The Influence of Tree Islands and Microtopography on Pedoecological Conditions in the Forest-Alpine Tundra Ecotone on Niwot Ridge, Colorado Front Range, U.S.A.

The extreme patchiness of plant communities and site conditions in the upper timberline ecotone is primarily controlled by the influence of microtopography and scattered tree islands on windflow near the soil surface, which in turn affects the distribution pattern of the winter snow cover. Snow accumulates inside the tree islands, and large snowdrifts build up behind their leeward end. Both depth and duration of the snow cover as well as vegetation influence other site factors such as soil moisture and soil temperature that are important to soil-forming processes. Moreover, deposition of alpine loess is enhanced by tree islands as revealed by the data on soil texture and Ca content. The study provides evidence for distinct differences in pedogenesis and soil properties on the windward, interior, and leeward sides of tree islands. Such differences are evident in contents of organic matter and nutrients (N, P, K), C/N ratio, cation exchange capacity, and pH. Different soil types belonging to different great groups of Inceptisols developed as a result of specific conditions on windward, interior, and leeward sites. Under tree islands, pedogenesis is deeply influenced by a thick O horizon. Thus, initial podzolization was observed in some cases. Soil-forming processes gradually change due to downwind migration of wind-exposed tree islands. Consequently, differences in pedoecological conditions on windward, interior, and leeward sites can only be explained in terms of microtopography and the long-term development of the migrating tree islands.