Effects of bison grazing, fire, and topography on floristic diversity in tallgrass prairie

  • Published source details Hartnett D.C., Hickman K.R. & Fischer Walter L.E. (1996) Effects of bison grazing, fire, and topography on floristic diversity in tallgrass prairie. Journal of Range Management, 49, 413-420.


In North American prairies, periodic wildfires and ungulate grazers have had a major influence in shaping plant community composition and structure. In this study, sites subjected to different prescribed fire regimes were studied on the Konza Prairie Research Natural Area (39º05’N, 96º35’W) in Kansas (central USA) after 4 years of bison grazing or grazing exclosure (1987-1991).

Vegetation was dominated by warm-season perennial grasses (i.e. big bluestem Andropogon gerardii, indiangrass Sorghastrrum nutuns, little bluestem Schizachyrium scoparium and switchgrass Panicum virgatum), with numerous subdominant grasses and forbs.

In 1987, 50 bison were stocked in a 469 ha area, expanded as the herd grew to include a further 480 ha in May 1992. They could move freely between areas subject to spring (April) burning at 1-, 2-, 4-, or 20-year intervals. In 1987, prior to bison release, eight 5 x 5 m exclosures were erected at different sites.
In this study, vegetation within six exclosure in an area burned annually and within six in an area burned at 4-year intervals was sampled in quadrats in June, July and September 1991. Within each quadrat, cover and frequency of each species was estimated using a modified point-frame method. Nearby grazed areas were likewise sampled.

Cover and frequency of cool-season grasses (e.g. Poa pratensis, Agropyron smithii), sedges Carex spp. and some forbs (e.g. snow flurry Aster ericoides and yellow woodsorell Oxalis stricta) were consistently higher in bison-grazed prairie compared to exclosures. The four dominant warm-season grasses and other forbs (e.g. Missouri goldenrod Solidago missouriensis) decreased under grazing.

Increased heterogeneity and higher average species richness in grazed areas (40 species/sample site) compared to ungrazed prairie (29 species/site) were considered probably due to greater microsite diversity created by bison activity. Preferential grazing of dominant grasses was probably responsible for increases in other species. Increases in plant diversity in bison-grazed sites were generally more pronounced in annually burned than in 4-year burned areas.
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