Variation in the density of the wood of Pinus radiata
D. Don was studied in a progeny trial comprising 26 open-pollinated families arranged in nine randomised blocks. Samples of wood taken from different radii at breast height on 692 trees were divided into inner, middle, and outer segments, each containing five annual rings. The density of each segment and a mean for all 15 rings were determined for each tree.
Analyses of variance showed highly significant differences between families and between blocks. Point estimates for heritability ranged from 0.51 to 0.72.
On the basis of common knowledge, it may be assumed that on average each radial sample within a stem showed a consistent trend of density increasing with age, as xylem was deposited further and further from the pith. If, then, the variance of density is regarded as that of a point anywhere within the breastheight cross-sections of some 600 trees, a large part of the statistical effect of this trend may be extracted from the data as a linear regression component. With that concept, the linear regression component in this experiment accounted for 65% of the total variance in wood density. The mean density/age gradient was 7.87 kg/m3/yr, with a standard deviation of 3.51 kg/m3/yr. There were highly significant differences between the families in the gradient: its heritability was estimated as 0.27.
Density showed the usual weak, negative phenotypic correlation with radial growth-rate. A multiple regression analysis, relating density to the radius and height of the stem jointly, accounted for 2.2% of the total variation (p < 0.001), the partial regression coefficients being negative for radius and positive for height. In a similar analysis of the pooled within-families variance the regression coefficients were again negative for radius and positive for height, and the joint regression accounted for 16% of the total variation (p < 0.001).
Genetic, environmental, and phenotypic correlation coefficients were estimated for 10 characters. There appeared to be little or no association between density and external characters affecting log quality, but the different determinations of density showed consistently strong positive genetic correlation amongst themselves. This suggests that in each tree the same genes were acting on wood density, and acting in the same way, at all stages during the 15-year period.