Interspecific competition in young Douglas-fir plantations of the Oregon coast range Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/x059cc570

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  • Improving vegetation management decisions in Pacific Northwest forests requires a better understanding of the interactions between planted conifers and associated vegetation. A practical index of interspecific competition, and quantitative models predicting conifer performance from such an index are particularly needed. To meet this need, neighborhood models of interspecific competition were developed for individual Douglas-fir (Pseudotsuqa menziesii Mirb. Franco) trees in the central Oregon Coast Range using a two-phase study. In the first phase, a retrospective analysis of two site-preparation experiments was used to develop preliminary models describing the relation between surrounding non-coniferous woody vegetation and the size of 4- to 9-year-old saplings. The importance of interspecific competition relative to other factors in the reforestation environment was examined. The second phase tested and refined these models by examining the growth of planted seedlings under a gradient of woody and herbaceous vegetation for 3 years. A systematic approach to evaluating the influence of the 1) abundance, 2) height, 3) distance, 4) azimuth, and 5) spatial arrangement of woody neighbors was used in both phases. The height and stem diameter growth of planted Douglas-fir were negatively correlated with all neighborhood measures of interspecific competition. Visual estimates of cover for woody species provided the best measure of neighbor abundance among 7 measures tested. Accounting for the height of the cover relative to the tree improved the models. Douglas-fir height growth was influenced primarily by woody neighbors that overtopped the tree, while stem diameter growth was influenced by all neighbors. Woody vegetation was more competitive than herbaceous vegetation. Results suggested that competitive influences on Douglas-fir height growth are one-sided or asymmetric, and more two-sided or symmetric on stem diameter growth. Light and soil water availability in the seedling environment were negatively correlated with neighbor abundance, and consistent with the best measures of interspecific competition predicting Douglas-fir growth. Seedling survival was not influenced by competition from woody or herbaceous vegetation.
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