Though studies have often inferred the impacts of invasive species, less has been done to measure those impacts directly. Microstegium vimineum is an annual grass that is exotic to North America and invasive throughout the region. We used data from the Buell-Small Succession Study, a permanent plot study, to document the impacts of this invader. M. vimineum was first recorded in the BSS in 1982, but only began to dominate in 2000 after a major drought. We documented changes in two, six-year time windows: 2001/2-2007/8 and 2007/8-2013/14. We related the change in M. vimineum cover to herbaceous understory species richness and cover in order to determine the initial impact of invasion during the first time window. During the second time window, plots that had not been invaded by the end of the first time period were analyzed separately to evaluate whether new invasions had similar impacts to those that occurred in the first period. To measure recovery, invaded plots where M. vimineum cover decreased during the second time window were also analyzed.
Results/Conclusions
The plots which became moderately to heavily invaded during the first time window overall showed a significant decrease in herbaceous understory species richness of 2.2 species (of 6.0 species on average) and herb cover by 10.4% per (of 30.6% cover on average) excluding M. vimineum. Plots that became invaded during the second time window showed no significant relationship between invasion and herbaceous understory richness or cover. In contrast, recovery only occurred in plots where M. vimineum cover collapsed, gaining 0.3 species on average. All other plots, including those which remained uninvaded, decreased in herbaceous richness. There was no significant recovery of herbaceous cover during the second time window. These results suggest that most of the impacts on understory richness and cover occur early in the invasion of M. vimineum, and that even where M. vimineum collapses and recovery does occur, it is relatively little compared to the initial impacts. However, the impacts of M. vimineum on the understory community appear to weaken as the invasion proceeds.