Per capita community-level effects of an invasive grass, Microstegium vimineum, on vegetation in mesic forests in northern Mississippi (USA)

Abstract

Quantifying per capita impacts of invasive species on resident communities requires integrating regression analyses with experiments under natural conditions. Using multivariate and univariate approaches, I regressed the abundance of 105 resident species of groundcover plants and tree seedlings against the abundance and height of an invasive grass, Microstegium vimineum, within 117 plots in four mesic floodplain forests in Mississippi (USA). Microstegium vimineum was most productive (i.e., tallest and most abundant) in canopy gaps in floodplains, and a significant amount of variation in resident species composition was directly explained by canopy gaps and stand age. The relatively small (but statistically significant) percentage of variation in resident species composition (1.8%) explained by M. vimineum in the multivariate analysis was attributable to significant relationships with a few common species. Most of these were negative relationships with shady mesic forest indicators. Most positive relationships were with infrequent disturbance indicators and with species with growth phenologies that differed from that of M. vimineum. Results of field competition experiments with the three most common species to show significant relationships with M. vimineum revealed asymmetric competitive effects of M. vimineum on Chasmanthium laxum and positive responses of Quercus alba seedlings and Leersia virginica adults to the removal of M. vimineum in one growing season. Results of this study suggest that negative per capita community-level effects of M. vimineum are likely to be greater in shady forests than in open floodplain forests due to the relative paucity of vulnerable species in the latter.