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An ecological study of the Sitka spruce forest on the west coast of Vancouver Island Cordes, Lawrence David

Abstract

A forest consisting of almost pure stands of Sitka spruce (Picea sitchensis) occurs in a narrow band varying in width from several feet to approximately 600 feet along the west coast of Vancouver Island. In this area spruce replaces western hemlock (Tsuga heterophylla) and western redcedar (Thuja plicata), the dominant species of the region. A comparison of the physical environment of the coastal Sitka spruce forest with that of the hemlock - redcedar forest revealed that ocean spray intensity was the major difference between, the two. Monitoring of ocean spray over a 17 month period indicated that incoming spray is highest during the fall and winter when major storms are frequent in the northern Pacific Ocean. However, the effects of ocean spray on vegetation were at a maximum during late spring and early summer when infrequent precipitation allowed salts to accumulate on the foliage. Young needles of coniferous species were killed by spray during this period. Sitka spruce was found to be much more tolerant than either western hemlock or western redcedar. During the spring and summer the amount of incoming ocean spray varied considerably from site to site depending on the configuration of the coastline, the type of shoreline and the degree of protection provided by vegetation in a seaward direction. The width of the Sitka spruce forest as well as canopy form were closely related to the amounts of incoming ocean spray. Measurements of salt concentrations in throughfall precipitation indicated that significant amounts of sodium, magnesium, calcium and potassium are added to the soil beneath the Sitka spruce forest. These additions improve the nutrient status of the soil but do not accumulate to high levels due to the high precipitation of the region. Retention of these cations in the soil varies considerably with calcium being the highest and sodium the lowest. Studies of the Sitka spruce forest have resulted in the delineation and characterization of seven ecosystematic units on the basis of vegetation, topography, parent material and historical factors. Comparisons of the environmental components of these units strongly suggest ocean spray intensity and water relations to be the most influential factors. An investigation of successional dynamics revealed that Sitka spruce was the dominant climax tree species in most of the forest type, although western hemlock shares this role with spruce in one type. Successional rates and stability of the different forest types are dependent on the geomorphic processes responsible for beach plain expansion and the degree of protection from incoming ocean spray provided by vegetation on the seaward side. Combining the forest types of the alluvial Sitka spruce forest, which were also studied, with those of the coastal Sitka spruce forest, a hierarchic system of classification consisting of plant associations, alliances and orders is proposed for the Sitka spruce forest of the Coastal Western Hemlock Zone on Vancouver Island.

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