The subdiscipline of genetics concerned with genetic variation and inheritance in forest trees. The study of forest genetics is important because of the unique biological nature of forest trees (large, long-lived plants covering 30% of the Earth's surface) and because of the trees' social and economic importance. Forest genetics is the basis for conservation, maintenance, and management of healthy forest ecosystems; and development of programs which breed high-yielding varieties of commercially important tree species.
Variation in natural forests
The outward appearance of a tree is called its phenotype. The phenotype is any characteristic of the tree that can be measured or observed such as its height or leaf color. The phenotype is influenced by (1) the tree's genetic potential (its genotype); and (2) the environment in which the tree grows as determined by climate, soil, diseases, pests, and competition with other plants.
No two trees of the same species have exactly the same phenotype, and in most forests there is tremendous phenotypic variation among trees of the same species. Forest geneticists often question whether the observed phenotypic variation among trees in forests is caused mostly by genetic differences or by differences in environmental effects. Common garden tests are often used to hold the environment constant and therefore isolate the genetic and environmental effects on phenotypic variability.
The term “provenance” refers to a specific geographical location within the natural range of a tree species. Natural selection during the course of evolution has adapted each provenance to its particular local environment. This means that there are large genetic differences among provenances growing in different environments. Provenances originating from colder regions, for example, tend to have narrower crowns with flatter branches better adapted to the dry snow and types of frosts in colder climates. To demonstrate that these differences are genetic in origin, common garden tests called provenance tests have been planted. That is, seed has been collected from several provenances and planted for comparison in randomized, replicated studies in various forest locations. The study of geographic variation through provenance tests should be a first step in the genetic research of any tree species.
In addition to genetic differences among provenances, there is usually substantial genetic variation among trees within the same provenance and even within the same forest stand. There are two reasons for this genetic diversity: (1) Different trees have different genotypes in most natural stands. (2) Each tree is heterozygous for many genes, meaning that a given tree has multiple forms (different alleles) of many genes. Population genetics studies patterns of genetic diversity in populations (such as forest stands). Results of many studies have shown that most forest tree species maintain very high levels of genetic diversity within populations. See Population genetics
Forest tree breeding
Beginning with the natural genetic variation that exists in an undomesticated tree species, tree breeding programs use selection, breeding, and other techniques to change gene frequencies for a few key traits of the chosen species. As with agricultural crops, tree breeders produce genetically improved, commercial varieties that are healthier, grow faster, and yield better wood products. After an existing forest stand is harvested, a new stand of trees is planted to replace the previous stand in the process called reforestation. Use of a genetically improved variety for this reforestation means that the new plantation will grow faster and produce wood products sooner than did the previous stand.
Several laboratory techniques promise to make major contributions to forest genetics and tree breeding: (1) Somatic embryogenesis is a technique to duplicate (or propagate) selected trees asexually from their vegetative (somatic) cells, and this allows the best trees to be immediately propagated commercially as clones without sexual reproduction (that is, no seed is involved). (2) Genetic mapping of some important tree species is well under way, and these maps will be useful in many ways to understand the genetic control of important traits, such as disease resistance. (3) Marker-assisted selection is the use of some kinds of genetic maps to help select excellent trees at very early ages based on their deoxyribonucleic acid (DNA) genotype as assessed in a laboratory (instead of growing trees in the field and selecting based on performance in the forest). (4) Functional genomic analysis is an exciting new field of genetics that aims to understand the function, controlled expression, and interaction of genes in complex traits such as tree growth. (5) Genetic engineering or genetic modification is the insertion of new genes into trees from other species.
Conservation of genetic resources
For commercially important species of forest trees, gene conservation is practiced by tree breeding programs to sustain the genetic diversity needed by the program. However, conservation of genetic diversity is a major global concern and is important for all forest species to maintain the health and function of forest ecosystems, and to sustain the genetic diversity of noncommercial species that may eventually have economic value. There are two broad categories of gene conservation programs. In-situ programs conserve entire forest ecosystems in forest reserves, national parks, wilderness areas, or other areas set aside for conservation purposes. Ex-situ programs obtain a sample of the genotypes from different provenances of a single tree species and collect seed or vegetative plant material from each genotype to store in a separate location (such as a seed bank in a refrigerated room). Both types of programs are important for conserving the world's forest resources. See Breeding (plant), Forest and forestry, Forest ecosystem, Genetic engineering, Plant propagation