. The Canadian field-naturalist. 238 The Canadian Field-Naturalist Vol. 86. Figure 5. A new beaver dam impounding water in a meander scroll depression in the Mackenzie Delta. Note drowned poplar stems cut by beaver. Step seven — Utilization of the Adjacent Poplar Community by Beavers The dammed meander scroll depression con- tains some usable vegetation in the form of willow and alder. The poplar community imme- diately adjacent to the swale provides the beavers' greatest food source, however. After beavers occupy such locations for a number of years, the stands show much evidence of selec- ti
. The Canadian field-naturalist. 238 The Canadian Field-Naturalist Vol. 86. Figure 5. A new beaver dam impounding water in a meander scroll depression in the Mackenzie Delta. Note drowned poplar stems cut by beaver. Step seven — Utilization of the Adjacent Poplar Community by Beavers The dammed meander scroll depression con- tains some usable vegetation in the form of willow and alder. The poplar community imme- diately adjacent to the swale provides the beavers' greatest food source, however. After beavers occupy such locations for a number of years, the stands show much evidence of selec- tive use; along the swale periphery, most of the poplar trees are felled (Figure 5), and secondary succession of less palatable alder and willow species occur. Aquatic succession within the impounded swale further alters the previous flora. Beavers normally then abandon the site, after which rapid clonal regeneration of poplar takes place. When the saplings reach sufficient maturity, the point bar may again attract beavers. This type of cyclic use was seen on a number of point bars as evidenced by a present beaver colony as well as very old beaver-cut stems. Summary and Conclusion The sequence of physical and biological events that create a distinct type of beaver habitat in the Mackenzie River Delta was traced. Helicoidal flow during channel shifting initiates sediment sorting and deposition of coarse alluvium on point bars; meander scroll depressions that impound local runoff are also formed during this process. Plant succession on coarse point bar deposits is such that species preferred as food by beavers colonize there. Beavers are thus attracted to point bars after which they may dam the adjacent scroll depres- sions and construct lodges near their food source. Selective use by beavers alters the flora of point bars, causing secondary succession to occur. If the site is occupied for a sufficient length of time, a zootic climax is established whereby alder and willow specie
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