The annual brome grasses, Bromus japonicus and B. tectorum, are common invaders of the Northern Great Plains. Our objective was to determine if these exotic plants were positively or negatively associated with particular plant species or functional types in a prairie/pine ecotone at Wind Cave National Park, South Dakota. We addressed this issue by sampling at two spatial scales—the landscape scale and the site scale. For the landscape we evaluated species associations across a 3800 ha ecotone using 90 transects. Annual bromes were positively associated with C₃ grasses, particularly Agropyron smithii and Stipa viridula, in addition to the shrub Symphoricarpos occidentalis. Annual bromes were negatively associated with trees, C₄ grasses, particularly A. gerardii, Bouteloua curtipendula and Schizachyrium scoparium, as well as the shrub Rhus aromatica. For the site scale we assessed relationships at a finer resolution within two 1 ha stands of grassland vegetation. Annual bromes were negatively associated with Poa pratensis in both stands. Results indicate that the bromes often dominate in portions of the landscape with C₃ grasses and Symphoricarpos occidentalis. Within individual sites, the bromes appear most limited by a competitive interaction with P. pratensis, resulting from phenological overlap.