Mountains of the Pacific Northwest, U.S.A.: A Study in Contrasts^∗

Abstract

The mountains of the Pacific Northwest generally evoke an impression of heavy precipitation, lush conifer forests, and high snow-covered volcanic peaks. While this is true for the Cascades, the region encompasses one of the most diverse collections of mountainscapes in the world. Three areas, all within the state of Oregon, have been chosen for study on a comparative basis—Cascades, Wallowas, and Steens. These mountain regions reach similar elevations (3000 m), but vary in mountain mass and are very different geologically, climatically, and biologically. The Cascades are constructed of extrusive volcanics consisting of both shield volcanoes and composite cones, the Wallowas are updomed and underlain by a batholith, and the Steens is fault-block in origin. Climatically, the western Cascades experience heavy precipitation and ameliorated temperatures with a strong marine influence, while the Wallowas receive only half as much precipitation and are much more strongly continental in regime. The Steens is in the most arid region of the Pacific Northwest. Vegetatively, the Cascades are dominated by mesic conifer species while the Wallowas share some of these but also contain several species found principally in the Rocky Mountains. In the Steens, conifers are unimportant and the upper timberline is formed by Populus tremuloides. More detailed investigation of the vegetation occurring above 1800 m in the three areas reveals that the Wallowas have the largest total number of species, followed by the Steens and Cascades. In addition, the Wallowas have the highest percentage of circumpolar and Rocky Mountain species whereas those in the Cascades have a greater affinity for Sierra Nevada and western California species. The Steens contains the heaviest presence of Basin and Range and Southwest Desert species. Within the study areas a number of species are found in all three mountain regions whereas others occur only in one or two of the areas. Reasons for these patterns are given and discussed.