Cities play an ambivalent role in climate change: They are responsible for an estimated 70% of the global carbon dioxide (CO2) equivalent emissions, yet they are particularly vulnerable to climate-related hazards such as heatwaves, flooding, and storms. This dual role positions cities and their inhabitants as drivers of climate change as well as victims of its consequences. As a result, cities present both a challenge and an opportunity for climate action through innovative urban development. A promising approach to mitigate climate risks while simultaneously strengthening urban resilience is integrating and regenerating green spaces into urban environments. These spaces enhance biodiversity, improve air and water quality, and provide environmental cooling. Among the various components of urban green spaces, trees stand out as a crucial part of a nature-based-solutions (NbS) planning that offer significant ecosystem services. However, the trees’ ability to deliver these benefits depends on their vitality. Trees in urban environments face stressors such as limited rooting space, reduced water availability, elevated temperatures, and anthropogenic activity – issues likely to intensify with increasing urbanization and frequency of extreme weather events. Understanding their hydraulic strategies and water uptake is crucial for maintaining their vitality and ensuring they continue providing their essential ecosystem services, despite the many urban stressors, including limited water availability and elevated temperatures due to the urban heat island (UHI) effect. Moreover, identifying the water sources that urban trees rely on and how they utilize it is key to effective urban planning and management, ensuring successful implementation of NbS. Thus, this study aims to quantify the water uptake depth and determine the contributions of different water sources to tree water uptake in urban environments. In 2024, we selected almost 90 deciduous trees in the city of Zurich (Switzerland), growing at street or park locations with varying levels of UHI effects. Prunus and Acer trees, selected for their abundance in the urban environment, were studied to investigate how the UHI affects their water uptake depth. We used stable water isotope analyses, a well-established method for tracing water movement through ecosystems, to partition the urban tree water sources. Soil and twig samples were collected and stored on ice to preserve their integrity. The soil and xylem water were isolated using cryogenic water extraction, a method that maintains the isotopic signatures intact. Measurement of the oxygen and hydrogen isotopic composition of the aqueous samples was done with an isotope ratiomass spectrometer (IRMS), allowing for differentiation among water sources. A Bayesian mixing model was employed to attribute the different soil water sources from various soil depths to the xylem sample, assuming it represents a mixture of all tree water sources. Trees were sampled three times during the 2024 growing season to obtain insights into potential phenological changes in water uptake. Preliminary results reveal significant differences in mean water uptake depth between Prunus and Acer (t-test, p<0.05). In relation to the UHI effect, both Prunus (t-test, p < 0.001) and Acer (t-test, p < 0.05) showed significant sensitivity to location, with trees in areas experiencing higher UHI effects exhibiting greater mean water uptake depths compared to those in areas with lower environmental heat stress. These findings support our hypothesis that trees subjected to UHI effects may adapt by developing deeper roots to access water from greater soil depths. Expanding our database with additional years and complemetary 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. This knowledge is essential to maximize trees’ potential as NbS and anticipate future climatic changes and proactively inform urban green space planning and city management.
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Jun 1, 2025
Yu Li + 9
Just Published
Open Access