Eucalypt hybrids

There is world-wide interest in developing hybrids between the major temperate plantation species Eucalyptus globulus and E. nitens for plantation development in areas too cold for E. globulus. A ten-year study of the performance of interspecific and intra-provenance F1 hybrid populations of E. globulus and E. nitens has shown that quantitative genetic models used for genetic evaluation of growth in pure species are not appropriate for interspecific hybrid populations. Additive genetic effects expressed in E. globulus intra-provenance populations for growth are also expressed in the inter-provenance hybrid population, with levels intermediate between the two parental populations. However there is no such association in an E. nitens x globulus hybrid population. Comparable estimates of these additive genetic effects are highly inflated in the hybrid populations, suggesting that different genes or gene interactions are affecting hybrid performance.

The field trial studied was established by CSIRO Forestry and Forest Products and North Forest Products (now Gunns Limited) in 1990 near Ridgley, Tasmania and has been regularly monitored by CRC scientists. It is one of the best hybrid eucalypt trials established and includes crosses within and between provenances of the pure species and interspecific hybrids, with the same parents being used for both within and between species hybridisation.

The interspecific hybrid population was characterised by atypically high levels of seedling abnormalities and high mortality in the nursery and field. The average performance of survivors was below the mid-parent value (i.e. negative heterosis). In contrast, hybrids between the Taranna and King Island provenances of E. globulus exhibited high survival and average performance well above the mid-parental value, although they did not perform better than the faster growing King Island crosses. The study showed large differences between the Taranna and King Island provenances in their propensity for hybridisation with E. nitens, with most of the poorer performing interspecific hybrid families derived from crosses with Taranna E. globulus. In contrast to growth, wood density measured indirectly using Pilodyn penetration exhibited more typical quantitative genetic behaviour in the inter-specific F1 hybrid and was always intermediate or comparable with one or other of the parent species. The inter-specific F1 hybrid did not show any significant advantage over either of the parental species at this site. They did not show any combination of superior growth or wood density that could not be found within the parental species even though there was one outstanding inter-specific F1 hybrid family for growth.