17 
The presence or absence of permafrost may have an im- 
portant bearing on plant growth. Permafrost is of general occur- 
rence throughout the cool microthermal zone, as indicated by 
Jenness (1949, Map 1). According to Johnston (1930), the depth 
of permanently frozen ground at Port Nelson averages about 30 
feet, and at Churchill ice has been found in cracks of the bedrock 
at a bore depth of 146 feet. According to Jenness, permafrost 
seems to affect vegetation mainly in two ways. First, where the 
non-frozen “active” layer of soil is thin, shallow-rooted tree 
species such as black spruce, white spruce, larch, and b&lsam 
poplar may grow, but deep-rooted species are excluded. Black 
spruce and larch also seem to be able to form auxiliary roots if 
their bottom roots are killed. Secondly, by providing an im- 
pervious base to subsurface water, permafrost confines drainage 
to the shallow active layer, producing extensive areas of low- 
lying muskeg dominated by the water-tolerant black spruce, with 
larch as a common associate. White spruce and balsam poplar are 
confined to the higher, better-drained sites. As aptly expressed 
by Ritchie (1956), “It is possible that, within the wider limits 
directly imposed by climatic factors, such edaphic factors as the 
presence of permafrost might determine the precise configuration 
of the tree line”. Ritchie (1957) has illustrated diagrammatically 
the relationship between permafrost and topography at Churchill. 
Wet depressions in a forested area east of the Churchill River 
estuary alternate with peat hummocks or small mounds overlying 
vertical extensions of permafrost. It is suggested by Ritchie that 
the formation of an insulating peat layer following colonization of 
the original calcareous glacio-fluvial mineral deposits by meadow 
and shrub phases of the vegetation raised the level of the perenni- 
ally frozen layer and that the resulting poor drainage produced the 
hummock-hollow topography. 
Ideally, in a region where moisture supply is not a limiting 
factor for plant growth, the tree line should extend northward to a 
boundary beyond which thermal efficiency is too low to support 
tree growth. However, it is generally recognized that climates are 
not yet static after the great disturbances of the Ice Age and that 
vegetation boundaries, too, are on the move, following in the wake 
of migrating climatic belts. Griggs (1937) notes that in southwest- 
ern Alaska the 50°F. (10°C.) isotherm for the warmest month stands 
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