Displays

Terrestrial vegetation is a main driver of ecosystem water fluxes, as plants mediate the water fluxes within the soil-vegetation-atmosphere continuum. Stable isotopologues of water are efficient tracer to follow the water transfer in soils, uptake by plants, transport in stems and release into the atmosphere through stomata. The development of in-situ methods coupled to isotope spectroscopy does now enable real-time in-situ water vapour isotopologue measurements revealing high spatial and temporal dynamics, such as adaptations in root water uptake depths (within hours to days) or the impact of transpirational fluxes on atmospheric moisture.

Examples will be given how isotopes can be used to inform the complex interplay between plant ecophysiological adaptations and hydrological processes. For example, root water uptake is not solely driven by soil water availability but has to be understood in the context of species-specific regulation of active zones in their rooting system determining the conductivity between soil and roots regulating uptake depths. The latter has also to be evaluated in context of the nutrient demand and the spatial nutrient availability. Similarly, plant water transport and losses are a fined tuned interplay between species-specific structural and functional adaptations and atmospheric processes.

Finally, first data of a large-scale ecosystem labelling experiment at the Biosphere 2 tropical rainforest of the B2 Wald, Atmosphere, and Live Dynamics (B2WALD) will be presented.

How to cite: Werner, C.: Tracing plant water fluxes in ecosystems by stable isotopes along the soil-plant and plant-atmosphere interfaces, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11289, https://doi.org/10.5194/egusphere-egu2020-11289, 2020.

Water uptake under variable soil water supply is highly critical for the functioning of trees and the services provided by forests. Current climate projections predict an increasing variability of precipitation and thus a higher frequency of droughts alternating with extreme precipitation events. Reduced water availability is the most critical driver for tree mortality and impairment of trees’ functions. Under variable water supply, both the ability of a plant species to utilize remaining water under drought and to immediately capitalize on soil rewetting from subsequent rainfall events will be crucial for its survival and competitiveness. High uncertainty still exists regarding the ecohydrological belowground interactions at the soil–root interface on short to seasonal time scales.

To overcome previous limitations, we carried out high-resolution in situ observations of δ¹⁸O in soil and xylem water to track the water uptake of beech trees based on the approaches of Volkmann et al. (2016a & b) in the hot dry summer 2018. We set up a laser isotope system to continuously probe the δ¹⁸O signature in the water vapor in equilibrium with the soil water at different soil depths and with the xylem of beech trees in a forest in Switzerland and applied a Bayesian isotope mixing model (BIMM) to resolve the origin of the water taken up. Moreover, we installed xylem flow sensors, dendrometers and soil moisture sensors in the trees.

Mid of June the drought period started with extended phases of high temperature and only infrequent precipitation. At the same time, soil water content sharply decreased, especially in the upper soil layers and transpiration as well as radial growth started to decline, and this pattern became more pronounced until the end of August. In the soil water, strong ¹⁸O enrichment in the upper 5 cm and slighter enrichment in 15 cm developed during this period. The BIMM results indicated that tree xylem water was made up by > 80% of shallow soil water (0-15 cm) at the onset of the drought and that this contribution continuously dropped to < 20% by the end of August, when deeper soil water and groundwater became more important. End of August, intensive rainfall events along with decreasing temperatures terminated the drought period when shallow soil water pools became partially replenished, and transpiration increased again. Within days, the contribution of shallow soil water to tree xylem water increased and reached a share of > 70% a couple of weeks after the end of the drought.  With the in situ method applied here, real-time information of the plasticity of soil water use becomes available and we can l trace the effect of drought and drought release on root activity of trees in different soil depths.

Volkmann THM, Haberer K, Gessler A, Weiler M. 2016a.High-resolution isotope measurements resolve rapid ecohydrological dynamics at the soil–plant interface. The New phytologist210: 839-849.

Volkmann THM, Kühnhammer K, Herbstritt B, Gessler A, Weiler M. 2016b.A method for in situ monitoring of the isotope composition of tree xylem water using laser spectroscopy. Plant, Cell and Environment9: 2055–2063.

How to cite: Geßler, A., Bächli, L., Treydtre, K., Saurer, M., Häni, M., Zweifel, R., Rigling, A., Schaub, M., Seeger, S., Herbsritt, B., Weiler, M., and Meusburger, K.: Where does the water come from? Variations in soil water uptake depth in a beech forest during the 2018 drought, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3451, https://doi.org/10.5194/egusphere-egu2020-3451, 2020.

Every drought period will eventually end and plants will have access to water again. This phase of “re-watering” is a critical point that will either ensure survival or collapse of ecosystems. The drought years 2018 and 2019 have laid bare how vulnerable Central European forest-systems are, even under short-term water scarcity. To understand the effects of repeated summer drought and its release on mature forest stands we investigated the recovery of a mixed forest stand. After 5 years of repeated experimental summer drought on roughly 100 trees (with n = 6 plots) the second phase of the Kranzberg Forest Roof (k.roof) experiment was started, which focuses on the re-watering with Deuterium labeled water (²H₂O) of the mature stand composed of European beech (Fagus sylvatica (L.)) and Norway spruce (Picea abies (L.)H.Karst.). According to our hypotheses the water household of the more anisohydric beech will recover faster and “stronger” (higher resilience) than the more isohydric spruce, due to the differences in stomatal control (hypothetical hydraulic regulation in beech vs. hormonal (ABA) control in spruce). We simulated a rainfall event to end our experimental drought and labeled the throughfall-exclusion (TE) and control (CO) plots of the k.roof experiment with roughly 13000 L for TE and 2000 L for CO of ²H₂O enriched water, i.e. δ²H 1500 and 400 ‰ respectively. We traced the ²H₂O signal along the soil-plant-atmosphere continuum (SPAC) from the soil through the stems and branches up to the leaves with conventional and real-time techniques (xylem sensors connected to CRDS system). Additionally, we measured leaf water potential and pressure-volume (PV) curves to assess the release of the drought stress. The distribution of the “new” water within the soil happened within a few days and we could not find any differences between the beech, mix or spruce dominated sites. However, the water uptake of the trees was significantly delayed in spruce compared to beech, evident from both the deuterium tracer signal (in stems and leaves) and leaf water potential. However, release of osmotic adjust was not different in the two species. The data allow for estimating the drought resilience of the water household of a mature forest stand after five-years of repeated summer drought and subsequent re-watering. While both species recovered their water household after several months to the same level as the control trees, we found beech to react faster and stronger than spruce.

How to cite: Hesse, B. D., Gebhardt, T., Hafner, B., Häberle, K.-H., and Grams, T. E. E.: K.ROOF II - Re-Watering after 5 Years of Repeated Summer Drought in Mature Beech and Spruce: Assessing Water Uptake and Allocation via Deuterium Labeling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7477, https://doi.org/10.5194/egusphere-egu2020-7477, 2020.

Labeling techniques have been widely applied in the literature to infer profiles of plant relative root water uptake (RWU). By enhancing the rather flat water isotopic composition gradient in soil with labeled water, relative RWU values from a set of soil water “sources” can be determined from inversion of isotopic data with greater confidence. This is usually done in the isotopic community through Bayesian multi-source mixing models. These models are not demanding in terms of data (only isotopic data is required) but do not incorporate knowledge water transport processes. Combined observations of water status and flow (e.g., soil water matric potential and transpiration rate) and soil-root hydrodynamics models allow mining deeper into isotopic data, and provide novel insights into the spatiotemporal dynamics of water transport across plants.

A one-dimensional and isotope-enabled soil-root physically-based model was used to simulate both water and isotopic measurements recorded during a 34-hour long intensive labeling experiment where a population of tall-fescue (Festuca arundinacae) was grown in a macro-rhizotron (0.2 m² surface area, 1.6 m depth). Above-ground data included tiller and leaf water oxygen isotopic compositions (δ_tiller and δ_leaf) as well as leaf water potential (ψ_leaf) and transpiration rate. As for below-ground data, profiles of root length density (RLD), soil water content and isotopic composition were destructively sub-hourly sampled. A first analysis of the results showed a striking decorrelation in temporal dynamics of water status and isotopic information.

There was no scenario in which the soil-root model could simulate both ψ_leaf and δ_tiller time series well. While the model-to-data fit for ψ_leaf was satisfying (R²=0.67), none of the tested root system groups of varying rooting depths could reproduce the measured temporal fluctuations of δ_tiller (R²=0.00). The model however showed the great sensitivity of δ_tiller to the population average rooting depth at the labeling point, thereby suggested spatial heterogeneity as the explanation for the observed temporal dynamics.

For comparison, one Bayesian mixing model was used and could successfully reproduce the δ_tiller high temporal dynamics induced by the labeling of deep soil water. If it succeeded in simulating RWU profiles, it was obviously at the expense of physical consideration: the strong variations in δ_tiller were translated into strong changes of RWU profile, which appeared not to be driven by environmental factors such as ψ_leaf and transpiration rate.

This study highlights the need for a holistic view, i.e., complement isotopic measurements with data on water status and calls for the use of physically-based soil-root model, especially in the context of labeling experiments.

How to cite: Couvreur, V., Rothfuss, Y., Meunier, F., Bariac, T., Biron, P., Durand, J.-L., and Javaux, M.: Disentangling temporal and population variability in plant root water uptake from stable isotopic analysis: a labeling study, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18071, https://doi.org/10.5194/egusphere-egu2020-18071, 2020.

How to cite: Miguez-Macho, G. and Fan, Y.: A Global Picture of the Depth and Origin of Soil Water Uptake by Vegetation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20821, https://doi.org/10.5194/egusphere-egu2020-20821, 2020.

