Data on the species composition of herb-layer assemblages recorded in 40 vegetation plots: 20 plots with the presence of Parthenocissus inserta and 20 plots without the species occurrence

Invasive woody climbers can substantially modify the structure and functioning of temperate forests, but the community-level processes facilitating their establishment remain insufficiently understood. We tested whether the probability of Parthenocissus inserta occurrence depends on the interaction between herb-layer functional structure and canopy closure, expecting that invasion patterns shift from dominance of habitat filtering in more open stands to stronger effects of niche differentiation under closed canopies. We established twenty paired plots with and without P. inserta in the Białowieża Forest and assessed invasion probability along gradients of herb-layer functional diversity and canopy closure using generalized linear mixed-effects models. The effect of functional divergence on invasion probability varied with canopy structure. Under low canopy closure, higher functional divergence increased the probability of P. inserta occurrence, whereas this relationship weakened as canopy cover increased, indicating a declining influence of habitat filtering in denser stands. Functional evenness showed contrasting responses, being negatively associated with invasion in more open stands but positively related to invasion probability under closed canopies. This pattern suggests stronger competitive constraints at low canopy cover and greater importance of niche partitioning in shaded conditions. Our results demonstrate that forest susceptibility to P. inserta invasion depends strongly on joint effects of canopy structure and associated community assembly processes. Trait based characterization of the resident herb layer vegetation can therefore serve as an early warning indicator by identifying tree stands that are more prone to invasive liana establishment and by allowing early recognition of increased invasion risk.