Genetic effects on stand-level uniformity and above- and belowground dry mass production in juvenile loblolly pine

Genetic differences in stand-level above- and belowground dry mass production in loblolly pine (Pinus taeda L.) may influence southern pine plantation productivity, sustainability and carbon (C) sequestration. Furthermore, deployment of more or less genetically homogeneous individuals could impact stand uniformity and ecosystem processes. In this study, we aimed to compare stand uniformity and-above and belowground dry mass production among loblolly pine genotypes of contrasting inherent genetic homogeneity. We hypothesized that stand-level uniformity would increase as within-genotype inherent genetic variation decreased (open-pollinated (half-sib) > full-sib > clone). To examine genetic effects on stand uniformity and productivity, we grew ten different genotypes (three open-pollinated families, three full-sib families, three clones, and one seed orchard mix variety) in a plantation setting for 4 years, at two different planting densities (~539 and 1077 trees ha-1), and used allometric relationships to estimate standing dry mass and annual dry mass production. In the low planting density treatment, age 3 total standing dry mass of the most productive genotype (5824 kg ha-1) was 82% higher than that of the least productive genotype (3207 kg ha-1). In the high planting density treatment, age 3 total standing dry mass of the most productive genotype (11,393 kg ha-1) was 110% higher than that of the least productive genotype (5427 kg ha-1). Genetic differences in annual dry mass production were of a similar magnitude with peak rates during the third year as high as 4221 and 8198 kg ha-1 yr-1 in the low and high planting density treatments, respectively. More genetically homogeneous genotypes did not show greater stand-level uniformity under operational management conditions. Over time, genotypes showed no consistent differences in the coefficient of variation (CV) for ground-level diameter; however, two full-sib and two half-sib families showed significantly lower CV’s for total tree height than all three clones. Moreover, genotypes with lower CV’s for height growth displayed greater stand-level dry mass production which supports the premise that greater stand uniformity will lead to enhanced productivity. Since uniformity and stand-level productivity of loblolly pine clones will be principally governed by environmental heterogeneity, our results highlight the need for silvicultural prescriptions that maximize site uniformity. In addition, our results demonstrate how the deployment of highly productive loblolly pine genotypes may provide a means of enhancing southern pine ecosystem sustainability by sequestering C in both harvestable aboveground biomass and woody belowground biomass.