GENOTYPE x ENVIRONMENT INTERACTION AND OPTIMAL NUMBER OF PROGENY TEST SITES FOR IMPROVING PINUS RADIATA IN NEW ZEALAND

A progeny test of 25 parents mated in a series of five, five-parent, disconnected diallels was established on 11 sites chosen to represent all major site types for growing Pinus radiata D. Don in New Zealand. Statistical analysis of an assessment at age 9 years suggested that for diameter, the most important trait in the New Zealand tree improvement programme, genotype x environment interaction was important. However, genetic gains predicted for diameter for several regionalisation options, using multi-site index selection, suggested that regionalisation of seed orchards would increase the average genetic gain in diameter over all sites only slightly. Further, many fewer than the full 11 sites were required for selection in order to capture essentially all of the predicted genetic gain for a national programme. Selection using the best site resulted in 90 % of the maximum possible predicted gain, but the poorest site only 23%. Sites which were "very good " for selection had rapid growth, high phenotypic variance, and high repeatability of general combining ability (GCA) effects. Sites which were "very poor " for selection had slow growth, low variance, and low repeatability.