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.
Highlights
1.1 Why is an improved understanding of tree water use needed?Ongoing climate change, through the combination of altered precipitation regimes and warmer temperatures, is affecting terrestrial ecosystems globally, promoting rapid and widespread changes in forest cover (e.g. Allen et al, 2015)
Our results show that δ2H offsets between xylem water and source water complicate the identification of plant water sources and the source contributions estimated by Bayesian isotopic mixing models (Fig. 7), a finding in contrast with recent studies (Evaristo et al, 2017)
Based on results from the Bayesian isotope mixing model with uncorrected δ18O and δ2H signals, we found that both F. sylvatica and Q. robur used a mixture of both top and deep soil water throughout the 2017 growing season, with marginal contributions of stream water (Fig. 6)
Summary
1.1 Why is an improved understanding of tree water use needed?Ongoing climate change, through the combination of altered precipitation regimes and warmer temperatures, is affecting terrestrial ecosystems globally, promoting rapid and widespread changes in forest cover (e.g. Allen et al, 2015). Allen et al, 2015) This is because soil–plant interactions and species-specific water use can provoke shifts in forest species’ distributions through readjustments in species abundance (Clark et al, 2016). In this context, it is a priority to better understand how spatial and temporal dynamics of water use by trees will be affected in the future. It is a priority to better understand how spatial and temporal dynamics of water use by trees will be affected in the future This will reduce uncertainties in the projections of forested areas This may help understand how climate refugia facilitate the persistence of important biodiversity hotspots (McLaughlin et al, 2017).
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