Light availability and anthropogenic stress shape plant understory invasions in understory of urban forests: A case study in Shanghai

Understory vegetation is a vital part of urban forests and provides a habitat for the sustainability of plant and animal biodiversity. Understory vegetation in urban forests is highly sensitive to anthropogenic stress and thus more prone to plant invasions. However, the influencing factors and invasion mechanisms of nonnative species (nonnatives) in the understory layers of urban forests remain poorly understood. Here, we studied the understory community structure and composition in 75 plots across 16 urban forests in Shanghai, China. We employed the random forest model, structural equation models and phylogenetic methods to explore the factors contributing to invasion success and impact of nonnatives, and also examined the phylogenetic distance and trait difference between native and nonnatives. We found that 19.4% of the species were nonnatives and Erigeron philadelphicus Willd. was the most frequent species, occurring in 33.5% of the plots. Socio-economic variables (gross domestic product, population, and distance from the city center), the understory cover of native species, and overstory canopy cover were the primary factors driving the invasion success and impact of nonnatives. The variation of the nearest phylogenetic distance between native and nonnative species indicated that both anthropogenic stress and light availability affect understory invasion success, but with different mechanisms. Specifically, an increase in light availability reduces the competition for light and provides invasion opportunities for nonnatives that are phylogenetically close to natives, while more habitat types created by human activities provide more invasion opportunities for species with distant phylogenetic relationships with the native species. We believe that these findings have important implications in the management and control of understory invasions in urban forests.