Associations Between Heterozygosity and Growth Rate Variables In Three Western Forest Trees

For each of three species, quaking aspen, ponderosa pine, and lodgepole pine, we determined the relationships between a ranking of heterozygosity of individuals and measures of growth rate. Genetic variation was assayed by starch gel electrophoresis of enzymes. Growth rates were characterized by the mean, standard deviation, logarithm of the variance, and coefficient of variation of annual ring widths measured from cores. In aspen, heterozygosity was associated with high mean growth rate, but in ponderosa and lodgepole pines heterozygosity was not associated with mean growth rate. High levels of heterozygosity were associated with high growth variability in aspen and ponderosa pine, and with low growth variability in lodgepole pine. S frequently observed phenomena suggest strong genetic influence on growth characteristics of forest trees. When trees from different origins are grown in common gardens they show marked differences in growth traits (Squillace and Silen 1962, Weidman 1939). These interpopulation differences are thought to be primarily the result of natural selection forming adaptations to local environmental conditions. Differences in growth charac­ teristics are seen also between trees within a single population. Genetic differences within a population arise from segregation and a variety of mating types; some individuals are produced by selfing, some by outcrossing between related individuals, and some by outcrossing between unrelated individuals. These matings produce progeny with relatively low, intermediate, or high levels of heterozygosity, respectively. One generation of selfing, for example, reduces heterozygosity by one-half relative to outcrossing. In selfed progeny, an increase in homozygo­ sity of deleterious recessive alleles may result in a lack of vigor (Fowler 1965, Sorensen and Miles 1974). In outbred progeny, however, increased vigor may result from heterosis because of a summation of heterozygote superiority over many loci. Heterosis for growth and viability is commonly observed in crosses between strains of domesticated plants and animals. This paper considers the relationships between hetero­ zygosity and growth rate variables within populations of three forest tree species. In this study, we used starch gel electrophoresis to determine heterozygosity of several protein genotypes and estimated mean growth rate from measurements of annual radial growth rings taken from