Investigating stable isotope signatures variability across tree compartments

Water stable isotopes are valuable proxies for tracing water fluxes within the critical zone, the Earth’s layer extending from vegetation through to deep aquifers. This technique has helped to develop conceptual models of water distribution across scales, making it essential to understand how trees regulate water stored within their internal compartments. To investigate this, we sampled a representative Pinus sylvestris tree within an ecohydrologically monitored forest plot in the Vallcebre research catchments (NE Spain). The primary aim of this sampling was to assess potential variability in the isotopic signatures across different parts of the tree to enhance understanding of soil-root-tree water uptake processes. Samples were collected from various soil depths (0–100 cm), woody tissues of twigs and branches (at 3 canopy heights), the stem (cores at 3 different heights), and roots in all four cardinal directions during two sampling days (July and September 2023). Water from soil and wood samples was extracted using: cryogenic vacuum distillation (CVD) and cavitron (centrifugation). Stable isotope ratios were measured for all samples using infrared laser spectrometry (Picarro). Additional data included long-term meteorological records, throughfall volumes and isotopic signatures, soil moisture content and potential, sap flow and tree water deficit rates (from adjacent trees). Results showed consistent patterns across sampling dates: twigs and branches displayed isotopic values close to those of soil and throughfall, whereas roots and stem tissues exhibited more depleted values, clearly distinct from soil, twig, and branch signatures. To determine whether these isotopic observed differences arise from methodological issues (differences between cavitron and cryogenic extractions and/or the part of the wood sampled) or reveal intrinsic processes within the tree, in a third sampling campaign (July 2024) we sampled soil, roots, stem, branches and twigs. From roots and branches we took samples for CVD and Cavitron extraction and from the stem we took heartwood and sapwood samples. In addition, selected samples from the third campaign will be analyzed by both Picarro and isotope ratio mass spectrometry (IRMS). This additional information promise new insights into the internal water dynamics of trees, clarifying if observed isotopic patterns reflect true physiological processes or methodological artifacts. This is critical for advancing our understanding of tree water dynamics and their role in critical zone water fluxes.