In plants, the constant demand for water driven by transpiration is supplied by uptake from the soil through the roots. Alternative water-uptake pathways through the leaves and the bark have been demonstrated for some species, mainly conifers. Alternative water-uptake pathways could allow plants to complement their water supply with canopy interception, fog or dew, sources often assumed unavailable as they are lost via evaporation before they can contribute to soil water recharge. Bark water-uptake has been putatively linked to repair of xylem embolism, although this has only been demonstrated in cut branches and/or under artificial conditions. We hypothesized that besides embolism repair, bark water uptake might also contribute to maintaining the transpiration stream in upper canopy branches when the xylem water column is subject to excess negative pressure, either because temperature drops, and water viscosity increases, or under high vapour pressure deficit and low soil water availability. We used a novel labelling methodology combining online measurements of the isotope composition (δ²H and δ¹⁸O) of the transpiration stream with analyses of δ²H and δ¹⁸O from leaf, bark and xylem water in Pinus sylvestris and Fagus sylvatica. We conducted sampling campaigns in two study sites: a boreal (northern Sweden) and a temperate (northern Spain) forest. We applied semi-permeable bandages injected with ²H-enriched water (0.8% ²H₂O), on intact upper canopy branches (7-13 m), and monitored δ²H and δ¹⁸O of the transpiration stream with a Cavity Ring-Down Spectrometer (CRDS) in three branches (only P. sylvestris in Sweden) for 24 h and then sampled branch segments 2 cm upstream and downstream of the bandage. We determined δ²H and δ¹⁸O of leaf, bark and xylem water from sampled segments with a CRDS after cryogenic extraction. Xylem, bark and leaf water from segments downstream of the bandage were enriched in δ²H with respect to their corresponding upstream segments. The δ²H and δ¹⁸O from leaf, bark and xylem water from upstream segments were similar to those of control branches (no bandages). Results were similar for both study species, sites and campaigns, indicating that bark water uptake is not restricted to gymnosperms and may be more ubiquitous than previously considered. Enrichment in δ²H in the transpiration stream was also detected in one out of the three continuously monitored pine branches within the 12 h following the bandage application. Our results demonstrate that water taken up through the bark may be incorporated into the transpiration stream and that transpiration might not solely rely on water absorbed through the roots and transported through the main stem. This could imply, for example, that sapflux measurements would underestimate canopy transpiration. Combining empirical flux measurements with stable isotopes and/or other atmospheric tracers could render more realistic estimates of transpiration and help constrain partitioning of evaporation and transpiration and its coupling to gross primary productivity.

How to cite: Gimeno, T., Saavedra, N., Barbeta, A., Stangl, Z. R., García-Plazaola, J.-I., Wingate, L., and Marshall, J. D.: Revealing the bark water uptake of isotopically-enriched water by intact branches in the field and its potential contribution (or consequences) to (or for) transpiration estimates, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16180, https://doi.org/10.5194/egusphere-egu2020-16180, 2020.

Fine-scale sampling of secondary growth, isotopic composition and wood anatomical features has enabled the study of intra-seasonal tree growth dynamics in response to environmental and ecophysiological processes, and this fine-scale sampling approach has led to a re-examination of our fundamental understanding of how environmental factors are recorded in δ¹⁸O of cellulose (δ¹⁸O_cell). High resolution xylem anatomical analyses, such as wood density, lumen area (LA), cell wall thickness (CWT), and blue intensity have also been used to understand tree response to climate. However, linking wood anatomical traits with their isotopic signature has not yet been explored, but can provide new insights on the interpretation of the δ¹⁸O_cell through time. In this study, we test the response of wood anatomical features in Pinus ponderosa and Pseudotsuga menziesii, including cell-wall thickness (CWT) and lumen area (LA), along with the oxygen isotopic composition of α-cellulose (δ¹⁸O_cell) to shifts in relative humidity (RH) in two treatments: one from high to low RH and the second one from Low to high RH. We observed a significant decrease in LA and a small increase in CWT within the experimental growing season in both RH treatments. The measured δ¹⁸O_cell along the tree ring was also responsive to RH variations in both treatments. However, estimated δ¹⁸O_cell did not agree with measured δ¹⁸O_cell when the proportion of exchangeable oxygen during cellulose synthesis (P_ex) was kept constant. We found that modeled δ¹⁸O_cell agreed with measured δ¹⁸O_cell only when P_ex increased through the ring formation; we also found that P_ex linearly decreased with an increase in standardized LA. Based on this varying P_ex within an annual ring, we propose a targeted sampling strategy for different hydroclimate signals: earlier season cellulose (larger LA) is a better recorder of relative humidity while late season cellulose (smaller LA) is a better recorder of source water.

How to cite: Hu, J., Szejner, P., Clute, T., Anderson, E., and Evans, M.: Reduction in lumen area increases the amount of δ18O exchange with source water during cellulose synthesis, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11215, https://doi.org/10.5194/egusphere-egu2020-11215, 2020.

Cities represent a significant source of atmospheric elemental carbon (EC), a minor constituent of particulate matter (PM) but a major climate-forcing agent and air pollutant. Urban trees scavenge PM and regulate material fluxes to the ground. As such, urban trees represent potentially important sinks—not only for PM but also for EC—in urban landscapes. Here we assess the magnitude and spatiotemporal drivers of EC removal by trees in urban atmospheres. We quantified foliar EC accumulation by, as well as throughfall EC flux under, the canopy of two oak species (Quercus stellata: post oak; Quercus virginiana: live oak), which are widespread across the southern United States. Sampling was conducted from March 2017 to March 2018 across the City of Denton, a city at the northern edge of the Dallas-Forth Worth metropolitan area in Texas. Over the year-long study period, we found that post oak tree canopies accumulated two times more EC (0.53 mg EC m^-2 leaf d^-1) than live oak trees (0.22 mg EC m^-2 leaf d^-1), with 95% of EC depositing to leaf surfaces as opposed to leaf waxes. Throughfall EC fluxes were also greater under post oak (0.15 mg EC m^-2 d^-1) compared to live oak (0.12 mg EC m^-2 d^-1) canopies, but these differences between post oak and live oak were far less pronounced than for foliar EC accumulation. These results suggest that considerable amounts of dry-deposited EC are retained in post oak canopies, reducing species differences in throughfall EC fluxes. Our findings also revealed strong, albeit, contrasting seasonal patterns for foliar EC accumulation and throughfall EC fluxes. For both tree species, EC accumulation on canopy surfaces increased, whereas throughfall EC fluxes decreased from spring to fall, providing additional evidence that EC retention on canopy surfaces results in decreased EC fluxes to the ground. In summary, our findings show that urban oak trees scavenge considerable amounts of EC from the atmosphere and that the magnitude of accumulation and delivery to soil vary by species and season. This research highlights the potential for urban trees and forests to contribute to climate and air quality mitigation.

How to cite: Ponette-Gonzalez, A., Rindy, J., Barrett, T., Chen, D., Elderbrock, E., Ko, Y., Lee, J.-H., Sheesley, R., and Weathers, K.: Oak trees are elemental carbon sinks in urban ecosystems: patterns and drivers, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10735, https://doi.org/10.5194/egusphere-egu2020-10735, 2020.

Stemflow production has been reported to be influenced by a suite of biotic and abiotic factors, and those factors would be quite different considering local and global scales. Although the number of published stemflow studies showed a steady increasing trend in recent years, the relative contributions of biotic and abiotic factors to stemflow production were still largely unclear due to the large number of influencing factors and the complex interactions among those factors. Here we present stemflow results conducted from both from local scale and global scale: (1) stemflow of nine xerophytic shrubs of Caragana korshinskii were measured in nearly nine growing seasons from 2010 to 2018 within a desert area of northern China, accompanying with observing on six biotic variables (shrub morphological attributes) and ten abiotic variables (meteorological conditions); (2) a global synthesis of stemflow production results (stemflow percentage was reported) derived from Web of Science for more than 200 peer-reviewed papers published in the last 50 years (1970-2019), and ten most reported biotic factors (vegetation life form, phenology, leaf form, bark form, community density, community age, vegetation height, diameter at breast height, leaf area index, stemflow measuring scale) and four abiotic factors (climate types, mean annual precipitation, elevation, mean annual temperature) were considered. We performed a machine learning method (boosted regression trees) to evaluate the relative contribution of each biotic and abiotic factor to stemflow percentage, and partial dependence plots were presented to visualize the effects of individual explanatory variables on stemflow percentage, respectively.

How to cite: Zhang, Y., Wang, X., Pan, Y., and Hu, R.: How biotic and abiotic factors affect stemflow production? Insights from both local and global scales, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6314, https://doi.org/10.5194/egusphere-egu2020-6314, 2020.

Despite the fact that stemflow is often a small percentage of precipitation, it is a concentrated flux of water, solutes, and particulates to near-trunk soils. As a consequence, per unit area, near-trunk soils receive water and nutrient inputs that largely exceed those received by soils in the distal zone via throughfall. This funnelling effect of trees can contribute to preferential flow and groundwater recharge and can have important biogeochemical implications. However, to evaluate the importance of this flux for near-trunk soils is necessary to quantify the magnitude of the stemflow infiltration area.

This study presents a stemflow simulation experiment with the objective of determining the stemflow infiltration area in near-trunk soils.  The experiment was conducted at the Fair Hill Natural Resources Management area in northeastern Maryland (USA). We selected four American beech (Fagus grandifolia Ehrh.) trees with a DBH of ~29 cm, growing in a loam soil. Each tree was equipped with a collar, built with a tube with small holes, and installed around the tree. This tube was connected with a hose to a 36.5 L container positioned ~ 1 m above the collar. The hose had two stopcocks to regulate the water rate. Before starting the simulations, litterfall was removed.

A total of thirteen simulations were run with differing simulated stemflow rates (from 30 to 290 L/h) and differing initial soil moisture conditions (mean soil moisture from 25 to 43 m³m^-3). Soil moisture was measured around the trees before each simulation with a TDR device. To further increase soil moisture between simulations, 40 L of water were carefully applied circumferentially around the trunk, at a maximum distance of 35 cm. Each simulation was performed with different colour dye tracer to enable accurate measurements of the stemflow infiltration area. After each simulation, the infiltration area was measured using a mesh grid of known area. At the end of the last simulations soil samples were taken around each tree.

The results show that in all cases the infiltration area is < 0.1 m², with a mean value of about 0.03 m². Likewise, there is a tendency to decrease the area of infiltration by increasing soil moisture. This trend seems to be modified for saturated conditions or when the stemflow rate is extreme. These small stemflow infiltration areas are explained by both the high infiltration rates of near-trunk soils in forests and the macroporosity produced by living or decaying roots. Moreover, these trees have slight buttressing that increase the perimeter of contact between the stem and the soil (with respect to the basal perimeter (calculated at breast height)), thus further promoting infiltration. Results suggest the importance of measuring the infiltration areas for different species and soil conditions to better evaluate the relevance of stemflow.

