Preprints
https://doi.org/10.5194/hess-2018-402
https://doi.org/10.5194/hess-2018-402
13 Aug 2018
 | 13 Aug 2018
Status: this preprint was under review for the journal HESS but the revision was not accepted.

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

Adrià Barbeta, Sam P. Jones, Laura Clavé, Lisa Wingate, Teresa E. Gimeno, Bastien Fréjaville, Steve Wohl, and Jérôme Ogée

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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Adrià Barbeta, Sam P. Jones, Laura Clavé, Lisa Wingate, Teresa E. Gimeno, Bastien Fréjaville, Steve Wohl, and Jérôme Ogée
 
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Status: closed
Status: closed
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
Printer-friendly Version - Printer-friendly version Supplement - Supplement
Adrià Barbeta, Sam P. Jones, Laura Clavé, Lisa Wingate, Teresa E. Gimeno, Bastien Fréjaville, Steve Wohl, and Jérôme Ogée
Adrià Barbeta, Sam P. Jones, Laura Clavé, Lisa Wingate, Teresa E. Gimeno, Bastien Fréjaville, Steve Wohl, and Jérôme Ogée

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Latest update: 19 Jul 2024
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.