Trait-mediated associations between Ligustrum lucidum invasion and functional diversity in urban Atlantic Alluvial Forests

Invasive alien species are drivers of biodiversity loss and can alter forest functioning. Despite the expansion of Ligustrum lucidum across South America, its effects on the functional organization of tree communities in urban Atlantic alluvial forests remain poorly understood. Here, we examined how variation in L. lucidum abundance relates to functional trait composition and diversity across a 103-plot permanent monitoring network in southern Brazil. We quantified leaf and structural traits, calculated community-weighted means and functional diversity indices, and combined multivariate analyses with structural equation modeling to evaluate direct and indirect relationships among invasion intensity, community assembly, and functional structure. L. lucidum exhibited trait values distinct from those of native species, occupying one extreme of the community functional space and accumulating substantially greater biomass. Increasing invader abundance was associated with declines in native species richness and with reductions in functional dispersion (FDis) and Rao’s quadratic entropy (RaoQ), indicating contraction of trait breadth and increasing similarity among native assemblages. However, invasion intensity was not directly associated with functional diversity. Instead, changes in FDis and RaoQ were mediated indirectly through shifts in species richness and dominant trait composition, demonstrating that functional reorganization emerges from trait-mediated community reassembly rather than direct suppression by the invader. Together, these findings indicate that L. lucidum acts as an ecological filter that reshapes trait structure, reduces functional redundancy, and may weaken ecosystem resilience in urban alluvial forests. Integrating functional metrics into monitoring and management is therefore critical for early detection, risk assessment, and restoration.