The effects of patch-burn grazing on a restored Minnesota tallgrass prairie
Conversion of prairies to agriculture, suppression of fire, and the near extirpation of bison have significantly altered prairie structure and function. Currently, less than 1% of native tallgrass prairie remains. Recent prairie restoration efforts, such as patch-burn grazing (PBG), have attempted to reestablish historic disturbance regimes. This strategy utilizes prescribed burning and large herbivore grazing simultaneously within the same paddock. In summer 2011, PBG was initiated in northwest Minnesota at Glacial Ridge National Wildlife Refuge, the site of the largest prairie and wetland restoration in the U.S. Objectives of this study are to assess and compare responses of prairie plant communities between PBG and a traditional burn-only approach. Specific questions include: (1) How does PBG affect spatial heterogeneity of the plant community vs. burning only? (2) How do plant community composition and species diversity respond to PBG vs. burning only? Twelve burn patches were established in an 800-ha unit that was stocked with 200 cow/calf pairs, and three patches are burned each year for four years (2011 – 2014). A control unit was established to compare PBG to burn only, and plant species composition, plant height, visual obstruction readings, ground cover, and litter depth were assessed in both units.
Cattle utilization was higher on recently-burned patches than on unburned patches, but fence lines and areas near water sources were also heavily grazed and/or trampled. Visual obstruction readings indicated that vertical plant structure was more heterogeneous in the PBG unit (4 – 43 cm) than in the control (36 – 51 cm). Average litter depth was lower in the PBG unit (2.5 ± 0.4 cm) than in the control (4.4 ± 0.3 cm), and frequency of bare ground was higher in the PBG unit (16.1% ± 3.4) than in the control (6.8% ± 1.2). Initial results support the assumption of PBG that livestock will graze primarily on recently-burned patches, which is likely a result of regrowth of more nutritious, palatable vegetation. Results also indicate that PBG increases spatial heterogeneity in terms of plant height, litter depth, and ground cover over traditional burn-only treatments. This has important implications for increasing diversity of ground-nesting birds and other wildlife in restored prairies. However, the higher amount of bare ground in the PBG unit may increase soil erosion compared to burn-only treatments, and the heavily-disturbed areas around water and along fence lines may be more susceptible to future invasion by exotic species.