Tree roots are responsible for the bulk of water and nitrogen (N) uptake by forests, but the detailed mechanism of resource uptake and their role in the spatial exploitation of water and nitrogen resources is incompletely understood. Theory and empirical evidence suggest that there are two processes delivering N to root surfaces for uptake: 1) convection of dissolved N carried in soil water as it is drawn toward the roots and 2) diffusion of dissolved N in still water toward the low concentrations at the root surface.  Quantifying these processes has, however, been difficult, particularly in forest trees. Recently paper by Henriksson et al. Our objective here was to make a similar comparison in an irrigation experiment.

We applied highly labelled ²H₂O and ¹⁵N on a small area at the beginning of the growing season a similar and tracked ²H and ¹⁵N in tissues of adjacent trees and shrubs, aiming to assess not only correlations between the water and N tracers at the end of the season, but also the dynamics of label passage and accumulation through repeated measurements. Synchronicity of the nitrogen accumulation with the water label passage would strengthen evidence for the role of water uptake in nitrogen uptake.

We found that the ²H-label in water, as measured in extracted water, passed through the system as a pulse, disappearing by late in the season. In contrast, the ¹⁵N label, as measured from leaf tissue, accumulated toward an asymptotic maximum. Among tree individuals, the rate of ¹⁵N increase was correlated with the plant-water ²H₂O labelling at any point in time, supporting the notion that the ¹⁵N uptake was predominantly driven by water uptake. No difference was detected between the irrigated and the non-irrigated plots, perhaps because high rainfall overrode any irrigation effects.

Next steps will be to also compare the cumulative δ²H of the wood in the new growth ring to the δ²H of the xylem water that passed through individual trees. If the correlation is strong, we will map the patterns of water and N uptake around the labelled plots to provide a detailed spatial description of the correlated water and N uptake processes. When combined with the current study, these results will show the spatial and temporal extent to which root water uptake facilitates nitrogen delivery to the roots.

How to cite: Marshall, J., Lehmann, M., Hagedorn, F., Saurer, M., Treydte, K., and Gessler, A.: Seasonal dynamics of 2H2O passage and 15N accumulation in a dual-label pulse-chase experiment in a mature, irrigated Scots pine forest, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7157, https://doi.org/10.5194/egusphere-egu23-7157, 2023.

Understanding the interactions within the soil-plant-atmosphere-continuum becomes more important considering the eco- and hydrological impacts of climate change. Especially stand specific flow pathways and the characteristic timescales of water movement potentially provide important information on drought resilience of different forest ecosystems. This study analysed tree stand specific water uptake dynamics through water stable isotopy at high temporal resolution for two isotopic labelling events (7mm with δ²H +1000 ‰ and 23mm with δ²H +800‰) during the 2022 drought in south-west Germany. Measurements in pure and mixed tree stands of European beech (Fagus sylvatica, n=18) and Norway spruce (Picea abies, n=18) included sap flow, in-situ water isotopy of soil and xylem water, radial stem growth and microclimatic conditions. Our central hypothesis is that species identity and water competition between tree species are major drivers for ecohydrological flux dynamics. The results of the two labelling events showed differences in label water uptake of the two tree species and the transit times of the label in the system. The labelling events showed different transit times in the tree xylem depending on the label intensity. P. abies showed a slightly higher uptake of shallow soil water label in mixed stands than in pure patches. When labelled water infiltrated into deeper soil layers water was taken up faster by F. sylvatica in mixed forest patches than in pure forest patches and showed a generally slower uptake in P. abies than in F. sylvatica. The faster response time in water uptake during the second labelling was supported through an increase in measured sap flux and modelled branch water potential. Those dynamics of water isotopy measured in a high temporal resolution allow for a better understanding of root water uptake dynamics and water use strategies but also show species interaction effects on ecohydrological fluxes.

How to cite: Kinzinger, L. L., Mach, J., Haberstroh, S., Dubbert, M., Weiler, M., Orlowski, N., and Werner, C.: Interspecific interaction and species identity effects on water uptake of beech and spruce trees by using stable isotope labelling, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-294, https://doi.org/10.5194/egusphere-egu23-294, 2023.

Urban green spaces are highly valuable in supporting the climate and infrastructure of cities through rainwater retention, evaporative cooling and shading. Investigating the diurnal and seasonal ecohydrological process dynamics in the urban soil-plant-atmosphere continuum is crucial to understand what types of landcover might best balance water re-distribution for a particular urban landscape providing cooling effects whilst not compromising groundwater recharge.