How to cite: Llorens, P., Latron, J., Carlyle-Moses, D. E., Näthe, K., Chang, J. L., Nanko, K., Iida, S., and Levia, D. F.: Stemflow infiltration areas into forest soils around American beech trees , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8813, https://doi.org/10.5194/egusphere-egu2020-8813, 2020.

Besides precipitation also atmospheric deposition is modified by canopy processes. By passing the canopy, precipitation water washes out those deposited compounds and creates substantial heterogeneity of input of dissolved matter at the forest floor. At the same time, both dry atmospheric deposition of aerosols and net precipitation are affected by canopy heterogeneity, like variation in canopy density. In consequence, spatial patterns of both dry and wet deposition are expected to vary strongly in space and to depend on canopy structure, which may lead to hotspots of input and deep drainage. However, few studies so far have investigated the spatial patterns of deposition of e.g. nitrogen compounds. In this research we investigated the spatial and temporal patterns of nitrogen deposition and export from the main rooting zone in a beech dominated forest in the Hainich National Park.

We find that below canopy spatial patterns of both canopy drainage and nitrogen deposition show some temporal stability. Spatial variation in canopy drainage also affected soil water percolation in 30 cm depth, with higher canopy drainage leading to higher soil water fluxes. Nitrogen deposition at the forest floor however, seemed rather driven by canopy exchange than by drainage patterns or dry deposition. On the other hand, at 30 cm soil depth nitrogen export in seepage water was driven by the soil water flow, indicating that spatial patterns of transport capacity, and not nitrogen availability in the soil, determined the export of nitrogen from the main rooting zone. Interestingly, spatial variation of soil water fluxes was not dampened, but rather increased by passage of the rooting zone. In other words, the origin of spatial patterns of water flow and nitrogen export below the main rooting zone lay already within the canopy, but was further enhanced in the soil. The next steps will be to understand why the heterogeneity of water fluxes propagates and increases during rooting zone transit and whether there is an interaction with soil development.

How to cite: Hildebrandt, A., Bock, S., Fischer, C., Metzger, J., Weckmueller, J., and Demir, G.: Do spatial patterns of water and matter fluxes below the main rooting zone depend on canopy processes?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9845, https://doi.org/10.5194/egusphere-egu2020-9845, 2020.

Naturally regenerating or planted forests of broadleaf and pine species occupy ca. 2.3 million ha (>52% of the total area) of the Mid-hills of Nepal and provide a range of forest products and ecosystem services to local and downstream populations. These forested catchments give rise to numerous rain-fed springs and streams that undergo wide fluctuations in seasonal flows because of the concentrated monsoon rainfall (June to September), steep topography and rapidly draining soils. However, current understanding of the hydrological functioning of these forested landscapes is limited, particularly concerning the changes in forest structure and composition through natural succession and anthropogenic disturbances related to forest use (mostly collection of firewood, litter and fodder). We measured gross rainfall (P), throughfall, stemflow and overland flow in three major forest types in a Mid-hills catchment of Central Nepal, viz.: a predominantly planted pine forest (PF, ca. 35 years old), natural broadleaf forest (BF) and a mixed pine-broadleaf forest (MF). Because of differences in the dominant products provided by the three forest types they are subject to different levels of use by the local population. The PF is used only occasionally (litter harvesting and leisure) while the MF and BF are used throughout the year for nearly all three products. For the period of study (June 2015 – December 2016), measured throughfall values for the PF, MF and BF were 77.5%, 73.7% and 72.0% of incident P, respectively, with corresponding stemflow values of 0.6%, 1.3% and 1.6%, resulting in rainfall interception values of 21.9%, 25.1% and 26.4% for the PF, MF and BF, respectively. Corresponding amounts of overland flow for the PF, MF and BF were 4.7%, 9.8% and 11.4% of net precipitation (throughfall + stemflow), reflecting the relative intensity of forest-use related disturbances. Consistent removal of biomass and associated effects on the forest floor were found to negatively affect soil hydraulic properties. Our results highlight the need to take into account the effects of differences in forest use intensity when evaluating the forest-water relationships in Nepal’s Mid-hills as well as other locally used or managed forested landscapes in similar environments.

How to cite: Badu, M., Ghimire, C. P., Nuberg, I., Bruijnzeel, L. A., and Meyer, W. S.: Rainfall partitioning in three major types of forests in the Mid-hills of Nepal, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22314, https://doi.org/10.5194/egusphere-egu2020-22314, 2020.

Surface water dynamics can impact ecosystem functioning, in particular in seasonally water-limited regions. The potential importance of dew and other non-precipitation water sources have long been recognized in arid biomes, however, their quantification is uncertain as the collection of precise data of these water sources has been difficult with typical measurement techniques.

The recent development of high precision shear-stress cells and validated data processing methods enables to detect water fluxes of 0.01 mm at the resolution of minutes from precision lysimeters. Thus, they outperform classical measurement devices like rain gauges and eddy covariance (EC) measurements regarding their accuracy in inferring dewfall or night-time evapotranspiration.

In this study, we analyze multiple (non-)precipitation water flux components with respect to their relative importance and corresponding inter-annual dynamics based on large high precision lysimeters. This is done in a Mediterranean tree-grass ecosystem.

Specifically, we concurrently analyze several years of lysimeter data with meteorological data and characterize how the different components of the water cycle respond in time to meteorological drivers and vegetation properties. The study is conducted at the experimental site Majadas de Tietar, Extremadura, Spain. Our results show that the fraction of non-precipitation water can account for more than half of the ecosystems water input during the dry months. We further investigate the micro-meteorological dimension of these processes in this ecosystem and changes of the flux components under exceptionally wet and dry years.

This study contributes to recent efforts to better understand the role of non-precipitation water sources in seasonally water-limited ecosystems of high seasonal precipitation dynamics. Furthermore, this investigation underlines the necessity to raise awareness of the limitations of EC and rain gauge devices in precisely quantifying all water fluxes.

How to cite: Paulus, S., El-Madany, T., Wutzler, T., Orth, R., Perez-Priego, O., Reichstein, M., Carrara, A., Moreno, G., and Migliavacca, M.: Hidden water fluxes in a Mediterranean ecosystem: new insights into seasonal dynamics from lysimeter data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13489, https://doi.org/10.5194/egusphere-egu2020-13489, 2020.

Dew and fog have proven to be essential water sources for plants across many Earth’s ecosystems. Under climate change with more frequent no-rain days expected in summer, the inputs of dew and fog on short-statured temperate grassland species are expected to become more important as an additional water source. In 2018, Switzerland experienced the driest April to August period of the last five decades. Our research using stable water isotopes investigates how dew and ground radiation fog affected Swiss grasslands in the extreme summer 2018 based on three intensive observation nights of dew and fog. Focusing on an intensively managed grassland located at a valley bottom close to Chamau (CH-CHA) in Switzerland, we measured the isotopic composition (δ²H and δ¹⁸O) of near-surface atmospheric water vapour with a cavity ring-down spectrometer (Picarro L2130-i), and the isotopic composition of the droplets on the leaf surface. Combining the water isotopes with eddy-covariance and meteorological measurements, we analysed the isotope dynamics and fractionation during these three dew and fog nights. Our results indicated that during dew and fog formation, water vapour δ²H and δ¹⁸O gradually decreased under saturated and even slightly supersaturated conditions, but fluctuated under unsaturated conditions. The isotopes of the sampled droplets on the leaf surfaces deviated from the expected isotopic composition based on water vapour under the equilibrium fractionation assumption. During dew and ground radiation fog nights, condensed water was a mix from two sources, atmospheric water vapour and vapour flux from the ground to the foliage. Condensation processes were accompanied by the evaporation from leaf surfaces and the diffusion in the supersaturated layer above the leaf surfaces. This caused non-equilibrium fractionation of water isotopes, seen in the fluctuation of water vapour isotopes and in the observed deviation in the sampled droplets from equilibrium liquids of water vapour. Thus, the isotopic approach is complementary to the often employed micro-lysimetric approach and helps to understand the dynamics and the sources of water vapour during dew and ground radiation fog formation.

How to cite: Li, Y., Riedl, A., Aemisegger, F., Buchmann, N., and Eugster, W.: Tracing dew and fog water inputs into temperate grassland using stable water isotopes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13628, https://doi.org/10.5194/egusphere-egu2020-13628, 2020.

The terrestrial water cycle partitions precipitation between its two ultimate fates: "green water" that is evaporated or transpired back to the atmosphere, and "blue water" that is discharged to stream channels.  Measuring this partitioning is difficult, particularly on seasonal timescales.  End-member mixing analysis 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, and this latter question is the one we seek to answer.  Here we introduce "end-member splitting analysis", which uses isotopic tracers and water flux measurements to quantify how isotopically distinct inputs (such as summer vs. winter precipitation) are partitioned into different ultimate outputs (such as evapotranspiration and summer vs. winter streamflow).  End-member splitting analysis has modest data requirements and can potentially be applied in many different catchment settings.  We illustrate this data-driven, model-independent approach with publicly available biweekly isotope time series from Hubbard Brook Watershed 3.  A marked seasonal shift in isotopic composition allows us to distinguish rainy-season (April-November) and snowy-season (December-March) precipitation, and to trace their respective fates.  End-member splitting shows that about one-sixth (18±2%) of rainy-season precipitation is discharged during the snowy season, but this accounts for over half (60±9%) of snowy-season streamflow.  By contrast, most (55±13%) snowy-season precipitation becomes streamflow during the rainy season, where it accounts for 38±9% of rainy-season streamflow.  Our analysis thus shows that significant fractions of each season's streamflow originated as the other season's precipitation, implying significant inter-seasonal water storage within the catchment, as both groundwater and snowpack.  End-member splitting can also quantify how much of each season's precipitation is eventually evapotranspired.  At Watershed 3, we find that only about half (44±8%) of rainy-season precipitation evapotranspires, but almost all (85±15%) evapotranspiration originates as rainy-season precipitation, implying that there is relatively little inter-seasonal water storage supplying evapotranspiration.  This proof-of-concept study demonstrates that end-member mixing and splitting yield different, but complementary, insights into catchment-scale partitioning of precipitation into blue water and green water.  It could thus help in gauging the vulnerability of both water resources and terrestrial ecosystems to changes in seasonal precipitation.

