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.. • ! - - -- 13 common graminoids include a sedge (Carex eleocharis), Canada bluegrass Poa compressa), and mountain muhly (MuhlenberQia montana) (U. S. Dept. of Energy 1994a). This plant association is ranked G2/S2 by the Natural Heritage Program network. The rank G2 indicates that good examples of this community are very rare, occurring in fewer than about 20 places worldwide. [n Colorado, we rank this plant association as S2, reflecting its very rare nature. We believe this community to be imperilled in Colorado and rare globally. Xeric tallgrass prairies have become extremely rare, due to building, mining, and grazing (Howe 1994). These types of grasslands once occupied expansive areas on the Great Plains but have been reduced to tiny remnants. The Rock Creek xeric tallgrass prairie occupies a large azea and is in good condition in places. Exotic plant species are common in patches throughout the community especially along roads or areas of disturbance. We recommend that it be included in a conservation site due to its size, fairly good condition and rarity. The xeric tallgrass prairie ecosystem is now fractured into remnants. The placement of several to many remnants such that genetic exchange of the associated organisms is fascilitated can aid in the long term persistence of the community. This may happen even though all of the individual components are insufficient. In this light, the setting aside of the Rocky Flats Site would fill a major gap between the Ciry of Boulder Open Space prairies (the nearest at the junction of Hwy 128 and 93) and the smaller patches near White Ranch in Jefferson County. The next closest patches are in the vicinity of Ken Caryl Ranch in southern Jefferson County. The true significance of this site is best viewed from the perspective of the remaining patches of this and associated grasslands irr Jefferson County and southern Boulder County. Ponions of this landscape aze r.=ported in Pague et al. (1993). Associated occurrences of the grassland communities are known from the adjacent areas. [t should be considered that these fragments are by themselves insufficient conservation units; however, perhaps with restored linkages, ~nre believe that they could provide community persistence. The Great Plains riparian community is characterized by a diverse mixture of trees, s::tntbs, graminoids, and forbs. Common species include plains cottonwood Po ulus deltoides ,coyote willow (Salix exiQUa), leadplant (Amorpha fruticosa), baltic rush (,Juncus balticus), and various sedges Carex species) and grasses. The vegetation along Rock Creek is dominated by a mosaic of several plant associations: Two of which are the Populus deltoides-Sa1ix amyQdaloides/S. exig_ua (Plains cottonwood riparian woodland) and the Amorpha fruticosa shrubland. The first is considered of global sig-rtificance by the Natural Heritage Network, G2G3/5253 (~Iobally very rare to rare and the same status in Colorado). This plant association is range-restricted and heavily impacted. The occurrence in the Rock Creek drainage is impacted, but potentially restorable. The Amor~ha fruticosa shrubland is ranked GU/SU by the Natural Heritage Network, indicating its poorly known status. The GU/SU rank indicates that ranking has not been attempted for this plant association and more information is needed to document its status. Taking a conservative