Genetic variation of wood density components in coastal Douglas- fir and their relationships to growth rhythm

Graduation date: 1991Overall wood density is a complex trait resulting from the interaction of three components: average earlywood density, average latewood density, and latewood proportion. In order to better understand the genetic control of wood density in juvenile Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), and to assess the utility of\ud information on its components for improving the efficiency of selection for overall wood density, wood density components were examined in 15-year-old trees of 60 open-pollinated families. Wood density traits were obtained on a ring-by-ring basis by X-ray densitometry of increment cores. In addition to providing information on overall wood density traits over the entire cores, these data made it possible to examine\ud changes in wood density traits with increasing distance from the pith.\ud Relationships between wood density traits and growth phenology, and their implications for tree breeding, were also examined.\ud Overall core density showed strong genetic control (h = 0.59) and a negative genetic correlation with bole volume (rA = -0.52). Wood density components were strongly genetically related with overall core density, but they were also strongly related among themselves and had lower heritability than overall core density. Therefore, information on density components would not be useful for improving the efficiency of\ud selection for overall core density, or for reducing the negative correlated response on volume growth. It is possible, however, to\ud substantially increase volume growth without loss in wood density, or even while slightly increasing wood density, when both traits are\ud included in selection indices. Genetic control of overall core density and its components increased with age (ages 7 to 15). Overall core density and its components at age 15 were also strongly genetically\ud correlated with their respective traits at all younger ages examined. Thus, early selection for overall core density at age 15 would be very effective. Including density components as secondary traits did not\ud enhance the efficiency of early selection for overall core density. Ring density and its components (adjusted by ring size) followed different age trends, but the slopes of the trends did not differ\ud significantly among families. A plateau in the earlywood density trend was observed around age 12 in some trees, and families varied in the proportion of trees having this plateau. This proportion was not\ud correlated with overall core density or with growth traits at age 15. It is hypothesized that the plateau in earlywood density trend might be related to an earlier juvenile-mature transition age. Overall core density and all its components were negatively\ud correlated with dates of cambial growth initiation and latewood transition, and positively correlated with date of cambial growth cessation. Thus, selection for higher wood density would cause a slight extension in the cambial growth period, and an earlier transition to latewood formation. Wood density was not genetically correlated with\ud the date of budburst