COMPARISON OF SELECTION METHODS FOR IMPROVING VOLUME GROWTH IN YOUNG COASTAL DOUGLAS-FIR

Data from a Douglas-fir [Pseudotsuga menziesii (MIRB) FRANCO var. menziesii] open-pollinated progeny test in Oregon were used for evaluating methods of selection for bole volume. Tree height and bole diameter at breast height (DBH) were measured on 12- to 13-year-old individuals from 90 families at each of three plantations, and bole-volume estimates were derived with a quadratic volume equation. Four methods were compared for both parental and progeny selection: (1) indirect selection based on height, (2) indirect selection based on DBH, (3) direct selection based on volume, and (4) selection based on an index that included all three traits. Two-stage selection was also explored, where test trees are culled on the basis of DBH in the first stage; and, height is measured only on the remaining trees, so that final selections (the second stage) are based on volume. Estimated genetic gains in volume from progeny selection were 8% to 11% greater than those from parental selection. The relative efficiencies of the various selection methods, however, were similar for parental and progeny selection. The greatest estimated gains in single-stage selection, which were achieved with the multitrait index, were only 1% better than those from selections based on volume alone. Indirect selection based on DBH produced about 90% of the gain achieved by direct selection for volume, and indirect selection based on height was about 92% to 94% as efficient as direct selection. With two-stage selection, up to two-thirds of the trees could be culled in stage 1 without significantly lower gains in bole volume than those expected if the height of all trees had been measured.