Cool Season Invasive Grasses in Northern Great Plains Natural Areas

Abstract

Native prairie sites within the Northern Great Plains are being invaded by cool season invasive grasses despite utilizing even the most conservation-oriented management. Native prairie sites of the Knife River Indian Villages National Historic Site, located near the confluence of the Knife and Missouri Rivers in central North Dakota, were reassessed 23 years after the first assessment. The native prairie sites were initially assessed the summer of 1984 utilizing a modified Daubenmire technique which entailed randomly placing twenty-five ¼ m2 quadrats along transects at each native prairie site. Prior to 1976, native prairie sites were grazed by livestock; and after the National Park Service took ownership, sites were not grazed by livestock and were essentially left idle. The same technique was repeated during the summer of 2007 on the same native prairie sites surveyed in 1984, and comparisons were made between species composition of those sites in 1984 and 2007. Species composition of several sites across the landscape changed from containing a high percentage of native graminoids and forbs to containing a high percentage of invasive species such as smooth brome (Bromus inermis Leyss.) and Kentucky bluegrass (Poa pratensis L.). A few of the more xeric native prairie sites maintained a high percentage of native graminoids and forbs with minor changes in coverage. This study illustrates the emergence of invasive species and species composition changes under historic land use and climatic conditions and the roles that disturbances, such as grazing processes and fire, may play in maintaining native plant communities.