Graduate Thesis Or Dissertation

 

A new index for describing the structural complexity of forests Public Deposited

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/d504rp19z

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  • Despite the critical ecological roles of structural features in forests, ecologically relevant quantitative measures of structure that allow comparisons among forest stands are still limited. A new index, the structural complexity index (SCI), was developed to characterize and compare the structural complexity of different forests. Point patterns of stem-mapped trees were converted into nearest neighbor triangles (with x, y, and z coordinates) by spatial tessellation, where x and y represent horizontal spatial location of points, and z represents a character of the point like height or diameter. The SCI was defined as the sum of the surface areas of a network of non-overlapping triangles, which form a continuous faceted surface in 3-D. Structural gradients were defined as the maximum size difference among the trees forming a triangle, with greater differences indicating higher structure. To compare structure at different vertical scales, size differences among neighbors that are below a certain magnitude (grain) are considered similar, trees sizes are set equal, giving a lower SCI limit of 1. The SCI was found to be strongly related to tree size variation, tree density, density of emergent canopy trees (> 100 cm dbh), and species mixture in multiple-aged stands. Different structural conditions were simulated by randomly assigning values of the tree size distribution to each tree position while holding the tree positions fixed, allowing an assessment of the maximum possible structural complexity (potential structure) given the observed tree size distribution. Based on the observed or effective structure and the potential structure, the 'maximum structure hypothesis for natural mixed conifer forests" was proposed, stating that in the absence of large-scale, stand replacement disturbances and in the presence of small-scale disturbances associated with mortality and replacement of individual trees, the effective structure is near the potential structure. There was strong evidence in young and mature stands in favor of the maximum structure hypothesis, but stands resembling old-growth structures had effective structures less that their potential. Although rejected in its simplistic form, the maximum structure hypothesis may prove to be a powerful concept in the analysis of forest ecosystems.
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