The role of landscape topography in mediating subsurface water availability and ultimately tree transpiration is still poorly understood. To assess how hillslope position affects tree water use, we coupled sap velocity with xylem isotope measurements in a temperate beech-oak forest along a hillslope transect in Luxembourg. We generally observed greater sap velocities at the upslope locations in trees from average-sized trees, suggesting the presence of more suited growing conditions. We found a lower difference in sap velocity among hillslope positions for larger trees, likely due to the exploitation of deeper and more persistent water sources and the larger canopy light interception. Beech trees exploited a shallower and seasonally less persistent water source than oak trees, due to the shallower root system than oak trees. The different water exploitation strategy could also explain the stronger stomatal sensitivity of beech to vapour pressure deficit compared to oak trees. Xylem isotopic composition was seasonally variable at all locations, mainly reflecting the contribution of variable soil water sources and suggesting that groundwater did not contribute, or only marginally contributed, to tree transpiration. Overall, our results suggest that trees along the hillslope mainly rely on water stored in the unsaturated zone and that seasonally shallow groundwater table may not necessarily subsidize water uptake for species that do not tolerate anoxic conditions. Contrary to previous studies, at our site, we did not find higher sap velocity downslope as the subsurface hillslope structure promotes vertical water flux over lateral redistribution in the vadose zone.

No abstract

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.