having contrasted root systems. We further aimed at evaluating the effect of such an association on water flow by monitoring root water uptake vertical patterns from isotopic analyses.

In a control environment, we ran experiments on soil columns (silty-clay) (diam=11cm, height=80cm) each planted with two wheat cultivars (“shallow-rooted” vs “deep-rooted”) until ear emergence. Six different modalities were tested, i.e., 3 crop types (2 shallow-rooted individuals / 2 deep-rooted individuals / combination of 1 shallow-rooted individual and 1 deep-rooted individual) x 2 treatments (well-watered conditions or water stress). We repeated the experiment six times to test all the different modalities mentioned above in triplicate. Profiles of root water uptake (RWU) relative fractions were statistically evaluated (with a Bayesian mixture model) at cm to dm vertical resolution from soil water and transpiration flux isotope data non-destructively using gas-permeable membranes and gas chambers coupled to a laser spectrometer. Plants were also monitored physiologically during the experiment (e.g., leaf area, chlorophyll content, root architecture by magnetic resonance imaging) and destructively (e.g., above-ground and below-ground biomass, root area, stomatal density).

We will present our observations on how the RWU profile are affected by soil water status, wheat phenotype and associated plant identity. Surprisingly, the deep-rooted phenotype individuals - which uptake more water than the shallow-rooted individuals at soil depth between 40cm and 80cm under water deficit condition - are also the most physiologically sensitive (reduction in leaf area, significant change in shoot/root biomass ratio) at the first reproductive stages. On the field scale, this could have later a negative impact on the yield of the deep-rooted phenotype monoculture, but this should be moderate in co-cropping situations, where the sensitivity of the whole is lower.

How to cite: le Gall, S., van Duschoten, D., Lattacher, A., Giraud, M., Harings, M., Deseano Diaz, P., Sircan, A., Poll, C., Lobet, G., Javaux, M., and Rothfuss, Y.: Investigating the impact on water fluxes and physiological development of the combination of contrasted root wheat cultivars from isotopic analysis, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15966, https://doi.org/10.5194/egusphere-egu24-15966, 2024.

Transpiration fluxes from land to the atmosphere hinge significantly on the degree to which trees opt to open their stomata to trade off water for CO2. Yet, the terrestrial ecosystem response to the changes in the atmosphere (CO₂, VPD, etc.) and the redistribution of water on land in an era of change are largely unknown. In addition, the effects of such long-term changes in trees’ adaptation strategies and resilience under short-term dry conditions are not yet fully understood. To address this issue we determined stable water isotopologues within a long-term (20-year) irrigation experiment in a drought-prone Scots pine-dominated forest in one of the driest areas of Switzerland, Pfynwald. Our sampling included plots with trees growing under naturally dry conditions (control), irrigated (from 2003 to present), and previously irrigated (irrigation stop; irrigated from 2003–2013; control condition since 2014). We have installed an in-situ high-frequency isotope measurement system in the field to sample stable water isotopologues (²H and ¹⁸O) in different soil depths, tree xylem, and in the atmosphere and to track tree water uptake dynamics at the control, irrigated, and irrigation stop plots. The sampling was complimented with manual extraction of soil and xylem water samples at the three treatment plots for isotope analysis in the lab. Our preliminary findings support the hypothesis that pine forests adjust their carbon allocation strategies during long-term wet periods, establishing a deeper rooting system to access deeper water sources. This adaptive mechanism enhances their resilience during short-term dry periods.

How to cite: R. Freund, E., Villiger, M., M. Lehmann, M., Hu, Z., Meusburger, K., and Gessler, A.: Scots pine response to short-term dry conditions after long-term soil moisture manipulation experiment  , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15354, https://doi.org/10.5194/egusphere-egu24-15354, 2024.

Soil-vegetation systems partition incoming precipitation into either the freshwater system ("blue water") or back to the atmosphere as evapotranspiration ("green water"). The isotope signatures of these fluxes are observed to be distinct. We investigate the partitioning of precipitation and illustrate one potential mechanism for this "apparent" isotopic fractionation of root water uptake by means of an isotope-enabled mechanistic water balance model [1].

