Homogenization of trees in urban green spaces along the moisture gradient in China

The species composition of urban green spaces (UGSs) in different climate areas has been found to be homog-enized compared with that of rural forests (RFs). However, the homogenization degree in response to moisture as a single ecological factor is still unknown. Here, we used 18 cities in the temperate zone of China to analyse the tree species composition and diversity index of UGSs and compare them with those of RFs. The results showed that (1) a total of 462 tree species were recorded in the UGSs in 18 cities, and 88 species were found in the UGSs of more than 9 cities. Among the 88 species, 47 species occur in UGSs in humid, semiarid and arid areas, including Ginkgo biloba, Salix babylonica, Acer mono, and Pinus tabuliformis. (2) The tree species richness and Simpson index of UGSs tended to converge (CVUGS approximate to 1/2 CVRF) along the moisture gradient (mean annual precipitation 200-800 mm). (3) The similarity of tree species composition among UGSs (J similar to 0.30) was signif-icantly higher than that among RFs (J similar to 0.07), revealing homogenization among cities in areas with different humidity conditions. (4) The similarity increased when the difference in moisture and per capita GDP (gross domestic production) of cities decreased, indicating that ecological and economic factors jointly affected the homogenization of UGSs. This study also provides guidance for the management of UGSs.