Selection differential

The second experiment using half sibs (progenies having one common parent) was used to calculate narrow sense heritability.38 Narrow sense heritability for woody tissue in the E. deglupta half sib families grown in the phytotron was found to be 0.44 with a standard error of 0.23. Therefore, wood density in E. deglupta appeared sufficiently heritable to enable its manipulation by selection. However, these small experiments using young seedlings and juvenile clones could only be used as a guide.

Heritability is a parameter arising from a particular experiment, applicable only for the specific environmental conditions and to the specific population from which the sample of parents is drawn. The precision of the heritability estimate is strongly influenced by the sample variance. A greater number of cloned and half sib plants than was used in the experiments described here would be required to improve the precision of the estimate of heritability.

Since vegetative propagation in conjunction with tree selection offered the opportunity to secure a high genetic gain more quickly than by sexual propagation, the relatively high broad sense heritability for wood density in E. deglupta, even though based on data from young plants, was particularly encouraging for pursuing a tree improvement programme for this species.

Selection differential

The selection differential is the difference of the random (base) population mean (of 125 trees) and the mean of parents selected for the breeding population (150 trees). As related already, there was a first stage selection based on their measurable appearance in the field. The breeding population had, on average, a 1.44-inch (3.66 cm) diameter at breast height, a 6.7 feet (2.0 m) total height and a 4.9 feet (1.5 m) bole length advantage over the random population. The breeding population trees were much straighter, and had less fluting of the stem cross-section. However, mean whole tree wood density was slightly lower in the breeding population trees (down from 0.36 to 0.34 gms/cc). Given that the phenotypic variation in whole tree wood density in the breeding population was more than 20%, this small drop was within the margin for error. More desirable trees had been obtained by the pre-selection based on tree size and form (critical wood volume production variables) for only an apparent 1% fall in the mean whole tree wood density.

38 Narrow-sense heritability is the ratio of additive genetic variance to the total phenotypic variance.