Abstract
Diffuse knapweed (Centaurea diffusa) was observed to invade a lightly grazed grassland in the interior Douglas-fir zone southeast of Kamloops, B.C. The presence of knapweed at densities of approximately 15 flowering plants/m2 reduced the growth of grass during the summer by about 2/3 and grass biomass by approximately 1/2. Knapweed seedling density dropped sharply over a distance of 10–20 m, but neither the ratios of seedling-to-rosette nor rosette-to-flowering plant changed significantly over this density gradient. Fertilization did not alter the grass or knapweed summer growth or biomass, but selective grazing by cattle might have obliterated any difference. The front of diffuse knapweed distribution advanced approximately 120 m during 3 yr, and scattered patches of plants occurred even further into the grassland. We conclude that overgrazing is not a necessary prerequisite to the invasion of diffuse knapweed into grassland.Key words: Knapweed, weed invasion, introduced weed, rangeland weed
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