ACCEPTABLE LEVELS OF SPOTTED KNAPWEED (CENTAUREA-MACULOSA) CONTROL

Competition between bluebunch wheatgrass and spotted knapweed was quan tified using three addition series experiments in an environmental cha mber. Using two density matrices, spotted knapweed plants were thinned by either 45 or 90% of their original densities 40 d after emergence. Another matrix of density combinations was not thinned. All plant mat erial was harvested 90 d after thinning. Spotted knapweed was about fo ur times more competitive than bluebunch wheatgrass. Reducing spotted knapweed by 45% did not alter the competitive relationship between the two species. We believe remaining knapweed individuals captured the m ajority of the newly available resources. Ninety percent reduction was necessary to shift the competitive relationship in favor of bluebunch wheatgrass. Successful integrated spotted knapweed management must ex ploit key mechanisms and processes directing plant community dynamics, in conjunction with weed density reduction, if communities are to be shifted toward those that are desired.