Plant and amphibian assemblages in zero-order basins in the Oregon Coast Range Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/x346d653q

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  • We have a poor understanding of the biotic communities in zero-order basins, drainages extending from ridgelines to the initiation of first-order streams. This study describes baseline plant and amphibian composition in unmanaged zero-order basins in the Oregon Coast Range. Specifically, I studied: i) the spatial distribution and diversity of species, including riparian-associates; and ii) the dominant environmental, spatial and geomorphic gradients in species composition The results of this research have implications for riparian management in steep, forested landscapes in the Pacific Northwest. The composition of tree and shrub layers in zero-order basins was more similar to upland areas than to larger-order riparian areas. Douglas-fir, western hemlock, and western red cedar had highest basal areas; bigleaf maple had highest hardwood density. Convergent areas (areas collecting surface flow) had significantly lower relative densities than surrounding slopes. I identified 138 forest floor herb, shrub, and seedling tree species in zero-order basins. Gradient analysis and empirical modeling suggested that the composition of forest floor plant assemblages was associated with environmental parameters related to geomorphic position and overstory characteristics, such as distance from basin center, basin aspect and overstory relative density. Vegetation types, developed using classification, followed similar environmental patterns. Vegetation types were useful in clarifying environmental gradients acting on groups of plant species, and in delineating the lateral extent of geomorphic and fluvial influences. Riparian-associated vegetation types were mostly restricted to valley floors and lower slope areas. Geomorphic and lateral surfaces were drivers of environmental gradients in zero-order basins; plant species composition followed these geomorphic gradients. Surfaces close to basin center ("inner gorges"), including valley centers, splash zones, and lower slope areas, supported the highest plant species diversity and most distinct plant assemblages. The spatial distribution patterns of amphibian species and assemblages were characterized along longitudinal and lateral gradients, and relative to three geomorphic surfaces (valleys, headmost areas and slopes), and empirical species-habitat models were developed. I identified eight amphibian species in zero-order basins (865 total captures), and analyzed data for six. Headmost areas supported a distinctive upland amphibian assemblage, while valley floors had the highest riparian amphibian diversity. Captures of three riparian species were higher in valley surfaces, within S m of the center of zero-order basins, while captures of three upland species were highest in areas 2-5 m from center. Upland-associated species were captured two times farther from basin centers than riparian-associated species. The best predictors of amphibian captures in empirical models were geomorphic, stability/ disturbance, moisture and overstory parameters. Ordination and indicator species analysis facilitated characterization of amphibian species assemblages within geomorphic surface zones, and suggested spatial compression of habitats and species in zero-order basins, in comparison to broader spatial extents in larger basins. Plant and amphibian assemblages in unmanaged zero-order basins were most similar to each other in their lateral and geomorphic spatial patterning, including the importance of inner gorge areas for support of diverse communities. Plant species were strongly associated with geomorphic position parameters, while amphibians had stronger ties to discrete microhabitat elements such as moisture levels and large substrate. Amphibians appeared to have a more distinctive assemblage in headmost areas than plant species. Results suggest that assemblages of plants and amphibians in these basins are distinct from both larger-order riparian assemblages and from vegetation in surrounding hillslopes. Riparian management designs could take these spatial patterns and habitat associations into account to maintain the ecological integrity of headwater communities. The longitudinal and lateral extents of landscape areas managed to minimize risk to persistence of zero-order basin plant and amphibian assemblages would need to incorporate both fluvial and hillslope disturbance regimes, and microhabitat features associated with them.
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