ENVIRONMENTAL CONTROLS INFLUENCING THE ALTITUDE AND FORM OF THE FOREST-ALPINE TUNDRA ECOTONE, COLORADO FRONT RANGE.

摘要

The forest-alpine tundra ecotone of the Colorado Front Range is a dynamic vegetative belt where environmental factors vary rapidly. Under the present climatic conditions the conifer trees appear stressed and seedling establishment does not coincide with the present upper limit of tree species. With the increasing population and recreation pressure within the Front Range there is need for concern that irreversible damage may occur to the forest-alpine tundra ecotone.;The interactions of the climate the environment and the survival of the conifer tree species within the forest-alpine tundra ecotone are indeed complex. A short, cool growing season, restricted by late-lying snow and early and late frosts, results in the tissues being unable to ripen and prepare adequately for winter hardiness. Seedlings find it impossible to establish and survive within the upper ecotone. The unpreparedness for such a harsh environment is evident during late fall when mild freezes may cause extensive damage to new growth, and during winter when many processes may weaken the tree. The winters on Niwot Ridge are long, with low temperatures, occasional days with high levels of radiation, strong winds and frozen soils. The foliage, if inadequately developed, survives only if protected by a deep snow cover. Low air temperatures, frost damage and winter desiccation appear to be primarily responsible for hindering growth in tree species within the ecotone. Of primary importance is the fact that these stress phenomena occur most often when the trees are least able to resist.;The forest-alpine tundra ecotone appears at present to be under great climatic stress. The uppermost part of the ecotone is no longer successfully regenerating by seed, but rather relies on vegetative reproduction. It becomes obvious that the climate, at some time in the past, was more conducive to seedling establishment and survival. Although the ecotone has 'held its ground' for a long period of time, if it were disturbed through burning, logging, or other human activities, it would not regenerate in its present form and location. It is concluded that the ecotone, and especially the tree species limit, is a relict of a former climate and may have been established several thousand years ago.;Meso-climatic parameters and conifer physiological responses were monitored throughout the year to determine what combinations of climate and inadequate physiological "preparedness" result in the limitation of tree growth in the alpine. The upper limit of seedling establishment was systematically determined, and detection of a climatic change and in which direction the ecotone responded to the change were noted. In order to provide a present day inventory of the forest-alpine tundra ecotone it was mapped in detail at 1:10,000 and 1:50,000. These maps also facilitated the determination of topo-climatic influences on its distribution and provided a historical document upon which to monitor future changes. Finally, they furnished a working tool for land use planners to develop management plans.