Gradient analysis of west central Vancouver Island forests

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Abstract
The objective of this study is to describe the structure, composition, and ecological relationships of old-growth forests of west central Vancouver Island. Data were obtained by sampling 172 plots, at elevations up to 1000 m, located within 13 drainage areas. Relationships between vegetation and environmental variation were examined using indirect and direct gradient analysis. Successive reciprocal averaging ordinations using data from the tree, sapling, seedling, shrub, herb, and bryophyte – lichen strata led to the recognition of six vegetation groups (floodplain, subalpine, Pinus contorta, Pseudotsuga, Thuja, Abies) and 23 community types. Vegetation groups are differentiated along climatic and soil parent material gradients. The floodplain group occurs on alluvial terraces and the Pinus contorta group on rock outcrops. The vegetation of the Pseudotsuga group, dominant inland and influenced by fire disturbance, appears to respond to gradients of elevation and soil moisture. The Thuja group is found only near the coast, and its vegetation varies along gradients of soil nutrients and elevation, soil moisture having little effect. Wind is the strongest disturbance factor in this group. The vegetation patterns of the Abies group are correlated with elevation and soil moisture. Most community types of this group are associated with cool microclimates.