Stable water isotope signatures were collected at a Swiss forest site dominated by beech trees (>60 cm DBH, 37m stand height) with a mean annual precipitation of ~1050 mm/year, at 800 m.a.s.l throughout two vegetation seasons. Up to bi-weekly samples of xylem and mobile soil water and six bulk soil water campaigns (down to 150 cm) were combined with continuous hydrometric measurements (down to 200 cm) to constrain modelled water fluxes such as preferential infiltration patterns and seasonal patterns of root water uptake.

During the model validation period in 2022, the model faithfully reproduced seasonal and vertical patterns in isotope signatures. The goodness of fits of time series showed δ¹⁸O RMSE smaller than 0.4‰ for mobile soil water at 50cm or 80cm or smaller than 1.0‰ for stem xylem water at breast height, while the goodness of fits of vertical profiles had δ¹⁸O RMSE smaller than 1.7‰ for bulk soil water profiles down to 150cm. Reduced soil water availability in the topsoil during the summer of 2021 led to a downward shift of the flux-weighted average water uptake depths of beech trees. However, while the relative contribution to water uptake of soil layers below 80 cm increased during the summer of 2021, their absolute contribution did not increase sufficiently to compensate the water missing in the topsoil layers where most roots are located. 

Modelled infiltration pathways and root water uptake illustrate how seasonal and vertical selectivity of root water uptake leads to distinct isotope signatures in the modelled green and blue water fluxes. This behaviour is obtained at this site without a two-domain representation of the soil domain nor preferential flow to deeper layers. Further, simulations with synthetic seasonal isotope patterns in precipitation demonstrate how this "apparent" fractionation factor depends on the timing of transpiration together with the seasonality of the precipitation isotope signature. In conclusion, this study highlights that the soil-vegetation system may fractionate heavier precipitation for the green water fluxes mainly because of the seasonal patterns in precipitation isotope signatures and transpiration rates.

[1] Fabian Bernhard. (2024). fabern/LWFBrook90.jl: v0.9.8 (v0.9.8). Zenodo. https://doi.org/10.5281/zenodo.10463109

How to cite: Bernhard, F., Lehmann, M. M., Gessler, A., and Meusburger, K.: How beech trees use isotopically heavier precipitation because of seasonality, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19346, https://doi.org/10.5194/egusphere-egu24-19346, 2024.

Trees are major regulators of the forest water balance. To stay vital, water loss via transpiration has to be compensated by root water uptake. However, it is often not clear where exactly the trees get their water from. This has become particularly relevant in central Europe, as forests are facing more frequent and intense droughts with climate change. It is known that water uptake strategies are species-specific, influenced by environmental conditions and, potentially, neighboring species. Yet, knowledge about species-specific patterns of water uptake depth and how it is affected by tree-species mixture is scarce. Stable water isotopes present a valuable tool to elucidate these belowground processes.

For our study, we selected mixtures of European beech (Fagus sylvatica), the dominant broadleaved tree species in central Europe, with native, but drought-prone Norway spruce (Picea abies), and non-native, but supposedly more drought-resistant Douglas fir (Pseudotsuga menziesii), as well as the respective pure stands. We aimed to uncover the effect of (1) species identity, (2) species mixture and (3) environmental conditions on water uptake depth.  

To achieve this, we conducted two sampling campaigns in climatically contrasting years: a natural abundance campaign in relatively wet 2021, covering 20 plots on 4 sites, and a tracer experiment focused on European beech and Douglas fir on a subset of plots, including weekly sampling of 12 trees throughout the drought summer 2022. 

We found species-specific patterns of water uptake depth, where Norway spruce tended to use the greatest share of shallow water, followed by European beech and Douglas fir. Within species, the data indicated differences in water uptake depth between pure and mixed stands, however, we did not detect a spatial differentiation between co-occurring species. Dry conditions tended to shift water uptake to deeper layers, with beech responding stronger than Douglas fir.

Our results corroborate that species-specific traits have to be considered when assessing forest water pathways, especially in mixed forests and under drought. Considering central Europe, our data supports the assumption that Douglas fir may be more drought resistant than Norway spruce by tapping deeper water sources. In mixture, both European beech and Douglas fir seem to exploit similar soil depths, while none of the species is limited to the drought-prone topsoil.      

How to cite: Hackmann, C., Paligi, S., Mund, M., Hölscher, D., and Ammer, C.: Water uptake depth of European beech, Douglas fir and Norway spruce – species-specific patterns, seasonal dynamics and mixture effects, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19455, https://doi.org/10.5194/egusphere-egu24-19455, 2024.

