In high-diversity communities, rare species encounter one another infrequently and therefore may compete more intensely with common species or guilds for limiting space and resources. In addition, rare species may be strongly recruitment limited because of their low abundances. Under these conditions, stochastic dispersal and immigration history can have an important influence on community structure. We tested the hypothesis that local immigration and competition from common, large-stature guilds interact to structure local biodiversity in high-diversity longleaf pine savanna groundcover assemblages (>30 species/m2). In two factorial field experiments, we increased local immigration by adding seeds of 38 mostly rare, small-stature forbs and sedges to plots physically dominated by either a common, large-stature bunchgrass or shrub species and to plots in which competition from these dominant guilds was reduced. We measured species richness and abundance at two spatial scales (0.01 and 0.25 m2) over two years. Immigration increased total species richness and richness of focal seed addition species regardless of levels of competition with bunchgrasses and shrubs, indicating that many rare, small-stature species can recruit in the face of potential competition from dominant guilds. Removal of dominant guilds increased total and focal species richness in shrub-dominated but not bunchgrass-dominated plots. In addition, competition from both dominant guilds had no clear effect on rank–abundance distributions of focal species. Our results suggest a key role for dispersal assembly in structuring local biodiversity in this high-diversity plant community, but the importance of this mechanism depends on the strength of local niche assembly involving competition from some, but not all, dominant guilds.