Investigating the Water Sources of Urban Trees

Urban trees play a vital role in resilient cities, providing essential ecosystem services such as cooling, carbon sequestration, and improved air quality. However, their ability to deliver these benefits depends on their resilience to urban stressors like limited rooting space, water scarcity, elevated temperatures, and anthropogenic activity. Understanding their hydraulic strategies and water uptake dynamics is therefore crucial for supporting effective urban planning and management. This study aimed to investigate the water uptake depth of urban trees in Zurich, Switzerland, using stable water isotope analysis. In 2024, nearly 90 deciduous trees were sampled at street and park locations. Prunus and Acer trees were studied along an urban density gradient to assess Urban Heat Island (UHI) effects on water uptake. Additionally, Quercus, Alnus, and Tilia trees were sampled to examine interspecies variations under similar environmental conditions. Soil and twig samples were processed using cryogenic water extraction, and Oxygen and hydrogen isotopic composition of the aqueous samples were measured using an isotope ratio mass spectrometer (IRMS). A Bayesian mixing model attributed soil water sources from various depths to xylem samples. Sampling occurred three times during the growing season to capture potential phenological effects. Preliminary results reveal significant differences in water uptake depths between Prunus and Acer. For Prunus, water uptake depth increased with urban density, indicating possible effects of UHI on tree hydraulic strategies. These findings suggest trees in UHI affected areas may adapt by developing deeper roots to access water. Expanding our database with additional years and complementary measurements, our findings will inform strategies to enhance the resilience and sustainability of urban green spaces, ensuring that trees can continue to play a vital role in mitigating climate risks and improving urban living conditions.