The genetic improvement of Sitka spruce (Picea sitchensis (Bong.) Cart) com
menced in 1963 with the objective of developing breeding populations well a
dapted to a range of site types, with improved stem form and growth potenti
al 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 dens
ity. If home grown Sitka spruce is to replace slower grown imported materia
l 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 str
ength.
Multi-trait selection index techniques were used to combine estimated breed
ing values for diameter, stem form and wood density based on the mean perfo
rmance of their progeny relative to the unimproved control of Queen Charlot
te Island origin after 15 to 22 growing seasons in replicated tests. Final
index rankings vary according to the economic weight applied to each select
ion trait. By applying a range of economic weights, breeders are now able t
o offer forest managers a choice in the genetic quality of planting stock a
ccording to their perceived relative values. The objective of the General P
roduction Populations is to maximize genetic gains for diameter and stem fo
rm while preventing a fall in wood density. Alternative production populati
ons offer further gains in wood density and stem form respectively, althoug
h managers must accept more modest diameter gains as a consequence.