Habitat, dispersal and propagule pressure control exotic plant infilling within an invaded range

Deep in the heart of a longstanding invasion, an exotic grass is still invading. Range infilling potentially has the greatest impact on native communities and ecosystem processes, but receives much less attention than range expansion. ‘Snapshot' studies of invasive plant dispersal, habitat and propagule limitations cannot determine whether a landscape is saturated or whether a species is actively infilling empty patches. We investigate the mechanisms underlying invasive plant infilling by tracking the localized movement and expansion of Microstegium vimineum populations from 2009 to 2011 at sites along a 100‐km regional gradient in eastern U.S. deciduous forests. We find that infilling proceeds most rapidly where the invasive plants occur in warm, moist habitats adjacent to roads: under these conditions they produce copious seed, the dispersal distances of which increase exponentially with proximity to roadway. Invasion then appears limited where conditions are generally dry and cool as propagule pressure tapers off. Invasion also is limited in habitats >1 m from road corridors, where dispersal distances decline precipitously. In contrast to propagule and dispersal limitations, we find little evidence that infilling is habitat limited, meaning that as long as M. vimineum seeds are available and transported, the plant generally invades quite vigorously. Our results suggest an invasive species continues to spread, in a stratified manner, within the invaded landscape long after first arriving. These dynamics conflict with traditional invasion models that emphasize an invasive edge with distinct boundaries. We find that propagule pressure and dispersal regulate infilling, providing the basis for projecting spread and landscape coverage, ecological effects and the efficacy of containment strategies.