A critical question in invasion biology involves the relative importance of propagule rain and community invasibility. For plant invasions, invasibility is often related to disturbance, but few studies of forest invaders have simultaneously investigated both canopy and ground-level disturbance. We investigated the relative importance of seed rain, canopy disturbance, and soil disturbance in a mature forest in Maryland on the recruitment of four invasive species: wine raspberry (Rubus phoenicolasius Maxim.), Japanese barberry (Berberis thunbergii DC), multiflora rose (Rosa multiflora Thunb.), and Japanese stiltgrass [Microstegium vimineum (Trin.) A. Camus]. Using complete censuses of a 9-ha plot at two points in time (2011–12 and 2014), we mapped new recruits, and related their locations to canopy and soil disturbance, as well as to a seed rain index based on locations of reproducing plants and seed-dispersal kernels.
We found that propagule rain, as measured by the seed rain index, was a significant predictor of recruitment for B. thunbergii, R. phoenicolasius, and M. vimineum. For R. multiflora, seed sources were not located, precluding assessment of propagule rain, but recruitment was linked to canopy disturbance, as was recruitment of M. vimineum. However, because reproduction of R. phoenicolasius and, in some years, of B. thunbergii is higher in treefall gaps, these gaps experience higher propagule rain, with the result that recruitment is indirectly associated with these gaps. Ground-layer disturbance was an important predictor of recruitment only for B. thunbergii. Our findings reveal that the importance of propagule rain is the most consistent driver of recruitment, but canopy or ground-layer disturbance promotes recruitment of some invasive plant species.