EFFECTS OF SHEEP GRAZING ON A SPOTTED KNAPWEED-INFESTED IDAHO FESCUE COMMUNITY

Spotted knapweed (Centaurea maculosa Lam,), a Eurasian perennial forb, is replacing many native perennial grasses, such as Idaho fescue (Fes tuca idahoensis Elmer,), in foothills of the Northern Rocky Mountain r egion, Our objective was to determine if 3 summers of repeated sheep g razing would reduce spotted knapweed without impacting the dominant, a ssociated native perennial grass, Each summer, small pastures were gra zed for 1-7 days in mid-June, mid-duly, and early September, Areas rep eatedly grazed by sheep had lower densities of seedlings, rosettes, an d mature spotted knapweed plants than ungrazed areas. In addition, the proportion of young plants in the population was less in grazed than ungrazed areas, Basal areas of spotted knapweed plants were greater in grazed (8.2 cm(2)) than ungrazed areas (4.0 cm(2)), There were fewer spotted knapweed seeds in soil samples from grazed areas (12 seeds m(- 2)) than from ungrazed (49 seeds m(-2)), Idaho fescue plant density in creased 40% in grazed areas from 1991 to 1994, but leaves and flower s tems on these plants were 38% and 17% shorter, respectively, than in u ngrazed areas. By 1994, frequency of Kentucky bluegrass (Pea pratensis L,) was 35% greater in grazed than ungrazed areas, Grazing did not al ter the amount of litter; however the amount of bare soil increased fr om 2.2 to 5.6% in grazed areas, while it decreased from 4 to 1% in ung razed areas, Three summers of repeated sheep grazing negatively impact ed spotted knapweed, but minimally affected the native grass community , A long term commitment to repeated sheep grazing may slow the rate o f increase of spotted knapweed in native plant communities.