in) in d.b.h.
Basal area of Oregon white oak stands has ranged from 8.0 to 60.8 m² /ha (35 to 265 ft²/acre), with up to 19.3 m²/ha (84 ft²/acre) additional basal area of other species present. In these and other stands averaging 10 cm (4 in) or more in d.b.h., number of oak stems ranged from 10 to 2,800/ha (4 to 1,133/acre) (1,4,31, 62,69,70,72,75). Volumes for stands on different sites and of different ages are not known. One 80-year-old stand that averaged 160 trees 9 cm (3.6 in) and larger in d.b.h. would yield about 94.5 m³/ha (15 cords/acre) (60).
Rooting Habit- Oregon white oak has a deep taproot and a well-developed lateral system; it is very windfirm even in wet areas. Fast taproot extension and sparse development of laterals are shown by seedlings in the first few weeks of growth. Despite formation of a deep taproot, a high percentage of oak roots are found in upper soil layers. Only 11 percent of the total number of oak roots were found below 76 cm (30 in) in deep Willakenzie soil (38). In contrast, 28 percent of the total Douglas-fir roots in the same soil were found below 76 cm (30 in).
Reaction to Competition- Oregon white oak has been classed as intermediate in tolerance, intolerant, and very intolerant of shade (47). Perhaps such a range of tolerance best describes its status in different situations. Clearly, it is not tolerant of over-topping by Douglas-fir and associated conifers. Dead oaks often found beneath Douglas-fir canopies bear witness that they could not endure the shade (40,72). In some locations and situations, Oregon white oak perpetuates itself, indicating that it can reproduce adequately in its own shade. Branch development on open-grown trees may be very dense. Sparse development of side branches in closed stands provides evidence, however, that it should be classed as intolerant of shade.
Oregon white oak functions as both a seral and a climax species. It is long lived, reproduces from both seeds and sprouts, forms nearly pure stands, and can endure great adversities. In fact, it rates as a climax species because it has greater ability than other species to establish itself and persist where yearly or seasonal precipitation is sparse, where soils are shallow or droughty, or where fire is a repeated natural occurrence.
Geologic and floristic evidence indicates that Oregon white oak associations have evolved through successive eras as components of relatively and pine-oak forests, have repeatedly advanced northward from a locus in the southwestern United States and northwestern Mexico, and have repeatedly retreated as North American climates warmed and cooled (16). The most recent northward advance ended about 6,000 years ago; the more and vegetation types, including oak woodlands, are now being replaced by conifer forest favored by the climatic trend toward cooler and moister conditions.
The seral role of Oregon white oak is illustrated by major changes occurring in the Willamette Valley. Open oak woodlands, savannas dotted with oaks, and grasslands were prominent and widespread before the territory was settled; fires-natural as well as those set by Indians-maintained these open conditions (30,31,36,44,61). Post-settlement exclusion of fire permitted development of closed-canopy white oak stands that are typically of two ages-large spreading trees, now 270 to 330 years old, are scattered among smaller trees of narrow form, 60 to 150 years old (73). Where not restricted by agricultural practices, young oaks continue to encroach into grassland. But, in turn, many oak stands are being invaded and superseded by bigleaf maples or conifers, mainly Douglas-fir (fig. 4). A similar sequence of events is occurring in the northern oak woodland, a distinctive Oregon white oak type in California (5,51,69). Unless steps are taken to reverse present trends, the Oregon white oak type will continue to become a less prominent part of the western flora. A reduction in species diversity will also occur, for open-canopy communities have a more varied composition than closed conifer communities (13).
Damaging A