BIOMASS AND CARBOHYDRATES OF SPOTTED KNAPWEED AND IDAHO FESCUE AFTER REPEATED GRAZING

Spotted knapweed (Centaurea maculosa Lam.), an aggressive Eurasian for b, is replacing many native perennial grasses such as Idaho fescue (Fe stuca idahoensis Elmer,) on foothills of the Northern Rocky Mountains, We assessed biomass allocation, carbohydrate reserves (total nonstruc tural 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 e arly May 1994, we excavated 30 spotted knapweed and Idaho fescue plant s previously exposed to repeated grazing and 30 ungrazed plants of eac h 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 cont rast, shoot (P = 0.26) and root biomass (P = 0.85) of grazed and ungra zed spotted knapweed plants were similar, Although grazing resulted in some minor differences in total non-structural carbohydrate concentra tions and carbohydrate pools of shoots, total nonstructural carbohydra te concentrations and pools of crowns and roots were similar for graze d and ungrazed plants of each species, Thus, carbohydrate concentratio ns or pools were not sensitive indicators of the response of Idaho fes cue or spotted knapweed to the cumulative effects of repeated grazing, In contrast, aboveground biomass could be used to indicate the respon se of Idaho fescue to repeated grazing, By reducing shoot and root bio mass of Idaho fescue but not spotted knapweed, repeated grazing may re duce the ability of Idaho fescue to compete with spotted knapweed when both species are grazed.