Establishment of an invasive grass in closed-canopy deciduous forests across local and regional environmental gradients

Abstract

Spread of Microstegium vimineum, an invasive exotic grass, in closed-canopy forests of West Virginia, U.S. was evaluated across a local (roadside to forest interior) and regional (across two geographic provinces) environmental gradient. Seed dispersal distances from roadside populations into forest interiors based on seed rain and soil seed bank data were determined. Colonization and extinction of M. vimineum patches were followed over three years (2005–2007), and spread rate was estimated using a reaction-diffusion model. Direct seed dispersal from the roadside populations occurred primarily adjacent to the maternal plants, indicating that the disjunct M. vimineum patches within the forest interiors occurred via secondary seed dispersal over longer distances. Patches of M. vimineum were found as far as 45 m into the forest interior, but M. vimineum seed in the seed bank were only found within 10 m of the roadside population. Colonization rates of the forest interiors were significantly higher for the more mesic sites than the more xeric sites. The same trend was noted for the spread rate. 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 (more mesic sites) and 59 (more xeric sites) years. 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.