Vegetation trends in tallgrass prairie from bison and cattle grazing

摘要

Comparisons between how bison and cattle grazing affect the plant community are understood poorly because of confounding differences in how the herbivores are typically managed. This 10-year study compared vegetation changes in Kansas (USA) tallgrass prairie that was burned and grazed season-long at a moderate stocking rate by either bison or cattle. We held management practices constant between the herbivores and equalized grazing pressure by matching animals so that the total body mass in all pastures was similar each year. Trends in species cover and diversity indices in the bison and cattle pastures were compared with ungrazed prairie that also was burned annually. We found that little bluestem (Schizachyrium scoparium) cover decreased over time in bison pastures, and big bluestem (Andropogon gerardii) cover increased over time in cattle pastures. Grazing by either herbivore increased the canopy cover of annual forbs, perennial forbs, and cool-season graminoids, but both annual and perennial forb cover increased at a greater rate in bison pastures than in cattle pastures. Missouri goldenrod (Solidago missouriensis) and heath aster (Symphyotrichum ericoides) were primarily responsible for the increased forb cover in grazed pastures. Species richness at both small (10 m(2)) and large (200 m(2)) spatial scales increased at a greater rate in bison pastures than in cattle pastures, but richness did not change through time in ungrazed prairie. The number of annual forb species was significantly higher in bison pastures than in pastures grazed by cattle. Residual graminoid biomass at the end of the grazing season was lower in bison pastures than in cattle pastures, whereas forb residuum increased over time at a greater rate in pastures grazed by bison than in pastures grazed by cattle. Although bison and cattle differentially altered some vegetation components, the plant communities in bison and cattle pastures were 85% similar after 10 years of grazing. We conclude that most measurable differences between bison-grazed and cattle-grazed pastures in tallgrass prairie are relatively minor, and differences in how the herbivores are typically managed may play a larger role in their impact on prairie vegetation than differences between the species.