Stable water isotopes are very useful tools to investigate these complex interactions between soil properties, plant physiology and atmospheric drivers. In 2022, we conducted an experimental study looking at the complex patterns of the urban soil-plant-atmosphere interface at high temporal resolution at an urban tree stand and a grassland in Berlin, Germany during an entire growing season. To assess atmospheric moisture demand and vegetation water dynamics, we performed novel in-situ and real-time sequential measurements in the field at different heights in tree xylem and in the atmosphere. This was complemented by destructive sampling of soil water isotopes from multiple depths and eddy flux measurements at an open field near the sites.

We could identify clear evaporative and drought signals in the different water cycle components, including the extensive summer drought across continental Europe in July and August 2022. Results showed faster and larger responses to precipitation inputs in the upper soil moisture signal at the grassland. Atmospheric fluxes indicated clear evaporative losses just above the grass (15 cm height). Underneath tree canopies, upper soils responded more slowly to precipitation inputs and the atmospheric profile showed more homogenous spatio-temporal distribution of water vapour signals. Xylem water dynamics revealed contrasting diurnal and seasonal variations in different tree species and tree heights. Plant water sources were mostly drawn from deeper soil (70 cm) horizons. During the extended period of water scarcity in August, drought signals came from enriched atmospheric water vapour and low sap flux.

This knowledge of the water dynamics under different drought-stressed urban vegetation is extremely useful for the development of isotope aided ecohydrological models and allows science-based evidence for sustainable urban planning tackling climate change and urban densification.

How to cite: Ring, A.-M., Tetzlaff, D., Dubbert, M., and Soulsby, C.: High-resolution water and stable isotope dynamics in drought-stressed urban vegetation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-110, https://doi.org/10.5194/egusphere-egu23-110, 2023.

Low temperature is the main driver behind the upper elevational and higher latitudinal distribution range of trees. As the limit for tree growth in general, the alpine treeline is traditionally in the focus of most studies on the effect of low temperature limits of trees, but all other tree species that reach their distribution limits below the treeline have species-specific low-temperature limits, as well (Körner, 2021). Restricted water uptake and deteriorated hydraulic relations at low root zone temperatures might be one of the drivers for the species-specific cold distribution limits of trees. Negative cold soil effects on the hydraulic conductivity of trees can thus potentially amplify the direct effects of cold temperatures on growth and contribute to the cold limit of temperate tree species. Thus, we put forward two hypotheses: (1) the natural cold distribution limit of temperate tree species is related to their capacity to take up water at cold root temperatures; (2) cold root temperatures lead to drought-like water limitations that are more severe in low- compared to high-elevation species. In this study, we investigated the low temperature sensitivity of root water uptake and transport in seedlings of 16 European broadleaved and conifer temeprate tree species, which reach their natural upper elavation distribution limits at different distances to the alpine treeline acsross a ca. 1500 m elevational range. We tested the temperature sensitivy of root water uptake and whole tree water transport by exposing the seedlings to three different constant root temperatures (15, 7 and 2°C) while all seedlings received the same warm abovground temperatures. To quantify the negativ low temperature effects on water transport, we used 2H-H2O pulse labelling of the source water in hydroponic systems. The result showed a correlation of the thermal distance to treeline of the natural upper distribution limits of each species with its relative water uptake at 7°C root temperature revealed a moderate but significant linear relationship, whereby water uptake and transport tends to be more limited the larger the species’ thermal distance to treeline (i.e. the lower the high elevation distribution limit). In contrast, 2°C root temperatures strongly reduced water uptake, with consequently no significant correlation between relative water uptake and the species-specific distribution limits. We concluded low root temperatures lead to species-specific restrictions of water uptake and drought-like stress with reduced water potentials and stomatal conductance. Furthermore, species that reach their upper distribution at higher elevations are less sensitive to low root temperatures and vice versa. Low temperature-caused hydraulic restrictions might thus contribute to the cold distribution limits of temperate tree species.

How to cite: Li, Y. and Hoch, G.: Physiological drought-like water limitation at low root temperature in temperate tree species with different elevational distribution limits, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15149, https://doi.org/10.5194/egusphere-egu23-15149, 2023.

Analyzing water stable isotopes in soils and plants is a key method to identify the water sources for transpiration. However, the spatial representation of such studies is often limited and typically data from one or only a few soil water isotope profiles are used for analyzing plant water sources for much larger areas.   

Contrary, it is well known from soil sciences that soil physical and hydraulic properties are highly heterogeneous, even over small areas. Only few studies have investigated the spatial variability of soil water isotopes, despite its potential importance for water uptake depth analysis. Goldsmith et al., (2018) showed that vegetation can have a substantial influence on the spatial pattern of soil water isotopes in a tropical cloud forest. We extend the hypothesis that vegetation does not only have an influence on soil water isotopes in wet environments, but also under dry conditions: The isotopic enrichment of soil water isotopes under steady-state dry conditions is controlled by vegetation (canopy parameters). 

