Phenotypic plasticity of fine root growth increases plant productivity in pine seedlings

BACKGROUND: The plastic response of fine roots to a changing environment is suggested to affect the growth and form of a plant. Here we show that the plasticity of fine root growth may increase plant productivity based on an experiment using young seedlings (14-week old) of loblolly pine. We use two contrasting pine ecotypes, "mesic" and "xeric", to investigate the adaptive significance of such a plastic response. RESULTS: The partitioning of biomass to fine roots is observed to reduce with increased nutrient availability. For the "mesic" ecotype, increased stem biomass as a consequence of more nutrients may be primarily due to reduced fine-root biomass partitioning. For the "xeric" ecotype, the favorable influence of the plasticity of fine root growth on stem growth results from increased allocation of biomass to foliage and decreased allocation to fine roots. An evolutionary genetic analysis indicates that the plasticity of fine root growth is inducible, whereas the plasticity of foliage is constitutive. CONCLUSIONS: Results promise to enhance a fundamental understanding of evolutionary changes of tree architecture under domestication and to design sound silvicultural and breeding measures for improving plant productivity

Carbon sequestration from 40 years of planting genetically improved loblolly pine across the southeast United States

Highly productive, widely deployed genetically improved loblolly pine (Pinus taeda L.) may play an important role in mitigating rising atmospheric CO2 via carbon (C) sequestration. To understand the role of loblolly pine genetic improvement in future C sequestration strategies, we examined the historical (1968-2007) impact of operationally deploying improved families of loblolly pine on productivity and C sequestration across the southeast United States. Since 1977, nearly 100% of loblolly pine plantations in the southeast United States have been established with genetically improved loblolly pine. In recent years, more than 400,000 ha of genetically improved loblolly pine are planted annually. Between 1968 and 2007, we estimate that genetically improved loblolly pine plantations have produced a total of 25.6 billion m3 of stemwood volume and have sequestered 9,865 Tg C in live and dead biomass. Our estimates also indicate that genetic improvement has resulted in an additional 3.7 billion m3 (17% increase) and 1,100 Tg C (13%) of volume production and C sequestration, respectively, relative to volume production and C sequestration with no genetic improvement. We expect that loblolly pine plantation C sequestration will increase as more productive families and clones are deployed and as currently deployed genetic material continues to mature. Together, genetic improvement, intensive silvicultural, and longer rotations aimed at producing long-lived wood products will be important tools for maximizing C sequestration in loblolly pine plantations

Carbon sequestration from 40 years of planting genetically improved loblolly pine across the Southeast United States

Highly productive, widely deployed genetically improved loblolly pine (Pinus taeda L.) may play an important role in mitigating rising atmospheric CO2 via carbon (C) sequestration. To understand the role of loblolly pine genetic improvement in future C sequestration strategies, we examined the historical (1968-2007) impact of operationally deploying improved families of loblolly pine on productivity and C sequestration across the southeast United States. Since 1977, nearly 100% of loblolly pine plantations in the southeast United States have been established with genetically improved loblolly pine. In recent years, more than 400,000 ha of genetically improved loblolly pine are planted annually. Between 1968 and 2007, we estimate that genetically improved loblolly pine plantations have produced a total of 25.6 billion m3 of stemwood volume and have sequestered 9,865 Tg C in live and dead biomass. Our estimates also indicate that genetic improvement has resulted in an additional 3.7 billion m3 (17% increase) and 1,100 Tg C (13%) of volume production and C sequestration, respectively, relative to volume production and C sequestration with no genetic improvement. We expect that loblolly pine plantation C sequestration will increase as more productive families and clones are deployed and as currently deployed genetic material continues to mature. Together, genetic improvement, intensive silvicultural, and longer rotations aimed at producing long-lived wood products will be important tools for maximizing C sequestration in loblolly pine plantations

Productivity differences among loblolly pine genotypes are independent of individual-tree biomass partitioning and growth efficiency

Genetic differences in individual-tree biomass partitioning, growth efficiency, and stem relative growth rate (RGR) could confer intraspecific productivity differences and might strongly influence forest ecosystem carbon storage. We examined the relationship between genotype productivity (stem volume), whole-tree biomass partitioning, growth efficiency (stem wood production per unit leaf area), and stem RGR among nine different loblolly pine (Pinus taeda L.) genotypes from three different genetic groups of contrasting inherent genetic homogeneity: three open-pollinated (half-sib) families, three mass-control pollinated (full-sib) families, and three clonal varieties. We hypothesized that genotype productivity would be positively associated with increased partitioning to stem wood relative to other plant parts, higher stem RGR, and enhanced growth efficiency. After 3 years under plantation conditions, genotypes showed significant differences in stem volume, percent stem wood, percent branch wood, and partitioning to fine roots, yet no differences in stem RGR or growth efficiency. Furthermore, genotypic differences in stem volume were independent of genotypic differences in biomass partitioning, and overall, we found no evidence to support the hypothesized relationships. Even so, the observed variation in biomass partitioning has implications for forest C sequestration as genotypes which partition more biomass to long-lived biomass pools such as stems, may sequester more C. Moreover, the lack of a genetic relationship between stem volume and belowground partitioning suggests that highly productive genotypes may be planted without compromising belowground C storage

Genetic effects on total phenolics, condensed tannins and non-structural carbohydrates in loblolly pine (**Pinus taeda** L.) needles

