This chapter begins with a brief discussion on the different types of forests in the world, including their scope and importance. Then, the causes of variation in forests are outlined. A brief history of forest genetics is also provided. Finally, the importance of forest genetics in both natural and managed forests is discussed.

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The Forest Biology Research Cooperative recently established a series of loblolly pine clonal trials known as CCLONES (Comparing Clonal Lines on Experimental Sites). There are three primary levels of genetic structure in this study (parental, full-sib family, clone) that strengthen the power of CCLONES for examining genetic mechanisms and interactions with cultural treatments and locations. A fourth level of genetic structure can be added by considering the provenance of the parents. This report includes some preliminary results from the genetic analyses of 2 nd year growth traits that were recently measured at the CCLONES loblolly pine trials. The specific objectives of this report are 1) to determine heritability estimates for various growth traits for loblolly pine clones and seedlings, 2) to compare the genetic correlations between parents and families when grown as cuttings versus seedlings, and 3) to determine the genotype x environment interaction by looking at the genetic correlations for parents, families, and clones for paired trials. MATERIALS AND METHODSThe parental population consisted of twenty first-generation and ten second-generation selections from a larger population that is part of the Loblolly Pine Lower Gulf Elite Population. In addition two slow-growing parents were included. These selections represent the Atlantic Coastal Plain (ACP), Florida (FL), and Lower Gulf (LG) provenances of loblolly pine. These thirty-two elite loblolly pine parents were mated in a partial diallel design and created 70 full-sib families from which a total of 2,000 vegetatively propagated clones were generated. Rooted cuttings from approximately 1,000 of these clones from 61 full-sib families and seedlings from the same full-sib families were established at seven field sites across the southeastern United States utilizing a resolvable incomplete block design (Tests A-G).Each growth variable (2 nd year height, height increment, and crown width) was analyzed for cuttings and seedlings simultaneously with a bivariate analysis in ASREML. Narrow-sense heritability ( 2 h ) was estimated using the corresponding variance components. Type B genetic correlations for general combining ability ( between cuttings and seedlings were estimated in order to compare parental and family performance between propagule types. In order to quantify the extent of genotype x environment interaction, type B genetic correlations across pairs of trials were estimated for the clonal data.

Data from 171 full-sib tests of slash pine (Pinuselliottii Engelm. var. elliottii) measured at multiple ages between 4 and 15 years, were used to obtain restricted maximum likelihood (REML) estimates of variance components. These tests included over 2100 full-sib families, from more than 700 first-generation parents, represented by some 170 000 individuals. Analyses were completed of each test, and using standardized data, of all possible pairs of connected tests (tests with five or more common parents). Heritability, proportion of dominance variance, type B genetic correlations (which examine genotype × environment interactions), and age-age genetic correlations were estimated from the REML variance component estimates. The average heritability of volume was 0.07 at 5 years, which increased to 0.12 at 11 and 14 years of age. These heritability estimates are similar to, though slightly smaller than, previous estimates obtained from open-pollinated tests. There was approximately 1.6 times as much additive variance as dominance variance at 5 years of age, increasing to more than 2 times at 11 or more years of age. Although the relative importance of dominance variance in tree volume apparently declines with age, the existence of dominance variance may warrant some changes to the current breeding strategy. Estimates of type B genetic correlations increased from around 0.6 at 5 years to over 0.8 at 14 years, and so the importance of genotype × environment interaction appears to decline with age. These estimates of type B genetic correlations and also the estimates of age-age genetic correlations are similar to estimates from open-pollinated tests.

Loblolly pine (Pinus taeda L.) exhibits genetic resistance to fusiform rust disease (incited by the biotrophic fungus, Cronartium quercuum f. sp. fusiforme) and pitch canker disease (incited by the necrotrophic fungus, Fusarium circinatum). In this study, a total of 14,015 loblolly pine cuttings from 1,065 clones were screened in controlled greenhouse conditions to identify phenotypes of clones, families, and parents that guide a genetic dissection of disease traits associated with pitch canker and fusiform rust. A total of 23,373 phenotypic data points were collected for lesion length (pitch canker) and gall score, gall length, and gall width (fusiform rust). We verified heritable fusiform rust and pitch canker traits and calculated parental, clonal, and full-sib family rankings for both diseases. Genetic correlations revealed that traits associated with fusiform rust are genetically distinct from one another, and that the genetic mechanisms underlying pitch canker and fusiform rust resistance are independent. The disease phenotyping described here is a critical step towards identifying specific loci and alleles associated with fusiform rust and pitch canker resistance.