The genetic consequences of self-thinning in two populations of loblolly pine (Pinus taeda L.)

Although many aspects of forest stand development have been studied, little is known of genetic changes that may accompany it. In this study, I explore the relationship between plant genotype, as assessed by genetic heterozygosity, and the intense competition trees experience during the thinning stage of forest stand development. The questions addressed by the study are (1) does density-dependent thinning act as a selective force for individuals with higher heterozygosity levels, and (2) does stand heterozygosity increase with self-thinning?; I use allozyme analysis to examine population-level changes in tree genetics during stand development in two populations. In the first, I grew loblolly pine seedlings at three density levels and recorded height measurements and mortality over a two-year span. In a second population, I examine a mature stand for which height and diameter data spanning 60 years were available.; In the seedling population, asymmetric competition for light caused the formation of a size hierarchy under high density conditions, which led to mortality of the shorter, light-suppressed individuals. Plant genotype was then related to growth and survivorship. Mean heterozygosity appeared to confer no advantage to height growth or survivorship among loblolly pine seedlings. However, single locus heterozygotes for two genotypes had higher survivorship rates than the alternate homozygous genotypes. This result indicates that heterozygous individuals have a growth and survivorship advantage during stand thinning.; Among trees which survived to become the mature population, diameter was significantly correlated with mean heterozygosity during early stand growth. However, this correlation diminished after the trees were approximately 18 years old. This result indicates that heterozygosity confers a growth advantage to individuals, but is less influential once competitive interactions between trees becomes intense. Although the differences were not statistically significant, mean heterozygosity was highest in mature stands of intermediate densities which suggests that heterozygosity increases due to self-thinning at intermediate levels of intra-specific competition.; The results of this study indicate that asymmetric competition for light, the development of a size hierarchy, and differential mortality can cause changes in heterozygote frequency in plant populations during the thinning stage of forest development.