1997-06-13_REPORT - M1981302 (11)

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(McClaren and Sundt, 1992) . In Colorado, the largest population of the orchid is on City of Boulder Open Space at the Van Vleet site, a floodplain meadow, which has been used agriculturally for the past 50-75 years. This site is still grazed each year in the winter from February to May, irrigated in the spring and early summer, and mown in the summer around the beginning of July. When these activities were discontinued at a similar site in Boulder, exotic species such as Canada thistle proliferated and the orchid disappeared. Resumption of traditional agricultural uses has since reduced the thistle infestation and the orchid has reappeared (Tamara Naumann, pers. comm. 1994) . Grazing and mowing seem to promote flowering, presumably by opening the canopy to admit more sunlight. However, these management practices may impede fruit set by directly removing flowering stalks, enhancing conditions for herbivory of fruits by small mammals such as meadow voles, or altering habitat required by bumble bees, the primary pollinator (Arft 1993) . What is known about the habitat preferences of the Ute ladies'-tresses is consistent with the following model for natural population establishment and maintenance. This model is based upon observations and recent research since listing the Ute ladies'-tresses as a threatened species: Ute ladies'-tresses habitat is found along freshwater streams emerging from the flanks of mountains where the streambed is beginning to level out and meander within a developing floodplain. These streams are very dynamic. They are subject to seasonal flooding from snowmelt and intermittent heavy thunderstorms. Due to variations in snowpack, these streams experience fairly frequent severe (overbank) flooding sufficient to cause movement of the stream channel within its floodplain. The orchid colonizes early successional riparian habitats such as point bars, sand bars, and low lying gravelly, sandy, or cobbly edges. As the stream channel chanaes location and depth, the orchid persists in those areas where the hydrology provides continual dampness in the rooting zone throughout the growing season. These areas include old oxbows, side channels, or alder stream channels that have been filled in with alluvial material but which still have a hydrologic connection, through groundwater, to the stream system. The orchid is tolerant of a mix of wetland forb and grass species, is not tolerant of long term standing water, and does not compete with emergent plant species (e.g. , cattails) or aggressive species that form dense monocultures such as Canada thistle or reed canarygrass. Throughout the historical range of the orchid, the lower mountain flanks and associated riparian areas provide winter range for native ungulates. It is likely that late winter and early spring grazing by native ungulates (bison, elk, and deer along the Front Range, big horn sheep, elk, and deer along the south slope of the Uintas and west slope of the Wasatch Range) in riparian areas historically helped maintain the vegetation community in a condition favorable for the orchid (i .e. , prevented excessive buildup of live and dead vegetation) . Native ungulates typically follow the snowline (greenline) upslope as spring arrives, thus historically did not stay in orchid habitat in large numbers throughout the summer. Predators also likely kept native 9