Mechanisms of age‐related changes in forest production: the influence of physiological and successional changes

Net primary production (NPP) declines as forests age, but the causal role of decreased gross primary production (GPP), or increased autotrophic respiration (R_a) is still a matter of debate. This uncertainty complicates predicted responses to future climate, as higher atmospheric carbon dioxide (CO₂) concentrations may amplify the carbon (C)‐sink in temperate forests if GPP controls the decline in NPP, but increased temperatures may decrease this C‐sink if R_a controls the NPP decline. We quantified NPP in forests dominated by loblolly pine (Pinus taeda) in North Carolina, USA that varied from 14 to 115 years old. We used a sap‐flow approach to quantify summer canopy photosynthesis by pines and later‐successional hardwood trees, and measured wood CO₂ efflux to investigate age‐related changes in pine R_a. Despite increasing production by later‐successional hardwoods, an 80% decline in pine NPP caused ecosystem NPP to decline with age by ～40%. The decline in pine NPP was explained by reduced stomatal conductance and photosynthesis, supporting the hypothesis that increasing hydraulic limitation and declining GPP drove the age‐related decline of NPP in this species. The difference between GPP and NPP indicated that pine R_a also declined with age; this was corroborated by measurements of reduced stem CO₂ efflux with increasing age. These results indicate that C cycling in these successional temperate forests is controlled by C input from GPP, and elements of global change that increase GPP may increase the C‐sink in aging warm‐temperate pine forests.     

Induction of somatic embryogenesis in loblolly pine (Pinus taeda L.)

Summary Several factors that may affect induction of somatic embryogenesis in loblolly pine (Pinus taeda L.) were investigated in 1994 and 1995. Megagametophytes containing immature zygotic embryos were excised from seeds as explants. Potassium chloride, silver nitrate, myo-inositol, coconut water, or polyamine was added to the control media (U.S. patent no. 5,036,007) to determine the effects of each single ingredient or their combinations on the initiation of embryogenic tissue. Supplements of myo-inositol at 22.2 mM resulted in increases in frequencies of cell mass extrusion and proliferation compared with the control media in consecutive years. Addition of silver nitrate showed the potential to promote initiation of embryogenic culture. The combination of 10 mM potassium with 29.4 μM silver nitrate achieved the highest frequencies in both extrusion and proliferation of embryogenic tissue. The combination of silver nitrate at 29.4 μM with addition of myo-inositol at 11.1 or 22.2 mM achieved a higher conversion rate from extrusion to proliferation. Polyamine did not significantly affect the induction of somatic embryogenesis, but coconut water was inhibitory. Published with approval of the Director of Arkansas Agricultural Experimental Station.     

Estimation of gene flow into two seed orchards of loblolly pine (Pinus taeda L.)

Summary Multilocus allozyme markers, which were present in background stands but not in two seed orchards, enabled estimation of gene flow into two seed orchards of loblolly pine. Estimates averaged 36% in two orchards over three years. When pollen pool frequencies in the background stands were used rather than gene frequencies in the non-orchard trees, the estimates averaged 60%. In one year, trees in the row on the edge of one orchard received significantly more pollen from background stands. Estimates of gene flow between the two 2-ha orchards average 10%. Implications of the observed levels of gene flow are discussed in terms of population genetics and seed orchard management.Scientific Contribution No. 1285 from the New Hampshire Agricultural Experiment Station. This work was part of McIntire-Stennis Project 11 Allozyme studies of coniferous forest tree populations     

Polyethylene glycol-promoted development of somatic embryos in loblolly pine (Pinus taeda L.)

