Definition: Matching tree genetics principles to silvicultural systems
Tree improvement refers to the application of forest genetics principles within a given silvicultural system for the purpose of improving the genetic quality of the forest. Its goal is to improve the genetic value of the population while maintaining genetic diversity. Meeting this goal means that genetic improvement is aimed at the population level, rather than improvement of breeds or inbred lines.
Tree improvement programs provide a known source of seed, seedlings or propagules for forest establishment. Worldwide, tree improvement programs are linked to a range of silvicultural systems but they are most commonly integrated with plantation silviculture (Zobel and Talbert 1984). As such, each tree improvement program must be designed to fit not only the life history and natural range of the species but also the organization’s planting schedule, annual budget and harvest goals. For example, the least intensive tree improvement programs provide a known seed source for a specific period of time. More intensive tree improvement programs typically have enough profit incentive to invest on a full-scale breeding program for generations to come. Today’s tree improvement programs vary widely as a result but they do share some components and concepts. To understand this, one must first find the historical roots behind the idea.
Tree improvement, as the application of genetic principles to silvilculture, is considered a 20th century idea but few realize that the concept of planting trees is ancient, harking back as far as the origins of Neolithic agriculture. The first records for planted tree farms date to the Ptolemy kingdom in Egypt circa 3rd century, B.C. (Perlin 1989 pp. 131-140). Written records exist because Ptolemy’s administrators, faced with a dwindling timber supply, wrote orders on papyrus for forest tree species to be planted along the rich flood plains of the Nile Valley. Forest plantings came from seedlings grown in gardens or nurseries. Unlike Fertile Crescent agriculture, this early concept of planted forests languished for centuries as a consequence of timber oversupply. Subsequent civilizations had unending sources of old-growth forests as a result of colonial expansion and native forest discovery until 19th and 20th centuries (Williams 1989 pp. 238-285).
With the rediscovery of Mendel’s principles at the start of the 20th century, crop breeders integrated genetic principles into agricultural systems. The tree improvement concept first surfaced in Nordic countries then it was adopted on an industrial scale along with plantation silviculture in the U.S. as expanding uses of southern yellow pine for pulp, paper and later rayon prompted widespread plantation forestry after World War II (Williams 1989 pp. 238-285). Reliable sources of known seed sources were in short supply. Establishing tree improvement programs became the U.S. timber industry’s answer.
Parts of an advanced-generation tree improvement program
Tree improvement rests on the reservoir of genetic variation inherent to a species (Zobel and Talbert 1984). This means that understanding the patterns of genetic variation is the cornerstone to matching a well-adapted seed source to the right physiographic region. Early tree breeders quantified patterns of genetic variation across the natural range of species using common garden trials. These trials, composed of a common set of provenances or seed sources planted across many different physiographic locations, often showed profound adaptive differences within a species. With knowledge of genetic variation patterns within a species, provenances could now be matched to a region, ensuring well-adapted forests in the future.
How to supply the right seeds or seedlings was the next challenge. Breeders began by selecting the best trees from natural stands or even from existing plantations (Figure 1). Some breeders collected seeds from their selections and other collected branches for grafting. These initial selections or founders were established into seed orchards (and breeding populations) either as seedlings or grafts. As the selections in the seed orchards reached reproductive onset, they supplied a known source of seeds for planting programs (Figure 1).
In many parts of the world, this initial seed orchard was the start and the end of tree improvement programs. As the least-intensive tree improvement programs, they meet the planting needs for many low-cost agroforestry species, species with limited planting demand, species which are poorly adapted to plantation silviculture or programs dedicated solely to gene conservation.
Recurrent or advanced-generation tree improvement programs continued with additional breeding cycles (Zobel and Talbert 1984). Here the initial selections were intermated to provide offspring. The offspring were planted into replicated field tests then measured for traits of interest. The value of the parent was assessed on the basis of its offspring’s mean trait value. If a parent’s offspring were subject to disease or characterized by slow growth or malformation then the parent was removed from the seed orchard (Figure 1).
Most forest tree species chosen for tree improvement programs have high levels of inherent genetic variation. Observed characteristics for a single tree is defined as the phenotype. An individual phenotype is determined both by its genetic constitution or genotype and its environment as described in the following equation:
P = G + E (Equation 1)
P = Phenotype
G = Genotype
E = Environment
This equation provides the rationale behind testing offspring from initial selections even for the least intensive tree improvement program. From Equation 1, one can see how a desirable phenotype (P) could have low genetic value if it grows in a favorable micro-site relative to surrounding trees. Conversely, one might overlook a genetically superior tree because it is growing in an unfavorable micro-site relative to surrounding trees. How to sort the genotypic value from the confounded environmental value? Make the initial selections then collect their seeds and test their offspring. If the offspring are also superior then the parent’s genetic value is also high. Testing the offspring is a critical step in any tree improvement program because this is the method for removing genetically inferior parents from the production population.
Branch tips or scions from each selection are grafted onto seedling rootstock. Another option is to start with seeds from each selection. In either case, the initial selections are established into a seed orchard. In many programs, controlled-pollinations are made among the select group of parents. Control-pollination starts with isolating female reproductive structures (female flowers for angiosperms and female strobili for conifers and other gymnosperms) with bags. Pollen from the selected paternal parent is injected into the bag. The choice of the paternal parent depends on the type of mating design (Figure 1). At the end of the pollination season, the bag is removed and the seeds mature. The resulting seeds from a controlled-pollination has a pedigree complete with a known mother and a known father. This is the start of the breeding population (Figure 1).
