Interspecific interaction and species identity effects on water uptake of beech and spruce trees by using stable isotope labelling

Understanding the interactions within the soil-plant-atmosphere-continuum becomes more important considering the eco- and hydrological impacts of climate change. Especially stand specific flow pathways and the characteristic timescales of water movement potentially provide important information on drought resilience of different forest ecosystems. This study analysed tree stand specific water uptake dynamics through water stable isotopy at high temporal resolution for two isotopic labelling events (7mm with δ²H +1000 ‰ and 23mm with δ²H +800‰) during the 2022 drought in south-west Germany. Measurements in pure and mixed tree stands of European beech (Fagus sylvatica, n=18) and Norway spruce (Picea abies, n=18) included sap flow, in-situ water isotopy of soil and xylem water, radial stem growth and microclimatic conditions. Our central hypothesis is that species identity and water competition between tree species are major drivers for ecohydrological flux dynamics. The results of the two labelling events showed differences in label water uptake of the two tree species and the transit times of the label in the system. The labelling events showed different transit times in the tree xylem depending on the label intensity. P. abies showed a slightly higher uptake of shallow soil water label in mixed stands than in pure patches. When labelled water infiltrated into deeper soil layers water was taken up faster by F. sylvatica in mixed forest patches than in pure forest patches and showed a generally slower uptake in P. abies than in F. sylvatica. The faster response time in water uptake during the second labelling was supported through an increase in measured sap flux and modelled branch water potential. Those dynamics of water isotopy measured in a high temporal resolution allow for a better understanding of root water uptake dynamics and water use strategies but also show species interaction effects on ecohydrological fluxes.