Improving the timber quality of Sitka spruce through selection and breeding

Abstract

The genetic improvement of Sitka spruce (Picea sitchensis (Bong.) Cart.) commenced in 1963 with the objective of developing breeding populations well adapted to a range of site types, with improved stem form and growth potential and wood qualities satisfactory for the sawn timber market. Assessments of wood density, diameter and stem form in progeny tests have determined a strong negative genetic correlation between diameter and wood density while stem form has no significant correlation with either diameter or wood density. If home grown Sitka spruce is to replace slower grown imported material in the construction markets, it is important that tree breeders retain a balance between improvements for diameter and wood density; diameter should not be increased at the expense of wood density, and ultimately timber strength. Multi-trait selection index techniques were used to combine estimated breeding values for diameter, stem form and wood density based on the mean performance of their progeny relative to the unimproved control of Queen Charlotte Island origin after 15 to 22 growing seasons in replicated tests. Final index rankings vary according to the economic weight applied to each selection trait. By applying a range of economic weights, breeders are now able to offer forest managers a choice in the genetic quality of planting stock according to their perceived relative values. The objective of the General Production Populations is to maximize genetic gains for diameter and stem form while preventing a fall in wood density. Alternative production populations offer further gains in wood density and stem form respectively, although managers must accept more modest diameter gains as a consequence.