Ecohydrological dataset of various continuous field measurements on Norway maple (Acer platanoides) and small-leaved lime (Tilia cordata) in the city of Freiburg, Germany

Urban hydrological processes are significantly influenced by human activities, particularly the expansion of impermeable surfaces, which impact urban meteorology and hydrology. Urban trees play a crucial role in enhancing both the urban water cycle and meteorological conditions. However, the complex and often stressful urban environment poses challenges to tree ecophysiology.
In the city of Freiburg, Germany, we explored the transpiration dynamics of Norway maple (Acer platanoides) and small-leaved lime (Tilia cordata) across different urban settings. Our findings revealed that small-leaved lime exhibited higher average daytime transpiration rates (1.76 ± 0.53 mm) compared to Norway maple (1.53 ± 0.51 mm). Additionally, surface sealing, such as large impermeable surfaces beneath tree crowns, reduced transpiration in both species, suggesting that surface sealing can inhibit root water uptake.
Solar radiation emerged as the primary driver of daily transpiration dynamics, followed by vapor pressure deficit and soil water content. Tree morphological traits, including leaf area index and density, along with the degree of surface sealing, were also significant factors affecting transpiration rates. A strong correlation was found between leaf area index and the extent of impermeable surfaces within the crown projection area, emphasizing the interconnectedness between tree characteristics and urban conditions.
Our research further examined the effects of drought periods on the transpiration processes of selected study trees, revealing significant reductions in transpiration during drought periods, particularly in areas with a high degree of surface sealing.
This study underscores the importance of considering surface sealing when analysing hydrometeorological-dependent transpiration dynamics in urban environments. By gathering data on typical urban tree species in Central European cities, we contribute to a broader understanding of tree transpiration processes. However, long-term field measurements are necessary, covering a variety of meteorological conditions, tree species, and urban settings, to fully quantify the role of tree water relations in the complex urban water cycle.