A Study of Population Variation and Inheritance in Sitka Spruce

This study investigated the potential benefits to a tree breeding programme of selection based on height in the original ortet population, and on diameter and wood density of grafted-ramets representing the same ortets growing in a clone bank. The trees studied were a large, randomly selected same-aged population of Sitka spruce (Picea sitchensis (Bong.) Carr.) individuals of known origin growing on a single site in North-West Scotland. Estimates of correlated response indicated the value of including ortet height when adjusted for an environmental variable (in this case height above sea level), and grafted-ramet wood density and diameter in the estimation of breeding values. A selection intensity of 1 in 20 for height amongst ortets in the forest or grafted-ramets in the clone bank each gave a correlated increase in mid-rotation diameter of around 6% in the next generation. A similar selection intensity for wood density in the grafted-ramet population would give a 15% increase of wood density in the next generation. The study indicates worthwhile gain can be achieved at the stage of plus-tree selection when moderate single tree heritability is combined with sufficiently high selection intensity.