Multidimensional Urbanization Effects on Spontaneous Plant Diversity in a Cold Climate Megacity

Urbanization profoundly transforms ecosystems, often resulting in habitat loss and biodiversity decline. Urban spontaneous plants, which are established naturally without human intervention, play a critical role in urban ecosystems by providing habitats, mitigating urban heat islands, and acting as indicators of environmental changes. Multidimensional urbanization, encompassing vertical and horizontal scale, exerts a significant influence on the biodiversity of green space. While previous studies have extensively examined the effects of horizontal spatial scales (such as land use and population density), the impacts of vertical spatial scales remain understudied. To elucidate the spatial patterns and driving factors of spontaneous plant diversity under multidimensional urbanization, we conducted a comprehensive survey of spontaneous plants across Changchun, a rapidly urbanizing city in northeast China. We established 1147 herbaceous plots within 245 urban green space patches across 38 sites and analyzed the effects of multidimensional urbanization metrics on spontaneous plant diversity. A total of 408 species of spontaneous plants were recorded, with herbs as the dominant life-form (89.2%), 322 are native species (78.9%), and 21.1% non-native species (of which 65.1% are invasive), primarily dispersed by autochory. Significant differences in plant diversity indices were observed across various urban green spaces and habitat types in Changchun, with native plant diversity generally highest in square green spaces and scrub gaps, while non-native plant diversity was most prominent in brownfield sites and showed no significant variation among habitat types. Regression analyses revealed that, in addition to patch characteristic factors (including patch area, perimeter-area ratio, and landscape shape index), the richness of total, native, and autochorous spontaneous plants was primarily influenced by vertical urbanization (as indicated by building volume), with building volume positively associated with species richness. In contrast, the richness of non-native and anemochorous plants was also significantly affected by horizontal urbanization factors, such as the proportion of impervious surface within a 100 m buffer zone and distance from patch to city center. The results reveal distinct spatial patterns of spontaneous plant diversity driven by both urbanization of horizontal spatial scales and vertical spatial scales. Our study provides new insights into the interplay between multidimensional urbanization and biodiversity, offering a theoretical foundation for integrating biodiversity conservation into sustainable urban planning and ecosystem management.