. The Canadian field-naturalist. 64 The Canadian Field-Naturalist Vol. 96 preceded the nonpioneer species, maple and ash, in colonizing the ridge forest, and had been more suc- cessful to date in establishing permanently, even though it showed a greater drop in density from size classes I to II (84% decrease compared to 60% for maple, 47% for ash), because it showed a greater density of large trees than the other two species. Sandbar Willow is usually considered a shrub rather than a tree, and would not, therefore, be expected to show the same size distribution as the tree species. On the stud
. The Canadian field-naturalist. 64 The Canadian Field-Naturalist Vol. 96 preceded the nonpioneer species, maple and ash, in colonizing the ridge forest, and had been more suc- cessful to date in establishing permanently, even though it showed a greater drop in density from size classes I to II (84% decrease compared to 60% for maple, 47% for ash), because it showed a greater density of large trees than the other two species. Sandbar Willow is usually considered a shrub rather than a tree, and would not, therefore, be expected to show the same size distribution as the tree species. On the study area it seldom exceeded 10 cm dbh, but because its life form here resembled that of a tree rather than that of a shrub, and because of its important role as a nest site for several passerine species, it was included as a tree species in the analyses. Topoedaphic Gradient and Vegetation Heterogeneity For much of the length of the study area, the extreme north edge of the dune ridge, between the foreshore vegetation and the ridge forest proper, is colonized by Sandbar Willow (Figure 2). Here, this species forms a low shrubby band and, in places, a narrow discontinuous zone (< 5 m wide) of dense saplings, too small to have been included in the sam- pling scheme. Where this zone occurs, it has tended to stabilize the edge of the dune formation; elsewhere, serious erosion is evident. Inward from the north edge, the density of Sandbar Willow decreases quickly, being replaced by Peach-leaved Willow and the more shade-tolerant species, Manitoba Maple and Green Ash. These species reach their maximum den- sity in the north half of the ridge forest, and decrease in density toward the south edge, being replaced even- tually by a broad zone of Sandbar Willow that becomes progressively more shrub-like until it disap- pears in the transition to marsh or wet meadow. A plot of the frequency distribution of the stems of these four species across the width of the ridge forest indicates a dif
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