In the past 45 years, China has experienced a rapid urbanization process (Yang et al., 2021a). This process has resulted in many forests, which were originally used for harvesting forest products, being divided by built-up areas, forming fragmented forest patches (Zhou et al., 2017). These residual patches are crucial for biodiversity conservation (Mitchell et al., 2022), but protecting them is extremely difficult (Nielsen et al., 2013; Lepczyk et al., 2017; Paquette et al., 2023). Transforming residual forest patches into urban parks or other green spaces is a way to protect and utilize these ecological patches (Wang et al., 2021). Urban parks and other urban green spaces serve as the ecological heart of the city, playing a vital role in mitigating urban ecological degradation, preserving native species, and maintaining ecological balance (Chen et al. 2018; Figueroa et al. 2018), thus contributing to the sustainability of urban ecology (White et al. 2013). However, due to the high degree of similarity in tree species composition among different parks and the pressing issue of plant homogenization in green spaces (Li et al. 2023), the uniqueness of urban parks and green spaces has recently gained increasing attention (Chan et al. 2015; Wang et al. 2019).
Due to the lack of evaluation indicators and methods, there are only a few studies on the uniqueness of urban parks. Chan et al. (2015) and Wang et al. (2019) studied the uniqueness of urban parks and green spaces in Hong Kong and Helsinki, respectively. Paquette et al. (2023) studied the uniqueness of plant communities at different sites in a large urban park in Quebec City, Canada. The main evaluation methods for the uniqueness of urban parks and green spaces used in these studies are two: one is to use a questionnaire to conduct a subjective survey of visitors, such as Chan et al. (2015), who designed a survey using the Likert six-point method to interview three types of people - inbound tourists, potential tourists, and local residents - to understand their views on the uniqueness of six types of urban green spaces in Hong Kong. Wang et al. (2019) used a public participation geographic information system to collect coordinates of urban green spaces that people think are unique in Helsinki. However, using interviews to draw conclusions is subjective and influenced by the characteristics of the interviewees. In particular, local people who are familiar with the park tend to evaluate its uniqueness lower (Wang et al. 2019), while outsider tourists often have an un-objective understanding of the park (Chan et al. 2015). The other method is to use beta diversity analysis in ecology to evaluate the uniqueness of plant communities. Specifically, it analyzes the contribution of a certain unit or species to the overall beta diversity, using it as an index of uniqueness for a certain unit area or species (e.g., Paquette et al. 2023). This method is commonly used in ecology to analyze the functional uniqueness of specific sites or species within an ecosystem or community (Xia et al. 2022; Wang et al. 2023). This method can evaluate the uniqueness of different parts within a whole, and its applicability in evaluating and comparing different park green spaces remains to be verified. However, the uniqueness of biodiversity is only one aspect of community uniqueness.
The woody plants in urban green spaces play a crucial role in carbon fixation, air purification, providing habitats for birds, and shading functions (Fini et al. 2023; Villaseñor et al. 2021; Yang et al. 2005; Xiao et al. 2016). Both the diversity of plant species and the spatial size of different species can influence the sensory experience and uniqueness of urban park green spaces. However, in ecological indicators, diversity indexes only reflect species richness and composition (Young and Swiacki 2006) and cannot account for the spatial occupation of species. The species importance value (IV, the average of relative abundance, relative frequency, and relative dominance) is a comprehensive ecological index that takes into consideration species diversity and spatial occupation (Aye and Shibata 2023). Therefore, we suggest the potential use of the IV for calculation to evaluate the uniqueness of urban park green spaces.
Qingdao, a coastal city in eastern China, boasts a typical coastal hilly terrain. The urban area of Qingdao is dotted with numerous hills that are unsuitable for agricultural or construction purposes, resulting in patches of preserved forests. In the process of urban construction and renewal in Qingdao, more than 60 forest patches have been transformed into parks. In this study, we propose two evaluation indicators for woody plant community uniqueness in urban parks based on importance value (IV, the average of relative abundance, relative frequency, and relative dominance): the mean non-overlapping IV (MNIV) and the local contribution to beta diversity based on IV (LCBD1). Subsequently, we investigated and evaluated the diversity and uniqueness of woody plant communities in eight hill parks in Qingdao. We also compared the differences between the results of evaluating park uniqueness using different indicators, and analyzed the relationships between the uniqueness of urban park green spaces and species diversity, park area and age, and the IVs of different plants. This aimed to provide a theoretical basis for the protection and utilization of residual forest patches and the construction of urban park forests.