Historical legacies, competition and dispersal control patterns of invasion by a non-native grass, Microstegium vimineum trin. (A. Camus)

Biological invasions can be controlled by a number of factors, and the importance of these factors for individual species can change depending on the spatial scale under consideration. Furthermore, the long-term legacies of past land use may be among the major factors structuring contemporary communities. This study was designed to merge these two lines of thought by observing the processes influencing invasion success of an exotic grass species, Microstegium vimineum. Plots within twenty hardwood forest stands of different ages (40 years old to >100 years) were surveyed in central New Jersey for the presence and abundance of this species, and the data was analyzed using simple, multiple linear and logistic regressions, as well as regression trees. At the stand scale, M. vimineum abundance was influenced by age, slope, shrub cover and forest connectivity, with far more predictable responses in younger stands than in old. On the plot scale, forest age and prior use were again the dominant influences on abundance of the invasive. Invasion in old forests was most promoted by thinner canopy cover, while younger stands were most influenced by soil type and distance to forest edge. Further studies along dispersal corridors (streams and trails) in these stands consistently showed more invasion in younger stands than older, but also found the intensity of invasion to be higher along such corridors than through areas without them. Observation of isolated patches confirmed that seed dispersal is mostly short-range without dispersal conduits, with most seeds germinating less than a meter from their parent patch boundaries. Finally, an experimental study of the role of understory shrub cover and litter depth on seed germination and seedling growth showed that shade from shrubs strongly limits survival and growth. Seeds also need to be in contact with mineral soil to establish, but otherwise litter depth does not influence growth. These studies demonstrate that the establishment and spread of this invasive species is affected by complex hierarchies of factors that differ in stands with different land use histories. Land managers can use these differences to design targeted control programs that are more effective and stand-specific.