Graduate Thesis Or Dissertation
 

The structure and dynamics of old-growth Sitka spruce (Picea sitchensis) forest of the Oregon Coast Range

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/k643b414q

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  • The objectives of this study are to: 1) study the variations among stands of old-growth Sitka spruce forest located within a relatively restricted geographical area, 2) determine the relationship between species distribution, vegetational pattern, and such site factors as slope and aspect, and 3) ascertain the abruptness of phytosociological change which might permit the recognition of separate vegetational units. The power function regression model has been used to describe the frequency distribution in size-classes of Sitka spruce and western hemlock on thirty-five plots from three coastal sites in Oregon. The diameter distribution and the reproductive potential of both spruce and hemlock seem to indicate that the forest stands at Cape Lookout, Cascade Head, and Cape Perpetua are in a steady-state condition regardless of the relatively low number of seedlings and saplings. Linear correlation and regression analyses indicate no significant relationship between species diversity and topographic variables, thus emphasizing forest homogeneity within the study area. Other stand characteristics, however, were significantly correlated with some of the topographic variables. For instance, the basal area of hemlock was negatively correlated with slope inclination, whereas, the density of spruce was positively correlated with relative moisture value derived from slope aspect. Pit-and-mound topography or microrelief plays an important role in forest dynamics by influencing species composition and structure. Since pits and mounds are the result of windthrows, wind becomes a very important allogenic factor in controlling cyclic microsuccession in the spruce forest. Clustering analysis was used to group stands into phytosociological units or community types. Stand ordinations using polar ordination, principal components analysis, and reciprocal averaging were employed to relate the spatial distribution of the vegetation units to environmental gradients. Six community types have been identified and described on the basis of the dominant species in each vegetation stratum, topographic preference, and soil physical characteristics. The first ordination axis of all three ordination techniques corresponded to a "complex" slope gradient but the second axis did not correspond to any of the measured or derived site factors.
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