Carbon allocation to soluble phenolics (total phenolics, proanthocyanidins (PA)) and total non-structural carbohydrates (TNC; starch and soluble sugars) in needles of widely planted, highly productive loblolly pine (Pinus taeda L.) genotypes could impact stand resistance to herbivory, and biogeochemical cycling in the southeastern USA. However, genetic and growth-related effects on loblolly pine needle chemistry are not well characterized. Therefore, we investigated genetic and growth-related effects on foliar concentrations of total phenolics, PA and TNC in two different field studies. The first study contained nine different genotypes representing a range of genetic homogeneity, growing in a 2-year-old plantation on the coastal plain of North Carolina (NC), USA. The second study contained eight clones with different growth potentials planted in a 9-year-old clonal trial replicated at two sites (Georgia (GA) and South Carolina (SC), USA). In the first study (NC), we found no genetic effects on total phenolics, PA and TNC, and there was no relationship between genotype size and foliar biochemistry. In the second study, there were no differences in height growth between sites, but the SC site showed greater diameter (diameter at breast height (DBH)) and volume, most likely due to greater tree mortality (lower stocking) which reduced competition for resources and increased growth of remaining trees. We found a significant site × clone effect for total phenolics with lower productivity clones showing 27–30% higher total phenolic concentrations at the GA site where DBH and volume were lower. In contrast to the predictions of growth–defense theory, clone volume was positively associated with total phenolic concentrations at the higher volume SC site, and PA concentrations at the lower volume GA site. Overall, we found no evidence of a trade-off between genotype size and defense, and genetic potential for improved growth may include increased allocation to some secondary metabolites. These results imply that deployment of more productive loblolly pine genotypes will not reduce stand resistance to herbivory, but increased production of total phenolics and PA associated with higher genotype growth potential could reduce litter decomposition rates and therefore, nutrient availability

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

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

Genetic effects on total phenolics, condensed tannins and nonstructural carbohydrates in loblolly pine (Pinus taeda L.) needles

Carbon allocation to soluble phenolics (total phenolics, proanthocyanidins (PA)) and total non-structural carbohydrates (TNC; starch and soluble sugars) in needles of widely planted, highly productive loblolly pine (Pinus taeda L.) genotypes could impact stand resistance to herbivory, and biogeochemical cycling in the southeastern USA. However, genetic and growth-related effects on loblolly pine needle chemistry are not well characterized. Therefore, we investigated genetic and growth-related effects on foliar concentrations of total phenolics, PA and TNC in two different field studies. The first study contained nine different genotypes representing a range of genetic homogeneity, growing in a 2-year-old plantation on the coastal plain of North Carolina (NC), USA. The second study contained eight clones with different growth potentials planted in a 9-year-old clonal trial replicated at two sites (Georgia (GA) and South Carolina (SC), USA). In the first study (NC), we found no genetic effects on total phenolics, PA and TNC, and there was no relationship between genotype size and foliar biochemistry. In the second study, there were no differences in height growth between sites, but the SC site showed greater diameter (diameter at breast height (DBH)) and volume, most likely due to greater tree mortality (lower stocking) which reduced competition for resources and increased growth of remaining trees. We found a significant site × clone effect for total phenolics with lower productivity clones showing 27–30% higher total phenolic concentrations at the GA site where DBH and volume were lower. In contrast to the predictions of growth–defense theory, clone volume was positively associated with total phenolic concentrations at the higher volume SC site, and PA concentrations at the lower volume GA site. Overall, we found no evidence of a trade-off between genotype size and defense, and genetic potential for improved growth may include increased allocation to some secondary metabolites. These results imply that deployment of more productive loblolly pine genotypes will not reduce stand resistance to herbivory, but increased production of total phenolics and PA associated with higher genotype growth potential could reduce litter decomposition rates and therefore, nutrient availability

Leaf-level gas-exchange uniformity and photosynthetic capacity among loblolly pine (Pinus taeda L.) genotypes of contrasting inherent genetic variation

Variation in leaf-level gas exchange among widely planted genetically improved loblolly pine (Pinus taeda L.) genotypes could impact stand-level water use, carbon assimilation, biomass production, C allocation, ecosystem sustainability and biogeochemical cycling under changing environmental conditions. We examined uniformity in leaf-level light-saturated photosynthesis (Asat), stomatal conductance (gs), and intrinsic water-use efficiency (Asat/gs or δ) among nine loblolly pine genotypes (selected individuals): three clones, three full-sib families and three half-sib families, during the early years of stand development (first 3 years), with each genetic group possessing varying amounts of inherent genetic variation. We also compared light- and CO2-response parameters between genotypes and examined the relationship between genotype productivity, gas exchange and photosynthetic capacity. Within full-sib, half-sib and clonal genotypes, the coefficient of variation (CV) for gas exchange showed no consistent pattern; the CV for gs and δ was similar within clonal (44.3–46.9 and 35.5–38.6%) and half-sib (41.0–49.3 and 36.8–40.9%) genotypes, while full-sibs showed somewhat higher CVs (46.9–56.0 and 40.1–45.4%). In contrast, the CVs for Asat were generally higher within clones. With the exception of δ, differences in gas exchange among genotypes were generally insignificant. Tree volume showed a significant positive correlation with Asat and δ, but the relationship varied by season. Individual-tree volume and genotype volume were positively correlated with needle dark respiration (Rd). Our results suggest that uniformity in leaf-level physiological rates is not consistently related to the amount of genetic variation within a given genotype, and δ, Asat and Rd were the leaf-level physiological parameters that were most consistently related to individual-tree and genotype productivity. An enhanced understanding of molecular and environmental factors that influence physiological variation within and between loblolly pine genotypes may improve assessments of genotype growth potential and sensitivity to global climate change