Author/Authors :
Spies، Thomas A. نويسنده , , Sheridan، Chris D. نويسنده ,
Abstract :
Zero-order basins, where hillslope topography converges to form drainages, are common in steep, forested landscapes but we know little about their ecological structure. We used indirect gradient analysis to characterize gradients in plant species composition and cluster analysis to characterize groups of plant species associated with specific geomorphic areas. We sampled vegetation within 63 randomly selected zero-order basins in the southern Coast Range of Oregon and collected data on herb, shrub, and overstory tree cover, as well as environmental conditions. Zero-order basin overstories were similar in tree composition to both first-order riparian and upland plant assemblages, but were intermediate in tree density. Shrubs in zero-order basins included both species associated with dry upland conditions and species associated with riparian conditions. Results suggest that understory plant species composition in zero-order basins follows gradients in geomorphic and overstory conditions. Furthermore, it appears that zero-order basins have distinctive geomorphology and fluvial regimes. These distinctive features appear to support both plant species associated with riparian conditions and species associated with upland conditions. Zero-order basins represent the farthest upstream extension of riparian plant species into upland areas, increasing plant species diversity in steep, forested landscapes.