Snow damage to the canopy facilitates alien weed invasion in a subtropical montane primary forest in southwestern China
Climate-mediated disturbance often promotes the invasion of non-native species, which impact local ecosystems by altering community structure and composition. In subtropical forests, the formation of canopy gaps, which allow for rapid regeneration in the understorey, strongly affects successional dynamics. However, the role of canopy gaps in promoting invasive species into forest interiors has not been extensively studied. We examined the relationship between canopy disturbance and the germination of Ageratina adenophora (Asteraceae), a non-native branched herb species, using seedling population data from a 20-ha forest dynamics plot in southwest China. The species was first recorded in 2015, after extensive snow-damage to the forest canopy. Our hypothesis was that canopy gaps increase light availability on the forest floor, thereby facilitating the germination and subsequent invasion by the non-native branched herb into the forest interior. Field measurements of the Leaf Area Index before and after the snow damage was combined with measurements of canopy gaps and associated light conditions. Biotic factors (tree seedling species richness, herb species richness and herb coverage), abiotic factors (elevation, slope, convexity and soil moisture), and the density and spatial distribution of A. adenophora were also measured ten months after the snow damage. Seedling germination experiments were conducted in the lab to test the relationship between light availability and the germination of the invasive branched herb, showing the branched herb to be light demanding. Using spatial statistical methods, we found significant relationships between densities of recruiting A. adenophora and canopy gaps, with high densities of the invasive branched herb recruiting into gap areas. We conclude that light availability shapes the distribution of A. Adenophora in the understorey in this subtropical evergreen montane forest. Our results illustrate that disturbances leading to canopy damage can promote the establishment and proliferation of invasive understorey species in forest interiors, providing a rapid route to colonization.