Trends in Genetic Parameters with Stand Development and Their Influence on Early Selection for Volume Growth in Loblolly Pine
Analysis was conducted on data from a 15-year-old progeny test of 11 open-pollinated families of loblolly pine. Plots consisted of 49-tree blocks of single families, and measurements were taken annually for the first 8 years and at 10 and 15 years. Three types of analysis were used to explore early selection for a more mature trait. These analyses included: age-related trends in genetic and environmental variances, juvenile-mature genetic correlations, and indirect selection. The age-related trends varied by trait. The trends for additive genetic variance ranged from flat (survival) to exponential (total plot volume) or concave (live crown ratio). However, for all traits, additive genetic variance dipped around age 7 or 8, presumably due to intensifying competition at crown closure. Genetic correlations between early traits (juvenile) and more mature traits (15 years) were generally high. The correlations of height or total plot volume at age 15 with juvenile traits improved rapidly from age 1 to 5. Diameter at breast height, in which there was little or no increase from ages 3 to 5, was the one exception. When correlating earlier height with height or plot volume at age 15, the pattern of correlation coefficients took a noticeable dip at ages 7 or 8. With total plot volume at age 15 (TVOL15) as the goal of selection, response to indirect selection for height and survival at various ages was predicted. The predicted, indirect response was expressed as a ratio of response to direct selection for TVOL15 and termed relative efficiency. The relative efficiency of selection by ages 3, 4, or 5 was 81, 82, and 85%, respectively. Forest Sci. 32:944-959.
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