Vertical thermal environment of subtropical broad-leaved urban forests and the influence of canopy structure

Abstract

Urban forests alter the surface energy balance and provide a comfortable thermal environment. However, little quantitative research exists on the relationship between their vertical temperature structure and influence on the urban thermal environment. In this study, we aimed to measure the differences and determine the correlations between the canopy temperature (T1), below-canopy air temperature (T2), below-canopy ground surface temperature (T3), and soil temperature (T4) of three forests: evergreen broad-leaved forest (EBF), deciduous broad-leaved forest (DBF), and mixed evergreen and deciduous broad-leaved forest (MBF) in Zhejiang Province, China. The thermal environment of DBF fluctuated greatly, whereas that of EBF and MBF was relatively stable. T1 was highly and significantly correlated with T2 (r > 0.850, p < 0.01) and T3 (r > 0.790, p < 0.01) in EBF and MBF; thus, T1 is a good predictor of T2 and T3 in EBF and MBF. T4 was susceptible to the external thermal environment and correlated relatively well with T2 (r = 0.879) in DBF. MBF and EBF had substantial cooling effects, with mean values of T2 and T3 2.4 and 2.3 °C, and 10.1 °C and 10.7 °C lower than the air and surface temperatures in the areas without canopy shade, respectively. Ambient temperature and humidity, solar radiation, canopy thickness and density, tree number, and tree species and height significantly influenced the thermal environment and cooling effect of the forests. The results of this study provide an important reference for the future design of thermally comfortable urban landscapes.