Effects of Herbicide on the Invasive Biennial Alliaria petiolata (Garlic Mustard) and Initial Responses of Native Plants in a Southwestern Ohio Forest Adriane M. Carlson 1 and David L. Gorchov 1,2 Abstract Restoration often includes control of invasive plants, but little is known about how native plant communities respond to this control. The biennial Alliaria petiolata (M. Bieb.) Cavara and Grande (garlic mustard) is one of the most prevalent invasive plants in forests of eastern North America. We investigated the effects of the herbicide Round-up (glyphosate) on Alliaria and the response of the forest floor plant community to the herbicide and the subsequent decline of Alliaria. In an old-growth Acer–Fagus stand and a second-growth Liriodendron-dominated stand in Hueston Woods State Nature Preserve, Ohio, United States, we spot applied Round-up in November 2000 and 2001 in 25 1 · 1–m plots and maintained 25 control plots. Herbicide decreased Alliaria density in both stands and reduced the density of other species in leaf during treatment (mostly exotic winter annuals) in the old-growth stand. Treatment did not affect the initial density of the Alliaria cohort that germinated in the spring of 2001, but decreased the 2002 cohort. Community differences were found in the old-growth stand after Alliaria reduction, specifically greater cover of spring ephemerals in the herbicide treatment. In the second-growth stand, herbicide treatment increased repro- duction of the late-summer perennial, Phryma leptosta- chya. These results indicate that glyphosate reduces Alliaria without negatively impacting native species and that some native species respond positively to a single-year reduction in this invasive biennial. Key words: alien species, DCA ordination, exotic species, glyphosate, herbicide, Hueston Woods State Park, invasive species, nonindigenous species, Round-up. Introduction Invasive plant species have the potential to negatively impact native plant species and communities, including reduction of biodiversity (Lodge 1993; Woods 1997), alteration of community structure, function, and composi- tion (Woods 1997), and changes in dynamic community properties (Huenneke & Mooney 1989). However, direct impacts of invasive plants on native plants have not been well studied (Parker et al. 1999; D’Antonio & Kark 2002), with some exceptions (Miller & Gorchov 2004). In the United States, nature preserves that have been invaded by exotic species are of special concern, as these preserves are virtually the only remaining representatives of intact communities (Westman 1990). Because control of invasive species is a major concern and cost of manage- ment of National Parks and other preserves, as well as a common component of restoration efforts, there is a need for better understanding of the impacts of invaders and the response of plant communities to eradication efforts (D’Antonio & Meyerson 2002). One of the most exemplary remaining stands of old- growth Fagus grandifolia (beech)–Acer saccharum (sugar maple) forests in the midwestern United States is in Hueston Woods State Nature Preserve (hereafter, ‘‘Hueston Woods’’) in southwestern Ohio (Runkle et al. 1984). The exotic biennial herb Alliaria petiolata (garlic mustard) has established there and is perceived to be a problem by the Ohio Department of Natural Resources (ODNR 1998). Since its introduction to North America from northern Europe in approximately 1868 for its medicinal and nutri- tional properties, Alliaria has spread into many portions of the northeastern and midwestern United States, including many parts of Ohio (Nuzzo 1993). In these areas, Alliaria is most common in damp, semishaded forests (Cavers et al. 1978; Nuzzo 1993; Byers & Quinn 1998). Alliaria has higher germination, growth, and reproduction in forest edges and mesic, lowland forests than upland forest inte- riors, but all of these habitats are susceptible to invasion (Meekins & McCarthy 2001). Although Alliaria has been labeled a severe threat to deciduous forests (Nuzzo 1994), there has been little study of the effects of Alliaria on native plants. A removal study showed that Alliaria had 1 Department of Botany, Miami University, Oxford, OH 45056, U.S.A. 2 Address correspondence to D. L. Gorchov, email gorchodl@muohio.edu Ó 2004 Society for Ecological Restoration International DECEMBER 2004 Restoration Ecology Vol. 12 No. 4, pp. 559–567 559