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Vegetation-environment relationships of Subalpine Mountain Hemlock Zone ecosystems. Brooke, Robert Charles

Abstract

Quantitative and qualitative vegetation and environmental data from one-hundred-fourteen sample plots were used to characterize and evaluate ecological relationships and dynamics previously little known for the Subalpine Mountain Hemlock Zone on the southwestern British Columbia mainland. Environmental analyses, presented in detail, include the description of thirty-six kinds of soil representing several major categories. From the analytical data, each sample plot consists of a single set of values representing local historically developed variations and patterns of vegetation, soil, microclimate and topography within the biogeoclimatic zonal concept of Krajina (1962). A combined vegetation-environment synthesis resulted in the characterization of ecosystematic units at several levels of generalization (zone, subzone, order, alliance, association, subassociation and variant) within the following classification scheme: (A) Parkland Subzone i) Chionophilous (Schneetalchen) units, with snow -duration of at least 9 months 1) Vegetation of unstable sheet-wash slopes or hamada-like surfaces 2) Sedge vegetation of semi-terrestrial basin habitats ii) Moderately chionophilous units with snow duration averaging between 8 and 9 months 1) Herbaceous vegetation of stream-edge, spring-line or semi-terrestrial habitats 2) Heath-like or low shrub vegetation of terrestrial mesic to hygric habitats iii) Chionophobous forested units with snow duration averaging about 8 months or less 1) Mesic habitats 2) Moderately dry habitats (B) Forest Subzone i) Forested units lacking a seepage influence or with only a temporary seepage influence 1) Moderately dry lithic habitats or habitats with shallow soils 2) Mesic habitats with shallow to deep soils 3) Hygric habitats with deep soils and a temporary seepage influence ii) Forested units of hygric habitats with a permanent seepage influence 1) Seepage fast-flowing in stream-edge or spring-line habitats 2) Seepage slow-moving or stagnating in depressions or spring-line habitats iii) Non-forested semi-terrestrial moor habitats As orders and alliances include units of lower rank with strong environmental and floristic similarities to those found in other biogeoclimatic zones and subzones and elsewhere, a new dimension is added to the organization of ecosystematic units. The Subalpine Mountain Hemlock Zone coincides with the main distributional area of Tsuga mertensiana -- an area with a cool, snowy forest climate (Dfc after Koppen), podzolization and gleization as the dominant soil-forming processes, and with the development of Humic and Humus Podzol soils with thick, acid accumulations of mycelial ecto-humus on mesic zonal habitats. Discontinuity in the forest cover coinciding with an increasing duration of snow provides a physiognomic, floristic and climatic basis for the recognition of the Parkland and Forest Subzones. Vegetation and soil patterns and relationships in the Parkland Subzone are evidently most influenced by snow duration, soil moisture regime, topography and microclimate, whereas soil moisture regime, land type and topography are important influences in the Forest Subzone. The interplay of compensatory influences may promote the development of similar floristic patterns on different topographic forms on an intra- and interzonal scale. The environmentally integrating influence of snow depth and duration in the Parkland Subzone results in sharp floristic and microclimatic patterns. Vegetation may have a strong autogenic influence on microenvironmental dynamics by hastening snow melt and extending length of the growing season over short distances. Dynamics within the zone favoring successional trends are proceeding at a very slow rate. Climatic changes shortening the duration of snow rather than cumulative autogenic influences would probably contribute most to vegetation changes at high elevations in the subalpine zone.

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