VEGETATION TYPE HIERARCHIES AND LANDFORM DISTURBANCE IN ARCTIC ALASKA AND ALPINE COLORADO WITH EMPHASIS ON SNOWPATCHES

Four-level Braun-Blanquet vegetation type hierarchies of an arctic (a section of the Arctic Coastal Plain at Atkasook, Alaska) and an alpine (Indian Peaks.area, Colorado Southern Rocky Mountains) area were compared through their vascular plant taxa composition. Vegetation types at all hierarchy levels were more numerous, less well defined, and less clearly distributed along the controlling environmental gradients in the Arctic than in the alpine. This greater environmental and vegetational heterogeneity and relatively poor definition of vegetation types are probably controlled by the surface disturbances of the thaw-lake cycle, rivers, cryoturbation, and wind, which cause significant horizontal movement of materials with low stability and rework small-to medium-sized surface patches in the Arctic in the intervals of several hundred to several thousand years. These surface disturbances are more frequent and less intense than the area-wide surface disturbances by glaciations in the Southern Rocky Mountains. More frequent surface disturbances seem to produce more numerous and less well defined and organized vegetation/landform types than less frequent disturbances, even when the less frequent disturbances are considerably more intense. Because most disturbances which interrupt the development of Alaskan Arctic vegetation/landform types and keep them relatively poorly defined occur every few thousand years, the time needed for a full development or a relatively good definition of vegetation/landform types seems to be longer than that. The correlation of high disturbance frequency and small to medium patch size with high vegetation type diversity at higher levels of the Braun-Blanquet hierarchy was confirmed in Atkasook snowpatches. Much higher disturbance intensity and greater development of snowpatches at Indian Peaks were reflected only in a higher number of associations.