In the Willamette Valley–Puget Trough–Georgia Basin ecoregion of the North American Pacific Northwest, there has been widespread encroachment of Douglas-fir (Pseudotsuga menziesii) upon Oregon white oak (Quercus garryana) savanna and woodland stands that were historically maintained by frequent anthropogenic fire. Restoration of these stands requires removal of the Douglas-fir overstory, although there is little information on how oak trees that have been suppressed for decades respond to release. Our objective was to evaluate the 10-year response of oak trees to three release treatments from overtopping Douglas-fir in a study replicated at four western Washington sites. Treatments were: full release (removal of all Douglas-fir within a radius equal to the oak’s height), half release (removal of Douglasfir within a half-height radius), and thin only (stand-level commercial thinning of the Douglas-fir overstory; oaks not targeted for release). Periodic diameter growth of oak trees in the full-release treatment was significantly greater (by as much as 243%) than that in the other treatments and increased with time since treatment. Oak height and crown area growth rates were not influenced by treatment, but larger pre-treatment crown size was associated with greater post-treatment diameter growth and acorn production. Full release increased acorn production compared to thin only in years when region-wide production was moderate to high (5 of 9 years); half release increased production relative to thin only in 2 of 9 years, but to a lesser degree. Height growth of residual Douglas-fir trees was greater than that of oak trees in all treatments. The increasing post-treatment growth rate of the fully released oak trees and their increased acorn production indicate the capacity of long-suppressed Oregon white oak to rapidly recover following complete removal of a conifer overstory. The oak trees showed no negative effects following the dramatic change in environment associated with this treatment. Thus, a single-entry, complete release from overtopping conifers was effective in restoring overstory structure and composition, which is an initial step in restoration of invaded oak woodland or savanna.
Cascadia Prairie Oak Partnership
Providing coordination for the technical prairie community in the Willamette Valley, Puget Trough, Georgia Basin (WPG) ecoregion and beyond.