Xylem water in riparian Willow trees (Salix alba) reveals shallow sources of root water uptake by in situ monitoring of stable water isotopes

Landgraf, Jessica; Tetzlaff, D.; Dubbert, Maren; Smith, Aaron; Soulsby, Chris

doi:10.5194/hess-2021-456

Cited by 9 publications

(18 citation statements)

References 67 publications

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“…Additionally, a previous greenhouse experiment that also investigated the cryogenic extraction impact on xylem water using S. viminalis cuttings (the same species used in this experiment) showed no bias for δ 2 H and only a small but consistent enrichment in δ 18 O above soil water (Newberry et al, 2017). In situ monitoring of xylem (e.g., Landgraf et al, 2021; Marshall et al, 2020; Volkmann, Kühnhammer, et al, 2016) and phloem water isotopic composition could likely improve uncertainties related to extraction method and would provide higher temporal‐resolution measurements. However, in situ monitoring of phloem (i.e., inner bark) has not been tested yet.…”

Section: Discussionmentioning

confidence: 99%

Phloem water isotopically different to xylem water: Potential causes and implications for ecohydrological tracing

et al. 2022

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The stable isotopes of hydrogen and oxygen in xylem water are often used to investigate tree water sources. But this traditional approach does not acknowledge the contribution of water stored in the phloem to transpiration and how this may affect xylem water and source water interpretations. Additionally, there is a prevailing assumption that there is no isotope fractionation during tree water transport. Here, we systematically sampled xylem and phloem water at daily and subdaily resolutions in a large lysimeter planted with Salix viminalis. Stem diurnal change in phloem water storage and transpiration rates were also measured. Our results show that phloem water is significantly less enriched in heavy isotopes than xylem water. At subdaily resolution, we observed a larger isotopic difference between xylem and phloem during phloem water refilling and under periods of tree water deficit. These findings contrast with the expectation of heavy‐isotope enriched water in phloem due to downward transport of enriched leaf water isotopic signatures. Because of previous evidence of aquaporin mediated phloem and xylem water transport and higher osmotic permeability of lighter hydrogen isotopologues across aquaporins, we propose that radial water transport across the xylem–phloem boundary may drive the relative depletion of heavy isotopes in phloem and their relative enrichment in xylem.

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Section: Discussionmentioning

confidence: 99%

Phloem water isotopically different to xylem water: Potential causes and implications for ecohydrological tracing

et al. 2022

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“…A lake water extraction facility (using bank filtration) is situated immediately north of the IGB site, and groundwater is ∼ 2.2 m below the ground surface with limited annual variability (< 0.1 m). A previous isotope-based study at the site has excluded groundwater and the nearby stream water as likely sources of water to the trees (Landgraf et al, 2021).…”

Section: Study Sitementioning

confidence: 99%

“…Willows are a riparian species which is well adapted to moderate and high moisture typical of riparian areas or those with high groundwater levels (Isebrands and Richardson, 2014). Additional site selection criteria are provided in Landgraf et al (2021). The willows have a similar age of ∼ 15 years with the willow in the north (Willow 1, Fig.…”

Section: Soils and Vegetationmentioning

confidence: 99%

“…Installation of polypropylene membranes (7 cm long, 0.2 µm pores, Kübert et al, 2020) for soil water vapour extraction at three depths (10, 40, and 100 cm) was conducted at the end of May 2020 (20 May 2020). Extracted in situ soil vapour samples were analysed every 2 h using a Picarro L2130-i cavity ring-down laser spectrometer (Picarro, Inc., Santa Clara, CA, USA), calibrated using the approach presented for this site in Landgraf et al (2021).…”

Section: In Situ Measurementsmentioning

confidence: 99%

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Modelling temporal variability of in situ soil water and vegetation isotopes reveals ecohydrological couplings in a riparian willow plot

