Gradient analysis of vegetation on the north wall of the Columbia River Gorge, Washington
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The Columbia River Gorge represents an ecocline of shifting environmental factors, species populations, and vegetation associations. Combinations of wind, temperature, and precipitation create different environmental regimes from the west to the east. The north wall of the Gorge supported four physiognomic associations: western conifer forest, central mixed forest, central broadleaf forest, and eastern steppe. Each association comprised species distributed individually along the ecocline. Douglas-fir (Pseudotsuga menziesii), western redcedar (Thuja plicata), bigleaf maple (Acer macrophyllum), dull Oregongrape (Berberis nervosa), sword fern (Polystichum munitum), bracken (Pteridium aquilinum), vine maple (Acer circinatum), and red bilberry (Vaccinium parviflorum) were centered in the western conifer forest. Garry oak (Quercus garryana), ponderosa pine (Pinus ponderosa), poison oak (Rhus diversiloba), and snowberry (Symphoricarpos albus) were centered in the central mixed forest. Cheatgrass (Bromus tectorum), northern buckwheat (Eriogonum compositum), Wyeth's lupine (Lupinus polyphyllus var. humicola), western serviceberry (Amelanchier alnifolia), mockorange (Philadelphus lewisii), and Sandberg's bluegrass (Poa sandbergii) were centered in the eastern steppe. Although the centre of distribution for each species was in one of the associations, few were confined to a specific association. The Columbia River Gorge is analogous to a mountain turned on its side, with the cool, moist summit to the west and the warm, dry base to the east. Plant species have migrated both eastward and westward along this ecocline during the lifetime of the Gorge, and each association has combinations of species derived from surrounding regions in Washington and Oregon.