Plains bison (Bison bison bison) are suspected to have played an important role in maintaining and structuring the plant communities throughout the Aspen Parkland. We examine the effects of a fenced bison herd on grassland plant communities in Riding Mountain National Park, Manitoba, Canada. We focus on the relationship between bison and woody plant encroachment along the forest-grassland ecotone, and on the effects that short- and long-term occupation by bison have on Festuca hallii grassland communities including the effects of bison reintroduction to a grassland area ungrazed by bison for 165 years. We found limited impacts of bison on the shrub community at the forest grassland edge. Bison rarely entered areas with dense, tall shrub understories, and thus bison cannot be a major factor controlling the ecotone. Shrub stem mortality due to fire may be a necessary precursor to bison effectively mitigating woody encroachment in the Aspen Parkland. We found limited short-term impacts of bison on grassland communities, with reduced sward heights but no change in composition. The long-term (>80 year) presence of bison was associated with greater species richness, reduced litter accumulation, and a distinct plant community composition compared to areas without bison impacts.