How to cite: Kirchner, J. and Allen, S.: Seasonal partitioning of precipitation between streamflow and evapotranspiration, inferred from end-member splitting analysis, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6054, https://doi.org/10.5194/egusphere-egu2020-6054, 2020.

Ecohydrological separation has been observed across climates and biomes, and at a fundamental level suggests that water in mobile versus immobile domains may resist mixing over varying periods of time; however little mechanistic evidence exists to explain this separation at a process scale. Non-equilibrium flow in the vadose zone may partially account for widespread perception of distinct hydrological domains yet no studies have weighed its contribution. Using a simple isotope mixing technique, we sought to determine the amount of preferential flow necessary to maintain a two water worlds scenario (i.e., physical separation between mobile and immobile water pools). We constructed 60 cm soil columns (20 cm-ID PVC) containing low soil structure (sieved soil material), subsoil structure (intact B horizon), and soil structure without matrix exchange (tubing reinforced macropores) to simulate multiple preferential flow scenarios. Columns were subjected to 3 rain storms of varying rainfall intensity (~2.5 cm h ^-1, ~5 cm h ^-1, and ~11 cm h ^-1) whose stable isotope signatures oscillated around known baseline values. Isotopic analysis was performed on collected leachate and matrix water sampled via direct vapor equilibration. Preliminary estimates of matrix water indicate up to 100% mixing with infiltrating rain water under low rainfall intensity (2.5 cm h ^-1) in subsoil structure columns, whereas high intensity rain (11 cm h^-1) produced clear separation between columns with intact or artificial soil structure and those controlled for structure (low structure treatment). This separation was confirmed by preferential flow estimates; however minimizing matrix exchange (via artificial macropores) reduced preferential flow by a factor of 4 compared to soil with intact structure. These data suggest that distinct domain separation may only be possible under extreme precipitation intensity; and that exchange with less mobile storage in the soil matrix produces more preferential flow. We intend to use these estimates of preferential flow as a benchmark to understand the prevalence, persistence, and plausibility of ecohydrological separation. As a next step, we will use this conceptual framework to define how recurrent drought, elevated CO₂, and warming may alter the partitioning of mobile and immobile water in mountain grasslands.

How to cite: Radolinski, J., Pangle, L., Klaus, J., Scott, D., and Stewart, R.: Simulating preferential flow in a two water worlds framework, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-646, https://doi.org/10.5194/egusphere-egu2020-646, 2020.

Recent advances in stable isotope measurements within the soil-plant-atmosphere continuum have paved the way to high-resolution sub-daily observations of plant water supply (Stumpp et al. 2018, Volkmann et al. 2016a, 2016b). It seems time is ripe for in-depth assessments of long-standing yet much-debated assumptions such as complete, homogenous mixing of water in the vadose zone (“one water world” versus "two water world") or absence of fractionation during root water uptake and vascular transport in plants.

Information on the nature of these processes contained in high-resolution data sets needs to be exploited. One way to test hypotheses and thereby advance our understanding of soil-plant water interactions is by analysing observations with numerical simulations of the system dynamics – a method also known as inverse modelling. By evaluating the model performance and parameter identifiability of different model structures, conclusions can be drawn regarding the relevance of the modelled processes for reproduction of the observations. Testing two different models allows thus to assess the impact of the difference.

We develop a framework for numerical simulation and model-based analysis of observations from soil-plant-atmosphere systems with a focus on isotopic fractionation. A central objective is to facilitate the evaluation of different model structures and thus test model hypotheses. This can assist development of models specifically tailored to the intended purpose and available data. The framework will first be tested with the "SWIS" model presented by Sprenger et al. (2018).

As an illustration of the framework, we will test the model performance on a dataset of continuous, in situ observations of stable isotopes in xylem water of beech trees and soil water in four depths combined with observations of soil water content. The model assumes one-dimensional soil water flow taking place in one or two separate flow domains for tightly and weakly bound pore water. These two water pools are separated by a matrix potential threshold and isotopic exchange is modelled only through the vapour phase. Root water uptake is parametrised using the Feddes-Jarvis model. First results allow to assess the relevance of the two-pore domain hypothesis for the different soil depths and xylem water.

Sprenger, M., D. Tetzlaff, J. Buttle, H. Laudon, H. Leistert, C.P.J. Mitchell, J. Snelgrove, M. Weiler, and C. Soulsby. 2018. Measuring and modeling stable isotopes of mobile and bulk soil water. Vadose Zone J. 17:170149. doi:10.2136/vzj2017.08.0149

Stumpp, C., N. Brüggemann, and L. Wingate. 2018. Stable isotope approaches in vadose zone research. Vadose Zone J. 17:180096. Doi: 10.2136/vzj2018.05.0096

Volkmann, T.H., K. Haberer, A. Gessler, and M. Weiler. 2016a. High‐resolution isotope measurements resolve rapid ecohydrological dynamics at the soil–plant interface. New Phytologist, 210(3), 839-849.

Volkmann, T.H., K. Haberer, A. Gessler, and M. Weiler. 2016a. High‐resolution isotope measurements resolve rapid ecohydrological dynamics at the soil–plant interface. New Phytologist, 210(3), 839-849.

How to cite: Bernhard, F., Seeger, S., Weiler, M., Gessler, A., and Meusburger, K.: Simulating soil-plant-atmosphere interactions for sub-daily in situ observations of stable isotopes in soil and xylem water to assess two-pore domain model hypothesis, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17975, https://doi.org/10.5194/egusphere-egu2020-17975, 2020.

Against the background of a future decrease in water availability, there is a need to use irrigation water with higher efficiency. To improve water management, it is crucial to clarify the role of irrigation water compared to soil water and additional water sources, including groundwater, which is often neglected by most water balance models.

We used deuterium-enriched water as tracer to distinguish irrigation water from soil water and groundwater and evaluate its contribution to the apple tree water uptake. The study was conducted in an apple orchard (Malus domestica, cv. Pinova) located in a flat area of the Venosta valley (South Tyrol, Italy) characterized by shallow groundwater (about 0.9 m from the ground). Before the experiment, the soil was covered for two weeks to prevent rain and irrigation from entering the soil. In July 2019, deuterium-enriched water (40 L/m², δ²H = 1500 ‰) was homogenously applied to the soil in four plots. In the proximity of each irrigated plot, not-irrigated trees were present (controls). From both irrigated and control plots, soil, leaf and shoot axis samples were collected starting from 2 hours until 7 days after the irrigation. Total tree and soil water was extracted through cryogenic vacuum distillation. Soil and plant water isotope composition was measured at the IRIS (Isotope Ratio Infrared Spectroscopy) and at the IRMS (Isotope Ratio Mass Spectrometry) analyzer, respectively. Reference ET for the period was 3.3 mm day^-1 on average.

Soil moisture in both irrigated and control soils decreased from the surface to 0.4-0.5 m soil depth and then progressively increased again until 0.8 m depth, in line with a maximum capillary rise of approximately 0.4 m estimated by models for a silty loam soil. In the upper 0.5 m soil layer, where around 80 % of total fine roots were concentrated, labeled irrigation water represented ca. 20 % of total soil water. The labeled water firstly appeared in the shoots starting from 8 hours from the irrigation (average δ²H = 27.4 ‰) and the deuterium concentration reached its maximum after 24-48 hours from water supply (δ²H = 68.1 ‰). At this time, irrigation water accounted for 8 % of the shoot extracted water. Considering the average deuterium abundance of the extracted water in the first 0.5 m soil layer, where labeled irrigation water mixed with soil water, we estimated that 35-40 % of the shoot water had been absorbed from such a layer. These preliminary results highlight the complexity of soil-water-plant interactions and call for additional investigation to understand the role of the soil water present before irrigation that could be preferentially taken up by roots. Additionally, the contribution of an upward flux from groundwater should be quantified.

How to cite: Aguzzoni, A., Engel, M., Zanotelli, D., Comiti, F., and Tagliavini, M.: Estimating irrigation contribution to apple tree water uptake by deuterium tracing, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13382, https://doi.org/10.5194/egusphere-egu2020-13382, 2020.

Stable water isotopes are promising tracers to study soil-tree interactions and root water uptake. Traditionally, destructive sampling techniques are applied to measure the isotopic signature in soils and plant tissues but these methods are limited in their temporal resolution. For calculating ecohydrological travel times from soil water to transpiration, high frequent isotope measurements are required. Recently, in-situ water isotope probes have been successfully applied in beech trees to yield high-frequent isotope measurements under field conditions but the complexity and heterogeneity of natural field conditions can make a systematical method testing difficult. Here, we test whether the new probes are capable of capturing tree species-specific differences in root water uptake and associated travel times.
We test this in a controlled experiment using large pots with three 4-6 meter high and 20 year old coniferous and deciduous trees: Pinus pinea, Alnus x spaethii and Quercus suber that are expected to have different water uptake strategies. We applied deuterated irrigation water to the homogeneous soils in the pots and traced the water flux from the soils through the trees with in-situ isotope probes in high temporal resolution.
This contribution presents preliminary results on ecohydrological travel times in relation to environmental parameters such as sap flow, photosynthetic activity, matrix potential, soil water content, water vapor pressure deficit and solar radiation.
Our in-situ isotope probes were capable to capture the breakthrough of the isotope tracer in all trees. The calculated travel times were shorter for the Pinus and Alnus compared to the Quercus which suggests differences in root water uptake. Detailed results from such controlled experiments are fundamental for testing new measurement techniques such as the in-situ isotope probes. Such results are important to better interpret results measured under natural and therefore more complex and heterogeneous field conditions.

How to cite: Mennekes, D., Rinderer, M., Seeger, S., de Boer, H., Orlowski, N., and Weiler, M.: Isotope labeling experiment to infer ecohydrological travel times, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-218, https://doi.org/10.5194/egusphere-egu2020-218, 2020.

