With the rapid growth of global urbanization and urban populations, many cities have been experiencing large-scale Land-use conversion (Ramalho, et al., 2014). Urban areas continue to expand to (semi-) natural areas outside their boundaries, and a large number of (semi-) natural areas are rapidly lost, split (Hayriye, et al., 2009), degraded or dispersed and embedded in a heterogeneous artificial built-up environment to form islands or island-like isolated "remnant habitats" (Fernández, et al., 2019; Han, et al., 2019). In most areas, urban expansion has destroyed (semi-) natural habitats (i.e., completely converted to the artificial environment), and few cities can retain these (semi-) natural habitats in the process of urbanization (Müller, et al., 2015). The process of habitat loss and fragmentation caused by the gradual transformation of intact habitat patches from natural land cover to impervious surfaces in urbanized areas usually has a large and negative effect on plant community species diversity(Fahrig, 2003; Kowarik & von der Lippe., 2018). Therefore, urbanization is considered to be the main cause of urban biodiversity loss and a serious threat to urban biodiversity conservation (McKinney, 2006). Urban remnant habitats are valuable ecological resources and important potential conservation core areas of species diversity in cities (Mona, et al., 2016; Alvey, 2006; Frank, et al., 2020). They can maintain urban native biodiversity and various ecosystem processes and provide ecological well-being for urban residents (Han, et al., 2019; Guirado, et al., 2006). Due to the complexity of the urban artificial environment, research on the maintenance mechanism of species diversity in urban remnant habitats has become a popular and difficult issue in global urban ecology (Ramalho, et al., 2014) and has attracted extensive attention in the field of urban ecology (Ramalho, 2012; Schlesinger, et al., 2008). However, the scope and mechanism of the effects of urban matrix environment composition and configuration on the plant diversity of remnant habitats and at different levels are still unclear.
The particularity of urban remnant habitats was that they are completely exposed to the heterogeneity of urban artificial built-up environments, and the matrix and the surrounding environment, in terms of both material exchange and spatial relationships, are relatively complex. In particular, the mutual influence between different landscape types and functions and changes in landscape structure and function of the influence of remnant habitat are complex and diverse(Patarkalashvil, et al., 2017; Shi, et al., 2020). Most previous studies have used the "gradient method" to explore how urbanization factors such as distance from urban centres and population density affect biodiversity in urban substrates (Kinzig, et al., 2005; Malkinson, et al., 2018). Several studies have also shown that the urban species richness had an obvious urban gradient effect(Breuste, et al., 2008; McDonnell & Hahs, 2008). However, other scholars regarded that urban biodiversity patterns (e.g., birds and plants) were influenced by the cultural and economic status of urban residents rather than by population density, distance from urban centres, or time after disturbance (Evan, et al., 2020; Luck, et al., 2009; Hope, et al., 2003). The urbanization gradient can be used as a proxy for causal mechanisms such as disturbance state, pollutant load or predation pressure, but it does not directly affect biodiversity(Kinzig, et al., 2005). To a greater extent, it could depends on the differences in adjacent patch characteristics, landscape fragmentation, Land-use composition, and structural changes in remnant habitats with different urbanization gradients.
