. The Canadian field-naturalist. . e twelve and twenty (ou nenlly placed thermiilofs w twenty lout irchei temperc Inch wos deteftnined by weekly â ed (torn the 32° , i loyet depths) 16 July August September Figure 3. Geotherms^'- indicating mean weekly soil temperature fluctuations in a poplar community during the 1967 summer period. banks is 60° F. (° C.) (Viereck, 1970) while Tununuk, in the northeastern Mackenzie Delta, has a July mean of ° F. (12° C.) (Abrahamsson, 1966). The two communities studied are similar in age, but tree height and density, and understory specie
. The Canadian field-naturalist. . e twelve and twenty (ou nenlly placed thermiilofs w twenty lout irchei temperc Inch wos deteftnined by weekly â ed (torn the 32° , i loyet depths) 16 July August September Figure 3. Geotherms^'- indicating mean weekly soil temperature fluctuations in a poplar community during the 1967 summer period. banks is 60° F. (° C.) (Viereck, 1970) while Tununuk, in the northeastern Mackenzie Delta, has a July mean of ° F. (12° C.) (Abrahamsson, 1966). The two communities studied are similar in age, but tree height and density, and understory species composition are dissimilar, which at least partially accounts for the differences in soil temperature. Further- more, temperatures were measured over sepa- rate summer periods, which may account for dissimilarities. Perhaps as important, however, the substrate of the Chena River site contains layers of moist silt, and is not sorted as uni- formly as the sandy substrate of the Mackenzie Delta stand. Soil moisture thus averages some 9% higher in the Chena River stand, which could individually account for the differences in soil temperature. The entire surface of the Mackenzie Delta is covered by recent alluvial deposits, thus in- dividual plant habitats are created largely by differences in flooding and sediment processes. Vegetation response to the relatively warm and mesic soil conditions created by the mechanics of point bar construction has been such that the prograding edge of virtually every point bar is colonized by a discrete successional com- munity dominated by the balsam poplar (Fig- ure 4). On many point bars this community functions as the pioneer stage (Figure 2). Even in permafrost areas the balsam poplar is a comparatively deep-rooted species, and it re- quires a soil which is relatively warm (Stoeck- ler, 1952). Balsam poplar also requires good soil aeration for the proper respiratory func- tioning of its roots, thus it benefits if there is a high pr
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