CHANGES IN THE FOREST-ALPINE TUNDRA ECOTONE: COLORADO FRONT RANGE

On Niwot Ridge, alpine climatic factors dominate today within the upper part of the forest-alpine tundra ecotone on the east slope of the Colorado Front Range. This study provides evidence that the climatic conditions controlling the distribution of the upper limit of tree species and growth forms within the ecotone have changed through time. It appears that much of the present forest-alpine tundra ecotone is a relict of past warm climate episodes. There is much evidence that, in the past, tree forms were more symmetrical and less deformed, tree trunks with larger diameters existed, and the trees were more extensive in areal coverage. However, no evidence exists to show that the upper limit of tree species growth was more than marginally higher in the past than today. The treeline appears to be stable to historical and Holocene climatic fluctuations, but the altitudinal limits of seedling establishment and survival are not stable. The liming of climatic changes in the Front Range and their possible influence on the distribution of the trees within the ecotone are reviewed. Because of its extreme easterly location, slight but significant climatic variations may be observed within the forest-alpine tundra ecotone of Niwot Ridge, but are not observed elsewhere.