Effect of Natural Inbreeding on Variance Structure in Tests of Wind-Pollination Douglas-Fir Progenies

Abstract

Studies of the mating habits of Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) have shown that wind-pollination families contain a small proportion of very slow-growing natural inbreds.The effect of these very small trees on means, variances, and variance ratios was evaluated for height and diameter in a 16-year-old plantation by systematically deleting from analyses the shortest individuals in wind-pollination (W) and diallel (D) progenies of six parent trees. The progenies were growing together in a common field test. The D population served as a control (no inbreds).The low-vigor trees in the W population type gave a downward bias to means and an upward bias to within-plot and family x block variance estimates. Amount of bias differed among W families. Before deletion, estimates of additive genetic variance and heritability for height in the W test were about one-third and one-sixth the estimates from the D test. Deleting the shortest 5 to 8% of the trees from the W population did not remove all the discrepancy, but it did result in much closer agreement between W and D tests. For the D test alone, deletion of the smallest trees had a negligible effect on estimates of additive genetic variance and heritability. For. Sci 34(1):102-118.