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Title: Sources of variation in the self-thinning boundary line for three species with varying levels of shade tolerance
Author: Weiskittel, Aaron; Gould, Peter; Temesgen, Hailemariam.;
Source: Forest Science
Publication Series: Scientific Journal (JRNL)
The species self-thinning boundary line has been widely analyzed with a variety of statistical techniques. There is growing evidence that the use of stochastic frontier analysis is an effective statistical means for objectively fitting the species self-thinning boundary line, offering the opportunity to test the influence of additional covariates. Using extensive even-aged coastal Douglas-fir (Pseudotsuga menziesii var. menziesii [Mirb.] Franco), western hemlock (Tsuga heterophylla), and red alder (Alnus rubra Bong.) data sets in the Pacific Northwest, we examined the assumption that the intercept and slope of the species self-thinning boundary line are insensitive to stand and site factors. Likelihood ratio tests indicated that site index, stand origin (natural versus planted), and purity (proportion of basal area in the primary species) significantly influenced the species self-thinning boundary line intercept for each of the species examined in this study. In Douglas-fir and western hemlock, the slope of the self-thinning boundary was also dependent on stand origin as well as site index in the case of Douglas-fir. In addition, the inclusion of site aspect and dryness index marginally improved the red alder model of self-thinning, but neither site soil nor climatic variables improved the models for western hemlock and Douglas-fir. Thus, the species self-thinning boundary line can differ significantly among stands, and its variation is driven by several distinct factors; stochastic frontier analysis is an effective tool for identifying these factors.
Keywords: Stochastic frontier analysis, stand dynamics, self-thinning, Douglas-fir, red alder, western hemlock, Pacific Northwest, stand density index
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Weiskittel, Aaron; Gould, Peter; Temesgen, Hailemariam. 2009. Sources of variation in the self-thinning boundary line for three species with varying levels of shade tolerance. Forest Science. 55(1):84-93.
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