et al. 2022

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Abstract. The partitioning of water fluxes in the critical zone is of great interest due to the implications for understanding water cycling and quantifying water availability for various ecosystem services. We used the tracer-aided ecohydrological model EcH2O-iso to use stable water isotopes to help evaluate water, energy, and biomass dynamics at an intensively monitored study plot under two willow trees, a riparian species, in Berlin, Germany. Importantly, we assessed the value of in situ soil and plant water isotope data in helping to quantify xylem water sources and transit times, with coupled estimates of the temporal dynamics and ages of soil and root uptake water. The willows showed high water use through evapotranspiration, with limited percolation of summer precipitation to deeper soil layers due to the dominance of shallow root uptake (>80 % in the upper 10 cm, 70 %–78 % transpiration/evapotranspiration). Lower evapotranspiration under grass (52 %–55 % transpiration/evapotranspiration) resulted in higher soil moisture storage, greater soil evaporation, and more percolation of soil water. Biomass allocation was predominantly foliage growth (57 % in grass and 78 % in willow). Shallow soil water age under grass was estimated to be similar to under willows (15–17 d). Considering potential xylem transit times showed a substantial improvement in the model's capability to simulate xylem isotopic composition and water ages and demonstrates the potential value of using in situ data to aid ecohydrological modelling. Root water uptake was predominately derived from summer precipitation events (56 %) and had an average age of 35 d, with xylem transport times taking at least 6.2–8.1 d. By evaluating isotope mass balances along with water partitioning, energy budgets, and biomass allocation, the EcH2O-iso model proved a useful tool for assessing water cycling within the critical zone at high temporal resolution, particularly xylem water sources and transport, which are all necessary for short- and long-term assessment of water availability for plant growth.

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“…Isotope measurements of the transpiration flux become more widely available due to lower costs and new technologies (Marshall et al, 2020;Volkmann et al, 2016), which allows sub-daily to daily xylem isotope analysis. However, such in-situ measurements have shown that xylem water isotope variation is by far less responsive than stream water isotopes (Gessler et al, 2021;Landgraf et al, 2021;Seeger and Weiler, 2021). Further, recent simulations with the EcH2O-iso model by 5 Knighton et al (2020) indicated that mixing of water in plants could conceivably dampen rootuptake signals, undermining the insights that could be gained by high frequently xylem sampling.…”

Section: Plant Water Isotopes Indicate That Trees Use Older Water Tha...mentioning

confidence: 99%

Precipitation fate and transport in a Mediterranean catchment through models calibrated on plant and stream water isotope data

Sprenger

Llorens

Gallart

et al. 2022

Preprint

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Abstract. To predict hydrologic responses to inputs and perturbations, it is important to understand how precipitation is stored in catchments, released back to the atmosphere via evapotranspiration (ET), or transported to aquifers and streams. We investigated this partitioning of precipitation using stable isotopes of water (2H and 18O) at the Can Vila catchment in the Spanish Pyrenees mountains. The isotope data covered four years of measurements, comprising > 550 rainfall and > 980 stream water samples, capturing intra-event variations. They were complemented by fortnightly plant-water-isotope data sampled over eight months. The isotope data were used to quantify how long it takes for water to become evapotranspiration or discharged as streamflow, using StorAge Selection (SAS) functions. We calibrated the SAS functions using a conventional approach, fitting the model solely to stream water isotope data, as well as a multi-objective calibration approach, in which the model was simultaneously fitted to tree xylem-water isotope data. Our results showed that the conventional model-fitting approach was not able to constrain the model parameters that represented the age of water supplying ET. Consequently, the ET isotope ratios simulated by the conventionally calibrated model failed to adequately simulate the observed xylem isotope ratios. However, the SAS model was capable of adequately simulating both observed stream water and xylem water isotope ratios, if those xylem water isotope observations were used in calibration (i.e., the multi-objective approach). The multi-objective-calibration approach led to a more constrained parameter space, facilitating parameter value identification. The model was tested on a segment of data reserved for validation, showing a Kling-Gupta Efficiency of 0.72, compared to the 0.83 observed during in the calibration period. The water-age dynamics inferred from the model calibrated using the conventional approach differed substantially from those inferred from the multi-objective-calibration model. The latter suggested that the median ages of water supplying evapotranspiration is much older (150–300 days) than what was suggested by the former (50–200 days). Regardless, the modeling results support recent findings in ecohydrological field studies that highlighted both subsurface heterogeneity of water storage and fluxes and the use of relatively old water by trees. We contextualized the SAS-derived water ages by also using young-water-fraction and endmember-splitting approaches, which respectively also showed the contribution of young water to streamflow was variable but sensitive to runoff rates, and that ET was largely sourced by winter precipitation, that must have resided in the subsurface across seasons.

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Xylem water in riparian Willow trees (Salix alba) reveals shallow sources of root water uptake by in situ monitoring of stable water isotopes

Cited by 9 publications

References 67 publications

Phloem water isotopically different to xylem water: Potential causes and implications for ecohydrological tracing

Phloem water isotopically different to xylem water: Potential causes and implications for ecohydrological tracing

Modelling temporal variability of in situ soil water and vegetation isotopes reveals ecohydrological couplings in a riparian willow plot

Precipitation fate and transport in a Mediterranean catchment through models calibrated on plant and stream water isotope data

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