Disturbances such as fire and grazing enhance plant biodiversity and maintain sustainable species composition in grasslands. Yet, when these disturbances increase in intensity or become too frequent, they can cause negative effects on vegetation. Management strategies such as patch burn grazing (PBG) use fire rotation and the fact that livestock concentrate in recently burned areas to implement naturally rotating fire and grazing pressure across the landscape. Currently, we have limited information about how the effects of grazing intensity are altered within this mosaiced landscape of fire. During the growing season of 2021, we implemented four levels of grazing intensity: no grazing, ambient grazing (cattle stocked at 3.2 ha per pair), heavy grazing (ambient + 50% clipped), and severe grazing (ambient + 100% clipped) within a long-term PBG experiment in tallgrass prairie (Konza Prairie Biological Station, eastern Kansas). This PBG system is divided into three watersheds, each undergoing a prescribed burn every three years in a three-year rotation with the annual burning and grazing (ABG) pasture as the control. Here, we address the overarching question: how does grazing intensity and fire frequency affect biodiversity, plant phenology, and community composition?
Heterogeneity of vegetation and soil moisture was substantially greater in PBG treatments compared with ABG. Plant species richness and evenness in this system were surprisingly resistant to grazing intensity in PBG, except for severe grazing in areas not burned recently where we found a reduction in richness when compared with the ABG pasture. Initial analyses of plant phenology traits suggest that severe grazing delays the natural seasonal cycle of grass species. Lastly, we used multivariate analyses to assess whole community changes to show that communities did not shift after one year of grazing manipulations.
Based on these findings, we suggest that heterogeneity created by PBG is a strong stabilizing factor for plant communities at large spatial scales. PBG and other management strategies that focus on maintaining and enhancing heterogeneity are likely to result in more sustainable landscapes. Additionally, these results will help determine the maximum grazing pressure that a PBG system in tallgrass prairie can withstand while remaining sustainable. The legacy effects of heavy or severe grazing on tallgrass prairie would be better understood by a multi-year study, but this study gives insight into the benefits of a PBG system across a variety of grazing pressures within one year.