Understanding the factors driving species composition similarity of urban spontaneous plants

Understanding factors that influence spontaneous plant species similarity across urban sites provides insights into species exchange processes in urban environments. We surveyed spontaneous plants in 30 urban sites in Nanjing, China, to investigate how seed capture opportunity, establishment resistance, and movement resistance affect species similarity between paired sites. Using various gravity models, we found that establishment resistance, measured by differences in land cover and building density between sites, explained 35.52 % of species similarity variation. Seed capture opportunity, calculated from paired sites' forest areas, explained 25.70 % of the variation. While Euclidean distance and land cover-based movement resistance showed no significant correlation with species similarity, building density-based movement resistance (measured within 50-meter buffers) explained 20.52 % of the variation. A combined model incorporating these factors achieved an R² of 47.93 %. Analysis of dispersal modes revealed that wind-dispersed plants showed the highest inter-site similarity, followed by animal-dispersed plants, with unassisted dispersal plants showing the lowest similarity. Establishment resistance strongly influenced unassisted dispersal plants, while wind- and animal-dispersed plants responded to combinations of all three factors. These findings highlight how establishment resistance, forest coverage, and building density patterns shape spontaneous plant distribution across urban landscapes.