Some of the case studies have used indicators of urbanization as potential drivers of urban biodiversity. For example, in a study on urban plant diversity in Wuhan, China, Yan et al. (2019) showed that urban impervious surface area was an important indicator that can be used to effectively measure the impact to urban plant diversity. When the proportion of urban impervious surface area exceeds 40%, urban plant diversity declineed sharply. Hayriye et al. (2009) analysed the impact of accelerated urbanization on open space protection in the Phoenix Metropolitan Area of Arizona, USA, using time series and three indicators of matrix effect, isolation and connectivity, and the results showed that increased Land-use intensity adjacent to the reserve may increase the edge effect and reduce the habitat value of the inner or core habitat area. De Souza et al. (2020) used fractional Brownian motion to study how patterns of dynamic habitat loss and fragmentation affected the level of biodiversity in an ecosystem, and the results showed that both patterns of habitat loss and environmental heterogeneity affected species distribution patterns. Ramalho et al. (2014) evaluated the impact of urban fragmentation on plant species richness in 30 Banksia remnant habitats in Perth, Australia, the results showed that the impact of rapid urban fragmentation on residual vegetation was great and complex, and these effects may take several decades to show a significant relationship. Fahrig et al. (2003) showed that multiple mechanisms may lead to positive or negative landscape scale responses to habitat fragmentation and may change with environmental change. Studies on the effects of urbanization on plant diversity in Africa, Europe and Asia have confirmed that patch size and adjacent Land-use type were important factors determining plant species composition and richness, but the direct or indirect effects of disturbance mechanisms and surrounding environmental changes may be different(Frank, et al., 2020; Guirado et al., 2006; He, et al.,2019).
Additionally, other scholars have begun to explore the potential relationship between urbanization and species diversity on a spatial scale. Fernández et al. (2019), using a multitemporal and spatial scale approach, assessed the effects of urban matrix vegetation patterns on the primary productivity of natural heritage habitats in 10 cities in Santiago (Chile), the results showed that the primary productivity of all remnant natural habitats decreased, and this decrease was spatially related to the change in vegetation cover in the surrounding urban matrix within 900 m and was more strongly related to the composition of the matrix. Peng et al. (2019) found that, during the urbanization process, landscape units with a radius of 600–700 m and different land uses had the most appropriate spatial scale range for the conservation of native plant diversity in the study on the optimal landscape pattern for the conservation of native plant diversity in Shunyi District, Beijing. Planchuelo et al. (2020) analysed urban matrix characteristics (e.g., impervious surface area, average floor area) within a 500-m radius around endangered plant species in Berlin, Germany, and revealed the negative impact of urbanization dynamics in Berlin (increase in the impervious surface) on the survival of endangered plant species. Although studies have revealed the potential relationships between some urbanization indicators/predictors and urban biodiversity patterns at different spatial scales, there is no unified and optimal methodological indicator to measure and evaluate the impact of urbanization development on urban biodiversity in all regions (Yan, et al., 2019). Therefore, the selection of appropriate urban matrix drivers and spatial scales could make it easier to collect data and accurately predict or reflect the effects and mechanisms of urbanization on plant diversity in remnant habitats.
The Karst area in southern China, centred on the Guizhou Plateau, is the most typical, complex and abundant Karst area in the world, as well as the largest and most concentrated ecologically fragile area (Wang, et al., 2015). Due to the special geomorphological form of solitary peaks and peak forest, in the process of urban expansion in this area, a large number of Karst hills of different scales were left as islands or island-like (hemi-) natural remnant habitats in the artificial built-up heterogeneous urban environment, formed the Karst “urban remnant hills" (URHs) habitat and the landscape mosaic of “city amomg the hills, hills in the city". The spatial form of the Karst hilly city embedded with the hills not only creates a unique city style but also forms the precious remnant ecological resources of the urban hills in the Karst hilly city(Ren, et al., 2018). Thus, this area provides an ideal research place for the ecological study of remnant habitat in an artificial urban environment.
Guiyang, located in the middle of Guizhou Province, is a typical Karst hilly city with a large number of URHs in the built-up area of the central urban area. In this study,15 URHs in the urban built-up area of Guiyang were selected as the research object, and four indicators of urban matrix characteristics, namely PTIA, Land use, VC and FI, were adopted to explore the following questions: 1) Are there any correlations between the urban matrix characteristics and the plant diversity at different levels of the remnant hills? 2) If there is a correlation between the two, what are the main influencing factors of the characteristics of the urban matrix on the plant diversity of the remnant hills? and what is the scale effect? 3) Are the responses of plant diversity at different levels to the characteristics of the urban matrix in the remnant hills consistent?