Summary The effect of polyethylene glycol (PEG) combined with abscisic acid (ABA) and KCl on somatic embryo development in loblolly pine was investigated. Two embryogenic cell lines, which had not produced cotyledonary stage embryos using previously published methods, were employed in this study. As a maturation medium, basal medium was supplemented with 0 to 10% PEG (MW 3350), 10 to 40 mg/l (37.8 to 151.3 μM) ABA, 0 or 10 mM KCl, 1.5 g/l activated charcoal, 30 g/l sucrose, and 6 g/l agar. Without PEG in the maturation medium, most somatic embryos at stage 1 could not mature further. Embryogenic tissues on the maturation medium with 5 to 7.5% PEG consistently produced stage 2 and stage 3 somatic embryos. PEG at 10% significantly decreased the number of stage 2 and 3 embryos compared to PEG at 5 to 7.5% ABA at 40 mg/l (151.3 μM), combined with 7.5% PEG and 10 mM KCl, gave the maximum number of stage 3 embryos in both cell lines. ABA at 10 mg/l (37.8 μM) induced an over-proliferation of embryogenic tissues and generally failed to produce mature embryos. KCl also significantly enhanced initial stage embryo formation and subsequent embryo maturation.     

The effects of UV-B radiation on epidermal anatomy in loblolly pine (Pinus taeda L.) and Scots pine (Pinus sylvestris L.)

The effects of enhanced UV‐B radiation on the needle anatomy of loblolly pine (Pinus taeda L.) and Scots pine (Pinus sylvestris L.) were studied in the field under supplemental UV‐B radiation supplied by a modulated irradiation system. The supplemental UV‐B levels were designed to simulate either a 16 or 25% loss of stratospheric ozone over College Park, Maryland. Enhanced UV‐B radiation caused different responses in these two species. The needles of loblolly pine had larger amounts of tannin in the lumen of epidermal cells and more wall‐bound phenolics in the outer epidermal walls of UV‐B‐treated needles, whereas the most pronounced effect on Scots pine needles was increased cutinization. In both species, the outer epidermal cell walls thickened and the needle cross‐sectional and mesophyll areas decreased (statistically significantly only in Scots pine). This suggests that more carbon may have been allocated to the protection mechanisms at the expense of photosynthetic area. The difference in response between these species suggests that the response to UV‐B radiation is not mediated by a single mechanism and that no generalization with regard to the effects of UV‐B on conifers can be made.     

High-throughput genotyping and mapping of single nucleotide polymorphisms in loblolly pine (Pinus taeda L.)

The development and application of genomic tools to loblolly pine (Pinus taeda L.) offer promising insights into the organization and structure of conifer genomes. The application of a high-throughput genotyping assay across diverse forest tree species, however, is currently limited taxonomically. This is despite the ongoing development of genome-scale projects aiming at the construction of expressed sequence tag (EST) libraries and the resequencing of EST-derived unigenes for a diverse array of forest tree species. In this paper, we report on the application of Illumina’s high-throughput GoldenGate™ SNP genotyping assay to a loblolly pine mapping population. Single nucleotide polymorphisms (SNPs) were identified through resequencing of previously identified wood quality, drought tolerance, and disease resistance candidate genes prior to genotyping. From that effort, a 384 multiplexed SNP assay was developed for high-throughput genotyping. Approximately 67% of the 384 SNPs queried converted into high-quality genotypes for the 48 progeny samples. Of those 257 successfully genotyped SNPs, 70 were segregating within the mapping population. A total of 27 candidate genes were subsequently mapped onto the existing loblolly pine consensus map, which consists of 12 linkage groups spanning a total map distance of 1,227.6 cM. The ability of SNPs to be mapped to the same position as fragment-based markers previously developed within the same candidate genes, as well as the pivotal role that SNPs currently play in the dissection of complex phenotypic traits, illustrate the usefulness of high-throughput SNP genotyping technologies to the continued development of pine genomics. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Polyethylene glycol and maltose enhance somatic embryo maturation in loblolly pine (Pinus taeda L.)

Summary A culture medium that can efficiently produce mature somatic embryos was developed for loblolly pine (Pinus taeda L.). The medium contained maltose as a carbohydrate source and polyethylene glycol as an osmoticum. This medium formulation significantly enhanced embryo maturation efficiency compared to a medium with only maltose, or with sucrose combined with polyethylene glycol. Maltose at 4% and polyethylene glycol at 6% resulted in the highest embryo maturation efficiency; an average of around 100 cotyledonary embryos were produced from 1 g of embryogenic tissue. These results suggested that previous ineffective embryo maturation in loblolly pine may be due to the lack of the proper combination of osmoticum and carbohydrate source. This embryo maturation method also improved morphology of cotyledonary embryos of loblolly pine.     

