Comparison of 17-year realized plot volume gains with selection for early traits for loblolly pine (Pinus taeda L.)

Diameter, height, survival, and per-hectare volume of loblolly pine (Pinus taeda L.) at 9 years of age were evaluated as traits for indirect selection to improve per-hectare volume at age 17. Competition-induced mortality begins between these two ages, and the concern is that early selection may not account for family differences in competitive ability and thereby not maximize gains in stand volume after the onset of competition. Eight half-sib North Carolina families and one Mississippi–Alabama commercial check were grown in replicated single-family blocks at three spacings (1.5 m × 1.5 m, 2.4 m × 2.4 m, and 3.0 m × 3.0 m), and actual per-hectare volumes were determined at age 17. Family selections based on age 9 measurements were compared with the average for all families and the check to determine “realized” gains in age 17 stand volume. Early selection on age 9 diameter or volume per planted tree yielded the greatest realized gain in volume per hectare at age 17. The lack of family diameter and survival differences at age 9, before the onset of competition, reduced the effectiveness of early selection. Results suggest planting progeny tests at closer spacings may allow earlier observation of family differences in competitive ability and thereby improve genetic gains in stand productivity from early selection.