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2000 was characterized by an unusually warm and dry spring, and plant growth stage appeared to be <br />somewhat advanced, as compared to the other years of the study. Density of noxious plants is subject to <br />less year to year variation due to climate, grazing, and other uncontrolled variables than is cover, and is a <br />more precise tool for assessing success of control efforts on target species. In 2001, a number of thistle <br />plants within Reference Area #1 were noted to be infested by the larvae of the painted lady butterfly, but <br />the percentage of plants with larvae was much lower in Reference Area #1 than in Reference Area #2 and <br />adjacent areas. <br />20 <br />15 <br />10 <br />5 <br />0 <br />Figure 2 - Sutey Reference Area #1 Noxious <br />Species Density <br />1998 1999 2000 2001 <br />—�— Thistle <br />• Thistle & <br />Houndstongue <br />Combined <br />Reference Area No. 2 <br />From 1998 to 2001, absolute cover of noxious species and non - noxious forbs declined, while grass cover <br />increased. 2001 total vegetation cover was significantly lower than in 1998, because the increase in grass <br />cover did not offset the decline in forb and noxious species cover (Figure 3). <br />60.0% <br />50.0% <br />40.0% <br />30.0% <br />20.0% <br />10.0% <br />0.0% <br />Figure 3 -Sutey Reference Area #2 Absolute <br />Cover Trends; 1998 -2001 <br />1998 1999 2000 2001 <br />— 0— Total Vegetation <br />Cover <br />T Non - noxious Forb <br />Cover <br />—* — Grass Cover <br />Noxious Species <br />Table 5 lists 2001 absolute and relative cover by species and category. Table 6 presents absolute and <br />relative cover sample means by category and year; 1998 through 2001. Absolute cover of noxious species <br />declined from 14.6% to 2.6% over the course of the study, while absolute cover of perennial grasses <br />increased from 4.4% to 20.2 %. Total vegetation cover remains low, at only 28 %. Non - noxious forb cover <br />declined from 37.2% in 1998, to 5.2% in 2001. <br />At the individual species level, cover of the noxious plumeless thistle declined from 9.6% in 1998, to 0.8% <br />in 2001. Impacts to other, less abundant noxious species were not evident from the cover data. Of the most <br />common perennial forbs, cicer milkvetch (Astragalus cicer) declined from 17.6% to 2.2 %, yarrow <br />4 <br />