In the nonglaciated Allegheny Plateau of northwestern Pennsylvania ground-layer plant communities of headwater riparian forests are among the most species-rich of the region. We examined the influence of inundation potential (geomorphic surfaces with high, moderate or low probability of seasonal inundation) and forest overstory (stems ≥2.5 cm dbh) characteristics (percent overstory cover, overstory basal area, overstory stem density and basal area of eastern hemlock, Tsuga canadensis) on species richness, biomass and cover of the summer ground-layer (all vascular plants ≤1 m tall) at six riparian sites. The study design used inundation as a class variable with sites or portions of sites within each class as replicates (n = 4 for each inundation class). Mean total species richness (7.3 ± 1.3 se species/0.25 m²) and biomass (51.0 ± 12.4 se gm dry weight/0.25 m²), forb species richness (4.3 ± 0.8 se species/0.25 m²) and biomass (9.3 ± 4.5 se gm dry weight/0.25 m²) and graminoid species richness (2.4 ± 0.3 se species/0.25 m²) and biomass (40.3 ± 8.7 se gm dry weight/0.25 m²) were significantly greater for geomorphic surfaces of high inundation potential than for those of moderate and low inundation potential. Overstory cover, overstory stem density and basal area of T. canadensis were not correlated with ground-layer species richness, biomass and cover within inundation classes. Wetland species (obligate facultative wetland species), chiefly forbs and graminoids, occurred most often on geomorphic surfaces of high inundation potential (66.7% of total species for which wetland indicator status has been determined); upland species (facultative facultative upland upland species), mainly tree seedlings, occurred principally on geomorphic surfaces of moderate and low inundation potential (81.5% and 74.3% respectively, of total species for which wetland indicator status has been determined). Differences in species composition, richness, biomass and cover suggest that the ground-layer of Allegheny Plateau riparian forests is composed of a series of wetland and upland species guilds that replace each other along the stream to mesic forest gradient.