Abstract
Size-density trajectories and self-thinning boundary lines were modeled for two diverse provenances of Pinus taeda L. and five open-pollinated families within each provenance. The data used come from a single site with replicated 64-tree block plots measured through age 25 years. The effects of provenance and family were tested using linear and nonlinear mixed-effects self-thinning models. The drought-hardy Lost Pines of Texas provenance displayed a greater predicted carrying capacity (almost 200 more trees per hectare at reference diameter 25.4 cm) and had a more abrupt approach to the self-thinning line than the widely planted Atlantic Coastal Plain provenance. However, the growth rate of the Lost Pines of Texas provenance was considerably slower and stem form was unacceptable for timber production. Families from the Atlantic Coastal Plain differed in their maximum stand density index predictions (from 1,118 to 1,282 trees per hectare at the reference diameter), suggesting there is an opportunity for artificial selection to change maximum stand density index in this breeding population of loblolly pine. A novel method for predicting the self-thinning boundary line using random effects inherent to the experimental design is presented and recommended for repeated measures data. Experimental design considerations for evaluating genetic differences in self-thinning are discussed.
Study Implications
Genetic improvement of growth rate in forest trees has resulted in large gains in plantation productivity, but the effect on carrying capacity has not been addressed. This study indicated that artificial selection on tolerance to competition in the widely planted Atlantic Coastal Plain provenance of loblolly pine can potentially increase harvest yield without sacrificing growth rate. The drought-hardy Lost Pines of Texas provenance displayed greater carrying capacity but had poor stem form and slow growth. The Lost Pines provenance may be attractive for aboveground carbon sequestration, since it sustained substantially more biomass because of greater maximum stand density and denser wood.