Tree crowns and their associated summertime microclimatic adjustment and thermal comfort improvement in urban parks in a subtropical city of China

Abstract

Trees have significant shading and cooling effects because of their crowns. This study investigated the cooling effects of their crown attribute on urban microclimate adjustment and thermal comfort improvement. These attributes included the ratio of crown diameter to branching position height (D/PH) and crown porosity, which were measured using the leaf area index and leaf gap ratio. Field measurements and a questionnaire survey were conducted in two urban parks in Deyang, a heavy industrial city in China, with a subtropical climate. D/PH had a significant microclimatic impact, which was evident in its strong correlation with mean radiant temperature and thermal sensation vote. On hot summer days, increased D/PH from 0 to 8 could help decrease mean radiant temperature by approximately 15 °C while improving thermal sensation from 2 (warm) to 0 (neutral). The microclimatic effect of crown porosity was found to be insignificant, since few gaps in the crown were found in the leafy season. The results provide important directions for urban planning and landscape design in terms of plant selection for improving the urban microclimate.