SMALL-SCALE COMMUNITY ANALYSES OF ALPINE RIDGE VEGETATION IN THE CENTRAL SIERRA NEVADA

Abstract

Most studies of alpine vegetation communities focus on the variation occurring across large scales, in which diverse environmental habitats are assessed across the landscape and detailed plant associations are described. However, community patterns occurring at more intricate scales are easily concealed by such a broad perspective. To complement large-scale studies, we assessed smaller-scale patterns occurring at the extremes of moisture and exposure gradients in an alpine environment. We studied the variation in plant community composition among and within seven ridges extending down two rocky metamorphic peaks in Yosemite National Park. Species cover and environmental factors (proximity to a summit, elevation, aspect, slope, and substrate size) were sampled in 281 2 × 2-meter plots. Variation among ridges was primarily determined by differences in aspect. Within the ridges, although the physical environment in which we sampled was seemingly homogenous, ordinations identified two distinct vegetation types: (1) high diversity and high cover vegetation (HD), dominated by cushion plants and (2) low diversity and low cover vegetation (LD), dominated by plants in large clumps or small rosettes. In general, LD vegetation was found nearer to the summit and was highly variable in its component species, while HD vegetation was found farther along the ridgeline and was relatively uniform. This non-intuitive pattern in which cushion plants do not dominate the most exposed habitat occurred independent of elevation. Instead, our data suggest that along with changes in rock size and aspect, proximity to a summit is a meaningful factor governing alpine community structure. We discuss the water limited conditions imposed by the absence of smaller rock structure at the summit and how this may allow for only the sparse LD vegetation to persist there.