Water isotope tracing techniques in combination with laser-based isotopic analyses have advanced our understanding of plant water uptake patterns providing opportunities to carry out observational studies at high spatio-temporal resolution. Studying these highly dynamic processes at the interface between soils and trees can be challenging under natural field conditions, as available water resources are difficult to control. On the other hand, the results of small pot experiments in the greenhouse using tree seedlings are often difficult to transfer to mature trees. Here, we setup a controlled outdoor large pot experiment with three different, 4-6 meter high and 20 year old trees: Pinus pinea, Alnus spaethii and Quercus suber. We took advantage of stable water isotope techniques by tracing plant water uptake from the root zone through the xylem via isotopically labelled irrigation water. We combined ecohydrological observations of sapflow, photosynthesis, soil moisture and temperature and soil matrix potential with high resolution measurements of water stable isotopes in soils and trees to understand how soil water is used by different tree species. We monitored the isotopic composition of soil and xylem water in high temporal resolution with in-situ isotope probes installed at different depths in the soil and different heights in the tree stem. We further compared the water isotopic composition of our in-situ monitoring setup with destructive sampling methods for soil and plant water (vapour equilibration method and cryogenic extraction).

Our results from the continuous monitoring showed a distinct difference in the xylem sap isotopic signature between Quercus on the one hand and Alnus and Pinus on the other hand. This is likely due to different water use strategies of these tree species. The tree xylem isotopic signature of Alnus and Pinus responded to the isotopic label within one day and six days at 15 cm and 150 cm stem height, respectively. The peak isotopic signature in the tree xylem due to the label application was similar to the isotopic signature of the soil in 30 cm (for Alnus) and 15 cm (for Pinus). Quercus showed a delayed and much slower increase in the xylem isotopic signature in response to the label and the highest values were significantly lower than the corresponding soil isotopic signatures. Our methodological comparison showed that the isotopic signature of the destructive samples (from both methods) had a larger spread and this spread tended to become larger with subsequent labeling. Destructive soil samples showed a wider isotopic variation than destructive xylem samples. The in-situ isotope measurements in comparison showed a relative constant small to medium spread for soil and xylem isotopic measurements. Our in-situ isotope probes therefore seem to be a potential alternative or supplement to destructive sampling offering much higher temporal resolution. The continuation of the labeling experiments in 2020 will allow us to further study tree-species specific water uptake strategies, which will become important under future climatic conditions in terms of development of adaptation strategies for sustainable forest management.

How to cite: Orlowski, N., Seeger, S., Mennekes, D., de Boer, H., Weiler, M., and Rinderer, M.: Monitoring tree species-specific water uptake strategies via continuous in-situ stable water isotope measurements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16786, https://doi.org/10.5194/egusphere-egu2020-16786, 2020.

Several ecohydrological problems such as when and where precipitation becomes the source of plant uptake are usually tackled through stable isotope measurements. Our ability to go after these questions is often limited by field conditions that cannot be controlled, but targeted manipulation experiments can go beyond some of these limitations by imposing known boundary conditions and allowing the experimental closure of the isotope balance. This contribution presents examples from existing experiments that aim to understand which water, in terms of age and tracer composition, is uptaken by vegetation or drained to deeper soil horizons. In particular, we illustrate the Spike II experiment, which was carried out on a large vegetated lysimeter within the EPFL campus (CH) in 2018. This experiment featured the application of 40 mm of isotopically-enriched water on top of the lysimeter and its tracking for 40 days through the soil water, the lysimeter bottom drainage and the plant xylem. A total of more than 900 water samples were collected to reconstruct the “story” of the labeled precipitation. The detailed results from such controlled experiments represent a fundamental “ground truth” for our understanding of root water uptake patterns in large and diverse landscapes.

How to cite: Asadollahi, M., Benettin, P., Nehemy, M., Rinaldo, A., and McDonnell, J.: Manipulation experiments to infer the age and tracer composition of hydrologic fluxes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8134, https://doi.org/10.5194/egusphere-egu2020-8134, 2020.

Plant water stable isotopes (δ¹⁸O, δ²H) have been used in eco-hydrological, biogeochemical and hydrological studies to e.g., quantify terrestrial water fluxes or to determine plant water sources. Current plant water extraction methods for isotope measurements are either expensive, labor-intensive or can lead to isotopic fractionation. Recent studies employed a new, extraction-free measurement method that was originally developed for the analysis of isotopes in sediment pore water: the water-vapor equilibrium method. It still needs to be tested if this method can be reliably used for isotope analysis of plant samples and how to best prepare the samples. Therefore, we investigated the effects of various preparation steps when measuring the plant water stable isotopes using this new method. We chose tomato and strawberry plants and prepared roots, shoots, leaves and fruits by either grinding or cutting them into pieces. Further, the necessary sample amount and the effect of equilibration time was evaluated. We investigated the effect of the preparation steps on mean values, standard deviations and a measurement device-specific value (LWV) that indicates a negative impact of volatile organic compounds (VOC) on reported isotope values. Results showed that an equilibration time longer than 24 hours is not advisable as the relationship between δ¹⁸O and δ²H of all plant samples worsened with R² declining from 0.97 to a minimum of 0.16. Additionally, the LVW indicated the influence of VOC with progressing equilibration time. Optimum amounts of plant material for roots were 3 g while for all other plant parts 5 g was necessary. In contrast to cut samples, kinetic fractionation effects were observed for grinded samples which could also be apparent fractionation effects because of the observed changes in LWV indicative of VOC interferences. For both plants the successive enrichment of the irrigation water from roots to leaves was observed. Fruits showed differences in their isotopic composition of the water stored inside the fruit compared to the water in the skin, with the inside water closer to the applied irrigation water. The intersection of the dual-isotope plot of all measured plant samples with the local meteoric water line was close to the applied irrigation water, making it theoretically possible to acquire information about the plant source water and enrichment factors in future studies when using the water-vapor equilibration method. From the findings of this study protocols can be established for sample preparation and plant water stable isotope analysis using the water-vapor equilibrium method.

How to cite: Stockinger, M., Santos Pires, S., and Stumpp, C.: Analysis of plant water stable isotopes using the water-vapor equilibrium method, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8859, https://doi.org/10.5194/egusphere-egu2020-8859, 2020.

Urban areas, more than many experimental catchments, are characterized by a markedly heterogeneous distribution of land covers, with different degrees of permeability that radically vary partitioning of precipitation into evapotranspiration (“green” water fluxes) and runoff and groundwater recharge (“blue” water fluxes). While the quantification of ecohydrological fluxes using stable isotopes in water as environmental tracers has been an established method for many years, surprisingly few studies have been applied to the highly complex urban water cycle. To determine the effects of representative urban green space “types” on water partitioning, we carried out plot-scale studies at a heterogenous field site in Berlin-Steglitz that integrates climate, soil moisture and sap flow data, with isotope sampling of precipitation and soil moisture on a regular basis. Soil moisture and isotope measurements were conducted at different depths and under contrasting soil-vegetation units (grassland, trees, shrub) with different degrees of permeability. Our investigations revealed uniformly decreasing soil moisture content during the dry summer of 2019, with only temporary re-wetting of the uppermost soil layers despite heavy convective precipitation events. Soils under trees were driest, whilst grassland soils were wettest, with shrubs intermediate. Isotope-based modelling indicated that this was the result, of greater interception, transpiration and – surprisingly – soil evaporation from forest sites. The isotope signatures of soil water also revealed stronger “memory effects” of summer drying in forest soils, which persisted until the major re-wetting of the system in autumn allowed drainage from the soil profile to contribute to groundwater recharge. Modelling showed that recharge under grasslands could be over 3 times higher compared to under trees and shrubs. Upscaling these findings with large-scale isotope studies of surface and groundwater across Berlin highlights the importance of the vegetation in urban green spaces to water partitioning in heterogeneous city landscapes and the need for careful integration of vegetation management in urban water and land use planning.

How to cite: Kuhlemann, L.-M., Tetzlaff, D., Kleinschmit, B., Vulova, S., and Soulsby, C.: Using stable isotopes to quantify ecohydrological flux dynamics at the soil-plant-atmosphere continuum in urban green spaces, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-160, https://doi.org/10.5194/egusphere-egu2020-160, 2020.

In the face of current global warming conditions, temperate forest ecosystems are expected to be strongly affected by temperature increase and more frequent and intense water shortage. This leads to severe stress for forest vegetation in many temperate systems. Therefore, understanding the vegetation water use in temperate forests is urgently needed for more effective forest management strategies. Root water uptake (RWU) is a species-specific trait (tree physiology and root architecture) and its spatio-temporal patterns are controlled by a range of site-specific (e.g., topography, geology, pedology) and meteorological factors (e.g., temperature, soil humidity, rainfall.

In the present study, we use stable water isotopologues as ecohydrological tracers combined with continuous measurement of hydrometeorological (weather variables, groundwater levels, soil moisture, streamflow) and physiological (sap flow, radial stem growth) parameters to investigate the spatio-temporal dynamics of water uptake for beech (Fagus sylvatica L.) and sessile oak (Quercus petraea (Matt.) Liebl) trees along a hillslope in a Luxemburgish catchment.

Fortnightly field campaigns were carried out during the growing season (April-October) 2019 to sample water from xylem, soil water at different depths, groundwater, stream water, and precipitation. Soil water isotopic composition and xylem water were extracted via cryogenic distillation. Grab sampling was performed for the other water pools. The isotopic composition was determined through laser spectroscopy and mass spectrometry (for xylem samples only).

From preliminary results, the isotopic composition of xylem water shows a marked seasonal variability suggesting a plasticity in RWU or a change in the isotopic composition of the water pools over the growing season. Moreover, beech and oak trees exhibit different uptake strategies when water supply is low. Within the range of observed groundwater variation topography does not play a statistically significant role on RWU.

How to cite: Fabiani, G., Penna, D., and Klaus, J.: How do meteorological variables and topography control species-specific water uptake strategy along a forested hillslope?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6954, https://doi.org/10.5194/egusphere-egu2020-6954, 2020.

Stable water isotopes have proven to be useful tracers to determine the origin of water taken up by plants, quantify the relative contributions of water sources to stream runoff and investigate water flow paths. However, the presence of different water pools in a catchment and soil water allocation complicates our understanding of water cycling, and calls for research on processes governing soil water movement and storage, as well as interactions between soil and plants.

