Stable water isotope dataset of precipitation, groundwater, soil water, stemflow and throughfall water, and xylem water of Norway maple (Acer platanoides) and small-leaved lime (Tilia cordata) growing at different urban sites in the city of Freiburg, Germany

Trees in urban ecosystems play a pivotal role in enhancing the climate resilience of cities through their ecohydrological processes, such as transpiration and interception, to mitigate the urban heat island effect and reduce the risk of flooding. However, the complex spatial heterogeneity in urbanized areas and the occurrence of extreme weather events, such as droughts, may disrupt ecohydrological dynamics of trees. In this study, we conducted an extensive field study on 25 urban trees growing in contrasting urban environments. Addressing the limited understanding of water source uptake dynamics of trees planted in complex urban ecosystems, we investigate the composition and temporal pattern of stable water isotopes of groundwater, urban soils, twig xylem water and precipitation to determine the proportional source water uptake over two vegetation periods. Bayesian mixing analyses with three potential sources, including young and old precipitation as well as groundwater, indicated that trees were mainly dependent on older precipitation events, occurred two to six months before the twig sampling events, while another mixing analysis, including groundwater and two soil layers, indicated soil water in 20-40 cm depth as the primary water source. Groundwater showed the lowest proportion in all mixing models variants. Despite some significant differences in the proportional uptake of tree water sources between study sites, the primary source remained consistent. The most significant contrast between sites was the reliance of trees in tree pits on soil water in 20-40 cm depth and older precipitation during drier periods. Our study generated a unique dataset on the soil-plant-atmosphere continuum within an urban ecosystem, which highlighted the dynamics of water uptake by urban trees. However, long-term investigations on different urban tree species are crucial to understand the mechanisms of tree water cycling, including influences of droughts and heat waves.