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Hydrology and Earth System Sciences An interactive open-access journal of the European Geosciences Union
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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 09 Jan 2019

Research article | 09 Jan 2019

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

Unexplained hydrogen isotope offsets complicate 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 water isotopes. It is generally assumed that no isotopic fractionation occurs during root water uptake, so that the isotopic composition of xylem water effectively reflects that of source water. However, this assumption has been called into question by recent studies that found that, at least at some dates during the growing season, plant water did not reflect any mixture of the potential water sources. In this context, highly resolved datasets covering a range of environmental conditions could shed light on possible plant-soil fractionation processes responsible for this phenomenon. In this study, the hydrogen (δ2H) and oxygen (δ18O) isotope compositions of all potential tree water sources and xylem water were measured fortnightly over an entire growing season. Using a Bayesian isotope mixing model (MixSIAR), we then quantified the relative contribution of water sources for 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 the considered water sources. We argue that an isotopic fractionation in the unsaturated zone and/or within the plant tissues could underlie this unexpected relatively depleted δ2H 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 mixing models was largely affected by this δ2H depletion. A better understanding of what causes this isotopic separation between xylem and source water is urgently needed.

Adrià Barbeta et al.
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Adrià Barbeta et al.
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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...