Abstract
Unstable roadside slopes in mountainous forested regions pose significant challenges for ecological restoration and the maintenance of forest ecosystem integrity. While passive (spontaneous) vegetation recovery offers a low-cost and sustainable strategy, the underlying ecological drivers-particularly the role of adjacent forests and propagule sources-remain poorly understood. In this study, we examined 14 naturally restored roadside slopes and their adjacent forests in the subtropical mountains of southwestern China. Across 126 slope plots and 252 forest plots, we recorded 249 vascular plant species and assessed species composition, dispersal traits, and phylogenetic similarity between habitats. We found that 95.9% of slope species were native, with 80.0% exhibiting long-distance dispersal traits, mainly through wind and birds. Approximately 65.3% of slope species were shared with adjacent forests, and the average phylogenetic similarity was relatively high (0.589), indicating that nearby forests act as key propagule sources. Structural equation modeling showed that species overlap and phylogenetic similarity were significantly influenced by forest species richness, forest cover, distance to core protected areas, and plant dispersal strategies. Phylogenetic dissimilarity between slope and forest communities was largely driven by lineage turnover, with nestedness contributing particularly in slope-forest pairs with close spatial proximity. These findings underscore the ecological importance of conserving adjacent forest habitats to facilitate spontaneous vegetation recovery on disturbed slopes. They also demonstrate that forest proximity, propagule pressure, and dispersal efficiency are critical to the success of passive restoration. From a management perspective, strategies should focus on maintaining forest connectivity and promoting landscape-scale seed flow to enhance biodiversity recovery and slope stability in mountainous forest ecosystems.