**1. Introduction**

City is a complex ecosystem including nature and society [1]. Different functional areas in the city are embedded with each other to form a complex system, which supports the city's demand for huge logistics, energy flow, information flow, and other flows. It also determines that the city has special habitat characteristics, such as obvious gradient change of environmental factors, apparent reduction of natural habitat areas, habitat fragmentation, and fragile ecosystem [2]. Urban biodiversity is not only the fundamental condition of urban survival but also the basis of human survival. At the same time, it has ecological, cultural, and esthetic values [3]. The protection and maintenance of urban biodiversity is the foundation of urban healthy development, the premise of improving the function of urban ecosystem and meeting the needs of human survival. Urban green space is an artificially constructed plant community, which affects the living environment of citizens, but it is disturbed by human activities to a great extent. Therefore, the protection of biodiversity in the city is extremely important and difficult simultaneously.

Urbanization is considered to be one of the main drivers of environmental change, which has a significant impact on the attributes of biological and abiotic ecosystems all over the world [4–7]. Driven by the tide of economic globalization and the requirements of improving people's living standards, the level of urbanization will be further improved. At the global scale, a widely used dataset for projecting the global urbanization level by 2050 is the national-scale 5-year-interval World Urbanization Prospects (WUP) data released by the Population Division of the Department of Economic and Social Affairs of the United Nations (UNPD) [8]. According to WUP, the world urbanization rate will reach 68.4% by 2050 [9]. Well-managed urbanization processes can help maximize the benefits of economic agglomeration while reducing environmental degradation and other potential adverse impacts [10, 11]. However, rapid urbanization will also bring many negative effects. For example, the urban expanded, which often grows faster than parks or reservations [12–14]. Among them, the impact of urbanization on biodiversity is the focus of attention and one of the hotspots in current ecological research [15]. Urban expansion leads to habitat fragmentation and invasion of alien species, thus reducing professional species and increasing common generalist species. This process is defined as biological homogenization [13, 16, 17]. Urbanization leads to the decrease of biodiversity. The invasion of alien species and the reduction of local species caused by urbanization lead to the homogenization of urban organisms [13, 17–19], and the homogenization of urban plants is also prominent [20]. Plants are the basis of all life and development, providing habitats for animals and microorganisms.

Plant diversity is the foundation of urban ecosystem stability and the guarantee of urban sustainable development. It plays an indispensable role in the stability and function of urban ecosystem. Plant diversity under the influence of urbanization is the basis and hotspot of current urban biological ecosystem research [21]. The research on urban plant diversity began in the 1840s in Germany, Britain, and other European countries, and in the 1980s and 1990s there were extensive studies in various developed countries [22–26]. In developing countries, urbanization often has two characteristics: First, the natural ecosystem is replaced by artificial buildings; second, the natural soil is replaced by the green space dominated by exotic ornamental plants, which has a great impact on biodiversity [27]. China has a relatively late cognition of urban plant diversity, and there are few studies. Since 2000, there have been studies on plant diversity in Zunhua, Shanghai, and other cities in China [28–31]. Beijing, which has developed rapidly urbanization, also has a certain research foundation [32, 33], including research on plant diversity in parks [34–36], research on plant diversity in residential green spaces [37], and research on exotic plants in residential green spaces [38]. However, there are relatively few studies on the impact of urbanization on plant diversity under different urban green space types [39, 40], and there is a lack of research on the impact of urbanization gradient on plant distribution.