Tree height and age-related decline in growth in Scots pine (<Emphasis Type="Italic">Pinus sylvestris</Emphasis> L.)

Growth and seasonal water use was measured amongst trees growing in an old growth Scots pine forest in the Scottish Highlands. Three sites which differed in their recent management history and contained old and naturally regenerated young trees growing together were monitored in the field. Our results showed a clear decrease in growth efficiency with age, from values of around 0.25 kg m⁻² leaves year⁻¹ in approximately 25-year-old trees to less than 0.1 kg m⁻² leaves year⁻¹ in trees over 200 years old. When the old trees in one of the field sites were released from competition by thinning, their growth efficiency reverted to that of coexisting young trees, indicating that the decline in growth was reversible. This is consistent with the results of a parallel study showing that cambial age had no effect on the physiology or growth of grafted seedlings originating from the same population studied here (Mencuccini et al. 2005). Our detailed study of tree water use in the field showed an overall decrease in whole-tree hydraulic conductance and stomatal canopy conductance with tree height in the unthinned stands, in agreement with the hydraulic limitation hypothesis. However, the effect of this reduction in hydraulic efficiency on growth was comparatively small, and old trees also showed consistently lower nitrogen concentrations in needles, suggesting that hydraulic and nutritional factors combined to produce the decline in growth efficiency with age observed in the studied populations.     

Agrobacterium-mediated transformation and assessment of factors influ-encing transgene expression in loblolly pine (Pinus taeda L.)

INTRODUCTIONRecombinant DNA technology is a powerful toolfor the introduction of foreign genes into longlivedperennials and fOr fundamelltal studies of gene expression. Using such techniques, we can overcomethe difficulties associated with the breeding of a long-lived perennial. At present, although considerablereseaxch effort has been devoted to the genetic en-gineering of fOrest trees, it has lagged behind ad-vances made in herbaceous crops due both to eco-nomics and the recalcitrant n…     

Correlation between infection by ophiostomatoid fungi and the presence of subterranean termites in loblolly pine (Pinus taeda L.) roots

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Promotion of hypocotyl elongation in loblolly pine (Pinus taeda L.) by indole-3-acetic acid

Elongation of excised loblolly pine ( Pinus taeda L.) hypocotyls was promoted by indole-3-acetic acid and the fungal metabolite, fusicoccin. Gibberellic acid, kinetin, zeatin, or zeatin-riboside were either without effect or promoted elongation only slightly. The most auxin-responsive tissue was just below the cotyledonary node, and elongation was confined to sections excised from the upper 2 cm of the hypocotyl. Indole-3-acetic acid induced elongation rates in the hypocotyl sections equal to those of intact hypocotyls when the sections were excised from young seedlings. Elongation rates decreased in intact hypocotyls before the excised tissues lost responsiveness to the auxin. Hypocotyl elongation in loblolly pine is discussed in relation to hypocotyl elongation in angiosperm species.     

Fine-root respiration in a loblolly pine (Pinus taeda L.) forest exposed to elevated CO2 and N fertilization

Forest ecosystems release large amounts of carbon to the atmosphere from fine-root respiration (R(r)), but the control of this flux and its temperature sensitivity (Q(10)) are poorly understood. We attempted to: (1) identify the factors limiting this flux using additions of glucose and an electron transport uncoupler (carbonyl cyanide m-chlorophenylhydrazone); and (2) improve yearly estimates of R(r) by directly measuring its Q(10)in situ using temperature-controlled cuvettes buried around intact, attached roots. The proximal limits of R(r) of loblolly pine (Pinus taeda L.) trees exposed to free-air CO(2) enrichment (FACE) and N fertilization were seasonally variable; enzyme capacity limited R(r) in the winter, and a combination of substrate supply and adenylate availability limited R(r) in summer months. The limiting factors of R(r) were not affected by elevated CO(2) or N fertilization. Elevated CO(2 )increased annual stand-level R(r) by 34% whereas the combination of elevated CO(2) and N fertilization reduced R(r) by 40%. Measurements of in situ R(r) with high temporal resolution detected diel patterns that were correlated with canopy photosynthesis with a lag of 1 d or less as measured by eddy covariance, indicating a dynamic link between canopy photosynthesis and root respiration. These results suggest that R(r) is coupled to daily canopy photosynthesis and increases with carbon allocation below ground.     

