Water stable isotopes reveal the ecohydrological importance of stemflow infiltration 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 whether and to what extent trees and their neighbours use stemflow for water uptake and supply. We applied deuterium-enriched stemflow equivalent to a 23-mm throughfall event to adult Fagus sylvatica trees to trace stemflow through the soil and trees. We continuously measured in situ water stable isotopes in soil and xylem water and destructively sampled xylem water in the crowns of adult labelled, nonlabelled F. sylvatica and nonlabelled Picea abies, complemented by destructive xylem water sampling of neighbouring juvenile F. sylvatica. Stemflow water supported up to 5.7 ± 2.1% of daily sap flux of adult F. sylvatica trees labelled at the stem base and was detectable in xylem water for several weeks. In the soil, deuterium-enriched stemflow was present at a maximum distance of 0.40 m from the labelled tree. Moreover, nonlabelled juvenile trees within a distance of 2 m showed uptake of labelled water, indicating rooting into soil compartments affected by stemflow. We demonstrate the importance of stemflow as a water source for both adult and neighbouring juvenile F. sylvatica, even when soil water is not limiting.