The impact of invasive plant species on native plants is largely assumed to be negative, but supporting evidence is sparse. A common control method of non-native plants is herbicide application, but little is known about the effects of these chemicals on non-target plant populations, or differences in these populations before and after control measures are taken. We examined the response of the forest floor plant community to herbicide-mediated reduction of Alliaria petiolata in an old-growth and a second-growth forest stand in Hueston Woods State Park, Preble and Butler Counties, OH. Fifty 1 × 1 m plots were established in each stand, and 25 plots per stand were treated with Round-up© each November 2000–2004, which reduced cover of adult Alliaria petiolata but did not suppress recruitment. Percent cover of herbs and woody plants ≤0.85 m tall was assessed in May and June, 2000–2005. To determine compositional differences between sprayed and unsprayed plots in each stand we ordinated plots based on peak cover of each species using nonmetric multidimensional scaling, tested for differences in community composition with a multiple response permutation procedure, and compared total cover of growth forms with Kruskal-Wallis tests.
Five years of Alliaria petiolata control only modestly affected the forest floor vegetation. Neither species richness nor diversity differed significantly between sprayed and unsprayed treatments in any year of the study. Community composition differed each year between stands, but treatments differed significantly only in 2002 (in the second-growth stand), and marginally in 2004 and 2005 (in the old-growth stand). Treatment affected cover of some growth forms during the study, but only in some years: in the second-growth stand sprayed plots had significantly greater cover of spring perennials and graminoids in 2003 and marginally lower cover of annuals in 2005; in the old-growth stand sprayed plots had marginally more spring perennials in 2005. Wintergreen species, particularly the exotic annual Stellaria media, had lower cover in sprayed plots in the old-growth stand in 2005. We attribute the compositional differences we observed in the forest floor community to competitive impacts of Alliaria petiolata, but suggest that effects were modest due to the persistence of rosettes in the sprayed plots.