In this study, we used isotopic data from a forested catchment in the Italian pre-Alps to i) investigate the spatial and temporal variability of the isotopic signature of various water sources, and ii) determine which waters are used by beech and chestnut trees in the study area.

Ecohydrological and hydrometeorological monitoring took place in the 2.4-ha Ressi catchment (Northern Italy). Elevations range from 598 to 721 m a.s.l., while average slope is 31°. Average annual precipitation is about 1695 mm, while average annual temperature is 9.7 °C. The entire catchment is covered by deciduous forest, with beech, chestnut, hazel and maple as the main tree species.

Water samples for isotopic analysis were taken monthly from bulk precipitation, approximately bi-weekly from stream water, groundwater and soil water by two suction lysimeter cups in the riparian zone. Bulk soil water samples and twigs for xylem water extraction by cryogenic vacuum distillation were collected starting in June, 2017. All water samples were analysed by laser spectroscopy, except xylem water that was analysed by mass spectrometry.

Stream water, groundwater and soil water extracted by suction lysimeters were isotopically similar to precipitation and aligned to the local meteoric water line. Bulk soil water obtained by cryogenic vacuum distillation showed an evaporation signature, especially on the hillslope where soil moisture was lower and soil water had been extracted by suction lysimeters only during or just after a large rainfall event. This indicates that soil water sampled by suction lysimeters and extracted by cryogenic vacuum distillation is stored differently in the soil layers due to the different soil tension, and hillslopes tend to store less mobile soil water compared to the riparian zone. At greater depths, bulk soil water extracted by cryogenic vacuum distillation was slightly less evaporated and less enriched in heavy isotopes compared to soil water extracted from shallower layers. The isotopic composition of xylem water had a large temporal and tree-species variability, with chestnut xylem water samples more enriched in heavy isotopes than samples obtained from beech trees. Xylem water was more similar to soil water obtained by cryogenic vacuum distillation, suggesting that in the study area trees likely use more bulk soil water than the mobile soil water, groundwater and stream water.

Keywords: stable water isotopes; soil water; xylem water; forested catchment.

How to cite: Zuecco, G., Marchina, C., Anam, A., Engel, M., Frentress, J., Gelmini, Y., Comiti, F., Penna, D., and Borga, M.: Origin of the waters sourced by trees in a pre-Alpine forested catchment, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12892, https://doi.org/10.5194/egusphere-egu2020-12892, 2020.

The vadose zone is a key component of the critical zone (CZ) and the interface of the atmosphere and the subsurface. A better understanding of critical zone hydrological processes is key for improving hydrological models and sustainable resource management. Isotopes of Hydrogen (²H) and Oxygen (¹⁸O) are a common tool to decipher hydrological processes in the CZ. However, there is still lack in understanding the spatiotemporal distribution of the soil water stable isotope composition (²H and ¹⁸O) at catchment scale. Until today, only a few studies evaluated long-term variability and spatial patterns. Here we present results of bi-weekly measurements of the soil water stable isotope over nine months. SWI composition were measured using direct vapour equilibration and accounted for different landscape elements (eight locations per campaign) in the forested Weierbach (~0.42 km²) experimental catchment in Luxembourg.

Preliminary results show that a strong similarity of δ¹⁸O depth profiles between different landscape elements at the same sampling date. However, after a snowmelt event we observed a much higher variability throughout the catchment likely from different melt, fractionation, and infiltration processes. The δ¹⁸O profiles throughout the landscape change consistently with time driven by a combination of rainfall and evaporation. Lc-excess data showed that soil water was experiencing kinetic evaporative fractionation in the top 30 cm of the soil throughout the year. The presented high frequent data on isotopic composition of soil pore water are useful to analyse spatial difference in vadose zone processes for better understanding soil-atmosphere interaction and flow processes. Eventually such data can be used for constraining spatially distributed hydrological models.

How to cite: Moussa, A., Fabiani, G., and Klaus, J.: Spatiotemporal patterns of soil water stable isotope composition in forested headwater catchment, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10288, https://doi.org/10.5194/egusphere-egu2020-10288, 2020.

Due to changes in the snowmelt timing and the potential shift towards less snowfall and more rainfall, infiltration patterns into the soil will increasingly be altered in a warming climate. Mixing and transport processes of water in the unsaturated topsoil layer regulate the subsurface transport and retention of solutes and contaminants, as well as the distribution of plant available water. Recent advances in soil isotope ecohydrology indicate that in some ecosystems, water in macropores largely bypasses soil matrix and rapidly percolates into the groundwater. Here we combine tracer experiments and geophysical surveys to explore soil water mixing in non-stratified till soil in the Pallas catchment located in sub-arctic conditions in Finnish Lapland. A 5x20 m plot at the Kenttärova hilltop was sprinkled with deuterated water (d²H 88‰) for two days (totally 200 mm at average intensity of 6.7 mm/h), until surface water ponding was observed on substantial share of the plot. Soil moisture dynamic were monitored by a network of soil moisture sensors and manual soil probe measurements. Soil water was sampled hourly with suction cup lysimeters at three (5 cm, 30 cm, 60 cm) depths and pan lysimeter at 35 cm depth in two soil profiles on the irrigation plot. Groundwater was sampled hourly, while xylem samples from spruce and birch trees in the plot were collected on each day of the experiment and on a weekly basis during the following month. Ground penetrating radar (GPR) survey and soil coring with window sampler down to 1 m depth were completed four times over the course of the experiment, and additional set of soil cores were taken two weeks after the experiment to inspect how natural precipitation events have infiltrated into the deuterium enriched zone. We investigate the mechanisms of soil matrix water replenishment by answering the following questions: i) Can all soil matrix water be displaced during high volume events and when does newly introduced soil matrix water become available to the plants?; ii) What is the extent of soil water mixing at different depths?; and iii) What is the effect of increased moisture content and groundwater table rise on soil water mixing? Due to paucity of field data sets and inability of most hydrological models to accurately simulate soil freezing and thawing effects, ecohydrologic partitioning has been barely studied in Northern regions with seasonal snow cover. We present a novel field data set that focuses on soil matrix water replenishment in glaciated till soil at sub-arctic conditions. Results support our understanding of ecohydrological processes in northern environments where hydrological cycle is dominated by intense infiltration events as it occurs during snowmelt.

How to cite: Muhic, F., Ala-Aho, P., Sprenger, M., Marttila, H., and Klöve, B.: Mechanisms of soil matrix water replenishment in a sub-arctic till soil based on an isotope tracer experiment, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10006, https://doi.org/10.5194/egusphere-egu2020-10006, 2020.

The interaction between aquatic vegetation and water flow is investigated here focusing on the drag coefficient. Compared with the standard drag coefficient of isolated cylinder, the phenomena of "blockage effect" and "sheltering effect" are put forward for vegetation clusters with different vegetation densities and Reynolds numbers. "Blockage effect" occurs when the drag coefficient of vegetation cluster is greater than the standard drag coefficient of isolated cylinder. The reason is that viscous boundary layer attached to the surface of vegetation items, resulting that the effective flowing width between adjacent vegetation items is less than the spacing of them, which brings a greater flow resistance and the drag coefficient of vegetation array is greater than the standard drag coefficient. On the other trend, "sheltering effect" is formed when the drag coefficient of vegetation array is less than the standard drag coefficient. This effect usually occurs for flow with large Reynolds numbers. In this case, Karman vortex streets forms and these vortexes are filled in the vegetation interval, thus causing the drag coefficient of vegetation cluster to be less than the standard drag coefficient of isolated cylinder.

How to cite: Wang, W.-J.: Blockage and sheltering effects of vegetation in turbulent flow, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20999, https://doi.org/10.5194/egusphere-egu2020-20999, 2020.

Rainfall interception by birch (Betula pendula Roth.) and pine (Pinus nigra Arnold) trees was measured in small urban park in the city of Ljubljana, Slovenia from beginning of the year 2014. Three and a half years of measurements of throughfall, stemflow and rainfall in the open were analyzed to estimate the influence of rain event characteristics on rainfall interception. A new approach using multiple correspondence analysis (MCA) was implemented. MCA is a multivariate statistical method for descriptive rather than quantitative variables, and can be used to estimate the relationship between the variables. The results are presented using diagrams, in which the proximity of the variables corresponds to their interdependence and the location of the variables (positive or negative domain) corresponds to their positive or negative correlation. The analysis included information from 176 events, showing the relationship between rainfall interception of birch and pine trees and rainfall amount, duration and intensity, wind speed and direction, drop number and median volume diameter (MVD), expressing raindrop size. The numerical values of the variables were transformed to the descriptive ones using classes regarding the threshold values of the variables (more or less than threshold), which was determined through sensitivity analysis. The thresholds were 6 mm for rainfall amount, 4 h for duration and 1.8 mm/h for intensity, 1.3 m/s for wind speed, 8 cardinal directions for wind direction, 1.5 mm for MVD and 10,000, 50,000 and 100,000 raindrops for their number. The MCA again showed the dominant influence of the rainfall amount, as the ratio of rainfall interception to rainfall amount decreases with increasing rainfall amounts. MCA including the wind characteristics gave a new insight into its influence on rainfall interception. The results expressed two new directions of occasional wind corridor according to the nearby buildings which were not visible using other methods of data analyses. The presented analysis, using MCA, confirmed results of previous analyses using other methods and offered a new insights into the process.

How to cite: Zabret, K. and Šraj, M.: Application of the multiple correspondence analysis for the evaluation of rain event characteristics influence on rainfall interception, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1575, https://doi.org/10.5194/egusphere-egu2020-1575, 2020.

The accurate estimation of the potential evapotranspiration (PET) is one of the key processes for water balance research and for determination of actual evapotranspiration (AET). The rate of PET is primarily affected by the amount of available water, climate conditions and surface characteristics. One of the main controlling factors is the radiation balance. Both shortwave and longwave radiations significantly influence the rate of PET. Since the longwave radiation is rarely measured, it has been computed. The computing approaches include several coefficients connected to specific climate conditions. The accuracy of original set of coefficients is questionable when applied in different sites. Here we present potential systematic error in estimating PET while using modelled longwave radiation. In our study, the use of original coefficient values in calculated longwave radiation resulted in differences from 20 to 80 mm of PET in the growing season. It decreased to less than 20 mm per season after parameter calibration.

