Soil water uptake strategies of European tree species: a comparative study in a drought-prone forest ecosystem

Understanding how tree species access and use water resources under conditions of more frequent and intense droughts is important for predicting the resilience of forest ecosystems to climate change. This knowledge is particularly needed in the light of recent droughts, which have led to unprecedented rates of tree mortality in European forests. The southern Rhine valley region of Rhineland-Palatinate and Hesse (Germany) was one of the most affected forest areas where massive crown dieback and tree mortality have been observed. The species-rich Lenneberg Forest in the centre of this region was selected for setting up an intensive monitoring plot for studying water use strategies in native broadleaved European tree species. The hydrological regime of this old-grown, species-rich forest is exclusively shaped by atmospheric precipitation and a deep reaching soil profile and thus offers ideal conditions for studying effects of drought on soil-plant water relations.

The research aim was to investigate the differences in soil water uptake depths among six co-occurring tree species (Fagus sylvatica, Quercus robur, Tillia cordata, Acer platanoides, Fraxinus excelsior, Prunus avium). Four sampling campaigns were carried out from 2023 to 2025 to collect soil water samples from various depths within the rooting zone of 57 selected trees. Thermal dissipation sap flow sensors were installed on all trees and the calculated sap flow velocities were used to estimate the water uptake time. Based on this, twigs for xylem water extraction were collected by tree climbers from the upper canopies. Stable isotope ratios of hydrogen (δ²H) and oxygen (δ¹⁸O) were measured in both soil and xylem water. Bayesian stable isotope mixing models were employed to estimate the relative contribution of each Root Water Uptake depth to the xylem water mixture on a species and individual-tree level. Electrical resistivity tomography was additionally used to visualize the spatial distribution of humidity across the soil profile.

During wet conditions the dominant source of xylem water was the topsoil layer (0-10 cm) across all species. However, with drying of the soil profile we found three different responses: (i) a pronounced downward shift in water uptake depth (Quercus robur, Fagus sylvatica, Fraxinus excelsior), (ii) slight shift toward deeper sources while maintaining primary reliance on the topsoil layer (Acer platanoides, Prunus avium), and (iii) continuous uptake from the topsoil (Tillia cordata). None of the species showed substantial contributions of deeper soil to the xylem water (20-30, 30-70 cm). This observation is consistent with a sustained depletion of water reserves in deeper soil layers during the vegetation period. Our findings contradict previous reports that trees continuously shift water uptake deeper into the soil profile with increasing drought. The consistent reliance on shallow soil water observed in this study highlights potential vulnerabilities of certain species to prolonged drought and underscores the need to integrate site-specific rooting and soil hydraulic constraints into forest management and climate adaptation strategies.