Drought Impact on Transpiration Dynamics of Common Deciduous Trees Growing at Contrasting Urban Sites

Urban trees provide essential ecosystem services, notably air cooling through transpiration, which helps mitigate the urban heat island effect and enhances cities' climate resilience. However, the complex spatial variability within urban areas and extreme weather events like droughts can disrupt trees' ecohydrological dynamics. In a study conducted in Freiburg, Germany, we investigated transpiration processes in Norway maple (Acer platanoides) and small-leaved lime (Tilia cordata) across diverse urban locations, including parks, parking lots, grass verges and tree pits. We assessed the effects of four distinct drought periods on transpiration and compared differences between tree species and growing sites. Small-leaved lime exhibited a 5% greater reduction in transpiration during drought periods compared to Norway maple, which experienced a 34% decline in transpiration during peak sap flow compared to nondrought periods. Tree pits with 90% surface sealing induced the most significant drought-induced transpiration reduction for small-leaved lime (58%), with both species displaying the lowest transpiration to potential evapotranspiration ratio in these locations. Significant differences were observed in the diurnal sap velocity patterns for both species. We highlighted the site-specific impact of surface sealing on transpiration during droughts, as well as the significant relationship between soil water deficit and relative transpiration rates. This study provides crucial insights into common urban tree species' responses to drought-induced transpiration across varied urban settings, emphasising the role of surface sealing. Continuous monitoring of diverse urban tree species is essential for building extensive databases and enhancing our understanding of tree water relations in diverse urban landscapes.