To assess the community-level responses of a New England forest to invasion by the Eurasian biennial Alliaria petiolata (garlic mustard), we conducted a vegetation census at twenty-four plots ranging from low to high invasive cover, and experimentally removed 0, 50, or 100% of garlic mustard from adjacent highly invaded plots at the same study site. Species richness did not respond to natural or experimental levels of invasion, but the Shannon diversity and equitability indices declined with increasing in situ densities of garlic mustard, and increased in response to removal of garlic mustard at the experimental plots. Individual species demonstrated variable responses to high-, intermediate-, and low-level invasion. Of all plant functional groups, tree seedlings declined most notably with increasing in situ levels of invasion. This functional group, and seedlings of three key canopy tree species within the group, increased in response to partial, but not full eradication of garlic mustard. Our results demonstrate that the effectiveness of full or partial removal depends on management priorities for promoting overall diversity, species richness, native species composition, and/or individual species performance within native communities.