Pine hybrids — a review of their use performance and genetics

Abstract

Inter-specific hybrid trees planted throughout the world on a commercial basis are most common in the genera Acacia, Eucalyptus, Larix, Picea, Pinus and Populus although the focal point of this review will include only the genus Pinus. The most important forest tree hybrids in this genera include the inter-specific Pinus hybrids in USA, Korea and Australia. Of those hybrids that are used commercially, by far the most common are first generation crosses. In particular, these hybrids appear to be most successful when they are planted in areas outside the normal plantation zones of at least one of the parent species. Generalisations about genotype×environment interactions across genera were not found, and interactions seemed to be specific to the hybrid concerned. Published estimates of hybrid genetic parameters were not common, but the following general trends were found: (i) heritabilities calculated from hybrid populations were usually larger when compared to pure species estimates; (ii) dominance variance varied with taxon, trait and age; (iii) epistatic variance, although only estimated in one case, was negligible when compared with the additive and dominance effects. It is possible that inflated heritability estimates were obtained because the infinitesimal model may not be appropriate in hybrid populations. Other alternative models should be investigated in the future. The ability to reliably predict hybrid progeny performance from general combining abilities of pure species populations depended on trait and hybrid, although it appeared that this ratio was greater for traits of higher heritability. No consistent evidence was found to support the hypothesis that hybrids may be more adaptable across environments due to their greater heterozygosity and therefore their ability to buffer different environmental stresses. Although there are still gaps in our knowledge of how hybrids behave, some hybrid combinations are without doubt highly successful, and the impact of the use of hybrids in commercial forestry should not be underestimated.