. The Canadian field-naturalist. 1982 MACKENZIE: DUNE-RlDGE FOREST, DELTA MARSH 65 30 E « ?J en 20 10 North Manitoba Maple Green Ash Peach-leaved Willow Sandbar Willow. Ridge Zone Midpoints Figure 3. Mean density of stems of the major tree species per zone of the dune-ridge forest, Delta Marsh, Manitoba, expressed as percent frequency. Each zone comprises approximately 10% of the width of the ridge forest. Zone midpoints correspond to width intervals of approximately 8 m. south zone, rather than by a gradual change in vegeta- tion composition and structure from north to south. The relatively g
. The Canadian field-naturalist. 1982 MACKENZIE: DUNE-RlDGE FOREST, DELTA MARSH 65 30 E « ?J en 20 10 North Manitoba Maple Green Ash Peach-leaved Willow Sandbar Willow. Ridge Zone Midpoints Figure 3. Mean density of stems of the major tree species per zone of the dune-ridge forest, Delta Marsh, Manitoba, expressed as percent frequency. Each zone comprises approximately 10% of the width of the ridge forest. Zone midpoints correspond to width intervals of approximately 8 m. south zone, rather than by a gradual change in vegeta- tion composition and structure from north to south. The relatively greater variability of the south-zone vegetation is due, in part, to the occurrence of clumps of Peach-leaved Willow, conspicuous among the Sandbar Willow, and also to localized stands of Cottonwood, but to a lesser extent. The reason for this clumping is unclear, but may be related to subtle topographic variation. In one segment of the study Table 2. Density of stems of tree species in north and south halves of the dune-ridge forest. Delta Marsh, Manitoba' area, the dune appears to have been levelled off some- what and the sand driven southward. Here, a stand of mature Manitoba Maple and Green Ash extends the full width of the ridge forest. The occurrence of mature trees of these species in the south half of the ridge forest also contributes to the greater heteroge- neity of the south zone. The increased elevation of the south zone resulting from this broad blanket of sand Table 3. Values for soil variables from plots in the north and south halves of the dune-ridge forest, Delta Marsh, Manitoba Signifi- Signifi- North South cance North South cance half half level half half level Species (N = 53) (N = 47) (a = ) Variable (N = 26) (N = 24) (a = ) Manitoba Maple ± ± P = Moisture (%) ± ' ± P< Green Ash ± ± P = Organic matter (%) ± ± pH ± ± P> Sand
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