Abstract: Cities are influenced by a wide range of human activities and are often hotspots of biotic homogenization, leading to a loss of biodiversity. To mitigate these negative impacts and promote urban sustainability, urban greening has been recognized as an effective strategy. However, urban greening can also lead to increased homogeneity across cities by selecting similar street tree species to provide urban ecosystem services for human health and wellbeing. The objective of this study is to (1) describe the species composition of commonly planted street trees in cities worldwide and compare their similarity, and (2) examine the relationships between street tree species and cities across different cultures, histories, and climatic zones. Most meta-analyses of urban biodiversity have been shown a pronounced geographical bias towards North American and European cities. Given that Asia and Africa are currently experiencing the fastest rates of urbanization, we put great efforts to collect urban street tree data across South Asia, Africa, and the Middle East. A total of 149 cities were included in this study, based on their provision of street tree inventory data (including richness and abundance), and observational studies between 2000 and 2022 in cities with populations over 300,000 inhabitants, or less than 300,000 inhabitants but serving as a capital city. The range of commonly street tree richness in cities varied from 3 to 21 species. Among 149 cities, only 19 cities adhered to the Santmour 10-20-30 rule. Notably, the commonly planted street trees in 12 cities were all non-native species. The percentage of native tree species in tropical and arid cities was only 29% and 32%, respectively. Acer was found as the most commonly planted tree genus and Fabaceae as the most prevalent tree family across the 149 cities. Similarity between street tree communities across all cities were determined by using Ward's linkage method. The cluster analysis of tree species composition in 149 cities revealed five distinct groups. The first group included North American and European cities, while the second group consisted of Asian cities and a few cities from South America and Africa. The third group comprised only Asian cities from Japan or South Korea, whereas the fourth group consisted of Asian and European cities. The fifth group comprised cities from five continents. These findings show that cities become increasingly similar in street tree composition not only due to climate or geographical distance but also because of other cultural and historical factors.
