Several studies have suggested that riparian plant communities are highly invasible due to the disturbance regime and the greater availability of limiting resources. However, very few of these studies have attempted to account for the role of exotic plant propagule supply. Invasibility is defined as the susceptibility of a given environment to exotic plant invasion or the probability of establishment and subsequent survival of individual plants per arriving propagule. Therefore, invasibility cannot be quantified without accounting for propagule pressure and most studies of riparian community invasion have actually reported the current level of exotic plant invasion rather than the susceptibility of the system to invasion. To accurately identify habitat attributes associated with invasibility we need to account for the effects of properties of the invading species, the number and distribution of arriving propagules (i.e., propagule pressure), and the community properties that influence susceptibility to invasion (i.e., ecological resistance).  In this study, we compared the invasibility of riparian and upland forest communities by measuring both the change in abundance of exotic plants and propagule pressure at the plot level over a five year period. To control for species effects, we focused on the three most abundant exotic plant species in our study sites: Japanese stiltgrass (Microstegium vimineum), garlic mustard (Alliaria petiolata), and Japanese barberry (Berberis thunbergii).

Results/Conclusions While the extent of invasion of all three exotic species was significantly greater in the riparian forest communities, the supply of arriving exotic propagules in the riparian sites was also significantly higher for both Japanese stiltgrass and garlic mustard. Although riparian forests had more than ten times as many Japanese stiltgrass and garlic mustard individuals as the upland forests, we found that, once propagule pressure was accounted for, the invasibility of these community types did not vary significantly for these species. In the studied system, the increased level of Japanese stiltgrass and garlic mustard invasion observed in the riparian forests resulted from the greater supply of propagules. However, for Japanese barberry, we did find the riparian forest communities to be more invasible.
Although riparian areas often have a greater abundance of exotic plants, they are not necessarily more invasible due to the supply of water dispersed propagules. This study highlights the need to quantify the role of natural variation in propagule pressure in attempts to compare the invasibility of different communities.