Serpentine barrens are globally rare, savanna habitats that support distinctive vegetation. In eastern North America, two major threats to serpentine barrens are the spread of invasive species and the encroachment of trees into open habitats. We assessed these threats by comparing serpentine savannas and woodlands in the State Line serpentine barrens, southeast Pennsylvania. We asked which invasive species were most frequent and abundant in each habitat and how they may affect plant diversity and community composition and how succession of savannas to woodlands may affect plant diversity and community composition. We compared species richness, evenness, diversity, and beta diversity between savannas and adjacent woodlands and quantified differences in community composition. Invasive species comprised 16% of total plant cover, and the most frequent invasive species were Microstegium vimineum (Trin.) A. Camus, Lonicera japonica Thunb., and Elaeagnus umbellata Thunb. Savannas had substantially greater species richness, diversity, and beta diversity than woodlands. Of all species we found, 31% were unique to savannas. The cover of invasive species and the native greenbrier Smilax rotundifolia L. were negatively associated with the cover of characteristic native grasses and forbs. Although invasive species threaten diversity in serpentine barrens, tree encroachment may pose the greater threat. Savannas that become woodlands lose diversity and change composition. Fighting tree encroachment and thickets of Smilax spp. is evidently necessary to maintain the distinctive vegetation of serpentine barrens. These habitats are worth protecting because they harbor many rare species, increase landscape-level beta diversity, and make a unique contribution to regional biological diversity.