The COST Action WATer isotopeS in the critical zONe (WATSON; https://watson-cost.eu/) aims to elucidate the interactions between groundwater recharge, soil water storage, and vegetation transpiration across various climatic settings. Within this framework, root water uptake by trees is crucial for understanding water partitioning and forest resilience to drought. While isotopic approaches have successfully revealed root water uptake strategies for different species, a comprehensive assessment across different climates and vegetation types is still missing (Beyer and Penna, 2021).

To address this research gap, we executed a synchronized, participatory sampling campaign by WATSON Action members in late spring and summer of 2023. Soil and vegetation samples were taken across 39 well-distributed forest sites encompassing 17 European countries. The samples were analyzed for the stable isotopes of oxygen and hydrogen at the WSL laboratory in Switzerland. The data enable us to investigate the spatial and temporal (spring vs summer) variability of root water uptake of the shallower-rooted spruce (Picea abies) and deeper-rooted beech (Fagus sylvatica) trees. We expect beech to exhibit a more pronounced shift to deeper water sources during summer than spruce due to its deeper rooting system. We also expect that the dominant root water uptake depth is influenced by site-specific factors (climate, elevation, latitude, soil type, level of understory cover) and tree characteristics (tree height, stem diameter). This presentation will describe the sampling campaign and the preliminary results on root water uptake for both species across Europe.

How to cite: Meusburger, K., Geris, J., Penna, D., Rothfuss, Y., van Meerveld, I., and Lehmann, M.: Investigating Large-Scale Variation in Plant Water Uptake Across European Climates and Vegetation Types – a WATSON Cost Action Initiative, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14819, https://doi.org/10.5194/egusphere-egu24-14819, 2024.

The ongoing climate change is significantly impacting forest ecosystems, intensifying environmental extremes such as droughts and heatwaves. The increased atmospheric evaporative demand, coupled with reduced soil water availability, poses a risk of water deficit to trees, threatening their health status. These extremes can amplify the thermal and hydrologic gradients along hillslopes, driving spatially variable thermal and hydraulic stress for trees. Until now, site-specific case studies have offered only a limited understanding of how hydrological processes at the hillslope scale influence tree water use. Comparative studies hold the potential to offer a more generalized understating of how trees growing in complex terrains respond to spatially varying growing conditions.

To address this, we set up a comparative study on two forested hillslopes located in the Weierbach catchment (Luxembourg) and the Lecciona catchment in Tuscany (Italy), respectively. The investigated sites differ in steepness, climate, geology, and soil characteristics, but both are dominated by beech (Fagus sylvatica L.) trees. We combined sap velocity, isotope measures, and wood moisture content with environmental monitoring (soil moisture, groundwater level, and hydro-meteorological variables) at different locations to capture beech trees’ water response to heterogeneous environmental conditions. This combination of measurements allowed us to link the transpiration response of trees to water availability along the two investigated hillslopes over the 2019 and 2020 growing seasons in the Weierbach catchment and over the 2021 growing season in the Lecciona catchment.

We observed that surface topography and hillslope structure result in differing sap velocities in response to environmental controls (i.e., vapor pressure deficit and relative extractable water), but not consistently across our study sites. In the Weierbach catchment, we noted a uniform physiological response to environmental controls among trees, even during the drier conditions of 2020, compared to 2019. We attribute this consistency to the homogeneous growing conditions across the slope. On the contrary, in the Lecciona catchment, trees located upslope displayed a more conservative hydraulic behaviour compared to the footslope location. This finding suggests a stronger edaphic and environmental gradient across the hillslope topography. Here, water redistribution through shallow subsurface flow results in more favourable growing conditions and longer growing seasons for trees at the footslope location, compared to the rather uniform growing season observed in the Weierbach catchment. These contrasting results between the two investigated hillslopes suggest that in landscapes where the hydraulic and climatic gradients are stronger, the physiological response among locations will be more spatially variable.

How to cite: Fabiani, G., Klaus, J., Pfister, L., and Penna, D.: The influence of topography on beech water use: evidence from a comparative study, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8308, https://doi.org/10.5194/egusphere-egu24-8308, 2024.