Water stable isotopes reveal the ecohydrological importance of stemflow for mature and juvenile European beech

Stemflow of forest trees can contribute a significant fraction of water to forest's water fluxes; however, it is still unclear if and to what extent trees use stemflow for water supply and how much stemflow is lost by percolating below the root zone. We applied deuterium-enriched stemflow equivalent to a throughfall depth of 23 mm to adult Fagus sylvatica trees to trace stemflow through the soil and trees. We continuously measured in-situ water stable isotope compositions in soil and xylem water and destructively sampled xylem water in the crowns of adult labelled (n=18), unlabelled F. sylvatica (n=15) and unlabeled Picea abies (n=9), complemented by destructive xylem water sampling of neighboring juvenile F. sylvatica (n=45). Stemflow water supported 3.9 - 14.0% of daily sap flux of adult labelled F. sylvatica trees. In the soil, deuterium-enriched stemflow was detectable at a max. of 0 - 0.40 m to the labelled tree. However, unlabeled juvenile trees within a distance of ~2 m showed label water uptake, indicating rooting into soil compartments affected by stemflow label. We demonstrate the importance of stemflow as a water source for both adult and neighboring juvenile F. sylvatica , strongly profiting from stemflow infiltration.