Seasonal Variation in Tree Water Uptake in a Pre-Alpine Catchment (Alptal, CH): A Stable Isotope Study

The transpiration of forests is a major component of the water cycle and is highly sensitive to climate change. The interactions between soil moisture contents and tree water uptake are complex, influencing not only local water dynamics but also the health and resilience of the ecosystem. This thesis investigates the spatial and temporal variability in water uptake depths (WUD) of Norway spruce (Picea abies) trees in a headwater catchment in the pre-Alpine Alptal valley. Twig xylem water, stem core xylem water, bulk soil water, and mobile soil water were sampled at five study sites on five days between July and October 2024. 2H and 18O were analysed in reference to the isotopic composition of precipitation, soil moisture contents, weather and groundwater data, as well as study site and tree characteristics. The contribution of shallow (5-20 cm) and deeper (25-50 cm) soil water to tree water uptake was calculated using the MixSIAR model. All water sources were enriched in heavy isotopes in August, which primarily reflected the impact of evaporation in summer and snowfall in autumn on the isotopic composition of precipitation. The significant differences in the isotopic composition of soil water between study sites as well as between elevation zones could not be attributed to particular study site characteristics. Twig xylem water samples had a positive lc-excess, which potentially resulted from the uptake of dew or fog drip from the unsampled topsoil. Contrarily, stem core xylem water samples had a negative lc-excess, indicating a high within-tree variability in water isotopes and high model uncertainty. The modelled WUD was primarily a mixture of shallow and deep bulk soil water; no temporal or spatial trends and no impact of soil moisture on the WUD could be detected. The high uncertainty of the model results can be attributed to the small sample size and limited number of study sites, combined with the high number of variables affecting the WUD and the unexpected isotopic composition of tree xylem water. Additionally, soil moisture was relatively high throughout the entire study period, especially at greater soil depths, which was responsible for a less distinct soil profile and a smaller range in soil moisture conditions. This thesis contributes to research by discussing key factors that influence the results of WUD studies in wet regions.