Genetic improvement of tropical acacias: achievements and challenges §
Three acacia species, Acacia auriculiformis, A. crassicarpa and A. mangium, dominate over 2 million ha of tropical acacia plantations worldwide. Species and provenance trials carried out in the 1980s and 1990s established the fastest-growing regions of provenance for each species, and most current breeding populations comprise open-pollinated families from several, or many, local provenances sourced from these favoured regions. Provenance–progeny trials are thinned to seedling seed orchards (SSOs), which yield both selected progenies for the next generation of breeding and operational seed for plantations. Clonal forestry has been successfully developed for A. auriculiformis and the hybrid between A. mangium and A. auriculiformis. In many instances production of improved seed in seed orchards has not been adequate to meet planting demand; strategies to address this have been (1) the establishment of additional unpedigreed seed production areas of broad and superior genetic base, and (2) clonal family forestry to multiply elite seed families. Controlled pollination of acacias is technically demanding, so is not suitable for advancing main breeding populations. Substantial (50–100%) gains in wood volume production have been demonstrated in trials comparing seed from first-generation SSOs or selected clones with inferior provenances and land races, but volume gains in subsequent cycles of breeding will be smaller. The potential for genetic improvement in form traits and wood properties has also been demonstrated. Genetic improvement objectives must now give heavy weighting to improving disease resistance and tolerance. Ganoderma root rot and Ceratocystis stem wilt have destroyed large areas of acacia plantations in Indonesia and Malaysia. Since clonal deployment of resistant genotypes is the quickest and most effective way of delivering resistant planting material, ongoing research to develop clonal forestry for A. crassicarpa and A. mangium is warranted. Novel interspecific hybrid combinations might deliver useful genetic variation for breeding.
Keywords: breeding objectives, clonal forestry, genetic gain, seed production