Genetic and environmental control of Douglas-fir stem defects

We studied the genetic and environmental control of Douglas-fir (Pseudotsuga menziesii var. menziesii) stem defects across 22 breeding programs in western Oregon and Washington. Our goal was to understand the relationships between stem defects (forks and ramicorn branches) and growth. We tested the hypotheses that stem defects are associated with progeny test site productivity and distance to the coast, and then obtained robust estimates of genetic variances, heritabilities, and genetic correlations that can be used to design effective breeding programs. Stem defects were more frequent on high productivity sites and near the coast. Compared to the least productive site within each breeding program, the proportion of trees with stem defects was about twice as great on the most productive site. For example, the proportion of trees with ramicorn branches increased from 11% to 24%, and the proportion of trees with forks increased from 5% to 14%, between the shortest and tallest plantations. However, the relationships between stem defects and growth varied substantially within and among breeding programs (R2 ⩽ 27%). Stem defects were also more frequent near the coast, but even harder to predict based on the locations of the plantations (R2 ⩽ 18%). Although stem defects are genetically variable, heritable, and have positive genetic correlations with growth, genetic variation and heritabilities for stem defects were low and highly variable. Nonetheless, stem defects can be reduced using direct backward selection, and are expected to increase only a small amount when genotypes are selected based on volume growth alone. The quantitative genetics of stem defects in Douglas-fir are generally consistent with what has been observed in other conifers. In general, focused breeding could be used to develop low-defect varieties, and these could be deployed to problematic sites. This approach might increase the value of reforestation programs overall, but it will be difficult to deploy low-defect varieties optimally because site productivity and distance to the coast are only weakly associated with stem defects. Although current multi-trait breeding approaches that consider growth and stem defects seem appropriate for most sites, controlled crosses between low-defect parents could be made in seed orchards, and the resulting seedlots could be deployed to sites that are known to be prone to defects (e.g., based on past data). Our results also suggest that (1) it might be possible to improve protocols and training for measuring stem defects, (2) breeders should monitor the among-site relationships between growth and stem defects in advanced generation breeding programs, and (3) low-defect genotypes should be identified and archived so they are available for future breeding.