In both field and experimental studies, there is increasing evidence for an isotopic mismatch between bulk water from plant twigs or stems and the potential mixture of waters from sources such as rain, soil, stream and groundwater. This poses a significant challenge in the interpretation of where plants access water spatially within the critical zone and temporally over the season with stable water isotopes. To date, a considerable number of studies have confirmed the existence of isotopic heterogeneity in soil water composition in association with variations in soil porosity. However, few studies have investigated the role of stem isotopic heterogeneity caused by plant internal water transport and redistribution on this isotopic mismatch. Recent experiments conducted on woody stems of Fagus sylvatica L., Quercus robur L. and Pinus pinaster Ait. have revealed a generalised isotopic heterogeneity when applying a novel water extraction technique based on the flow-rotor method known as Cavitron used to induce and measure embolism in plants (Barbeta et al., in prep). With this method, stems are centrifuged at different rotor speeds to generate emboli in the center, that expands through the longitudinal axis causing the stepwise displacement of vessel water outside the stem where it can be collected for stable isotope analysis. In addition, the water that remains in the stem tissues after cavitation (tissue water) can be extracted using a cryogenic vacuum distillation approach. In this presentation, we will show data that demonstrate that tree phenology has an important role in explaining the physiological mechanisms underlying differences in the stable isotope composition of vessel and tissue water and its use over the season. In addition, we will expose different water isotope datasets collected over multiple growing seasons and experiments using this technique and describe the advances in our understanding of the isotopic mismatch between stem and soil water composition that help to reconcile the growing number of studies that have observed this discrepancy.< Réduire