Interception describes the amount of water captured by vegetation. Captured water often evaporates back to the atmosphere, thus it doesn’t participate in surface runoff or infiltration of water to the soil. Therefore the rate of interception loss hasn’t an impact only on evaporation but also on other components of water balance. As the interception is often neglected, we decided to compare observed and modelled values of interception loss. Five different modelling approaches were selected and discussed against measured values. Resulting interception loss differences were in range from 1 to 60 mm per growing season. The differences in the rate of interception led to overall variations in predictions of discharge, groundwater height and soil moisture content modelled by HBV model.

How to cite: Kofroňová, J., Šípek, V., and Tesař, M.: The importance of an accurate estimation of the radiation balance and interception loss for the evaluation of evapotranspiration, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22315, https://doi.org/10.5194/egusphere-egu2020-22315, 2020.

Can old growth alpine forests be biophysical barriers against current heat waves?

The current climate crisis requires an urgent understanding of ecosystem features dampening and alleviating the increasing radiation forcing. To this end, emission of latent heat from forests emerges with its relevance among the terrestrial ecosystem properties. It is not clear, however, if the different forest structures and ages act similarly, depending on the species composition, or if their structure has a role.

We performed a research on the hydrological cycle in the highly instrumented research facility in Renon forest, Italian Alps, belonging to the ICOS European infrastructure. The site is covered by a dense but structurally heterogeneous spruce forest, characterized by a young sector, with 30 years trees and an old forest sector composed by 200 years old trees.

Energy and water balance are quantified by eddy covariance instrumentation, 12 sap flux sensors in trees representative of the forest tree ages and 20 below-canopy pluviometers in each of the two forest structures. With these pluviometers, we quantified the relative role of canopy interception as a function of LAI density, precipitation intensity and duration. Water discharge and fog interception measurements allowed the closure of the water cycle at catchment scale.

Interestingly, we found that the water cycle is largely decoupled from the ground. In the old forest section, the fraction of water reaching the ground in the old sector is the 0.42±0.17 (vs. 0.67±0.17 in the young sector) of incoming precipitation. This suggests that in old alpine forests the hydrological cycle takes place largely in the crown and the old forest is using a large fraction of precipitation to dissipate heat.

Our results support the view of stand age as emerging property in the atmosphere-biosphere interaction and highlight the relevance of old forests in dampening the recurrent heat spells spreading across Europe, with the Alps and their remaining old growth forests standing as biophysical barriers.

How to cite: Montagnani, L., Obojes, N., Wolf, G., and Garcia Santos, G.: Can old growth alpine forests be biophysical barriers against current heat waves?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17882, https://doi.org/10.5194/egusphere-egu2020-17882, 2020.

Black spruce (Picea mariana) dominated peat plateaus are an important component of northwestern Canada’s heterogeneousboreal landscape. Threats to these ecosystems, including permafrost thaw and wetland expansion, could impact hydrological fluxes therefore, it is essential to understand the factors affecting the hydraulic function of black spruce in these rapidly changing landscapes. Sap velocity (V_s, cm·hr⁻¹) is the movement of water and minerals through tree stems during the growth period and can be used as an indicator for plant water use and the quantification of tree transpiration. Here, we identified the meteorological variables driving daytime and nighttime V_s in Picea mariana (black spruce) trees growing across a 21 hectare (20 m² grid) subarctic boreal peatland complex underlain with discontinuous permafrost, ~630 km west of Yellowknife, Northwest Territories (61°18'N, 121°18'W; ForestGEO Plot). For two consecutive growing seasons (2017 and 2018), eighteen black spruce trees were instrumented with sap flow sensors using the heat-ratio method to measure V_s. Meteorological variables including vapor pressure deficit (VPD) and photosynthetically active radiation (PAR) accounted for 57 and 73% of the variance in daytime mean V_s in 2017 and 2018, respectively, while VPD, PAR and air temperature accounted for 26 and 40% of V_s variance at night. VPD and PAR were the strongest meteorological drivers of black spruce V_s in the ForestGEO Plot. An increase in either variable corresponded to an increase in V_s across various time periods (day/nighttime). In addition, we investigated how daytime seasonal mean/maximum V_s for black spruce was affected by local environmental factors including fibric layer depth, organic matter decomposition, black spruce density, black spruce basal area, phosphorus supply rate (P) and soil water content (SWC) when physiological traits of black spruce, including diameter at breast height and crown area, were considered as covariables. It was hypothesised that stand density and basal area would affect V_s, but results indicated that only P and SWC had a (weak) influence on black spruce Vs. The variables P and SWC had a greater influence on the amplitude (seasonal daily maximum) of V_s over the sampling period. Overstory vegetation in Canada’s Northwestern boreal forest is important for the terrestrial water cycle through tree water storage, and transpiration, therefore the quantification of black spruce transpiration and an improved understanding of the environmental controls of black spruce V_s in boreal peatlands would be a natural next step for this research.

How to cite: Perron, N., Pappas, C., Baltzer, J., Dearborn, K., and Sonnentag, O.: Environmental controls of Picea mariana water use in a boreal subarctic peatland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12443, https://doi.org/10.5194/egusphere-egu2020-12443, 2020.

Determining the water partitioning in the critical zone, and how the biotic and abiotic factors affect these processes, is crucial to improve the comprehension of hydrological processes. Adequate field measurements of water partitioning in forested areas are challenging. Especially, continuous forest litter interception measurements are difficult to obtain. Therefore, we developed an equipment (named Litter Interception Device - LID), composed of a weighting system that contains a load cell with a resolution of 1 g, for continuous measurements of forest litter interception. The study was carried out in a Cerrado woodland forest (Cerrado sensu stricto) since 2017 in the State of São Paulo, Brazil. Following the continuous monitoring, we observed eventual weight gain during the nights. We analyzed the measurements for possible accumulation of dew in two LIDs between August 2018 and August 2019. We first carried out laboratory tests to check the possibility of measurement errors due to temperature shifts on the load cell. A maximum of 3 g error measurement after 10 °C temperature reduction was observed. We also estimated the dew point temperature for the study area during the monitoring period, based on temperature, relative humidity and rainfall data of sensors installed outside the Cerrado’s forest. In the forest, we monitored the temperature using a thermocouple installed in the forest litter sample. All sensors’ data were stored in a datalogger every 10 min. The dataset was analyzed in daily periods between the 9:00 pm and 7:00 am of the subsequent day. To check for dew accumulation on the forest litter, we defined the following minimum criteria to be considered dew, for each interval of our analysis: (a) the total mass gained could not be less than 2 g (equivalent to 0.0125 mm moisture accumulation); (b) the maximum temperature variation on forest litter 7 °C (considering that daily temperature variations close or above 10 ºC could introduce more errors); (c) there was no rainfall from 9:00 pm to 7:00 am of the subsequent day. On 204 days dew point temperature was reached, from which 76 days at least one of the LIDs registered a weight gain. During the study period, the temperature on the forest litter presented a maximum and mean variation of 6.7 °C and 2.5 (±1.2) °C, respectively. The data analysis indicated on average 4.59 mm of dew in one year. This average corresponded to 0.35% of the total rainfall for the study period (1206 mm) and 3.74% of the total average forest litter interception (133 mm). In tropical forests like the wooded Cerrado presented here, rainfall is the major input of water; otherwise to arid regions, were studies have shown that dew is the major input (i.e. Negev Desert). In our case, despite the low percentage related to the total rainfall, dew should not be neglected. As the LID measures all the mass inputs, including forest litter’s deposition, dew must be considered to correctly determine the hydrological processes at different time-space scales.

How to cite: Rosalem, L., Anache, J. A. A., Coenders, M., and Wendland, E.: Are dew measurements relevant for forest litter interception on a Cerrado woodland forest?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-231, https://doi.org/10.5194/egusphere-egu2020-231, 2020.

Dew and fog occur rather frequently in ecosystems all over the world. Still, water from dew and fog is often not considered in ecohydrological budgets. One reason is that there is no reference standard instrument to measure those water inputs into ecosystems. Another reason is that the water input from dew and fog is, compared to the water input from precipitation, a rather small amount at most locations, which makes it difficult to be measured accurately.

We developed a custom-made measurement system for quantifying dew and fog water inputs to temperate grassland ecosystems. The system consists of three high accuracy weighing micro-lysimeters composed of a plant pot which stands on a weighing platform with additional sensors. The weighing micro-lysimeters were designed to quantify even small water gains caused by dew formation on grasses with unprecedented accuracy.  Some former studies on micro-lysimeter design for dew measurements used small-size plant or bare-soil pots in combination with low capacity load cells, which allowed high accuracy measurements, but these systems were not able to mimic natural field conditions in terms of thermal behaviour and plant development. Other studies used large lysimeter systems which were better capable to simulate natural conditions, but required substantial infrastructure for installation and often showing too low accuracy, because of a trade-off between load cell accuracy vs. capacity.

Inside the micro-lysimeter plant pots, we installed soil moisture and temperature sensors to compare thermal and moisture conditions inside the plant pots with sensors installed in a control field plot at 1 m distance. A further component of the measurement system is a visibility sensor which allows to determine if water inputs originate solely from dew or from dew and fog in combination (fog: horizontal visibility < 1000 m). A leaf moisture sensor gives a redundant measurement to sense if leaves are really wet and for how long they stay wet. 

We set up a measuring network with the beforementioned system at eight sites in Switzerland and an additional site in South Tyrol (Italy). The sites were selected to gain representative measurements over an extended elevational gradient (from 500 to 2000 m a.s.l), within areas prone to fog (Swiss Plateau) and rather unlikely fog occurrence (Alps), as well as with low and high precipitation amounts (from 500 up to over 1500 mm/year).

Measuring dew and fog water inputs is expected to be important, as grassland species are able to take up water via foliar water uptake. Thus, dew and fog water can be important water inputs, especially in dry periods during fair weather summer conditions.

How to cite: Riedl, A., Li, Y., Buchmann, N., and Eugster, W.: High accuracy measurement system for dew and fog water quantification in temperate grassland ecosystems , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4535, https://doi.org/10.5194/egusphere-egu2020-4535, 2020.

