What are the consequences of growth selection on wood density in the French maritime pine breeding programme?

Volume and stem straightness were the main selection criteria for the first two generations of the French maritime pine (Pinus pinaster Ait.) breeding programme. In this article, we investigate the consequences of this selection on wood quality. Wood density, as a predictor of wood quality, is studied both in the breeding populations and in commercial varieties. Phenotypic and genetic correlations between wood density and growth traits are investigated in successive breeding populations with three genetic field experiments of respectively 30, 29 and 12 years old. Correlation estimates were either slightly negative or non-significantly different from zero depending on the test considered. Consequently, a low impact of growth selection on wood quality should be expected in improved seed sources. However, we observed a significant wood density decrease in two improved varieties as compared to unimproved seed sources at age 15. In addition to this first effect on wood density, growth improvement is also expected to reduce the rotation age and thus increase the proportion of juvenile wood, which is known as having a lower density than mature wood. This change was studied and quantified using a growth model. Finally, a wood density decrease reaching up to 6% was predicted in the improved varieties compared to unimproved material, when both the observed decrease in wood density and the predicted increase in juvenile wood proportion were taken into account. Implications for the breeding programme were considered.