The impact of urban forest structure and its spatial location on urban cool island intensity

Urban forest can help decrease land surface temperature (LST) and create urban cooling effect (UCI) to mitigate urban heat island (UHI). However, it is still unclear how urban forest structure and its location affect UCI, particularly under different seasons. In this study, with plot-based urban forest structure and UCI intensity extracted from Landsat-7 Enhanced Thematic Mapper Plus (ETM+) thermal data, we first conducted correlation analyses between UCI and different forest structures (crown closure, tree height, leaf area index, basal area, stem density and diameter at breast height, etc.) and spatial location (distances from buildings and from water bodies and elevation) attributes, and we then carried out quantitative regression analyses between them. Our results indicate that (1) Urban forest could create “urban cool islands”, which were higher in summer than those in autumn.(2) UCI could be significantly affected by urban forest structural attributes, especially by crown closure and LAI. All urban forest structural attributes had positive linear relationships with UCI except for LAI and basal area which had positive non-linear relationships with UCI.; (3) UCI in urban forest could also be affected by its spatial location but not by its elevation. The UCI non-linearly decreased with decreasing distance from buildings and with increasing distance from water bodies. The threshold values of DB for significantly affecting UCI variation is approximately between 100 m and 300 m in summer and autumn, respectively; and (4) the relationships between UCI and urban forest structure and its location attributes were complex and seasonal dependent. Urban forest attributes had greater effects on increasing UCI in summer than those in autumn. These findings would deepen our understanding of interactions between UCI and urban forest attributes and provide urban planners with useful information about how to design urban forest to effectively mitigate UHI effects.