Movement corridors as avenues of invasion of a deciduous forest by a non-native annual grass, Microstegium vimineum

Shade-tolerant invasive plant species are threatening deciduous forest communities of the eastern United States, but their method of invasion remains unclear.  We test the idea that arrival of one such species, the non-native annual grass Microstegium vimineum, is facilitated by movement of humans or animals by direct observation of distributions along movement corridors over two years.  Weak colonization of experimental plots shows that Microstegium is limited by seed availability, and to a lesser extent litter cover, in the absence of human or animal movement.  Extension of populations along off-road-vehicle trails, small roads, and stream sides implies movement of seeds by vehicles, road graders, and water movement, with maximum rates approaching 80m per year.  Saturation of isolated roadside sites demonstrates that within-site colonization is very rapid relative to colonization between sites.  Little extension along footpaths and horse trails implies that pedestrian and horse movement are not important mechanisms of invasion.  These results suggest that Microstegium invasions are spatially hierarchical, with different mechanisms at different scales, and that rapid within-site movement is caused by substrate-moving processes.