Abstract
Roadside grassland vegetation in a Quercus garryana forest, that is one of the dominant species in west Cascade, on the Oak Creek Wildlife Area, Naches, Washington, USA, was investigated to determine the patterns of human impact on the vegetation along mountain trails. Vegetation and environmental data were collected on forty-eight 50 cm × 50 cm plots. Plot cover ranged from 3 to 100% (1 to 8 species) and most of the cover was from exotic species. In order to explore vegetation patterns the following environmental variables were measured: slope, tree canopy area, bare area, distance from road, and litter thickness. The vegetation data and environmental variables were examined using Canonical Correspondence Analysis (CCA). CCA showed that canopy area played important roles in vegetation development. Litter thickness and soil firmness also seemed to be related to the vegetation pattern. Distance from the road affected plant cover but was not related to canopy area, and litter thickness, suggesting that the distance was not a prime determinant on the vegetation pattern a priori. Species richness was the highest in mid-vegetation cover plots, i.e., 40-60%. An exotic plant, Sisymbrium officinale, increased in frequency with a decrease in vegetation cover, and two exotic plant species, Achillea millefolium and Anthoxanthum odoratum, occurred with S. officinale. S. officinale may be a good indicator for evaluating environmental deterioration. The preservation of canopy cover is of prime importance for nature conservation in forested recreational areas.