Wood density variability in successive breeding populations of maritime pine.

Growth and form are the two main traits used for genetic improvement of maritime pine (Pinus pinaster Ait.) in the southwest of France. In this paper, wood density is studied to answer two main questions: Is there a general trend for density variability throughout tree development and has selection indirectly reduced wood density variability over breeding populations, owing to genetic unfavourable correlation with growth? Wood density and its components were studied in three polycross tests, each representative of one of the successive breeding populations. Wood density was measured with an X-ray densitometer in approximately 50 families per test with >1900 trees. A preliminary study showed that bark-to-pith ring indexing allows for a better estimation of genetic effects than does pith-to-bark indexing. Genetic variability of wood density appears to be highly dependent on the year considered and no general pattern can be detected over time. Whereas the variability of selected traits is known to have decreased over breeding populations, no significant change was found for variability of wood density. [ABSTRACT FROM AUTHOR]