Factors Influencing Soil Moisture and Plant Community Distribution on Niwot Ridge, Front Range, Colorado, U.S.A.

Slope-aspect influenced evapotranspiration and soil desiccation in the Colorado alpine tundra via control over radiation and wind speed. South- and east-facing sites upon a fellfield knoll received 18 and 14% more net radiation than the north-facing slope on clear sky and more typical summer days, respectively. West-facing slopes experienced moderate radiation loads but highest wind speeds. Water loss during two drying sequences for east-, west-, and northfacing slopes was 80, 80, and 600%o, respectively, of evapotranspiration from the south-facing slope. Consequently, soil at the south-facing site dried faster than soil on other slopes of the knoll. Dryas octopetala prefers more moist northern slopes on Niwot Ridge, illustrating the importance of topoclimatic controls over soil moisture to vegetation distribution in the fellfield. Water relations during the growing season influence vegetation distribution within topographic depressions on Niwot Ridge. Where meltwater is supplied throughout the growing season wet meadow and moist shrub tundra occur. Evapotranspiration from the wet meadow was 1.5 times greater than from other communities in the Saddle area during a drying period. Contrasts in snow cover depth between windward and leeward slopes is an important location factor for dry fellfield, dry meadow, moist meadow, and snowbed vegetation.