Modeling Tree Shade Effect on Urban Ground Surface Temperature

There is growing interest in the role that urban forests can play as urban microclimate modifiers. Tree shade and evapotranspiration affect energy fluxes and mitigate microclimate conditions, with beneficial effects on human health and outdoor comfort. The aim of this study was to investigate surface temperature (Ts) variability under the shade of different tree species and to test the capability in predicting Ts of a proposed heat transfer model. Surface temperature data on asphalt and grass under different shading conditions were collected in the Cascine park, Florence, Italy, and were used to test the performance of a one-dimensional heat transfer model integrated with a routine for estimating the effect of plant canopies on surface heat transfer. Shading effects of 10 tree species commonly used in Italian urban settings were determined by considering the infrared radiation and the tree canopy leaf area index (LAI). The results indicate that, on asphalt, Ts was negatively related to the LAI of trees (Ts reduction ranging from 13.8 to 22.8°C). On grass, this relationship was weaker probably because of the combined effect of shade and grass evapotranspiration on Ts (Ts reduction ranged from 6.9 to 9.4°C). A sensitivity analysis confirmed that other factors linked to soil water content play an important role in Ts reduction of grassed areas. Our findings suggest that the energy balance model can be effectively used to estimate Ts of the urban pavement under different shading conditions and can be applied to the analysis of microclimate conditions of urban green spaces.