Available Nitrogen as a Factor Influencing the Occurrence of Sitka Spruce and Western Hemlock Seedlings in the Forests of Southeastern Alaska

Abstract

The region known as Southeastern Alaska lies in the northwestern part of North America between latitudes 54? and 60? N. and longitudes 130 and 138? W. It bounds northern British Columbia on the west, extending from the peaks of the coast range over a belt of mainland and myriads of islands to the open Pacific. The central part of the coast range throughout its length is composed of an immense core of granite and diorite, which is evident also in the central portion of the larger mountainous islands. Bands of metamorphosed strata, often folded, and in places several miles in width, border this core and are in turn bordered at lower elevations by beds of sandstones, shales, limestones, volcanic lavas and tuffs. The climate is moderate with heavy precipitation (table I) well distributed throughout the year; this results in a dense forest growth which covers the mainland and islands from sea-level to approximately 1500 feet elevation. The entire panhandle of Southeastern Alaska, with insignificant exceptions, is included within the Tongass National Forest. The fact that the region under consideration is geologically young partially explains the shallowness of the soils. An extensive survey by the U. S. Forest Service in Southeastern Alaska indicates that less than one-tenth of one per cent of the area may be classed as potential agricultural land. Areas of possible agricultural value are generally small alluvial deposits located at the mouths of streams. In general the soils supporting commercial forest stands may be classified as to parent material either as mainland soils developed from igneous rocks, granites and diorites, or the lower slope soils of the islands which commonly have been derived from limestones. The recently formed glacial moraines have been derived from igneous rocks and usually do not support commercial timber, the lack of large trees on such areas being chiefly due to their extreme youthfulness. Because of the relatively deep character of the soil material glacial moraines are potentially good forest sites. As the soil body develops under the influence of climate and the pioneer vegetation, a 1 An abstract from a dissertation presented to the faculty of Yale University in partial fulfillment of the requirements for the degree of Ph.D.