Accuracy of genomic selection methods in a standard data set of loblolly pine (Pinus taeda L.)

Genomic selection can increase genetic gain per generation through early selection. Genomic selection is expected to be particularly valuable for traits that are costly to phenotype and expressed late in the life cycle of long-lived species. Alternative approaches to genomic selection prediction models may perform differently for traits with distinct genetic properties. Here the performance of four different original methods of genomic selection that differ with respect to assumptions regarding distribution of marker effects, including (i) ridge regression-best linear unbiased prediction (RR-BLUP), (ii) Bayes A, (iii) Bayes Cπ, and (iv) Bayesian LASSO are presented. In addition, a modified RR-BLUP (RR-BLUP B) that utilizes a selected subset of markers was evaluated. The accuracy of these methods was compared across 17 traits with distinct heritabilities and genetic architectures, including growth, development, and disease-resistance properties, measured in a Pinus taeda (loblolly pine) training population of 951 individuals genotyped with 4853 SNPs. The predictive ability of the methods was evaluated using a 10-fold, cross-validation approach, and differed only marginally for most method/trait combinations. Interestingly, for fusiform rust disease-resistance traits, Bayes Cπ, Bayes A, and RR-BLUB B had higher predictive ability than RR-BLUP and Bayesian LASSO. Fusiform rust is controlled by few genes of large effect. A limitation of RR-BLUP is the assumption of equal contribution of all markers to the observed variation. However, RR-BLUP B performed equally well as the Bayesian approaches.The genotypic and phenotypic data used in this study are publically available for comparative analysis of genomic selection prediction models.     

Genetic mapping of expressed sequence tag polymorphism (ESTP) markers in loblolly pine (Pinus taeda L.)

The development and mapping of genetic markers based upon expressed sequence tag polymorphisms (ESTPs) in loblolly pine (Pinus taeda L.) are reported. The new markers were generated by PCR-amplification of loblolly pine genomic DNAs with primers designed from sequenced cDNAs. The cDNA libraries were constructed from RNAs expressed in the needles of loblolly pine seedlings or in the xylem from young trees. DNA polymorphisms were identified by analyzing the amplified products for differences in fragment size or restriction sites, or by examining mobility differences using denaturing gradient gel electrophoresis (DGGE). DGGE revealed more DNA polymorphisms than the other two methods. Fifty six ESTPs were mapped using either of two mapping populations and positioned onto a loblolly pine consensus genetic map. Unlike many other markers commonly used in forestry, ESTPs can be used as orthologous markers for comparative mapping, to map genes of known function, or to identify candidate genes affecting important traits in loblolly pine. Received: 10 April 2000 / Accepted: 13 July 2000     

Reducing airborne ectomycorrhizal fungi and growing non-mycorrhizal loblolly pine (Pinus taeda L.) seedlings in a greenhouse

Atmospheric spores of ectomycorrhizal (ECM) fungi are a potential source of contamination when mycorrhizal studies are performed in the greenhouse, and techniques for minimizing such contamination have rarely been tested. We grew loblolly pine (Pinus taeda L.) from seed in a greenhouse and inside a high-efficiency particulate air-filtered chamber (HFC) constructed within the same greenhouse. Seedlings were germinated in seven different sand- or soil-based and artificially based growth media. Seedlings grown in the HFC had fewer mycorrhizal short roots than those grown in the open greenhouse atmosphere. Furthermore, the proportion of seedlings from the HFC that were completely non-mycorrhizal was higher than that of seedlings from the greenhouse atmosphere. Seedlings grown in sterilized, artificially based growth media (>50% peat moss, vermiculite, and/or perlite by volume) had fewer mycorrhizal short roots than those grown in sand- or soil-based media. The HFC described here can minimize undesirable ECM colonization of host seedlings in greenhouse bioassays. In addition, the number of non-mycorrhizal seedlings can be maximized when the HFC is used in combination with artificially based growth media.