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The Vegetation of Alberta: IV. The Poplar Association and Related Vegetation of Central Alberta

E. H. Moss
Journal of Ecology
Vol. 20, No. 2 (Aug., 1932), pp. 380-415
DOI: 10.2307/2256085
Stable URL: http://www.jstor.org/stable/2256085
Page Count: 40
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The Vegetation of Alberta: IV. The Poplar Association and Related Vegetation of Central Alberta
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Abstract

The poplar association comprises a large part of the natural vegetation of central Alberta. This association is characterised by somewhat over 200 species, of which only about 50 vascular plants and a very few mosses are important constituents. Two consociations are recognised in the association; these are the balsam poplar and aspen consociations, the former generally confined to rather moist situations, the latter occurring in drier places and being much more widely distributed in the region. The composition of these communities is set forth in a table showing constancy values and frequency indices of species. In central Alberta, the balsam poplar rarely exceeds 135 years, the aspen 120 years, in age. On the better sites, these species attain a height of about 27 metres in 70 years. Poplar stands are almost invariably even aged; this is due to the fact that the trees of a stand start as suckers shortly after a complete burning off of the parent stand. Prevailing ages of older stands in the region are 30-38, 50, 70 and 110 years. These ages give a clue to dates of general burning and hence to periods when dry climatic conditions prevailed. On the basis of these considerations, there is proposed a hypothesis concerning 20-year climatic cycles. Northern and western parts of the region have soils of the grey or wooded type, while central and southern parts are characterised by soils that have developed under a grassland type of vegetation. The evidence provided by soils is, therefore, that much of the poplar region was at one time occupied by prairie; however, the probability is that poplars have dominated in most parts of the area for at least a few centuries. The aspen consociation tends to invade the prairie to the south and east. This tendency is counteracted mainly by fires, which favour the prairie association and prevent the aspen from occupying the drier situations. As a consequence there occurs between the poplar region and the prairie a broad transition belt, the parkland, consisting of groves of trees in depressions and on north-facing slopes, and patches of prairie vegetation on south-facing slopes and other dry situations. This transition belt contains within it thousands of narrow tension lines or ecotones, these occurring wherever poplar and prairie communities meet. A typical ecotone of this kind is dominated by grasses and shrubs, of which Symphoricarpos occidentalis is the most common species. Consideration is given to the question of the establishment of aspen in grassland by seed, particular reference being made in this connection to the role of the shrubs, Symphoricarpos and Elaeagnus, and certain animals, especially "gophers" and badgers. A very few species are common to the northern prairie community and the aspen consociation, and of these only two, Agropyron tenerum and A. Richardsoni, are abundant in both communities. A graphical representation is given of the relative abundance of leading grasses and sedges in southern prairie, northern prairie and aspen communities. White spruce is almost certainly the climax species of northern and central parts of the poplar regions, while aspen is probably the climax of the southern part of that region and of the parkland. However, succession to spruce has been effectively retarded by certain factors, especially burning. The white spruce association is characterised by a "feather" moss stratum; otherwise it has much in common with the balsam poplar consociation. Consideration is given to hydroseres and xeroseres of the poplar region, with particular reference to the place occupied by the poplar association in these seres.

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