A MULTISCALE ASSESSMENT OF THE OCCURRENCE OF EXOTIC PLANTS ON THE OLYMPIC PENINSULA, WASHINGTON

Exotic plants were surveyed in 208 plots within the Dungeness and Hoh river watersheds on the Olympic Peninsula, Washington, USA. Landscape patch types included uplands (clearcuts, young and mature forests) and riparian zones (cobble bars, shrub patches, riparian forests. and ald er flats). Patterns of exotic plant invasions were assessed between wa tersheds, between riparian and upland areas, among patch types, and wi thin clearcuts. 52 exotic plant species were encountered, accounting f or 23 % of the flora in each watershed. In both watersheds, exotic spe cies richness was approximately 33 % greater in riparian zones than in uplands, and mean number and cover of exotic species were > 50 % grea ter in riparian zones than in uplands. Among landscape patch types, ri chness and mean number and cover of exotics was highest in young ripar ian patches, intermediate in clearcuts and riparian forests, and lowes t in young and mature forests. The exception to this was Hoh alder fla ts, which had the highest mean cover of exotic plants. Cover of exotic plants peaked in uplands 3 to 7 yr after clearcutting, then decreased with increased canopy closure. Disturbance type and time since distur bance were major factors influencing invasibility. Landscape patch siz e, position within watershed (distance from patch to human population centers, major highway, or fiver mouth), and environmental variables ( slope, aspect, and elevation) were not important indicators of landsca pe patch invasibility. Riparian zones facilitated movement of exotic p lants through landscapes, but did not appear to act as sources of exot ic plants for undisturbed upland areas.