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Variation in growth efficiency of selected western hemlock (Tsuga heterophylla (RAF.) Sarg.) Nelson, Gary Lee

Abstract

Eighty western hemlock trees, in the age range of 15 to 48 years, were selected on three Crown Zellerbach tree farms in northwestern Oregon and southwestern Washington to sample the range of variation in growth efficiency. Growth efficiency is defined as the ability of the crown to produce the maximum amount of wood in relation to its crown surface area. Selection of the trees was based on the crown index ratio (live crown length/ crown width). The objectives of the study were to estimate: 1) the range of variation in growth efficiency of individual trees, 2) how variation in growth efficiency of individual trees could be utilized to maximize volume on a unit area, and 3) the efficiency of narrow crown western hemlock trees as wood producers. Results from regression analysis showed that there was sufficient variation in growth efficiency, with a range of the standardized residuals exceeding at least ± 2.0 standard errors of the estimate for all three regression models. Based on this range it is suggested that selection of ten year basal area increment or gross stem volume for western hemlock in relation to crown surface area or sapwood basal area may be worthwhile. The significance of the variation in growth efficiency becomes apparent when the higher growth efficiency classes are selected. It is estimated that selection of the higher growth efficiency classes rather than the average may increase ten year basal area increment/hectare by 39 to 45 percent. It appears from the trees measured that there is little relationship between growth efficiency and the degree of slenderness of the crown.

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