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Density of 23 perennial grass species was measured in 25 permanent plots nine times between 1972-2000. Grass density was not related to the intensity of livestock grazing. Only one species expressed a difference between the summer rest and no summer rest with heavier stocking grazing treatments: bush muhly (Muhlenbergia porteri Scribn. ex Beal) density was less under the no summer rest with heavier stocking treatment. Beginning in 1975, the non-native Lehmann lovegrass (Eragrostis lehmanniana Nees) spread from distant seedings to one plot, and by 1991, it was the dominant species on most plots. The density of native species was not related to the length of time that the non-native lovegrass was present on a plot. In general, native species declined prior to the arrival and increase of the non-native lovegrass. (C) 2001 Academic Press.