Quercus garryana communities in the Puget Trough, Washington.

Abstract

Among the legacies of the Vashon Glaciation are Oregon white oak (Quercus garryana), prairie, wetland, and Douglas-fir (Pseudotsuga menziesii) communities arrayed in a mosaic in the Puget Sound Area (PSA). Much of this mosaic has been destroyed. The largest remaining portion is on Fort Lewis Military Reservation. We examined oak communities on Fort Lewis to assess encroachment by exotic plants and by Douglas-fir, to determine amounts of regeneration of oak and other tree species, and to compare oak community diversity with that of nearby Douglas-fir forests and glacial till prairies. For the 22 largest communities, we determined densities of trees, distributions of tree diameters and heights, amounts of regeneration for each tree species, evidence of exogenous disturbances, and covers of vascular understory species. For study sites, we calculated basal areas of tree species, richness and diversity of vascular plants, and percentages of species that were exotic. We constructed species accumulation curves for oak communities, Douglas-fir forests, and prairies. We performed Bray-Curtis and weighted averaging ordinations for 176 sampling plots from the 22 sites. Oak communities were typically more diverse than either Douglas-fir forests or prairies and were transitional in species composition between them. However, oak communities contained numerous exotics, particularly Scot’s broom (Cytisus scoparius) and colonial bentgrass (Agrostis capillaris). Most oak communities contained large-diameter Douglas-firs and other tree species and appeared to be transforming to conifer or conifer/mixed hardwood forests. With succession, exotic species become less prevalent, but the extent and abundance of oaks is diminished. Maintenance of oak communities, and the PSA natural mosaic, may require tree-density management in oak stands, removal of Douglas-fir, development of replacement oak sites, prescribed burning, and mechanical suppression of exotics before burning.

Citation

Thysell, D.R.; Carey, A.B. 2001. Quercus garryana communities in the Puget Trough, Washington. Northwest Science. 75(3): 219-235