The breeding cycle within a tree improvement program is a succession of breeding, selection and testing activities in order to upgrade the genetic value of the breeding population (Figure 1) Only the very best of the breeding population are selected for the production population or the trees which provide the seeds, seedlings or propagules for planting. Tree improvement is only one of many silvilcultural tools available for forest management but its benefits are among the most enduring.
Safeguarding genetic diversity
Tree improvement programs improve the genetic value of the population while maintaining genetic diversity (Namkoong et al. 1988). How to maintain genetic diversity given selective breeding is narrowing the population size?
Early generations of recurrent genetic improvement maintained high levels of genetic diversity by keeping large population sizes and by maintaining a high degree of unrelatedness among the selections (Williams et al. 1995). This could change in the future as more intensive breeding programs face a tradeoff between enhanced genetic gains and decreased genetic diversity. One solution is to safeguard genetic diversity using grafted archives. Conservation archives protect the original selections, a type of insurance against widespread loss due to pests, pathogens, extreme weather events, climate change and encroaching demands for arable land. Monitoring genetic diversity after large shifts in breeding or production population size becomes important particularly in light of climate change, reductions in wild forest populations and use of fewer genotypes in forest plantations.
The best trees are selected from natural stands or plantations. Breeding or intermating this select group of trees is followed by testing their offspring. The next set of selections are made among the tested offspring (forward selections) although some of the best parents might also be included here (backward selection). This select group of selected trees is the breeding population. The three-step process or breeding cycle is repeated for each successive generation. The goal of recurrent breeding is to raise the average value of the breeding population with respect to a desired trait. Examples of traits include disease resistance, growth, form or wood quality.
The seed orchard serves as the production population. Its purpose is to meet demands for seed, seedlings or propagules. Production populations are upgraded as better selections become available from the breeding population. With completion of a breeding cycle, new selections from breeding population become available for the next production population.
Tree improvement programs, aimed at genetic improvement at the population level, require large amounts of land, highly skilled labor and annual expenditure on a long timeframe. In the short term, forest managers are rewarded with a reliable supply of seed from a known source. In some cases this is adequate. But with patience, breeders of recurrent tree improvement reap substantial genetic rewards which increase through time. In these programs, molecular domestication places a premium on maintaining genetic diversity because genetic diversity measures the reservoir for future adaptive variation.
BOX 1: A LEXICON FOR TREE IMPROVEMENT
Allele: Variants of genes, defined as alleles, range from common to rare.
Breeding cycle: Selective breeding followed by testing.
Breeding population: A group of individuals selectively bred, tested and culled in order to increase their mean genetic value for a desired trait.
Chromosome: A chromosome consists of a long, continuous strand of DNA and associated proteins. It resides within the nucleus of a cell. Each parent contributes one chromosome to each offspring, so an individual receives half of its chromosomes from its mother and half from its father.
Genes: A gene is a region of DNA on a chromosome which codes for biological information. A gene refers to a functional unit composed of coding DNA sequences, non-coding introns and its regulatory DNA sequences. A gene corresponds to a sequence used in the production of a specific protein or another nucleic acid, RNA.
Genetic diversity: Allelic richness, reservoir for future adaptation to changing environments.
Genetic testing: Planting a replicated trial using offspring or other relatives then measuring their trait values as a method of estimating genetic value for a particular individual in a breeding or production population.
Genetic variation: Refers to genetic information segregating within a species. The genetic information is coded by genes which reside on chromosomes.
Heterozygosity: The state of having two different alleles or DNA polymorphism at a single gene locus residing on a chromosome.
Homozygosity: The state of having two of the same alleles at a single gene locus residing on a chromosome.
Mating design: Intercrossing male and female parents in a systematic pattern. Examples include diallels, factorials, polymix or nested mating designs.
Propagule: A production population vegetatively propagated in order to provide planting stock.
Provenance: Place of geographic or physiographic origin.
Scion: A branch tip, usually grafted onto rootstock.
Seed orchard: Production population managed for seed production.
Seed source: A generic term referring to seed collected from trees within a region especially in the case of exotic introductions where true provenance may not be known.
Selection: Artificial selection, refers to choosing a phenotype to adapt and propagate for human use.
Testing: see Genetic testing
References and Further Reading
Cubbage Frederick, Wear David and Zohra Bennadji. Economic prospects and policy framework for forest biotechnology for the southern United States and South America. Ed. C.G. Williams. In: Landscapes, Genomics and Transgenic Conifers. Dordrecht: Springer Press, 2006. p. 191-207.
Namkoong Gene., Kang Hyun.C. and Jean S. Brouard. Tree Breeding : Principles and Strategies. New York: Springer-Verlag, 1988.
Perlin, John. A Forest Journey. Cambridge: Harvard University Press,1989.
Williams, Claire G., James L. Hamrick and Paul O. Lewis. “Multiple-population versus hierarchical breeding populations: a comparison of genetic diversity levels”. Theoretical Applied Genetics 90 (1995): 584-594.
Williams, Michael. Americans and their forests. New York: Cambridge University Press, 1989.
Zobel, Bruce and John T. Talbert. Applied Tree Improvement. New York: Wiley Press, 1984.
Posted: April 2007
Updated: 22 August 2007; V3