In order to test this hypothesis, we undertook a spatial sampling of ten soil water isotope depth profiles (at 6 depths up to 2m depth) and ~60 evergreen and deciduous trees at the peak of dry season in February 2019 in a tropical dry forest in the northwest of Costa Rica. We then correlated the spatial patterns of water content and isotopes of the soil with 12 vegetation indices and surface (leaf/soil) temperature derived from UAV (Unmanned aerial vehicles; drones) overflights (Jan-Apr 2019) in order to investigate if spatial patterns of soil water isotopes can be predicted using additional information. Finally, we interpolated (external drift kriging) the soil water isotope values using the highest correlated vegetation indices in order to provide a spatially distributed map of soil water isotope depth profiles.

Our findings indicate that i.) soil water isotopes are (highly) spatially heterogeneous, even under steady-state conditions (no rain); ii.) this heterogeneity is particularly pronounced for the near-surface soil (first 50 cm) and diminishes with soil depth; iii.) there is a significant correlation between soil water isotopes and multiple vegetation indices. Surprisingly, the highest correlations (0.82 for water content, 0.75 for 𝛿²H and 0.62 for 𝛿¹⁸O, all for 10 cm soil depth) were found for indices based on color infrared (CIR) and the red-edge triangular vegetation index (RTVI), and not NDVI (Normalized Difference Vegetation Index).  We proved the theoretical concept (more vegetation cover = lower soil temperatures = less fractionation) to hold true by correlating the soil surface temperatures at each sampling location to the water isotope values (R² = 0.75 for both 𝛿²H and 𝛿¹⁸O at the soil surface).

This research demonstrates that classic approaches of assigning one or few soil water isotope profiles for characterization of water uptake depths of larger areas are highly error-prone. Vegetation and soil water isotopes affect each other and need to be incorporated into spatial analyses. The interpolated soil water isotope depth profiles we provide can act as a baseline for more robust spatial investigations of soil water uptake depths in highly heterogeneous environments.

How to cite: Beyer, M., Gerchow, M., Iraheta, A., Kuehnhammer, K., Koeniger, P., Sanchez-Murillo, R., Dubbert, D., Dubbert, M., Callau-Beyer, A. C., and Birkel, C.: Vegetation controls spatial patterns of soil water isotopes in a tropical dry forest and UAV’s can help to predict them, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10902, https://doi.org/10.5194/egusphere-egu23-10902, 2023.

With the intensification of the hydrological cycle, the identification and assessment of factors controlling catchment climate resilience are key. A major obstacle to the design and implementation of precautionary measures against ‘once in a lifetime’ flood events is the still very limited understanding of the hydrological mechanisms involved. Along similar lines, the clustering of extreme events remains elusive until this day.

Stable isotopes of O and H in streams and precipitation are cardinal tools for investigating questions related to water source, flowpaths and transit times. However, their spatial and temporal variability remain largely unknown – essentially due to the limited availability of long historical time series of O-H isotope signatures in stream water, as opposed to the multi-decadal records in precipitation.

Based on their quality as natural archives of in-stream environmental conditions, freshwater mussels have been recently used for complementing stream water δ¹⁸O isotope records. Their potential is far from being exhausted, with nearly 1200 freshwater bivalve species inhabiting a large variety of river systems and lakes around the globe. Their average life span is ca. 10 years, even though many species can live much longer (up to 200 years for the freshwater pearl mussel). Here, we introduce an innovative avenue for pushing the boundaries in hydrological time series reconstruction even further. Our proof-of-concept work from the Our River (Luxembourg) is geared towards widening the portfolio of analysed proxies in shells, eventually extending the high-resolution (seasonally to annually resolved) reconstruction from stream water δ¹⁸O to river discharge.

Note that the newly gained knowledge on multi-decadal and centennial changes in streamflow generation is of direct relevance for freshwater mussel population dynamics – an issue that is at the heart of all past and ongoing projects for the protection of freshwater molluscs.

How to cite: Pfister, L., Schöne, B., Guilhem, T., Christoph, G., Frankie, T., Christophe, H., François, B., and Loic, L.: Reconstructing the history of flowing waters and stream water isotopes from freshwater mussels, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14839, https://doi.org/10.5194/egusphere-egu23-14839, 2023.

