- Below-canopy cooling benefits of tree species can vary depending on weather types.
- We studied air temperature from the tree canopies to the ground under tree shades.
- 20 Robinia pseudoacacia and Tilia cordata trees were studied during the summer 2016.
- Shading is the prominent cooling benefits when the days are very hot.
- Transpirational cooling from trees and grasses are prominent in mild or summer days.
Moderation of thermal energy balance through the canopies of urban trees is well known. However, a more functional and quantitative view of the heterogeneous urban environment and their influence on the below-canopy vertical air temperature gradients is largely missing. Throughout the summer 2016 we continuously measured air temperature at three different heights (at 1.5, 3 and 4.5 m from the ground) under the canopies of two common but contrasting street tree species in respect of eco-physiology and morphology in Munich, Germany: Robinia pseudoacacia L. (ring porous) and Tilia cordata Mill. (diffuse porous). Along with air and surface temperature we also measured meteorological and edaphic variables and categorized summer time as cool, mild, summer and hot days. Global radiation, vapour pressure deficit and soil temperature increased as the days got warmer but precipitation, soil moisture and wind speed showed the reversed pattern. Overall, T. cordata trees with higher leaf area index and sap-wood area provided three times more transpiration than R. pseudoacacia. On an average air temperature gradient of outside to inside canopy dropped from 1.8 °C to 1.3 °C for T. cordata but from 1.5 °C to only 0.5 °C for R. pseudoacacia as the days got warmer. Vertical decline of air cooling effect was around 1 °C from canopy to the near-ground (1.5 m). Lower soil moisture but higher soil temperature suggested that cool air from the canopy mixed with a higher amount of sensible heat flux under the canopies of T. cordata compared to the R. pseudoacacia as the days got warmer. The study indicated a threshold for extreme hot days when grass surface evapotranspirational cooling will not be as effective and act like built surfaces rather deep shading from tree canopies will be important.