Honey mesquite (Prosopis glandulosa) has been shown to modify grasslands in the southwestern United States by altering soil characteristics, modifying microclimate, and facilitating the migration of understory shrubs. However, there has been little investigation of mesquite at the northern edge of its range. We investigated a mesquite savanna within native mixed grass prairie in southwestern Oklahoma. The vegetation at the site included dense, closed canopy mesquite patches, scattered mesquite individuals, and open grassland. Vegetation and environmental data was collected from 15 m transects that were located: within a mesquite patch, in the open grassland, and across grassland-mesquite patch transition. TWINSPAN identified six vegetation types along the grassland-mesquite patch transition. A runs test determined that the distribution of the vegetation types was non-random. Results from detrended correspondence analysis (DCA) illustrate the overlapping nature of the six vegetation types in ordination space, but distinct changes in plant community composition were indicated. Species richness and diversity decreased with increased mesquite cover. Mesquite canopy cover was positively correlated to exotic species cover and cactus cover. Mesquite may be facilitating these changes, but the mechanisms at this site are yet to be determined.

Key words: diversity, mesquite, Oklahoma, Prosopis glandulosa, woody plant invasion