Vegetation characteristics strongly influence interception loss as well as the redistribution of precipitation. In contrast to forests, net precipitation is rarely measured in grasslands. Over the long term, grasslands are often assumed to evapotranspire less than forests because of their shallower root structure and smaller leaf area, although some studies also indicate the opposite.

This research focuses on interception by grass canopies. We propose a new way to measure net precipitation in a grassland. We designed an in-situ net-precipitation measurement tool for the low vegetation, named interception grid. It consists of four half-pipes connecting to a sheltered sampling funnel per sampling point. The small size of the sampler allows for natural growth of the grass canopy. The spatial dimension of each interception grid is approximately 1m. This method does not separate stemflow and throughfall. We applied the new interception device in a grassland and compared results obtained with those from conventional throughfall samplers in an adjacent forest.

The research area is located in central Germany as a part of Hainich Critical Zone observatory. We conducted field observations for net precipitation synchronously in an adjacent forest (25 location, 1 ha) and grassland (25 samplers, 0.047 ha) plots in 2019 (March- August). We measured gross precipitation above the canopy (ca. 1 meter). Care was taken that the extent of sampling points was similar in both grassland and forest (1 m).

During nine of the total 22 measurement weeks, the grass was short and left the grids uncovered. This provided a chance to compare gross precipitation both by the grid and by the dedicated gross precipitation samplers.  These data suggest that the grids were accurate up to 30 mm precipitation per week. However, heavier precipitation was underestimated. Further work is currently underway to understand the reason for the underestimation.

During thirteen weeks, the grids were covered by the grass canopy, and interception data were acquired. Preliminary data show that the grassland exhibited a similar level of interception loss as the forest (34 % for the forest and 28 % for the grassland). Surprisingly, for weeks with gross precipitation higher than 5 mm, measures of spatial variation of throughfall in both land uses were similar in magnitude. The coefficient of variation of net precipitation in the forest varied between 0.06 and 0.16, and in grassland between 0.05 and 0.14. Both average interception and spatial variation in throughfall decreased with increasing gross precipitation for both vegetation types. An overall taller grass cover (later in the growing season) increased interception and increased the spatial variation of net precipitation.

In the long term, by these measurements, we aim to understand the influence of vegetation-induced water input on percolation at the plot scale with the help of intensive field observations.

How to cite: Demir, G., Metzger, J. C., Flipzik, J., Guswa, A., Michalzik, B., and Hildebrandt, A.: Comparative study: Do grasslands canopies create less spatial heterogeneity in net precipitation than forest?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20447, https://doi.org/10.5194/egusphere-egu2020-20447, 2020.

The first contact between precipitation and the land surface is often a plant canopy. The resulting precipitation partitioning by vegetation returns water back to the atmosphere (evaporation of intercepted precipitation) and redistributes water to the subcanopy surface as a “drip” flux (throughfall) and water that drains down plant stems (stemflow). Prior to the first benchmark publication of the field by Horton in 1919, European observatories and experimental stations had been observing precipitation partitioning since the mid-19th century. In this paper, we describe these early monitoring networks and studies of precipitation partitioning and show the impressive level of detail. Next to a description of the early studies, results included in this synthesis have been digitized and analyzed to compare them to recent studies. Although many early studies lack modern statistical analyses and monitoring tools that have become standard today, they had many strengths (not necessarily shared by every study, of course), including: A rigorous level of detail regarding stand characteristics (which is often lacking in modern ecohydrological studies); high-resolution spatiotemporal throughfall experiments; and chronosequential data collection and analysis. Moreover, these early studies reveal the roots of interest in precipitation partitioning processes and represent a generally forgotten piece of history shared by the hydrology, meteorology, forestry, and agricultural scientific communities. These studies are therefore relevant today and we hope modern scientists interested in plant-precipitation interactions will find new inspiration in our synthesis and evaluation of this literature.

How to cite: Friesen, J. and Van Stan II, J. T.: Early European Observations of Precipitation Partitioning by Vegetation: A Synthesis and Evaluation of 19th Century Findings , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9078, https://doi.org/10.5194/egusphere-egu2020-9078, 2020.

We aim to discuss topics and questions raised by the recent book published under this presentation's title. The book presents research on precipitation partitioning processes in vegetated ecosystems, putting them into a global context. It describes the processes by which meteoric water comes into contact with the vegetation's canopy, typically the first surface contact of precipitation on land. It also discusses how precipitation partitioning by vegetation impacts the amount, patterning, and chemistry of water reaching the surface, as well as the amount and timing of evaporative return to the atmosphere. Although this process has been extensively studied, this is the first review of the global literature on the partitioning of precipitation by forests, shrubs, crops, grasslands and other less-studies plant types.  

How to cite: Van Stan, J., Gutman, E., and Friesen, J.: Precipitation partitioning by vegetation: A global synthesis, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19838, https://doi.org/10.5194/egusphere-egu2020-19838, 2020.

Although recognized as a hotspot, being one of the most diverse biomes in Brazil and responsible for recharging the main aquifers in South America, the Cerrado has been suffering from intense deforestation. Since rainfall, after reaching the forest canopy, has its physicochemical features altered by the metabolites leaching from the leaves tissues, branches, and stem, this study was developed in order to obtain information about the hydrological processes in the biome and the potential of nutrient input by their forest species. There is a lack of studies as proposed in natural environments such as Cerrado. Based on this, we have evaluated the relative importance of stemflow and throughfall solute concentrations to the soil surface in a Cerrado forest in Brazil and also the potential of stemflow by 8 Cerrado species to soil nutrient input. The following chemical aspects from rainfall, throughfall and stemflow were determined: Na²⁺, K⁺, hardness (Ca²⁺ and Mg²⁺), Cl^-, , PO^3- and  on a liquid chromatograph Metrohm ECO IC during august to december 2018. The comparison between mean concentration, showed that most of the elements and compounds were more concentrated in throughfall and stemflow, except for Na²⁺ and Ca²⁺, which were more concentrated in rainfall (p <0.05). While the amount of stemflow channeled to the tree trunks comprised approximately 4% of rainfall, some nutrients in stemflow were enriched up to 10-fold in comparison to throughfall and rainfall. When we have discriminated the solute concentration by stemflow between 8 forest species from Cerrado, we have noted that each species has a specific contribution to the stemflow nutrient and for most of the species, the ion concentrations in the stemflow water is higher than those found in the rainfall and throughfall. Xylopia aromatica has shown the major difference between the solute concentrations when compared within other species. The total input of nutrients fluxes, as the amount of rainfall loading had been ranked as follows: K⁺ > Ca²⁺ > Mg²⁺ > NO₃ > > > Cl^- > Br^-. The highest nutrient input by stemflow was for K⁺, which ranged from 7.91 (H. ochraceus) to 114.08 (X. aromatica) kg ha^-1.These results highlight the importance of investigating the individual contribution of each species in the stemflow in Cerrado forest, suggesting a variety in nutrient input through the biogeochemical cycle and could be a strategy to accommodate the species for soil recovering. The knowledge of the biogeochemical dynamic helps to understand the processes that are responsible for the sustainability of forest ecosystems and the forest ecosystem plays an important role in water balance, not only in terms of water quantity (volume) but also in the distribution of the chemical elements.

How to cite: Tonello, K. C., Rosa, A. G., Guandique, M. E. G., Pereira, L. C., Matus, G. N., Lima, M. T., and Balbinot, L.: Nutrient fluxes in throughfall and stemflow in forest Cerrado species, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2465, https://doi.org/10.5194/egusphere-egu2020-2465, 2020.

Pollen is known to affect forest throughfall biochemistry, but underlying mechanisms are not fully understood. We used generalized additive mixed modelling to study the relationship between long-term series of measured throughfall fluxes in spring (April–June) at forest plots and corresponding airborne pollen concentrations (Seasonal Pollen Integral, SPIn) from nearby aerobiological monitoring stations. The forest plots were part of the intensive long term monitoring (Level II) network of the UNECE International Co-operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests (ICP Forests) with dominant tree genera Fagus, Quercus, Pinus and Picea, and were distributed all across Europe. We also conducted a 7-day laboratory dissolution experiment with bud scales and flower stalks of European beech (Fagus sylvatica L.), pollen of beech, common oak (Quercus robur L.), silver birch (Betula pendula L.), Scots pine (Pinus sylvestris L.), Corsican pine (Pinus nigra Arnold ssp. laricio (Poiret) Maire), Norway spruce (Picea abies (L.) Karst.) and sterilized pollen of silver birch in a nitrate (NO₃^--N) solution (11.3 mg N L^-1). Throughfall fluxes of potassium (K⁺), ammonium (NH₄⁺-N), dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) showed a positive relationship with SPIn whereas NO₃^--N fluxes showed a negative relationship with SPIn. In years with massive seed production of beech and oak SPIn and throughfall fluxes of K⁺ and DOC were higher, but fluxes of NO₃^--N were lower. The experiment broadly confirmed the findings based on field data. Within two hours, pollen released large quantities of K⁺, phosphate, DOC and DON, and lesser amounts of sulphate, sodium and calcium. After 24-48 hours, NO₃^--N started to disappear, predominantly in the treatments with broadleaved pollen, while concentrations of nitrite and NH₄⁺-N increased. At the end of the experiment, the inorganic nitrogen (DIN) was reduced, presumably because it was lost as gaseous nitric oxide (NO). There was no difference for sterilized pollen, indicating that the involvement of microbial activity was limited in above N transformations. Our results show that pollen dispersal might be an overlooked factor in forest nutrient cycling and might induce complex canopy N transformations, although the net-impact on N throughfall fluxes is rather low.

How to cite: Verstraeten, A., Gottardini, E., Bruffaerts, N., Cristofolini, F., Vanguelova, E., Neirynck, J., Genouw, G., Waldner, P., Thimonier, A., Nussbaumer, A., Neumann, M., Benham, S., Rautio, P., Ukonmaanaho, L., Merilä, P., Saarto, A., Reiniharju, J., De Vos, B., Roskams, P., and Cools, N. and the ICP Forests - Aerobiology: Impact of pollen on throughfall biochemistry in European temperate and boreal forests, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12994, https://doi.org/10.5194/egusphere-egu2020-12994, 2020.