Background/Question/Methods Within the Central Great Plains of North America, changes in fire frequency can result in transitions of grasslands to shrublands or woodlands. These potentially represent alternative states, making it difficult to reverse woody plant expansion. Fire plays a key role in mesic grasslands, affecting plant diversity and transitions between these states. However, it’s unknown how native grazers, such as the American bison (Bison bison), affect these states and transitions between them. For instance, bison are categorized as grazers but could affect woody plants through cryptic browsing, rubbing on trees, and wallowing. We examined the effect of bison at Konza Prairie, a mesic grassland in the Central Great Plains, on the plant diversity and woody plant cover within a woodland state with a 20-year fire return interval. We compared catchments with this similar fire history and the presence/absence of bison since 1991. To quantify large-scale shrub and tree cover changes, we used a remote sensed product that categorizes 2-m pixels into grass, shrub, deciduous tree, or evergreen tree, with >90% accuracy. To quantify changes in diversity, we used long-term monitoring plots (n=20). Finally, we used a seedling trial to determine if and how bison affect the mortality of evergreen tree saplings. Results/Conclusions Thirty years of bison reintroduction had noticeable effects on vegetation at all scales. Based on remote sensing, we found that bison grazed woodland areas have lower tree cover (5% grazed vs. 14% ungrazed) and may be promoting a shrub-dominated system. There was no statistical difference between shrub cover (~45% for both) and grass-dominated cover (50% grazed and 39% ungrazed) between the two treatments. Furthermore, bison limited evergreen tree cover (eastern red cedar, Juniperus virginiana) to approximately 0% cover, compared to 36% of the total tree cover in the ungrazed woodland. Plant diversity is twice as high in bison-grazed areas, and community composition differs significantly from the non-grazed treatment. Grazer reintroduction is often thought to favor woody plants by reducing competition with grasses and decreasing the intensity of fires when they occur. We found that bison—arguably the most iconic grazer in North America—had the opposite effect at low fire frequencies. We suspect that several more cryptic but nonetheless important behaviors account for this pattern, including trampling and cryptic browsing. From a conservation perspective, bison might be a useful tool for decreasing woody plant expansion in the absence of fire and increasing plant diversity.
