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Spotted knapweed (Centaurea maculosa Lam.), an aggressive Eurasian forb, is replacing many native perennial grasses such as Idaho fescue (Festuca idahoensis Elmer.) on foothills of the Northern Rocky Mountains. We assessed biomass allocation, carbohydrate reserves (total nonstructural carbohydrate concentrations - TNC), and carbohydrate pools (TNC X biomass) as indicators of cumulative effects of 3 summers (1991-1993) of repeated sheep grazing on spotted knapweed and Idaho fescue. In early May 1994, we excavated 30 spotted knapweed and Idaho fescue plants previously exposed to repeated grazing and 30 ungrazed plants of each species. On grazed Idaho fescue plants, shoot (P < 0.02) and root (P < 0.06) biomass were 38 and 27% less than on ungrazed plants. In contrast, shoot (P = 0.26) and root biomass (P = 0.85) of grazed and ungrazed spotted knapweed plants were similar. Although grazing resulted in some minor differences in total nonstructural carbohydrate concentrations and carbohydrate pools of shoots, total nonstructural carbohydrate concentrations and pools of crowns and roots were similar for grazed and ungrazed plants of each species. Thus, carbohydrate concentrations or pools were not sensitive indicators of the response of Idaho fescue or spotted knapweed to the cumulative effects of repeated grazing. In contrast, aboveground biomass could be used to indicate the response of Idaho fescue to repeated grazing. By reducing shoot and root biomass of Idaho fescue but not spotted knapweed, repeated grazing may reduce the ability of Idaho fescue to compete with spotted knapweed when both species are grazed.

Biomass and Carbohydrates of Spotted Knapweed and Idaho Fescue after Repeated Grazing