EFFECTS OF BISON GRAZING, FIRE, AND TOPOGRAPHY ON FLORISTIC DIVERSITYIN TALLGRASS PRAIRIE

Grazed and ungrazed sites subjected to different fare frequencies were sampled on the Konza Prairie Research Natural Area in northeast Kansa s after 4 years of bison grazing (1987-1991), The objective was to stu dy effects of bison grazing on plant species composition and diversity components (plant species richness, equitability, and spatial heterog eneity) in sites of contrasting fire frequency, Cover and frequency of cool-season graminoids (e.g. Poa pratensis L., Agropyron smithii Rydb ,, Carer spp,) and some forbs (e.g. Aster ericoides [A, Gray] Howell, and Oxalis stricta L.) were consistently higher in sites grazed by bis on than in ungrazed exclosures, whereas the dominant warm-season grass es (Andropogon gerardii Vitman, Sorghastrum nutans [L.] Nash, Panicum virgatum L., Schizachyrium scoparium [Michx.] Nash) and other forbs (e .g. Solidago missouriensis Nutt.) decreased in response to bison, Plan t species diversity (H') and spatial heterogeneity in all areas sample d were significantly increased by bison, Increased heterogeneity and m ean species richness in grazed prairie (40 species per sample site) co mpared to ungrazed prairie (29 species per site) were likely a result of greater microsite diversity generated by bison, whereas preferentia l grazing of the dominant grasses and concomitant increases in subordi nate species resulted in an increase in equitability of species abunda nces, Species/area relationships indicated greater effects of bison on plant species richness with increasing sample area, Increases in plan t diversity components associated with bison grazing were generally gr eater in annually burned than in I-year burned sites, Effects of ungul ate grazers on floristic diversity have important implications given r ecent evidence that plant species diversity and the compositional and production stability of grassland plant communities are positively rel ated.