Plant associations in the Subalpine Mountain Hemlock Zone in Southern British Columbia

The vegetation of the Subalpine Mountain Hemlock Zone was studied on 130 sample plots with analytic and synthetic methods of the Zürich-Montpellier school of phytosociology. This thesis describes 14 plant associations, predominantly of vascular plants, from two altitudinal subzones of the Subalpine Zone. Published radiosonde temperature data, in combination with thermograph data from Mount Seymour, were used to characterize the climate of the zone. In winter, which is the season of maximum precipitation, the freezing isotherm most frequently occurs at altitudes near the lower limit of mountain hemlock. A climatic result is the sharp increase in snow accumulation and duration near the lower limit of this species; an ecological result is the relatively sharp boundary between the Subalpine Mountain Hemlock Zone and the Coastal Western Hemlock Zone. Thus, the lower limit of the zone is indicated by the presence of mountain hemlock as a component of the forests and the upper limit is marked by the altitudinal "tree limit" of mountain hemlock. The zonal limits were placed at 3000 and 5000 feet in the Seymour - Grouse - Hollyburn - Cathedral Mountain area near the Strait of Georgia, and at 3700 and 5500 feet in the Paul Ridge - Diamond Head portion of Garibaldi Park. The lowest 600 to 800 feet of the zone are covered with continuous forest of mountain hemlock, amabilis fir, yellow cedar and western hemlock. This continuous forest is designated as the lower subzone. The upper boundaries of the zone, in contrast to the lower, are irregular as a result of topographic influences on snow duration. Snow accumulation increases with altitude so that near the tree limit mountain hemlock can grow only on prominences or ridges where snow duration is less. Most early stages of vegetation appear to develop towards the Phyllodoce - Cassiope association in the upper subzone. At altitudes of approximately 5000 feet and over (Alpine Zone), this association occupies mesic habitats where the relief is flat or convex and without seepage. In contrast, this same association occupies concave topographic positions, with temporary seepage, in the Subalpine Zone. Snow duration is approximately the same on this association in both bioclimatic zones. However, because of its occurrence on two distinct topographies it is chionophobous in the Alpine Zone but moderately chionophilous in the Subalpine Zone, when considered in relation to adjacent associations. The Vaccinium membranaceum - Rhododendron association is the most successionally advanced in the upper subzone. Near its lower limit, this association occupies mesic habitats but at its upper limit in the Alpine Zone it becomes a 'topographic climax' restricted to warmer exposures or to ridges between areas of Phyllodoce and Cassiope. In the lower subzone, the Vaccinium alaskaense association is successionally most advanced. Even if a distinct climatic "climax" association is recognized for the lower subzone, there are no tree species limited specifically to this altitudinal level. Both subzones are unified by the same tree species into one Subalpine Zone. Within this altitudinal belt most zonal features of the vegetation are related to the intensity, quantity and duration of snow.