The temporal origin of root water uptake and drainage provides insights into the impact of natural and anthropogenic disturbances on plant resilience and aquifer vulnerability. In-situ and virtual tracer experiments provide information on rainfall partitioning and transit times, which help to enhance the understanding of the hydrological response of a soil-plant-atmosphere continuum (SPAC) to climate variability and contaminant transport. A virtual tracer experiment was carried out in a 150-cm-thick soil lysimeter planted with winter rye in Austria. Water flow and tracer transport dynamics were simulated in the SPAC using HYDRUS-1D, previously calibrated with hydrochemical measurements. The root water uptake (τ_R) and drainage (τ_D) transit times (τ) were assessed by identifying arrival times when a prescribed percentage of the tracer mass breakthrough curves was reached. The estimates of τ_R and τ_D were compared to those derived from a particle tracking algorithm that simulates the particle trajectories subject to convective transport. The tracer-based arrival times were in close agreement with those determined using particle tracking when 50% of the tracer output flux was reached. On average, it took 19 or 234 days for water originating from rainfall in the growing season to be taken up by roots (21%) or exit as drainage (36%), respectively. In contrast, in the dormant season, 10% of rainfall water was taken up by roots after 245 days, while 79% became drainage after 241 days. The results from each daily event can be aggregated at any desired temporal resolution to investigate the effects of climate seasonality on water balance and timing. The temporal origin of water can be explored in other plots using the standard guidelines proposed in this study.

How to cite: Nasta, P., Zhou, T., Stumpp, C., Simunek, J., and Romano, N.: Assessing the temporal origin of root water uptake and drainage water using a virtual tracer experiment in HYDRUS-1D, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4203, https://doi.org/10.5194/egusphere-egu23-4203, 2023.

How, why, and what water flows through the soil-plant continuum are quite complex questions that are not yet well understood quantitatively. Soil and plant-induced heterogeneity, soil evaporation, and root water uptake are some of the main controlling factors of water flow dynamics in the soil-plant continuum. Coupling these processes is thus of quite importance to advance our understanding of subsurface mixing and soil-plant interaction and, especially, water sources used by trees. In this study, we combine hydrological and stable water isotopes (²H and ¹⁸O) field data in an integrated flow and transport model to investigate which water sources are used up by trees under different wetness conditions.

We conducted a field experiment on two sets of three Scots pine trees (Pinus sylvestris) in a forested plot within the Vallcebre research catchments (NE Spain). The experiment was carried out from May to September 2022. We monitored throughfall, sap flow, and dial stem diameter variation, as well as soil water potential and soil water content (in vertical profiles down to 70cm) at high temporal (5min) resolution. Furthermore, we sampled weekly water from the different water pools (throughfall, soil water (bulk and mobile), groundwater, and xylem water (twigs)) for isotopic analysis. The analysis of these data helped in clarifying the interaction between the different water pools and the effect of soil water potential and soil water content dynamics on the isotopic signals in the soil-plant continuum.

To further analyze the field data, we developed a numerical model using R-SWMS to simulate the flow in the vadose zone by solving Richards equation coupled with root water uptake, soil evaporation, and isotopic fractionation. To achieve this, we created a 3-D heterogeneous soil matrix that contains a root system. Field data (soil water retention and conductivity curves, initial water content, environmental conditions) from this and previous studies conducted in the catchment were used as the input data. The root system and its hydraulic properties were determined from theoretical values from literature. The isotopic fractionation during evaporation was modelled using the Craig-Gordon model. The model was used to estimate root water uptake distribution, soil water potential, soil water content, and isotopic composition distribution.

How to cite: Alharfouch, L., Llorens, P., Hidalgo, J. J., Poyatos, R., Saurat, P., and Latron, J.: Stable isotope-based understanding of water fluxes in the critical zone at the forest plot scale under Mediterranean climate, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-166, https://doi.org/10.5194/egusphere-egu23-166, 2023.

Plant water use in hydrologic, land-surface, and earth system models is frequently estimated by a series of equations reliant on unknown model parameters controlling plant hydraulic function. Estimating these plant hydraulic traits is critical for accurate simulation of terrestrial water storage, flow paths, tree resistance to drought, and ultimately, ecosystem response to climate change. Despite the prevalence of δ_XYLEM observations, few studies have used δ_XYLEM to estimate plant traits numerical ecohydrologic models We calibrated EcH₂O-iso, an isotopic-enabled, fully distributed ecohydrologic model, with δ_XYLEM observations of 30 Eastern Hemlock (Tsuga canadensis) trees across seven months. Calibrated values for maximum stomatal conductance, canopy light interception, and rooting depths were validated with independent datasets of latent heat flux, canopy light interception, and δ_XYLEM from a nearby hemlock stand. Results indicate significant correlations between tree diameter (DBH), topographic position, and the calibrated values of several vegetation traits. Our results demonstrate that δ_XYLEM data can be used to accurately parameterize plant traits; however, the locations and sizes of the sampled trees should be considered when upscaling measured or calibrated plant-traits from individual trees into larger horizontal scales.

How to cite: Li, K., Kuppel, S., and Knighton, J.: Integrated Xylem Water Isotopic Observations and Process-based Ecohydrological Model Simulations Reveal Species-Level Plant Hydraulics, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7685, https://doi.org/10.5194/egusphere-egu23-7685, 2023.