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Discussion papers | Copyright
https://doi.org/10.5194/hess-2018-402
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 13 Aug 2018

Research article | 13 Aug 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Hydrology and Earth System Sciences (HESS).

Hydrogen isotope fractionation affects the identification and quantification of tree water sources in a riparian forest

Adrià Barbeta1, Sam P. Jones1, Laura Clavé1, Lisa Wingate1, Teresa E. Gimeno1,2,3, Bastien Fréjaville1, Steve Wohl1, and Jérôme Ogée1 Adrià Barbeta et al.
  • 1INRA, UMR ISPA, F-33140, Villenave d’Ornon, France
  • 2BC3 – Basque Centre for Climate Change – Klima Aldaketa Ikergai, E-48940, Leioa, Spain
  • 3IKERBASQUE, Basque Foundation for Science, 48008 Bilbao, Spain

Abstract. We investigated plant-water sources of an emblematic refugial population of Fagus sylvatica (L.) in the Ciron river gorges in South-Western France using stable isotopes. The stable isotopes of water are a powerful tracer of water fluxes in the soil-plant-atmosphere continuum. It is generally assumed that no isotopic fractionation occurs during root water uptake, and that xylem water isotopes effectively reflect source water isotopes. However, recent studies showed that under certain conditions the isotopes in plant water do not reflect any of the potential sources considered. Highly resolved datasets covering a range of environmental conditions could shed light on possible plant-soil fractionations processes. In this study, the hydrogen (δ2H) and oxygen (δ18O) isotope compositions of all potential tree water sources and xylem water were measured bi-weekly over an entire growing season. Using Bayesian isotope mixing models (MixSIAR), we then quantified the contribution of the considered sources to xylem water of F. sylvatica and Quercus robur (L.) trees. Based on δ18O data alone, both species used a mix of top and deep soil water over the season, with Q. robur using soil water relatively deeper than F. sylvatica. The contribution of stream water appeared to be marginal despite the proximity of the trees to the stream, as already reported for other riparian forests. Xylem water δ18O could always be interpreted as a mixture of deep and shallow soil waters, but the δ2H of xylem water was often more depleted than any other possible water source. We argue that an isotopic fractionation in the unsaturated zone and/or within the plant tissues could underlie this unexpected δ2H depletion of xylem water, as already observed in halophytic and xerophytic species. By means of a sensitivity analysis, we found that the estimation of plant-water sources using isotope mixing models was largely affected by this isotopic δ2H depletion. A better understanding of what causes this isotopic separation between xylem and source water is urgently needed.

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Short summary
Plant-water sources of a beech riparian forest were monitored using stable isotopes. Isotopic fractionation during root water uptake is usually neglected but may be more common than previously accepted. Xylem water was always more depleted in δ2H than all sources considered, suggesting isotopic discrimination during water uptake or within plant tissues. Thus, the identification and quantification of tree water sources was affected. Still, oxygen isotopes were a good tracer of plant source water.
Plant-water sources of a beech riparian forest were monitored using stable isotopes. Isotopic...
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