Comparing the impacts of patch-burn grazing on vegetation in two northern tallgrass prairies

The management practice of patch-burn grazing varies grazing pressure across a site by rotating burn locations, thereby creating spatial heterogeneity in vegetation height and density (structure). Patch-burn grazing increases the range of habitats available for different wildlife species, but it may also unintentionally affect plant invasion and plant biodiversity. We evaluated the effects of patch-burn grazing on plant communities in two northern tallgrass prairies. Both dry-mesic prairie sites were in Minnesota, USA, on similar soils and undergoing invasion by the non-native, cool-season grass smooth brome (Bromus inermis). The sites had different cattle stocking rates and burning practices (3 or 5 burn units). We established 15-20 pairs of plots per site with a fence around one member of each pair. We measured vegetation structure, native and non-native plant richness, smooth brome frequency, and frequency-weighted mean coefficients of conservatism (mean C) over 5-6 years across two treatments: patch-burn grazing and burning-without-grazing. At the site with a lower stocking rate and more burn units, grazing promoted spatial heterogeneity by reducing vegetation structure 18-65 percentage points in some units and some years. In both treatments, native richness increased 15% over 6 years, but smooth brome frequency increased over 200%, suggesting that adjustments in management are needed to suppress smooth brome. At the site with a higher stocking rate and fewer burn units, grazing reduced vegetation structure 37-78 percentage points in all units and all years, but native richness was maintained over time. Grazing also increased non-native richness 38 percentage points and reduced mean C by 6 percentage points over 2 years. Smooth brome frequency increased 2% over 2 years in both treatments. Patch-burn grazing at this site may have increased richness of annual or biennial non-native plant species. At both sites, long-lived perennials may drive the resilience of native plants to periodic grazing impacts.