Willamette Forest-Praire Zone Complex
Confined to bottomland and adjacent slopes of the Willamette Valley is a mosaic of forest, woodland, open savanna, and prairie. Prairie and oak savanna at the time of first settlement was maintained by aboriginal burning. Woodlands dominated by Oregon white oak have since been invaded by Douglas-fir and grand fir, with bigleaf maple important on north-facing slopes. Grasslands maintained by grazing include many introduced species and occupy drier sites. Lacing this mosaic of forest and prairie are bands of riparian woodland in which Oregon ash (Fraxinus latifolia), black poplar (Populus trichocarpa), and willow (Salix spp.) are prominent.