The Quercus Garryana Forests of the Willamette Valley, Oregon

Quercus garryana forests, prominent at low elevations throughout the Willamette Valley, Oregon, have developed from oak savanna subsequent to settlement of the valley in the mid—nineteenth century. Interruption of the ground fires that were common in the pre—settlement environment probably caused the change. The understory of the oak forest is dominated by shrubs, and well—defined strata are present. Four plant communities occur: (1) Quercus garryana/Corylus cornuta var. californica/Polystichum munitum (most mesic); (2) Quercus garryana/Prunus avium/Symphoricarpos albus; (3) Quercus garryana/Amelanchier alnifolia/Symphoricarpos albus; and (4) Quercus garryana/Rhus diversiloba (most xeric). All are in seral condition because of their relatively recent development and because they have been disturbed throughout their existence by man's activities. The soils supporting the oak forest are generally deep and well drained and have developed profiles with illuvial horizons and acidic reaction. They are derived from sedimentary and basic igneous rocks and old valley—filling alluvium. Seven established soil series are present: Steiwer, Carlton, Peavine, Nekia, Dixonville, Olympic, and Amity. The Steiwer series and its catenary associate, Carlton, are the most common soils.