Tree- and stand-scale variability of xylem water stable isotope signatures in mature beech, oak, and spruce

In ecohydrology, water isotopes are used to study root water uptake by plants from potential source waters by comparing xylem water signatures with source water signatures. Such comparisons are affected by variability and the uncertainty of the measurements in plant and source waters. Tree-scale and stand-scale variabilities of the isotope composition in stem xylem waters are often unknown but important for sampling design and uncertainty estimation when assessing the sources of tree water uptake. Here, we quantified tree-scale and stand-scale variabilities of isotopic signatures in xylem waters in beech, oak, and spruce trees in a mature forest on the Swiss plateau. For stem xylem water, sub-daily replicates and replicates in different cardinal directions showed no systematic differences, but we found systematic differences with sampling height. The observed variability of isotopic signatures at different heights along the stem suggests that water residence times within trees, along with their effects on the isotopic signatures in different compartments of the tree (stem, branches, leaves), need to be considered. Further, concerning the hydrogen signatures, we found height and species-specific offsets (SW-excess δ H). Stem xylem waters’ tree-scale variability was similar in magnitude to their stand-scale variability and smaller than the variabilities in branch xylem and bulk soil water around each tree. Xylem waters from stem cores close to the ground, therefore, can give a more precise estimate of the isotopic signal of the most recent root water uptake and constitute a more accurate source water attribution.