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The measurement and statistical analysis of data from eight Eucalyptus nitens trials established in the summer rainfall forestry region of South Africa during the 1980s and 1990s, have enabled the characterisation of the Institute for Commercial Forestry's breeding population. Provenance testing showed that the more northerly New South Wales (Australia) Eucalyptus nitens provenances of Barren Mountain and Barrington Tops are distinctly better suited to the summer rainfall areas of South Africa than the southern New South Wales provenances and the Victorian provenance, Penny Saddle.Generally, the species was not badly affected by Coniothyrium canker. High Type B genetic correlations for all sites pairs, except one comparison, ranged from 0.75 to 0.99 for dbh, indicating very little, or no, genotype-environment interaction for diameter at 1 breast height for the genotypes tested in this study. Narrow sense heritability coefficients ranged from 0.01 to 0.34, indicating that the species generally exhibits sufficient breeding opportunity for improvement of diameter growth. High genetic correlations of greater than 0.90 between diameter measurements at 52 to 62 months after establishment and diameter measurements at 94 or 113 months were found, indicating that selections can be reliably made at five or six years.Predicted genetic gains were highest in the trials at Goedehoop and Arthur's Seat, with increases in diameter at breast height of 3.07 cm (17.1%) and 3.17 cm (20.7%), respectively, at full rotation.

It is important to have an understanding of the population genetics and validity of the pertinent underlying assumptions of a species in order to design an effective breeding strategy. In a South African breeding population of Eucalyptus nitens, various scenarios investigating a range of assumptions were developed and used to predict genetic gain in the F2. These were compared with realised gains achieved in a series of genetic gain trials. In the two scenarios using firstly, actual flowering for family (provenance) and, secondly, estimated flowering after 30% roguing of poor families, a coefficient of relationship of 0.33 resulted in predictions closest to realised gain on average. The statistical information suggested that outcrossing in the seed orchards was greater than 80%. Indications were that the effects were additive, and that very little or no heterosis had occurred, due to the still significant provenance effects and the lack of provenance rank changes in the F2.The custom of assuming a degree of inbreeding (and using a coefficient of relationship of 0.33) and of including provenance effects in the models on average resulted in genetic gain predictions which were very similar to the realised genetic gains in this population of Eucalyptus nitens.1

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The sub-tropical species,Eucalyptus longirostrata(formerlyE. punctatavarlongirostrata) andCorymbia henryiwere investigated as alternative species for growth on the Zululand coastal plain in South Africa. Provenance/progeny trials were established in 2001 at two sites, namely, Nyalazi and Kwambonambi. The seed material purchased from the Commonwealth Scientific and Industrial Research Organization in Australia in 2000 included six provenances ofE. longirostrataand five provenances ofC. henryi. Six-year diameter at breast height measurements were completed in 2007. Individual narrow-sense heritability coefficients for diameter growth varied from 0.30 to 0.58 for both species, with heritabilities being higher at the drier Nyalazi site. Heritabilities and breeding values were calculated prior to making selections in field. A total of 143 selections were made in theE. longirostratatrials, and 113 in theC. henryitrials during 2008. Predicted gains for the next generation range from 2.8 cm (20%) to 6.1 cm (61%) increase in diameter forE. longirostrata, and 3.4 cm (23%) to 5.4 cm (49%) increase forC. henryi, depending on site and selection scenario. Provenance differences were evident inE. longirostrataat both sites; however, there were no significant differences between theC. henryiprovenances of Australian origin. The top families of both species performed better than the hybrid controls at the Nyalazi site, indicating that bothE. longirostrataandC. henryiare viable alternative species for successful growth on the drier sites of the Zululand coastal plain.