Aspen Bibliography

The forests of the Cypress Hills, Alberta and Saskatchewan, Canada

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American Midland Naturalist





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The forests of the Cypress Hills are outliers of the aspen parklands and coniferous forest zones that border the Northern Great Plains of Canada. Their dominants, Picea glauca, Pinus contorta var. latifolia and PopuIus tremuloides, and their understory components, however, suggest a greater affinity to western cordilleran than to boreal vegetation. To determine the structure of these forests and to examine the pattern of floristic variation within them, 79 stands, incorporating the widest possible range of floristic and habitat combinations, were selected for study. Tree frequency, density and basal area were sampled by the point-centered quarter method, which was also employed wherever possible to measure sapling frequency and density. Where sapling density was too sparse to permit use of this method, 50-ft (15-m) arms-length quadrats were used. Seedlings and understory species were sampled with a 0.5 X 0.5 m quadrat. Seedlings were counted in each quadrat, but shrub and herb species were checked only for frequency. Increment cores were collected in 29 stands for growth layer analysis, and a soil moisture study was conducted in four pine and four aspen stands.

A 2-dimensional ordination based on tree, shrub and herb species demonstrates a vegetational continuum that reflects, primarily, a canopy gradient and that fails to satisfactorily order the understory species. An ordination based on frequency of herb species orders all species satisfactorily and also displays a vegetational continuum. Its first two dimensions emphasize topographic, geographic and physiographic differences; its third dimension re-establishes a canopy gradient comparable to that displayed by the first ordination. Analysis of the ordinated patterns leads to the formulation of a habitat index that incorporates stand density together with an evironmental index based on moisture availability. Use of the habitat index proved instructive in the behavior of both canopy and understory species.

Aspen reproduction in the Cypress Hills is restricted to suckering from root buds; in forested situations it propagates best under its own canopy. Lodgepole pine reproduction in the absence of fire is negligible. White spruce reproduces successfully under both pine and aspen and, in the absence of disturbance, will probably succeed these more pioneer species. It cannot, however, reproduce under its own influence and with human intervention the future of forests in the Cypress Hills is problematical.

Forest dynamics are analyzed in terms of three factor groupings: (1) successional influences (autogenic factors), (2) the physical environment (allogenic factors) and (3) the circumstances that attend forest initiation (alphagenic factors). The term "alphagenic" was coined to emphasize the effects of the circumstances that attend the initiation of forest stands, particularly those with a pyric origin.

The Cypress Hills' forests are immature. Ordination patterns and the behavior of species along the habitat index gradient suggest that alphagenic factors exert primary control over forest structure. Forests will probably expand at the expense of other communities if allowed to do so, but the limits of expansion cannot yet be estimated.