Background/Question/Methods:  Invasive plant species are an increasing threat to biodiversity and have been found to severely alter ecological systems. Most studies of invasive species prevalence have taken an area- or community-based approach to measuring abundance. However, a plant-centric strategy may be a more direct and relevant approach to quantifying the impact invasives have on native plant species. One particular native plant, American ginseng (Panax quinquefolius), is an economically and socially important understory species found throughout the interiors of eastern deciduous forests. The aim of this study was to determine the level of exposure of individuals and populations of P. quinquefolius to invasive plant species through a plant-centered survey over two growing seasons (2006-2007). More than 4,000 plants in 30 natural ginseng populations within the eastern United States were surveyed using the plant-centered approach. Invasive, herbaceous plants were recorded as present if they fell within a 2 m radius of a ginseng plant, while shrubs were recorded as present if within 5 m, and trees if within 10 m.

Results/Conclusions:  A high level of invasion was found with 63% and 70% of ginseng populations containing at least one invasive species in 2006 and 2007, respectively. Approximately one-third of all ginseng individuals were found in close proximity to invasive plants in 2006 and 2007. Twelve invasive species were found, 7 of which have been reported to alter soil properties. The most prevalent invasive species among populations and near ginseng individuals were multiflora rose (Rosa multiflora) and Japanese barberry (Berberis thunbergii). The exposure of size classes to invasives varied among populations. While previous work has suggested that invasive species were common, the abundance of invasives plants within forest interiors and within competitive range of this valuable medicinal herb suggests that the economic and ecological costs of competitive interactions with native species could be high.