Urban and meteorological factors controlling transpiration dynamics of two common deciduous tree species in the city of Freiburg, Germany

Urban hydrological processes are mainly influenced by anthropogenic activities, such as the expansion of impermeable surfaces which reduces infiltration of rainwater, increases runoff generation processes, and reduces evapotranspiration. Urban trees contribute positively to moving the urban water cycle closer to a natural one through their ecohydrological processes (e.g. transpiration). However, high heterogeneity within urban environments and increasing drought periods create multiple stressors to trees’ ecophysiology. We conducted intensive field measurements on Norway maple (Acer platanoides) and small-leaved lime (Tilia cordata) in the city of Freiburg, Germany, to advance our process understanding of transpiration behaviour of urban trees at contrasting growing and microclimatic environments and to determine the main hydrometeorological factors influencing transpiration. Soil water content, sap flux, crown solar radiation transmissivity, and meteorological measurements within the crown were carried out on 11 trees per species on sites with different degrees of surface sealing underneath tree crowns, in particular parks, parking lots, grass verges and tree pits.

Throughout the investigation period (2021-2022), average daily transpiration rates during the growing season were higher for small-leaved lime (1.76 ± 0.53 mm) than for Norway maple (1.53 ± 0.51 mm). We observed significantly reduced daily transpiration rates (1.13 ± 0.31 mm) at tree planting sites (e.g. tree pits) with 90% impermeable surface underneath the tree crowns. On average, the main hydrometeorological drivers for day-to-day transpiration dynamics were solar radiation (39.7%), followed by vapour pressure deficit (22.4%), and soil water content (4.8%). Additionally, tree morphological traits, such as leaf area index (LAI) and leaf area density (LAD), as well as the degree of surface sealing affected transpiration significantly (p-value < 0.05) among sites. Furthermore, LAI is significantly correlated with the proportion of surface sealing within the crown projection area. With this study, we created a highly needed dataset for the main urban tree species in Central European cities and provided a solid knowledge base for transpiration processes of trees in various urban environments. The study revealed that long-term field measurements with multiple tree species under contrasting urban growing conditions are a necessity to quantify tree transpiration dynamics and their contribution to the urban water cycle and to the cooling potentials of urban trees. In addition, relevant factors to plan resilient urban ecosystems can be extracted with such datasets and analysis.