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• Mesic Drainage (Figures 5, 6, and 14) <br />In the Supplemental Area, total top layer vegetation cover in the Mesic Drainage vegetation type <br />amounted to 75.4 percent (Table BS), compared to 83.9 percent (Table AS) in the 1990 Baseline <br />Area sampling. Bare soil and litter in the Supplemental Area were 4.4 and 20.2 percent, <br />respectively, compared to figures of 3.6 and 12.4 percent, respectively, in the Baseline Area. <br />Species density was 28.3 species per 100 sq.m. in the Supplemental Area data, compared to 24.1 <br />species per 100 sq. m. in the Baseline Area data. The average production estimated for 1990 was <br /> <br />found to be 1970 Ib/acre (Table A11). Production was not assessed in the Mesic Drainage <br />vegetation in 1992. Total woody plant density averaged 7,317 stems per acre in the Supplemental <br />Area, compared to 3,699 stems per acre in the Baseline Area; both silver sagebrush and mountain <br />snowberry were more abundant in the Supplemental Area. <br />The following discussion describes species dominance in the Mesic Drainage vegetation type based on <br />the all-laver relative cover values. All percentages given in the following three paragraphs refer <br />to this cover statistic. <br />• The Mesic Drainage vegetation type consists of a mixture of growth forms with (in ascending order) <br />top cover dominance by trees with 4.7 percent cover and 26.7 percent frequency, shrubs with 20.0 <br />percent cover and 86.7 percent frequency, combined forts with 30.2 percent cover and 100 <br />percent frequency, and combined grasses with 45 percent cover and 100 percent frequency. <br />Introduced species were a significant component of the total cover, with 25.2 percent cover and <br />100 percent frequency. The combined annual and biennial (orbs cover was 5.9 percent and 93.3 <br />percent frequency. The comparatively high values for introduced and annual /biennial species <br />indicates a higher level of disturbance and availability of introduced species, probably due to <br />grazing pressure and habitat preference by cattle. Species diversity and cover was more evenly <br />distributed among the growth forms in this vegetation type when compared to the other Yoast <br />vegetation types. Native perennial forts and grasses still provided the majority of the species <br />richness. <br />In the Baseline Area, box-elder (Neaundo aceroides ssp. violaceusl dominated the tree cover with <br />4.1 percent cover. Quaking aspen contributed 0.6 percent cover. The shrubs were dominated by <br />mountain snowberry with 13.7 percent cover. Other shrubs that contributed between 1 and 1.5 <br />percent cover were gooseberry (Rites inermel, mountain sagebrush, and chokecherry. Native <br />• perennial grasses were dominated by Agassiz bluegrass with 18.3 percent cover. Other species <br />which contributed between 1 and 4 percent cover were, Nebraska sedge (Carex nebrascensis), giant <br />13 <br />