Old-growth northern white-cedar (Thuja occidentalis L.) stands were examined in the Mid-Boreal Lowlands of west-central Manitoba. These stands represent the NW distribution limit of the species, which occurs disjunct from its continuous range limit in North America. The objectives involved characterizing the vegetation-environmental relationships, forest structure, and old-growth attributes of these stands. The environmental control exerted on these communities was explored with both cluster analysis and canonical correspondence analysis. Three T. occidentalis stand types were identified: xeric, mesic, and hydric — each representing a specific moisture regime and successional pathway. In the xeric sites, T. occidentalis co-occurred with jack pine (Pinus banksiana Lamb.), whereas in the mesic sites it co-occurred with white spruce (Picea glauca (Moench) Voss) and balsam fir (Abies balsamea (L.) Mill.). In the hydric sites, tamarack (Larix lancina (Du Roi) K. Koch) was the companion species. All stand types also had a significant component of black spruce (Picea mariana (Mill.) B.S.P.). The existence of these stands, containing trees upwards of 250 years of age, suggests a long return interval between stand replacing disturbances allowing T. occidentalis to subsist in the landscape. These old-growth T. occidentalis stands differed from one another in composition, tree size distribution, amounts and sizes of coarse woody debris, and snags. The uniqueness of these stands (in space, age, and composition) in central Canada stresses their need for conservation.
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Vol. 32 • No. 3