Genetic Effects on Long-term Growth in Loblolly Pine

Virtually all loblolly pine (Pinus taeda L.) seedlings currently planted in the southeastern United States have been genetically improved through selection and breeding. When deployed operationally, many of these seedlings are planted in single-family blocks. Optimal deployment of improved planting stock requires an understanding of how trees respond at both the individual-tree and stand level. Three tests were utilized to examine the growth of trees in single-family block plantings.;The first study utilized the SETRES-2 test in North Carolina. Ten open-pollinated families from two very different provenances, Atlantic Coastal Plain and Lost Pines Texas, were grown in single-family block plots to test for growth differences between provenances and among families under severely deficient and optimal nutrition regimes on a nutrient-deficient and dry site. The three-parameter Chapman-Richards function was fit to plot means over time by both provenance and family by nutrition treatment. Models with provenance- or family-specific parameters of the Chapman-Richards were tested for significant improvement over the use of global parameters.;Significant nutrition by provenance interactions were found for stand-level traits of basal area per hectare and volume per hectare, as well as significant family differences. Mean individual tree size was significantly higher in the Atlantic Coastal Plain provenance, but higher mortality in the fertilized treatment resulted in the provenance by nutrition treatment interaction at the stand level.;In general, provenance- or family-specific asymptotic parameters were most significant in accounting for differences in growth over time. Several traits required the use of local asymptotic and rate parameters in the fertilized treatment only. For modeling growth, a growth multiplier would be sufficient for the majority of traits. In some cases, adding a time multiplier could improve model predictions, but would likely be needed one for only a few families.;Linear regressions were fit to selected subsets for various genotypes at the maximum size-density frontier. Significant differences in the intercept parameters were found between the provenances, but no difference existed in the slope parameters for the provenances, indicating a difference in carrying capacity. The Lost Pines Texas provenance had a higher carrying capacity. Tests of the five open-pollinated families of the Atlantic Coastal Plain provenance showed the intercepts differed for the maximum size-density lines of most families, but the slope of only one family differed from the other four families. Results showed that genotypes with superior individual-tree growth may not have the highest stand-level yields, particularly if not managed properly.;The effectiveness of early, indirect selection for breeding programs is dependent upon the association between the selection trait and the traits that are of interest at harvest, and the presence of phenotypic differences among genotypes at harvest. Two tests established by MeadWestvaco in South Carolina with first- and second-generation families were examined to address these concerns.;Significant differences between genotypes planted in single-family blocks were found at all ages tested through 23 and 24 years. Differences in individual-tree characteristics of height, diameter and volume were highly significant at ages 6, 12, 17, 23, and 24. However, in both tests differences in the stand-level traits of volume per hectare and basal area per hectare were significant at age 6, but differences lost significance over time. Although the interaction of family by deployment type was significant for many of the traits examined, it was not for individual-tree volume at age 6, indicate that improvement in selection at age 6 would not be realized by changing progeny tests from mixed-family plantings to single-family blocks.