A multi-scale assessment of the occurrence of exotic plants on the Olympic Peninsula, Washington

Abstract

. 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 alder flats). Patterns of exotic plant invasions were assessed between watersheds, between riparian and upland areas, among patch types, and within clearcuts. 52 exotic plant species were encountered, accounting for 23% of the flora in each watershed. In both watersheds, exotic species richness was approximately 33% greater in riparian zones than in uplands, and mean number and cover of exotic species were > 50% greater in riparian zones than in uplands. Among landscape patch types, richness and mean number and cover of exotics was highest in young riparian patches, intermediate in clearcuts and riparian forests, and lowest in young and mature forests. The exception to this was Hoh alder flats, 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 disturbance were major factors influencing invasibility. Landscape patch size, position within watershed (distance from patch to human population centers, major highway, or river mouth), and environmental variables (slope, aspect, and elevation) were not important indicators of landscape patch invasibility. Riparian zones facilitated movement of exotic plants through landscapes, but did not appear to act as sources of exotic plants for undisturbed upland areas.