Homogenization of tree species diversity in urban green spaces along a temperature gradient in eastern China

The construction of urban green spaces usually leads to the replacement of native species by alien species, resulting in the homogenization of species composition across cities in different climatic zones. However, along a temperature gradient, whether tree species diversity and life form composition (evergreen vs. deciduous) are homogenized across cities has not been verified. In this study, we first compared the tree species diversity, life form composition, and species composition in green spaces among 29 cities in eastern China and then compared them with natural forests outside the cities. The results showed that (1) the increasing rates of tree species diversity (Shannon-Wiener and Simpson indices) in urban green spaces along the temperature gradient were less than half those in natural forests, indicating the convergence of tree species diversity across cities; (2) the similarity of tree species composition in green spaces across cities was higher than that in forests across natural regions, showing the homogenization of tree species composition across cities; (3) the similarity of tree species composition in green spaces between cities decreased with an increase in the temperature difference; (4) the increasing rate of the proportion of deciduous broad-leaved trees in the green spaces along the temperature gradient was similar to 24 % slower than that in the natural forests, showing the homogenization of life form composition of trees across cities; and (5) there were 88 tree species widely used in more than half of the studied cities, and 60 % of them were used in cities outside their natural distribution areas. This study is helpful for understanding the effect of human activities on the biotic homogenization of green spaces and provides guidance for the policymaking of the construction and management of urban green spaces.