. The Canadian field-naturalist. 50 The Canadian Field-Naturalist Vol. 86 ".^- °MJ^*^'^-"*'^"^ r'-N '-•. Figure 4. A lag gravel ridge summit in the Pelly Lake area (right) with a wet swale in a depression (left center). decumbens, Cassiope tetragona, and Vaccinium Vitis-idaea. The three latter species appear to occupy the areas where micro-variations in the terrain have produced moisture retention great- er than over most of the area, and in such areas Alectoria ochroleuca appears to be more abundant in terms of cover than A. nitidula. These species, for example, are found in th
. The Canadian field-naturalist. 50 The Canadian Field-Naturalist Vol. 86 ".^- °MJ^*^'^-"*'^"^ r'-N '-•. Figure 4. A lag gravel ridge summit in the Pelly Lake area (right) with a wet swale in a depression (left center). decumbens, Cassiope tetragona, and Vaccinium Vitis-idaea. The three latter species appear to occupy the areas where micro-variations in the terrain have produced moisture retention great- er than over most of the area, and in such areas Alectoria ochroleuca appears to be more abundant in terms of cover than A. nitidula. These species, for example, are found in the slight depressions of the polygon net and in the shallow, poorly developed drainage lines. In such areas, too, there appears to be a larger pro- portion of finer material in the soil profile. A sandy surface is not essential for the develop- ment of this community, as is shown by tran- sects #9-17 and #9-18 which were run on a rocky summit of the plain between two major drainage lines, one of which is now occupied by a stream, the other showing evidence of having been active in immediate postglacial time and which is now occupied by small lakes and ponds. A series of beach ridges is also ap- parent here, having been formed as the water in the basin drained and declined to successive- ly lower levels, apparently in response to melt- ing of glacial ice dams blocking the Back River basin (Craig 1964). On these ridges, com- munities of a composition intermediate between the rock fields and dry slopes are to be found. (Stand #9-20). The ecology of the Alectoria is of particular interest since the black coloration must have adaptive and survival value. It was noted that in early morning the mat of Alectoria has absorbed sufficient moisture to have become quite soft and pliable, in contrast to the singularly crisp, brittle state often encountered during the day. It is conceivable that the high absorptivity of radiation by black thalli raises the temperature of the lichen, even un
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