Determinisms of Douglas Fir heartwood
The first phase of the project consisted of exploring the determinisms of heartwood formation in Douglas fir by working with clones present on different sites representative of Douglas fir production areas. It emerged that the quantity of heartwood is primarily correlated with the diameter of the tree. Moreover, there is a significant clonal effect to explain the variability of heartwood content. In contrast, heartwood content is neither related to morphological characteristics nor to the growth and status (dominant, dominated) of the tree. This result is also true at clonal level. The effect of the study site was not significant but it should be noted that all were of similar fertility although representing distant geographical areas.
The gene expression approach did not reveal any significant differences between study sites or clones, and the comparison of this data set with other existing data shows that there does not seem to be any effect of silviculture on heartwood content. Concerning the effect of age, the heartwood content seems to increase until 20-30 years and then stabilises (at least until 50 years, the maximum age in our sample).
The second phase of the project made it possible to validate the genetic effect at the level of the progeny and to evaluate the feasibility of integrating the heartwood rate criterion into the Douglas Avenir improvement programme (as requested by the industry following the survey carried out by France Douglas on the wishes in terms of variety creation).
The absence of correlation between the heartwood rate and the growth of the progeny is a first result that allows us to affirm that, a priori, the heartwood rate has not been degraded by the importance given to growth. And since the amount of heartwood remains strongly related to the diameter of the tree, improving growth also increases the amount of heartwood produced.
Nevertheless, the variability of heartwood content is still limited and only 25-30% can be explained by genetics. The creation of heartwood-oriented varieties remains realistic, provided that there is considerable effort in measurement and strong selection pressure on rate and vigour.