Quercus garryana communities in the Puget Trough, Washington

Among the legacies of the Vashon Glaciation are Oregon white oak (Quercus g arryana), prairie, wetland, and Douglas-fir (Pseudotsuga menziesii) communi ties 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 Militar y Reservation. We examined oak communities on Fort Lewis to assess encroach ment by exotic plants and by Douglas-fir, to determine amounts of regenerat ion of oak and other tree species, and to compare oak community diversity w ith that of nearby Douglas-fir forests and glacial till prairies. For the 2 2 largest communities, we determined densities of trees, distributions of t ree diameters and heights, amounts of regeneration for each tree species, e vidence of exogenous disturbances, and covers of vascular understory specie s. 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-fi r forests, and prairies. We performed Bray-Curtis and weighted averaging or dinations for 176 sampling plots from the 22 sites. Oak communities were ty pically 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) a nd colonial bentgrass (Agrostis capillaris). Most oak communities contained large-diameter Douglas-firs and other tree species and appeared to be tran sforming to conifer or conifer/mixed hardwood forests. With succession, exo tic 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, dev elopment of replacement oak sites, prescribed burning, and mechanical suppr ession of exotics before burning.