The Structure and Origin of the Organic Soil Cover of Amchitka Island, Alaska

Abstract

The topographically low areas of Amchitka and many other islands of the western Aleutian chain are covered with a blanket of moss-lichen-sedge peat. A close relationship exists between the toposequence or drainage position of this peat, its chemical and physical characteristics, and the vegetation it supports. In terms of the U.S. Department of Agriculture's Histosol Supplement to the 7th Approximation, the organic soils of Amchitka can be separated by the relative proportions of the three decomposition states of the peat and the position of the soil in the drainage hierarchy. Those peats, which occupy the areally small, least well-drained positions in the toposequence such as drainage channels, are composed almost entirely of little-decomposed, coarse, fibrous (Fibric) peat several meters thick. The peat is strongly acid and contains >80% organic matter. Field moisture values frequently exceed 2000%. In slightly better drained, topographically higher positions in the toposequence, or on broad interfluve flats, are the areally extensive Blanket bogs. These bogs contain 1 to 2 m of both coarsely fibrous (Fibric) peat and peat of intermediate decomposition state (Hemic). These highly organic and strongly acid peats have an average field moisture of 650%. A thin gelatinous layer is just below the lichen-dominated vegetation. Traction applied to the surface of these soils can cause extensive and probably irreversible damage. It results in a tearing of the vegetation, compaction of the Hemic peat, and ponding. On still better drained, often very steep, positions of the toposequence, soils consist of Hemic peat to a depth of nearly 1 m. Some amount of littledecomposed Fibric peat or highly decomposed Sapric peat may be included, depending on local drainage. These soils are only slightly acid and, because of greater organic decomposition, contain about one-half as much organic matter as the wetter peats. The density of these soils increases and the field moisture