The Vegetation of the Willamette Valley
Johannessen, Carl L. , William A. Davenport, Artimus Millet, Steven McWilliams. The Vegetation of the Willamette Valley. Annals of the Association of American Geographers 61 (2), 286–302. 1971.
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ABSTRACT: The vegetation of the Willamette Valley, Oregon, has been modified by man for centuries. Thc earliest white men described the vegetation as extensive prairies maintained by annual fires set by Indians. The cessation of burning in the 1850s allowed expansion of forest lands on the margins of the former prairies. Today some of these forest lands have completed a cycle of growth, logging, and regrowth. Much of the former prairie is now in large-scale grain and grass seed production and is still burned annually. The pasture lands of the Valley are still maintained as open lands with widely scattered oaks. KEY WORDS: historical vegetation, Indian burning, prairies, vegetation change, Willamette Valley.
The vegetation of the Willamette Valley, Oregon, has changed significantly under human influence. The Indians of this area, at the time of contact with white settlers, set prairie fires annually, which created a prairie/open woodlands complex. The new settlers, who increased rapidly in the mid-nineteenth century, forced the Indians to leave. Their practice of annual burning was temporarily discontinued. White settlers brought modifications of the habitat with their livestock and cropping, and more recently, forestry systems…
The fire-tolerant, widely-spaced oak, fir, or pine seeded the so-called openings to form thickets that have grown to dense woodlands and forest. Firs now dominate these woodlands, because the firs are able to continue vertical growth and reach light more effectively than the broadleaf trees. A complete cycle has occurred in some locations. Mature 70 to 100-year-old fir trees have been harvested from formerly open prairie and parkland, and now new crops of seedlings
have invaded the logged-over areas…
In a township transect across the mid-Willamette Valley taken from 1853 survey maps, Habeck listed the vegetation types when the Valley was originally surveyed in the early 1850’s. (J. R. Habeck, “The Original Vegetation of the Mid-Willamette Valley, Oregon,” Northwest Science, Vol. 35 (1916), pp. 65-77.) The types of vegetation Habeck found are the same ones that we use. The term oak openings was widely used in early surveys. The oaks were usually described as being large and solitary, though occasionally they were in clumps. The most complete discussion of the Oregon white oak (Quercus garryana) association has been Thilenius’ rather detailed ecological study of the species. (J. F. Thilenius, “The Quercus Garryana Forests of the Willamette Valley, Oregon,” Ecology, Vol. 49 (1968), pp. 1124-33.)…
Habeck, op. cit., detailed quantitative data about forest regeneration on a single township (J. R. Habeck, Forest Succession in Monmouth Township, Polk County, Oregon since 1850. Montana Academy of Science, Vol. 21 (1962), pp. 7-17). Our field observations further south in the Valley corroborate his findings that: with settlement, the fires were controlled, and large portions of the oak openings have developed into oak forests. There is indication that these oak forests, in the absence of further disturbance, will be replaced by Douglas fir and/or big-leaf maple. He pointed out the validity of using tree habit, open-grown oaks, fir, and maple, as indicators of a former park landscape, and tall spindly habit as an indicator of closely spaced forest regeneration.
Sprague and Hansen reported that fire scars on tree rings and age classes of trees indicate that the vegetation of the western side of the Valley was frequently burned prior to the arrival of pioneer settlers, but burning decreased
sharply after 1848…
Changes in Forested Lands
A frequent misinterpretation of the current forest resources of the area is that the surrounding forests were always as dense as they are now. In woods that have remained essentially unharvested over the last 100 years, in fact, crown densities are several times higher than was the case at contact by white settlers. The most striking difference in density is probably on the slopes of hills facing the open prairie, where firs now dominate the former oak woodlands and park landscape…
The Douglas fir trees, which now make up fifty percent or more of all trees in most of the low elevation forests, have approximately eighty whorls, which means that these trees have had approximately eighty seasons of growth and are predominantly postcontact. Interspersed in this younger forest grow older [contact] trees with large lower limbs characteristic of open stands; the long lower limbs are dead in the shade of the more recent invaders.
Summary
The broad, relatively flat terraces of the Willamette Valley were prairie lands at White contact. The Indians of the Valley burned these prairies and the adjacent woodlands in late summer and early fall, thereby producing plant associations in which woody plants were widely spaced. The closely spaced plants, presumably, had been burned when the round was not moist enough to inhibit the passage of the fires. The dominant prairies of the early days have been occupied by agricultural, urban, and industrial activities, and the former association of prairie grasses has generally disappeared or been highly modified by introduced species. The bottomlands near the rivers have been cleared of most of their forest of black cottonwood, Oregon white ash, bigleaf maple, red alder, and Douglas firs. The loamy bottomland soils have provided rich agricultural harvests since severe flooding has been controlled by dams on the rivers. The poorly drained swales along the yazoo-type streams remain clogged with ash and bordered by oaks. Sedges, rushes, and occasionally willows indicate the waterlogged soils of the former wet prairies.
Dense oak woodlands developed from the park landscape of oak openings that bordered the Valley flats and extended nearly up to the crests of the surrounding hills. After the initial expansion of the Oregon white oak and black oak, Douglas fir was able to invade most of these hillslopes when they were dappled with shade by oak trees. Douglas fir also expanded onto previously grassy hillslopes when they were protected from fire by the colonists. The result was that, after fifty to seventy-five years, lumbering companies were able to harvest Douglas fir forests from the former grasslands and oak openings. With this harvesting of conifers (including much outright destruction of Ponderosa pine), the deciduous broadleaf components again expanded their domain, with oaks on the lower slopes and bigleaf maples on the higher wetter slopes or on shady sites on the north facing slopes.
Higher slopes with east and north facing aspects in the hillls east of Coburg are regenerating coniferous forests of Douglas fir, western hemlock, and red cedar on logged-over areas. A cyclic logging system was imposed on these coniferous forests that had increased in density since cessation of burning. This continues to the present. Some 200 to 300-year-old trees with dead lower branches still stand among the crop of tall but younger trees.
The vegetation existing today is analogous to what existed at the time of the original survey, although the species composition has significant differences of emphasis and extent. The invasion of prairie vegetation by trees and their persistence in the area, despite clearing and grazing, is one more argument that trees can grow on the Valley soils, and that the original prairies were artificially generated and maintained by fire.
