Landscape Relationships of Soils and Vegetation in the Forest-Tundra Ecotone, Upper Firth River Valley, Alaska-Canada

Drew, James; Shanks, Royal

In North America, a northern extension of white spruce into the Tundra occurs in the valley of the Firth River, a river which flows northeast from its headwaters in the Davidson Mountains of northeastern Alaska to Herschel Island at the Arctic Coast of Canada. Studies were undertaken in the Firth River Basin, a high interior valley basin at the Alaska-Canada border (about 70 mi south of the Arctic Ocean), to describe the soils and vegetation in this northern extension of the forest-tundra ecotone. The recognition of a number of landscape units in the Firth River Basin, each having characteristic topography, vegetation, and soil conditions, provided a framework for studying the relationships of soils and vegetation. The Firth River Basin consists of low terraces and a braided network of stream channels bordered on the south by uplands consisting of high bedrock terraces, and is surrounded by mountain slopes and rocky crags. Essentially all of the country rock in the vicinity consists of limestones of Carboniferous age. Soils in the valley basin are commonly alkaline, and are formed largely in residuum derived from limestone on mountain summits or slopes, and in alluvium derived from limestone on valley terraces. Strongly acid soils occur, however, in certain bog meadow areas and on the bedrock terraces. Soil drainage conditions range from well-drained soils of the mountain summits and certain low terrace edges to very poorly drained soils of the valley floor where free water is present at the soil surface. A frozen layer occurs within 2 ft of the surface in most of the poorly drained soils. On well-drained sites, factors of land surface age and calcareous soil material have prevented the development of acid conditions necessary for the formation of podzolic soils. The general pattern of soils in the Firth River Basin is one of interfingering areas of Calcareous Regosol, Alpine Rendzina, Tundra, and Bog soils formed under tundra vegetation, and Calcareous Regosol (mor surface) and Calcareous Low Humic Gley soils formed under spruce stands. Vegetation is sparse on the gravel flood plain and on the steep mountain crags surrounding the Firth River Basin. Tundra plant communities cover mountain summits and broad terrace areas within the valley basin, however, and the pattern of these communities is controlled largely by factors of land surface age, soil drainage, snow cover, soil moisture, and microrelief. Scattered spruce and open spruce woodlands occur on south-facing slopes and certain low terrace edges, and local areas of spruce forest grow adjacent to major streams. White spruce trees at least 250 years in age occur in the spruce forest areas, and the impression of these stands is one of present stability with no indication of higher regeneration, replacement, or stand degeneration as yet. A number of factors may be significant in controlling the northern occurrence of spruce in the Firth River Valley. Prevailing southerly winds may allow warmer summer temperatures from the interior of Alaska and Canada to prevail down the Firth River Valley. In the Firth River Basin, spruce occurs on calcareous soils, soils which are reported to be favorable for growth of white spruce at its northern limits. Moreover, the Firth River Valley is unglaciated, and may have constituted a refugium for tree-form conifers during the pleistocene. Within the valley basin itself, however, the direction of site exposure and soil conditions (as they influence the depth to which the soils thaw and their moisture-holding capacity) are important in the survival of spruce. Soils of south-facing slopes, and moderately fine textured soils underlain by gravel at positions near terrace edges or adjacent to streams provide sites where spruce survives. Several lines of evidence suggest that spruce stands growing on suitable soils in the upper Firth River Valley are in equilibrium with the existing climate.

Landscape Relationships of Soils and Vegetation in the Forest-Tundra Ecotone, Upper Firth River Valley, Alaska-Canada

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