Height-Diameter Relationships for Different Genetic Planting Stock of Loblolly Pine at Age 6

Data from nine genetic entries of labially pine (Pinus taeda L) at two different spacings (1.5 x 6.1 m, 1076 trees ha(-1) and 3.0 x 6.1 m, 538 trees ha(-1)) were analyzed to evaluate the effect of genetic entry and spacing on the asymptote and slope parameters of commonly used height-diameter functions. The genetic entries included one seed orchard mix, three open-pollinated (half-sib) families, three full-sib families, and two different clones. Genetic entry affected both the asymptote and the slope of the curves, which indicates that growth modeling may need to consider both genetic differences in height-age relationships and stem form differences to make sound predictions. In addition, there might be a stronger need for more genetically specialized models as genetically more homogeneous stands are planted. The primary reason for the significant differences in the asymptote and slope parameters was the clones. When the clones were excluded from the analyses, no significant effect for the asymptote or the slope parameter was found. More studies are needed to determine whether these findings are typical. Furthermore, spacing affected the slope parameter of the curves, indicating that the trees had lower height/diameter ratios at wider spacings. No significant interaction between genetic entry and spacing was found for the asymptote or the slope parameter.