The topography of the permafrost table in the Mackenzie River Delta is remarkably uniform. However, differences in active layer thickness are characteristically found around the stems of white spruce trees where conical depressions occur in the permafrost table. The locally increased active layer thickness appears to result from the interaction of the following factors, all of which cause greater heat diffusivity into the soil near tree stems: (1) some 25% of the gross rainfall is intercepted by individual spruce crowns, which causes a corresponding decrease in soil moisture below the tree; (2) accelerated sediment deposition around spruce stems during the spring flood creates small alluvial deposits that provide a locally better drained site; (3) the growth of insulative mosses around tree stems is also retarded by the increased sediment deposition; and (4) the low-albedo slopes of alluvial deposits surrounding tree stems intercept more solar radiation than the normal flat surfaces away from trees.

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