CALLAHAN, PRISCILLA H.1,2*, BRUCE HOAGLAND1,3, and PHILLIP T. CRAWFORD2. 1Department of Geography, University of Oklahoma, Norman, OK 73019; 2Department of Botany and Microbiology, University of Oklahoma, Norman, OK 73019; 3Oklahoma Biological Survey, University of Oklahoma, Norman, OK 73019. - The effects of mesquite (Prosopis glandulosa) encroachment on species diversity and composition of a mixed grass prairie.
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