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species density in all reclaimed areas exceeded that in the Alkali Sagebrush Reference Area and in the Sagebrush-Gravel Reference Area. The Sagebrush-Grass Reference Area had a total species density of 19.9 species/100 square meters that exceeded the species density of all reclamation units. Native perennial fortis are well distributed in both reclaimed and reference areas. The lowest density (3.6 species/100 square meters) of native perennial forb species was observed in the Alkali Sagebrush Reference Area and the highest density (5.06 species/100 square meters) was observed in the 1996 Reclamation Area. The density of native perennial cool season grasses was greater in all reference areas than in any reclamation unit. In this regard, the Alkali Sagebrush Reference Area was the least diverse and had 6.10 species/100 square meters. The greatest diversity of native perennial cool season grasses in a reclaimed area was in the 1995 Reclamation Area which had 5.65 species/100 square meters. Noxious Weeds Of plant species observed during 1999 sampling, Canada thistle (Breea arvensis aka Cirsium arvense) and musk thistle (Carduus nutans ssp. macrolepis) are listed among the top ten prioritized weed species for Colorado (C.R.S. 1996). Bull thistle (Cirsium vulgare), fiixweed tansymustard (Descurainia sophia), Russia thistle (Salsola australis aka Salsola iberica), and cheatgrass (Anisantha tectorum aka Bromus tectorum) are also listed as noxious. Canada thistle was found in all reclaimed areas and in the Sagebrush-Grass Reference Area. Musk thistle was found in the Pre-1986 Reclamation, 1996 Reclamation, and the Alkali Sagebrush Reference Area. Bull thistle and flixweed tansymustard were found in all reclaimed areas. Russian thistle and cheatgrass were found only in the 1995 Reclamation and in the 1996 Reclamation. The frequency (number of samples in which the species was found/ total number of samples) with which these species were found while conducting cover and total annual production sampling is summarized in the following two tables and provides and indication of the distribution of each Fre uenc of Noxious Weeds in Cover Sam les AREA Common Name Pre-1986 Alkali Sagebrush- Sagebrush- 1995 Rec. 1996 Rec. Sagebrush Rec. Grass Ref. Gravel Ref. Ref. Bull thistle 4.76 29.41 26.67 ---- ---- ---- Canadathistle 57.14 70.59 60 ---- 10 -•-• cheatgrass ---- 11.76 13.33 ---- ---- ---- Flixweed tansymustard 4.76 5.88 53.33 ---- ---- ---- Muskthistle 4.76 ----. 6.67 10 ---- ---- Russian thistle ---- ---- 20 ---- ---- ---- Fre uenc of Noxious Weeds in Production Sam les AREA Common Name Alkali pre-1986 Sagebrush- Sagebrush- - 1995 Rec. 1996 Rec. Sagebrush Rec. Grass Ref. Gravel Ref. Ref. Bull thistle 2.76 11.11 ---- ---- ---- ---- Canadathistle 19.44 5.56 ---- ---- ---- ---- cheatgrass ---- 5.56 11.11 ---- ---- ---- Flixweed tansymustard 8.33 11.11 11.11 ---- ---- ---- Muskthistle ---- ---- ---- ---- ---- ---- Russian thistle ---- 2.78 16.67 ---- ---- ---- species. IS