Using land-use history in combination with local factors to understand invasion patterns of an exotic grass

Explanations of plant invasions commonly focus on contemporary conditions of population, community and environmental factors. However, historical conditions may have long-lasting effects that constrain community response to the current environment, as recent studies of soils and plant communities show. We hypothesized that these legacy effects would affect the amount of invasion by an abundant, widespread exotic plant, Microstegium vimineum, and would constrain its response to contemporary environmental factors within the forest environment. We quantified plant communities and landscape structure in pairs of adjacent young (forested post-1935) and old (forested pre-1935) stands of mixed hardwood forest within a geologically-uniform region of New Jersey in 1267 circular plots (each with a 5m radius).  M. vimineum abundance was most strongly affected by canopy density, age of the stand, presence of dispersal vectors, and native shrub cover, in descending order. Furthermore, the spatial pattern of invasion was found to be quite different between young and old forests, with plants more widespread in the former and concentrated along dispersal corridors (streams and roads/trails) in the latter. Vernal pools and ATV trails were similarly invaded in both young and old stands, while stream banks and foot trails were both significantly more invaded in younger stands. Areas of vernal pools typically had the highest abundance of any sub-habitat, followed by streams, sites of historic human activity, and active trails.   These results clearly demonstrate that land-use history modifies the responses of an invading plant to environmental features, producing very different patterns of invasion in young and old forest stands.