Measurement of native and invasive plant cover changes to improve planning and management in a rare Virginia wetland

Background/Question/Methods

Documenting changes in exotic and native plant populations is an essential component of  the formation and assessment of resource management plans and efforts. Shenandoah National Park is home to Big Meadows Swamp (BMS), a globally rare Blue Ridge Mafic Fen, which supports a unique plant community. Monitoring performed in 2006 and 2008 focused on 13 target plant species, including five invasive exotic  plant species that threaten to impact this community and eight species of state rare native plant. This study sought to estimate the changes in cover of the 13 target species, identify priority areas for exotic plant control, and assess the effect of current exotic plant control measures. A 10 x 10 meter grid was used to generate 978 points in BMS that were sampled in 2006 and 2008. At each point, percent cover was visually estimated for  each of the 13 target species in a 1 x 1 meter quadrat. In two areas of greater rare plant abundance equaling 17% of the study area, 145 transects were also sampled in both years. Hand-pulling as a means of control for three target exotic species was also begun in these areas after 2006 sampling.
Results/Conclusions

Analyses of changes in plant cover using the quadrat and transect data indicated significant changes among three exotic and four native plant species.  Among exotic plants, Microstegium vimineum cover increased significantly, showing an  increase of over 300% throughout the entire study area, while Alliaria petiolata exhibited a significant decline.  Polygonum caespitosum exhibited a significant decline in only controlled areas.  Among native plants species, Epilobium leptophyllum cover increased significantly only outside of controlled areas while Sanguisorba canadensis and Carex buxbaumii declined significantly within controlled areas. Only the native Iris versicolor increased significantly within the controlled areas. The results suggest that present control methods are effectively halting the spread of P. caespitosum and may be preventing significant increases in M. vimineum within controlled areas. The decline of A. petiolata throughout the study site cannot be explained by control efforts. The trends in native plant cover suggest that only the native I. versicolor is expanding in controlled areas while other native species such as C. buxbaumii and E. leptophyllum may be negatively affected by disturbance resulting from mechanical control efforts.