CHANGES IN VEGETATION AND SOIL FERTILITY ALONG A PREDICTABLE SNOWMELT GRADIENT 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 multivariate techniques to document these changes at a microgeographic scale and to measure predictability of snowmelt patterns. This paper describes concordant changes in microenvironmental factors and plant species abundances among > 200 evenly spaced microsites in a large alpine snowbed in the Mosquito Range, Colorado. The snowmelt gradient was highly consistent: averaged over all microsites, the mean date of snowmelt varied by only 1 wk from 1989 to 1992, and relative date of snowmelt among microsites was highly predictable (R(2) > 90% among years). Soil in later melting microsites had reduced organic content, water content, nitrogen, phosphorous, and acidity. Total soil nitrogen and water content decreased through the growing season, while pH increased significantly. Species richness and total vegetation cover were significantly greater in early melting microsites, and soil disturbance by pocket gophers and erosion peaked in moderately late-melting areas. Canonical correspondence analysis (CCA) showed that microgeographic changes in 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 snowbed tended to decrease in abundance in later melting snowbed sites. Other species attained peak abundance in the less productive late melting sites, and a few appeared to be associated with soil disturbance. Plant community differences between paired sampling units (quadrats) were 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 interactions and physiological tolerances, rather than by historical accident and limited dispersal.