Nonnative M. vimineum has been expanding rapidly in the eastern United States, where it can negatively affect plant communities. Locally, the species is assumed to spread from roadsides into nearby forests, where it can form dense populations after disturbances, especially in light gaps. Using microsatellite markers, we quantified patterns of genetic variation and structure among populations at nine sites in West Virginia. We then examined patterns of local dispersal within each population, focusing on subpopulations along the roadside, those coalescing nearby along the forest edge, and subpopulations in the interior forest. We found that levels of genetic variation of M. vimineum were relatively low overall across populations but with genetic structure present among populations (Fst = 0.60). Within populations, subpopulations along the roadside were genetically variable, containing 4 to 22 unique, multilocus genotypes. Many of these genotypes were also identified in the adjacent forest, consistent with local, diffusive spread from the roadway. However, several genotypes in the interior forest were unique to the population, indicating that dispersal from other sites may also occur. Overall, it appears that genetic diversity and structure in M. vimineum reflects a variety of processes, including localized dispersal and long-distance migration.
Nomenclature: Japanese stiltgrass, Microstegium vimineum (Trin.) A. Camus.
Management Implications: Our research highlights the importance of evaluating local and regional patterns of genetic diversity when defining management strategies of an invasive plant. Microstegium vimineum has low genetic diversity, which could potentially make it susceptible to disease or unable to adequately respond to other stochastic events. Despite this paucity of diversity, M. vimineum has evolved within its invasive range, and the presence of at least one effective pathogen (Bipolaris spp.) has not forestalled its rapid spread.
Our results show that roadsides, despite having a relative abundance of chasmogamous seeds, are not a significant source of genetic variation. Instead, any new genotypes in an area are likely the product of long-distance dispersal. Second, our results indicate that forest interior subpopulations are more often not the product of spread from the immediately adjacent roadside but, instead, may come from a long-distant source. Finally, our study shows there is genetic differentiation among regional populations, which could serve as a source of new or highly fit (i.e., able to withstand a range of environments) genetic information.
The key to stopping this influx of new genotypes into a site appears to be stopping or slowing the long-distance dispersal of the seed. Effective preventative measures could include strategically located cleaning stations (e.g., highway intersections, rest areas, gas stations) for vehicles and equipment and the use of certified clean gravel and other road construction/maintenance materials. Based on our results, priority locations would be intersections between the regionally different populations.
Although our research highlights the complexity of the success of M. vimineum as an invader, it also supports a focus on preventative management to reduce long-distance spread.