Establishment, spread, and seed viability of Microstegium vimineum, an invasive exotic grass, in closed-canopy U.S. eastern forests were evaluated across a local and regional environmental gradient in West Virginia. Colonization and extinction of M. vimineum patches were followed over three years (2005–2007), and spread rate was estimated using a reaction diffusion model. Chasmogamous and cleistogamous seed were collected from roadside and forest interior plants in 2005 and 2008, Seeds were tested for viability and germinated. Non-germinating seeds were tested for viability as a measure of dormancy.
Results confirmed Microstegium vimineum’s reduced reproductive capacity in the forest interior compared to the roadside. Patches of M. vimineum in the forest interiors across the regional gradient were best defined by high native plant richness. Colonization rates of the forest interiors were significantly higher for the mesic sites than the xeric sites. Radial spread rates ranged between 0.16 and 0.50 m year-1 and forest interiors were estimated to become saturated with M. vimineum in anywhere between 10 (mesic sites) and 59 (xeric sites) years. Cleistogamous seed, followed by forest interior seed, were significantly less viable than chasmogamous seed in both 2005 and 2008. Seed from the driest habitat had significantly lower germination rates and longer dormancy values than the mesic sites in 2008. Viability dropped off sharply after 2007. These results support the possibility of accelerating spread rates in forest interiors caused by more long-distance dispersal events, but slowed by a reduction in M. vimineum fitness in shaded or relatively dry environments.