The diversity of herbaceous plant communities in urban forests faces threats by the establishment of non-native plant species. This poster describes the possible effects of one such invasive woody species, Rhamnus cathartica, commonly referred to as common buckthorn, on the diversity of herbaceous layer plants in an urban forest. Although the mechanism is not well understood, it has been found that Rhamnus possesses potentially allelopathic properties that have been shown to have a negative impact on native forb diversity. The study takes place in two habitats, one open canopy grassy area called the mounds, and an area dominated by woody species in a nearly closed canopy called the woodlands, at Tifft Nature Preserve, a 106 ha urban forest in Buffalo. Plots surveyed along transects running approximately north and south indicate differing effects on herbaceous plant diversity per habitat. Herbaceous diversity in the woodlands descreases significantly with Rhamnus seedling abundance whereas herbaceous diversity in the mounds increases significantly with the basal area of canopy trees . These results seem to indicate that in a nearly closed canopy, the establishment stage of Rhamnus recruitment coupled with the abundance of Rhamnus seedlings is an important influencer on diversity in herbaceous plant communities.