DISTRIBUTION OF HERBS AND SHRUBS IN RELATION TO LANDFORM AND CANOPY COVER IN RIPARIAN FORESTS OF COASTAL OREGON

In this study we characterized the distribution of herb and shrub spec ies relative to landform and forest canopy attributes of streamside fo rests in the moist, conifer-dominated mountains of coastal Oregon. Spe cies cover and environmental data were collected along transects at 94 sites. Species with relatively similar distributions were classified into 10 species groups to identify major patterns in the vegetation. A lthough these patterns were highly variable, ordination and gradient a nalyses indicated that vegetation composition is ordered along a compl ex environmental gradient running from streamside to hillslope. Simila rly, species diversity followed a decreasing trend from active fluvial surfaces to lower hillslopes. Vegetation patterns were related to spe cific landforms, topographic positions, microsites, and coniferous tre e cover within the trans-riparian gradient. We hypothesize that the en vironmental features correlated with these patterns are surrogates for the underlying mechanisms responsible for them. These are (i) hillslo pe processes and associated moisture gradients; (ii) hydrological dist urbance; (iii) tolerance of saturated, valley-floor soils; (iv) shade tolerance; and (v) mineral soil disturbance. This study indicates that valley-floor and lower-slope plant communities are distinct elements in these forest landscapes, supporting the assumption that riparian zo nes require a different management and conservation strategy than upla nd forest communities.