Historical Vegetation of the Willamette Valley, Oregon, circa 1850

Abstract

Land survey data recorded by the General Land Office between 1851 and 1910 were used to map historical vegetation in the Willamette Valley, Oregon. Of the 202 townships included in our study area, 148 (73%) were surveyed between 1851 and 1855. Widespread but dispersed and small-scale Euro-American settlement preceded the surveys by 5 years or more, but census records indicate that farming and logging at the time of survey had affected less than 4 and 0.5 percent of the valley, respectively. Native habitats presumably were grazed to an unknown extent by free-ranging livestock, but not otherwise disturbed by drainage or plowing. Ten vegetation classes were mapped, comprised of 66 subclasses. Prairie covered the largest area, followed in diminishing order by upland forest, savanna, woodland, riparian and wetland forest, water, shrubland, emergent wetlands, unvegetated, and herbaceous upland. Distribution of vegetation classes reflected gradients in precipitation, hydrology, soil moisture, topography, and fire frequency. In general, prairie occupied a central position in the valley surrounded by more or less concentric bands of savanna, woodland, and closed forest. Prairie and savanna dominated the southern and central valley, while forest and woodland were more abundant in the northern portion of the valley. To some extent, woodland may have been savanna under an earlier Native American fire regime. All subclasses except coniferous forest have declined in area since 1850.