International gene pool experiments in Pinus radiata: patterns of genotype-site interaction.
In 1970 an international exchange of 319 select lots of Pinus radiata was arranged among several countries. Most lots were open-pollinated progenies, with 31 pair-crosses, 26 poly-crosses, and 2 bulked lots. Field experiments were planted with varying field layouts and variable representation of lots. Trials were assessed mainly at 6-8 yr from planting, with one trial measured at 14 yr and one remeasured at 15 yr. The main emphasis was on stem diameter at breast height over bark (dbhob), stem straightness scores, branch habit scores (primarily relating to a short-internode light-branching ideal), and malformation, with varying scales used for tree-form traits. Pairwise genetic correlations between sites were studied, trait by trait, genotype-site interaction being manifested as departures from perfect correlation (rg=1) between sites. Usable data came from 9 sites, 1 each in New Zealand and South Africa, and 7 in Australia (New South Wales 3, South Australia and Victoria 1 each, and Western Australia 2). The data came from 47 lots at one site and from 126-238 lots elsewhere. Within sites, repeatability of lot means was good (generally >0.5) for dbhob, stem straightness score, and branch habit score, but sometimes near zero for malformation. Estimates of between-site genetic correlations (rg) for dbhob generally exceeded 0.5, but tended to be lower where the 2 Western Australian sites were involved and were often still lower for the South African site. With an adjustment for assessment-age differences between sites, however, many rg values were increased, although not radically. Some results could be interpreted, but there was often no obvious coherent pattern. For straightness, rg generally exceeded 0.6, the main exceptions involving 1 Western Australian site. For branch habit score rg tended to be still higher, except at the South African site where the scoring criterion was rather different. For malformation, rg was often very imprecise, but mostly very high and positive. For some sites or pairs of sites inclusion of some native-population lots inflated both lot means and rg, especially for dbhob. It appears that international exchanges of select genetic material can be beneficial. However, for boosting short-to medium-term genetic gain, imported material should be chosen on appropriate selection criteria, and quite heavy culling from testing may often be needed.