COS 39-5 - Native megagrazers act as a surprising inhibitor of woodland expansion in a Great Plains grassland

Abstract: Within the Central Great Plains, reductions in fire frequency are well-known to increase the invasion of woody plants, known as woody encroachment. Grazers should theoretically increase encroachment as well, but in this region most work on this topic has focused on non-native grazers (cattle). We examined the effect of a native megagrazer, Plains Bison (Bison bison) at Konza Prairie Biological Station, comparing areas of tallgrass prairie with and without bison that went largely unburned for 30 years. Based on remote sensed land cover, we found that areas with bison have lower tree cover (5%) than ungrazed areas (14% ungrazed) and have especially low evergreen tree cover (~0% cover), compared to 36% of the total tree cover in the ungrazed treatment. However, shrub cover (~45% for both) and grass-dominated cover (50% grazed and 39% ungrazed) were not statistically different between treatments. Plant diversity was 170% higher in bison-grazed areas (based on long-term monitoring), and community composition differed significantly from the non-grazed treatment, driven by an increase in forbs and a lack of tree species.Grazers are thought to favor woody plants by reducing competition with grasses and decreasing the intensity of fires when they occur. We found that bison had the opposite effect at low fire frequencies. We argue that cryptic browsing and trampling account for this pattern, which is supported by seedling trial where nine times more evergreen seedlings died in the presence of bison. Our results suggest that bison might be a useful tool for decreasing woody plant expansion and increasing plant diversity at low fire frequencies.