Significance On October 12, 2021, at the Leaders' Summit of the 15th Meeting of the Conference of the Parties to the Convention on Biological Diversity, China announced that it would set up a system of national botanical gardens in Beijing and Guangzhou, to strengthen ex situ conservation of endangered plant species in China. In April and July 2022, Beijing National Botanical Garden and South China National Botanical Garden were inaugurated successively. The building of the national botanical garden system is an important initiative to practice Xi Jinping's Thought on Ecological Civilization, and a concrete action to advocate the harmonious coexistence between human and nature at the national level, and to strengthen biodiversity conservation and implement the United Nations Convention on Biological Diversity. Under the background of national biodiversity conservation and national botanical garden system building, urban biodiversity becomes increasingly important, considering the coming of the Anthropocene and the intensification of urbanization which have brought new challenges to the conservation of biodiversity. Protecting and enhancing urban biodiversity is not only beneficial to the urban environment and residents' quality of life, but also an indispensable part of global biodiversity conservation. It is imperative to find ways to conserve and enhance biodiversity in such venues as urban environment and agricultural landscape that are less biodiverse than protected areas. As a professional institutions, botanic gardens possess a unique set of skills that encompass identifying, collecting, conserving and enhancing plant diversity across the entire taxonomic spectrum. By using the wealth of species, data, facilities, expertise and technology accumulated during ex situ conservation research and practice, the national botanical garden system of China can serve to enhance urban biodiversity and urban as one of its operational priorities. This research may provide reference for the functional orientation and development direction of China national botanical gardens.

Progress This research lists main problems existing in urban plant diversity, and briefs the botanical gardens' role in supporting the diversity of urban plants by introducing, domesticating, screening and breeding plant species, improving plant cultivation techniques, exploring the scientific arrangement of plants in urban green spaces, and investigating the planning and evaluation of urban ecosystem construction. Shanghai Chenshan Botanical Garden is taken as an example in this research. By plant collection, selection and demonstration, Chenshan Botanical Garden has enriched the species of urban plants, increased the choices of urban horticultural plants, developed the horticultural applications of plants and, to some extent, solved the problems of similarity and identification loss of urban plant landscape. Based on the systematic research on solutions to urban greening in the built environment from the aspects of rebuilding the habitats for plant growth, screening plants suitable for different urban environments and improving plant cultivation and management, the garden makes more plants adapt to the urban habitats, and further expands urban greening space by helping plants take root in spaces such as roofs and vertical walls, thus helping avoid the excessive occupation of the natural ecological space and strengthening the connectivity of ecological spatial pattern. In terms of alien invasive plants, the garden pays continuous efforts to monitor new or potential invasive plants in Shanghai and Eastern China and to assess the risk of plant invasion, so as to provide monitoring and assessment methods for urban plant diversity conservation, thus ensuring urban ecological security.

Conclusions and Prospects Botanical gardens can support the enhancement of urban plant diversity at four levels: gene, species, community and ecosystem. At the gene level, botanic gardens breed new plant species to create unprecedented plants. At the species level, botanical gardens collect and screen a large number of plant species suitable for urban environment, functioning as an urban plant germplasm bank that can provide strong support for urban ecological environment construction. At the community level, botanical gardens explore the structure of urban plant communities, guide the allocation of green spaces, and form a low-maintenance, high-benefit and sustainable urban plant landscape. At the ecosystem level, botanical gardens participate in the construction of urban ecosystems based on the research on plant conservation, phytoremediation and urban horticulture, and explore planning and evaluation methods for maintaining the sustainability of urban ecosystems to improve urban plant diversity and urban biodiversity. At present, national botanical gardens should take the research on and conservation of urban plant diversity as an important development direction. These gardens can serve cities in urban biodiversity survey, urban plant information sharing platform, urban plant screening, high-efficiency carbon sink plant selection, urban invasive plant list, ecological restoration, diversified ecological landscape design, biodiversity assessment, awareness enhancement in urban biodiversity conservation and social participation, etc. It is expected that national botanical gardens will transform cities into fronts to stop global biodiversity loss and better fulfill the function of biodiversity conservation.