Inbreeding in Pinus radiata. I. The effect of inbreeding on growth, survival and variance

The effects of inbreeding on growth, survival and variance in a 12-year-old radiata pine trial were studied in five populations each inbred to one of five different levels: outcross (F = 0), half-sib (F = 0.125), full-sib (F = 0.25), selfing (S-1, F = 0.5), and two-generations of selfing (S-2, F = 0 .75). These five populations were derived from a founder population of eigh t clones. Inbreeding reduced diameter, growth, and survival but increased t he variance for diameter. Inbreeding depression at F = 0.125, 0.25, 0.5, an d 0.75 was 5%, 6%, 15%, and 19% respectively for DBH; -3%, 1%, 7%, and 11% respectively, for survival. The standard deviation for diameter increased b y 10%, 10%, 30%, and 25% respectively for F = 0.125, 0.25, 0.5, and 0.75 an d, similarly, the coefficient of variation increased by 17%, 16%, 53%, and 55% respectively. There were significant differences among the eight rounde r clones in their response to inbreeding. The best clone in the trial showe d no inbreeding depression. Overall, inbreeding depression was found to be linearly related to the inbreeding coefficient F with no significant quadra tic effects for ally trait at any population level. However, two individual clones had a quadratic relationship with F for DBH and one clone had a sim ilar relationship for survival. A significant correlation (r = 0.96) betwee n S-2 and the breeding values of founder clones was observed while the corr elation (r = 0.58) between S-1 and breeding values was insignificant. The l ow inbreeding depression in radiata pine relative to other conifers may ind icate that historical purging of detrimental alleles through small geograph ic populations, a higher degree of population subdivision, and the relative high fecundity of inbred progenies has rendered radiata pine an ideal spec ies to use inbreeding as a breeding tool.