Abstract: Vegetation is a key factor to mitigate the urban heat island effect because it can form the local cool island. Previous studies focused on the local cooling effect of vegetation and water bodies in urbanized regions. However, few studies have figured out whether such effect would vary along the urbanrural gradient, as well as the relationship between such variations and the geometry characteristics of urban forest patches. In this study, we first retrieved the land surface temperature (LST) and the land use from Landsat 8 TIRS and GF-2 PMS imagery, respectively, and then analyzed the cooling effects of urban forest patches in Shanghai via buffer analysis method. Different urban development zones with various urbanization levels were used to represent the urban-rural gradient. Results showed that LST of the urban forest patches was 0.55 ℃ lower than the mean LST in Shanghai, indicating the role of forest patches as cool island in urban thermal environment during the daytime in summer. However, such effect of forest patches was not consistent along the urban-rural gradient. Strong cooling island effect was found for those in the downtown, with a reduction of LST up to 1.67 ℃. Forest patches did not serve as the cool island in the outskirt, such as in Chongming District. In each region, the area and shape characteristics of forest patches exerted great influences on the cool island effect. The patches with large size and regular shape had stronger cool island effect compared with those with small size and irregular shape. Compared to those in the region with low urbanization level, the small and irregular shape forest patches in the region with high urbanization level were apt to serve as cool islands. These findings can provide scientific support for future urban greening and ecological construction.

Key words: urban heat island, cooling amplitude, urbanization, urban forest, thermal environment.