Multi-Scale Variations in Understory Community Diversity and Their Driving Mechanisms Under Urbanization Pressure: A Case Study of Shanghai, China

Understory community diversity in urban forests is crucial for maintaining urban ecosystem functions and enhancing urban resilience, but it is threatened by rapid urbanization. Currently, there remains a knowledge gap regarding the multi-scale responses and driving mechanisms of understory community diversity along urbanization gradients, which hinders its scientific conservation and management. This study was conducted in Shanghai, a highly urbanized metropolis in China, at both plot and site scales. A total of 75 plots and 380 quadrats were established across 16 urban forest sites. Five key environmental factors were selected, including distance from the city center, visitor intensity, non-native species richness, overstory coverage, and forest area. Using taxonomic and phylogenetic diversity, regression models and null models were employed to analyze the multi-scale patterns and underlying assembly processes of understory plant communities. The results showed that the effects of environmental factors were scale-dependent, with environmental filtering as the core assembly mechanism. At the plot scale, the distance from the city center exhibited a U-shaped relationship with species richness (p = 0.005), while visitor intensity displayed a unimodal pattern with both species richness (p < 0.001) and Faith’s phylogenetic diversity (PD, p = 0.029). Increased non-native species richness intensified phylogenetic clustering (p < 0.05), and environmental filtering was the dominant process of community assembly. At the site scale, the β-diversity of non-native species drove the increase in phylogenetic the β-diversity of understory communities (p < 0.001); geographical distance had a significant positive effect on βMNTD (p = 0.002); and differences in non-native species could weaken biotic homogenization (p < 0.05). This study clarifies the multi-scale response patterns and driving mechanisms of understory community diversity and structure, providing a scientific basis for optimizing the conservation and management of understory vegetation in urban forests and enhancing urban ecosystem stability. Future work calls for long-term monitoring and broader environmental indicators.