Inbreeding in Pinus radiata. IV: The effect of inbreeding on wood density

Abstract

The effects of inbreeding on basic wood density in a 17-year-old radiata pine trial were studied using five populations, each inbred to one of five inbreeding levels: outcross (OC, F = 0), half-sib (HS, F = 0.125), full-sib (FS, F = 0.25), selfing (S1, F = 0.5) and two-generations of selfing (S2, F = 0.75). These five populations were derived from a founder population of eight clones. Although inbreeding resulted in slightly depressed wood density (inbreeding depression was 1.47%, 2.50%, 1.65%, 0.02%, respectively at F = 0.125, 0.25, 0.50 and 0.75), the effects were not significant. However, the variation of wood density among trees was increased by inbreeding, by 3.70%, 3.40%, 15.74%, and 29.01% respectively for populations at F = 0.125, 0.25, 0.5, and 0.75. In all five populations, the basic wood density increased linearly from 400 kg m-3 at age 4 (the earliest age for most samples) to about 525 kg m-3 at age 12 and stabilized with some fluctuation thereafter. There were significant differences among pedigrees in response to inbreeding for wood density. The pedigrees can be divided into three classes according to their response patterns to inbreeding: no decline of wood density under any inbreeding level; a linear decline from F = 0 to F = 0.75; and an initial decline at mild inbreeding levels contrasted with an increase in selfed generations. The lack of significant inbreeding depression of wood density at the population level combined with increased variation in wood density in the inbred populations suggests that it will be possible to quickly develop inbred lines with high wood density. The combination of low inbreeding depression for growth with a lack of inbreeding depression for wood density makes radiata pine a species ideally suited for the use of inbreeding as a breeding tool.