Soil Development in Relation to Vegetation and Surface Age at Glacier Bay, Alaska

Crocker, Robert; Major, Jack

The development of certain soil properties in relation to the broader features of vegetation dynamics and as a function of time has been studied in the extensive recently deglaciated areas of the eastern shores of Glacier Bay, Alaska. The physical nature of glacial till makes it a difficult medium for studies of this type but it has been possible to demonstrate a number of interesting effects of plants on soil formation. In the pioneer stages of succession, as the rate of change in soil properties is dependent upon the actual micro-pattern of plant colonization, the accidents and factors of dispersal and establishment are highly significant. The areal pattern of soil properties is very variable at this stage. The more or less continuous development of the alder thicket leads to greater uniformity. Under alder the reaction of uppermost horizons of the glacial till is reduced from pH 8.0 to less than pH 5.0 within 35-50 years. During the same period calcium carbonate in the fine earth is reduced from initial values of the order of 5% to negligible quantities. Under other species, the rate of change is much less, and on areas devoid of vegetation it is still slower. Within 40-50 years 5-6 kg./sq.m. of above-surface organic residues, 6-7 cm. deep, with pH 4.2-4.6 accumulate under Alnus. The 18 in. (45 cm.) deep mineral-soil profile and forest floor combined have accumulated almost 4.0 kg. of organic carbon and 0.3 kg. of nitrogen per sq.m. beneath 50-year-old alder. The Dryas mat is also capable of accumulating considerable quantities of nitrogen, but Dryas is not nearly as effective as alder. Significant nitrogen accretions also occur in the absence of macroscopic vegetation. The chronosequence as a whole covers the main phases in vegetation changes from the initial colonization of the bare surfaces to the establishment of the spruce (Picea sitchensis) dominated forest. The changes in the mineral-soil properties of bulk density of the fine earth, reaction, organic carbon, calcium carbonate and total nitrogen are traced, and related to broad vegetation change. The amount of forest floor material, its reaction, and its carbon and nitrogen contents were also determined. An absolute loss of nitrogen from the mineral soil-forest floor system is recorded, with the elimination of alder and the emergence of the spruce forest. Carbon-nitrogen ratios for the upper mineral soil and forest floor increase from the order of 12-16 to more than 30 over this stage. Organic carbon increases throughout the whole sequence. Soil reaction (pH) declines rapidly at first but apart from a downward extension of acidity does not change much after the alder thicket stage. The bulk density of the fine earth fraction of the surface horizons appears to fall throughout, but is more significant during the early stages.

Soil Development in Relation to Vegetation and Surface Age at Glacier Bay, Alaska

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