Quercus garryana communities in the Puget Trough, Washington
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Oak (Quercus garryana) communities at Fort Lewis, Washington, USA, were examined to assess encroachment by exotic plants and by Douglas fir (Pseudotsuga menziesii), to determine the amounts of regeneration of oak and other tree species, and to compare oak community diversity with that of nearby Douglas fir forest and glacial till prairies. Tree densities, distributions of tree diameters and heights, amount of regeneration for each tree species, evidence of exogenous disturbances, and cover of vascular understorey species were determined for the 22 largest communities. Basal areas, species richness and diversity of vascular plants, and percentages of exotic species were calculated for the study sites. Results showed that 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 (Cystisus 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 became less prevalent, but the extent and abundance of oaks is diminished.