Investigating the impact of bison grazing on native tallgrass prairie plant communities in northeastern South Dakota

Tallgrass prairies in the central United States have been fragmented and degraded due to conversion to managed rangeland and cropland. Bison, as historical grazers of tallgrass prairie, may serve as a means to restore native plant communities. This study sought to demonstrate the viability of bison grazing as a restoration method for tallgrass prairies through the investigation of plant community diversity and species abundance changes, as well as changes to soil C and N, under different grazing treatments.

By comparing plant communities and soil samples from fields under bison and cattle grazing in Sisseton, South Dakota, along with an unmanaged control field at Waubay National Wildlife Refuge, I investigated the effects of bison and cattle grazing on native plant abundances. I hypothesized that bison grazing would increase the abundance of forbs, increase N in soils and plant biomass, and promote the return of native warm-season tallgrasses. I used Braun-Blanquet community composition data from 2018 to 2021 from experimental plots in South Dakota, as well as bulk vegetative biomass and soil cores for C and N analysis. Bison grazing promoted lower graminoid cover, lower dead graminoid biomass, greater graminoid richness, higher forb biomass than cattle-grazed plots, and a greater proportion of native species in the plant communities. Bison and cattle grazing promoted higher soil C and N, and a lower C:N ratio for the grazed plots than the control plots. Over the three-year study period (2018-2021), there was an increase in total plant biomass, a decrease in forb biomass, an increase in native species area cover and the proportion of native species per plot, as well as a decrease in soil C:N regardless of grazing treatment. This study provides support for the use of bison grazing as a restoration method for degraded native tallgrass prairie plant communities dominated by non-native species and graminoids.