Unexplained hydrogen isotope offsets complicate the identification and quantification of tree water sources in a riparian forest

Abstract

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 deeper soil water 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 strongly affected by this δ2H depletion. A better understanding of what causes this isotopic separation between xylem and source water is urgently needed.

Depth distribution of soil water sourced by plants at the global scale: A new direct inference approach

“…On the basis of its assumptions, this approach has the advantage to quantitatively assess the contribution of each of the identified water sources even if one or more sources are missing. However, this approach relies on the assumption that no fractionation occurs at the soil-root interface or within plant woody tissues that is being increasingly questioned (Barbeta et al, 2019). Hence, these results should be used with caution, and extended analyses on various plant species and in different climatic contexts are needed to further test this method and to evaluate the differences compared with widely used mixing model results.…”

Borehole Equilibration: Testing a New Method to Monitor the Isotopic Composition of Tree Xylem Water in situ

“…The source for transpired water is the water taken up by the roots and transported through the tree xylem. In contrast to the strong fractionation that occurs during transpiration, xylem water is generally taken up and transported without fractionation (Wershaw et al, 1966), though a growing number of exceptions are reported (Lin and Sternberg, 1993;Ellsworth and Williams, 2007;Vargas et al, 2017;Barbeta et al, 2019;Poca et al, 2019). One might expect that the xylem water would be identical to precipitation, but precipitation varies over the year, creating depth profiles in the soil.…”

Investigating the root plasticity response of Centaurea jacea to soil water availability changes from isotopic analysis

“…It has been shown for a number of plant species that RWU as well as convective transport in xylem vessels induce no measurable isotope fractionation (Wershaw et al, 1966;Dawson et al, 2002). However, for cases in which isotope fractionation was observed see for example Lin & Sternberg (1993) and Barbeta et al (2019). The isotopic composition of xylem water usually reflects the mixture of RWU and thus across potential soil water sources.…”