Ecological - Second Order
Exotic annual grass invasion and dominance of rangelands is a concern across western North America and other semiarid and arid ecosystems around the world. Postfire invasion and dominance by exotic annual grasses in sagebrush communities is especially problematic as there are no cost-effective control strategies available for the vast acreages already invaded. However, fall-winter grazing by cattle has been suggested as a potential tool to decrease exotic annual grasses and encourage native perennial vegetation, but to date its efficacy has not been tested. We evaluated fall-winter grazing over 4 yr after wildfire in Wyoming big sagebrush (Artemisia tridentata Nutt. subsp. wyomingensis Beetle & Young) steppe invaded by exotic annual grasses. Fall-winter grazing reduced exotic annual grass and annual forb cover and density and increased the native perennial bunchgrass, Sandberg bluegrass (Poa secunda L.). Exotic annual grass cover and density were 1.5 × greater in ungrazed compared with fall-winter grazed areas after 4 yr. At this time, Sandberg bluegrass density and cover were 1.6 × and 2.3 × greater in fall-winter grazed compared with ungrazed areas. Large perennial bunchgrasses and perennial forbs did not increase with fall-winter grazing because either grazing did not facilitate their increase or they were slow to respond to decreases in exotic annuals. Fall-winter grazing likely decreased exotic annual grass by defoliating it during its early growth in the fall and late winter and by reducing ground cover that facilitates exotic annual grass emergence and growth. Fall-winter grazing clearly reduced exotic annual grasses, but its effects on native perennial vegetation were not conclusive. Careful application of fall-winter grazing appears to be a valuable tool for managing exotic annual grass cover and abundance, but longer-term research is necessary to determine if it can facilitate the return of native perennial dominance.