Tree breeding and silviculture: Douglas-fir volume gains with minimal wood quality loss under variable planting densities

Validating performance of genetically-selected trees under realistic planting scenarios is essential for confidence in tree breeding programs. Quantifying the relative impact of genetic selection and initial planting density on tree size and quality can further guide operational forest practices. We evaluate volume gains, survival, stem quality and wood quality traits on 20-year old trees representing three levels of genetic selection that were grown under four initial planting densities. Working in a realized gain trial for coastal Douglas-fir (Pseudotsuga menziesii var. menziesii (Mirb.) Franco) on five replicated sites, we ask: (1) Do predicted stand productivity levels materialize as expected under different planting densities? (2) Are single-tree plot designs, simulating progeny trials, capable of producing reliable results relative to large-block designs, simulating realistic planting scenarios? (3) If trees selected for volume gain show declines in wood and stem quality relative to wild-stand controls, can this be effectively managed by altering stocking densities? Because young progeny trials are used to estimate genetic gain in tree volume at a rotation age of 60, we use a growth and yield model calibrated to wildstand controls to assess whether genetically-selected families meet projections at age 20. On average, observed stand volumes exceeded projections on four out of five sites, and in three out of four initial planting densities. At the level of site by planting density, the moderate genetic-gain population (mid-gain) exceeded projections 13 out of 20 times while the top genetic-gain population (top-crosses) exceeded projections 11 out of 20 times. This fits expectations for breeding values, which are designed to reflect general performance averaged across all environments. Using a different validation approach, large-block designs showed better performance relative to simulated progeny trial designs. Very high planting densities (1890 + stems/ha) may minimize wood quality losses of genetically-selected planting stock but effects are relatively minor, while good performance among all traits was observed at operational planting densities (similar to 1189 stems/ha). Wood density and microfibril angle proxy measures in top-crosses showed relatively minor and non-significant losses (- 1.1 to - 4.0%) compared to major and significant gains in volume per hectare at age 20 (29.0%) when averaging values at 1189 and 1890 stems/ha. Altogether, the genetic selection systems produce reliable results across a range of site qualities.