Quantifying the hydraulic safety of urban trees: Do street environments impose greater strain than parks?

Source abstract

Urban trees are a primary nature-based solution, playing a key role in mitigating the urban heat island effect, yet their long-term health in dense urban environments remains a critical concern. It is widely assumed that street trees experience greater physiological stress than their parks counterparts due to harsh growing conditions, such as limited rooting volume and increased radiant heat. This study challenges that paradigm by quantitatively comparing the physiological status of four common species ( Pistacia atlantica , Brachychiton populneus , Dalbergia sissoo , and Ficus microcarpa ) in street and park microhabitats within a coastal Mediterranean city. We measured midday leaf water potential (Ψ leaf ), stomatal conductance (g s ), and turgor loss point (TLP) to calculate hydraulic safety margins, alongside branch xylem anatomical properties. Counter to our initial hypothesis, park trees consistently exhibited similar or significantly more negative Ψ leaf and lower g s than street trees, indicating higher levels of chronic water stress in the parks. Consequently, most park trees operated with narrower hydraulic safety margins, as exemplified by in the native P. atlantica . The anatomy of the park trees, characterized by significantly smaller xylem vessel diameters, further reflected structural acclimation to these drier conditions. These findings suggest that impervious surfaces may buffer street trees from seasonal soil drying by reducing surface soil evaporation, while open park soils may experience higher evaporative demand. Future management strategies must be tailored to the unique ecological dynamics of distinct urban microhabitats, recognizing that parks can be zones of significant, underestimated physiological stress in dry Mediterranean climates.