Sapwood and heartwood are not isolated compartments: Consequences for isotope ecohydrology

In most tree species, xylem consists of two different functional parts: sapwood and heartwood. While sapwood, as the flowpath for sap, has received more attention in isotope studies assessing water sources accessed by trees (e.g. soil water from different depths, groundwater, stream water or a mixture of these), much remains unknown about heartwood and the possible water exchange between the two functional parts. We investigated four tree species (Fagus sylvatica, Quercus petraea, Pseudotsuga menziesii and Picea abies) characterised by different xylem anatomy and timing of physiological activity to evaluate the degree of differentiation in isotopic composition of water between sapwood and heartwood on a biweekly time scale. We found that the sapwood and heartwood of all species displayed a concurrent variation in their isotopic composition throughout the growing season and on a daynight scale suggesting that the two are not isolated compartments. While the two functional parts display a consistent difference in isotopic composition in conifers, they are characterised by more similar values in broadleaved species in broadleaved species, suggesting a higher degree of water exchange. Furthermore, we have also observed a progressive change in the isotopic composition in broadleaved species with sampling depth rather than functional parts of xylem. Our study highlights the value of accounting for radial isotopic variation, which might potentially lead to uncertainties concerning the origin of the extracted water for water uptake studies.