Variation in outcrossing rates and growth in Eucalyptus camaldulensis from the Petford region, Queensland; evidence of outbreeding depression

Comparison of growth rates of Eucalyptus camaldulensis Dehnh. in provenance/progeny trials in Thailand has revealed significant differences among families. One possible cause of differential family performance in eucalypt species with mixed mating systems is variation in the level of inbreeding. Outcrossing rates were estimated for ten trees from each of four populations in the Petford region of north-east Queensland using allozymes. They were amongst the highest recorded in eucalypts (mean t_m = 0.95) with relatively little variation among families (t_m = 0.60-1.0). Regression analyses revealed a significant association between family outcrossing rates and growth which varied among populations. A positive association was observed in one population; negative relationships in the other three populations may reflect outbreeding depression associated with hybridisation. Differences in outcrossing rates did not explain a significant level of variation in seedlot viability or survival assessed at two years of age. The high mean outcrossing rates for the four populations of E. camaldulensis, together with the low proportion of variation in growth attributed to outcrossing rate, suggest that inbreeding is unlikely to be a problem in the first generation of tree improvement using open-pollinated families sourced from natural populations in the Petford region.