CHANGES IN VEGETATION AND SOIL FERTILITY ALONG A PREDICTABLE SNOWMELTGRADIENT IN THE MOSQUITO RANGE, COLORADO, USA

Changes in edaphic conditions and vegetation along snowmelt gradients are well known in many alpine areas, but very few studies have used mu ltivariate techniques to document these changes at a microgeographic s cale and to measure predictability of snowmelt patterns. This paper de scribes concordant changes in microenvironmental factors and plant spe cies abundances among > 200 evenly spaced microsites in a large alpine snowbed in the Mosquito Range, Colorado. The snowmelt gradient was hi ghly consistent: averaged over all microsites, the mean date of snowme lt varied by only 1 wk from 1989 to 1992, and relative date of snowmel t among microsites was highly predictable (R(2) > 90% among years). So il in later melting microsites had reduced organic content, water cont ent, nitrogen, phosphorous, and acidity. Total soil nitrogen and water content decreased through the growing season, while pH increased sign ificantly. Species richness and total vegetation cover were significan tly greater in early melting microsites, and soil disturbance by pocke t gophers and erosion peaked in moderately late-melting areas. Canonic al correspondence analysis (CCA) showed that microgeographic changes i n the plant community were associated with four environmental factors: snowmelt date, rock cover, soil disturbance, and soil organic content . Plant species that are common in the vegetation surrounding the snow bed tended to decrease in abundance in later melting snowbed sites. Ot her species attained peak abundance in the less productive late meltin g sites, and a few appeared to be associated with soil disturbance. Pl ant community differences between paired sampling units (quadrats) wer e determined more by differences in their snowmelt date than by their spatial separation, suggesting that plant species distributions along this alpine gradient are determined principally by ecological interact ions and physiological tolerances, rather than by historical accident and limited dispersal.