Programmes of forest genetic improvement should progress through several clearly defined stages, as shown below:
Plus tree selection
Seed stand establishment
Seedling or clonal seed orchards and controlled pollination
Advanced breeding techniques
Appropriate selection of species is the essential foundation to any genetic improvement programme, followed by selection of the most suitable provenances of that species. If not already known, these will have to be determined by carefully planned and assessed species and provenance trials. Such trials aim to get as good an idea about the genetic make-up of the trees - genotype - as possible, by ensuring that the effects of soil, climate, age etc - i.e. the trees' environment - is as uniform as possible, and then observing their outward appearance - phenotype. This is based on the relationship: Phenotype = Genotype x Environment, that is fundamental to selection and breeding. The importance of this level of selection has already been mentioned under SELECTING SPECIES AND SOURCES.
Selection of individuals - within the best provenances, genetic improvement can begin by ensuring that seed or vegetative material is collected only from individuals of the most suitable phenotype (i.e. superior or plus trees). In the case of seed, there will be no knowledge of the pollinator's phenotype, but at least it is a step in improvement.
Seed stand selection - further improvement can be made by identifying stands which have a higher proportion of trees with a suitable phenotype, thus further increasing the likelihood of improving the genotype. If these stands are managed to remove inferior trees of the desired species, then both quality and quantity of seed can be further improved, as the genetic quality of the pollen improves.
Plus tree selection - more active improvement begins with the selection of superior or plus trees from which seed or vegetative material is collected to establish the next stage:
Clonal or seedling seed orchards - in these orchards the parent tree identity is maintained, and different degrees of control of inter-pollination is exercised. This can lead to:
Advanced generation breeding - where both parents are known, propagation techniques may be specialised, and performance of progeny is carefully tested.
Genetic engineering - the most advanced stage of improvement, involves an understanding of the whole set of genes (genome) of the species, and which genes control which characteristic. The genes and/or the chromosomes on which the genes are found are modified using biochemical techniques so as to either remove or switch off genes controlling undesirable traits, or to introduce desirable ones. Such a process of genetic engineering produces so-called genetically modified organisms (GMOs), or transgenic species.
Hydrids - crosses between species can be produced naturally or artificially. Such hybrids often show desirable traits that can be exploited in breeding programmes, but they may be sterile and have to be propagated by vegetative means.
See SELECTED REFERENCES - IMPROVEMENT for sources of information