Methods for Tree Improvement and its Application

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 (William, 2007). Tree Improvement includes the practices of silviculture and tree breeding meant to enhance the total yield. Sometimes, it refers to an additional tool of silviculture that deals with the genetic makeup of the tree to be used in forest practice. It may link to a range of silvicultural systems but they commonly integrated with plantation silviculture. The success of tree improvement depends on tree breeding, silviculture, and genetics of species or its particular varieties. Tree Improvement fully relies on understanding and using a variation that naturally occurs in tree populations (Charvin, 2016).

Three main terms of tree improvement
1. Forest tree breeding
2. Forest Genetics
3. Forest tree improvement
Tree Improvement = Tree breeding + Genetics

Tree improvement relies on traditional breeding techniques – Selection of superior (plus candidate) trees for volume and stem straightness and grafting these into breeding orchards. Doing so, the process leads to improving the genetic quality of a tree species. The forest under management is still genetically close to their wild state forest in their natural range. However, there still exits variation in economic traits such as growth rate, stem form and wood quality between different populations within species, and also within individual trees within populations. This situation clearly reflects the opportunities to increase the silvicultural value by identifying the best wild source and developing varieties that are considerably better than the wild materials. Tree improvement does not include genetic engineering which is the intentional change of genetic structure or introduction of novel genes into plants (Oregon State University, 2007).

Tree improvement increases the value of a tree species by
1. Selecting the most desirable trees from natural stands or plantation
2. Breeding or mating these select trees
3. Testing the resulting progeny (Charvin, 2016)

Tree improvement includes a recurrent cycle of selection and recombination.
The general objective of tree improvement is to improve the genetic value of the population while maintaining genetic diversity. It helps in the sustainable management of genetic variation to produce, identify and multiply for the operational planting of well-adapted genotypes of the desired quality. Genetic improvement is aimed at the population level rather than of breeds or inbred lines. Developing individual trees, varieties or populations more suited for human needs. Providing a known source of seed, seeding or propagates for forest establishment (Williams, 2007). Besides increasing yield and shortened rotation of tree improvement technique loss of genetic diversity, higher production costs and requirement of constant upgrading are the major hindrances of tree breeding.

Needs of Tree Improvement technique for massive plantation

Methods for Tree Improvement

Two different methods of tree improvement are mainly used:
• Provenance studies – identify best wild population
• Tree breeding – to select and breed from the best individuals within the best populations. (Forestry Focus, 2019).

Provenance Studies
Many common forest trees have adapted a wide natural range covering many different habitats and climatic zones. Forest trees have developed characteristics that allow them to survive and grow in different conditions. Trees adapted in various conditions, as a result, they have very different characteristics depending on the original seed sources. Depending on which region the seed originated, the performance of these seed sources Can be very different when grown in plantation sites.
Provenance studies carried out on desired species across a range of sites and observations on survival, growth rate, stem form, résistance to drought, frost, disease and damaging agents such as insect pest. Seed from best sources is collected from areas of similar elevations, climatic and latitude. For the establishment of initially improved populations, species and provenance testing should be carried on as well as the development of breeding and gene conservation populations.

Tree Breeding:
Tree breeding is one of the important components of tree improvement that mainly involves the application of genetic principles for the mass production of seedling of desired traits in order to achieve higher productivity, better adaptability of the environment and vigorous growth rate (Thakur et. al., 2013). Trees take a very long time to grow and conventional breeding with them is logistically complex, time-consuming and expensive. The type of cross-breeding and genetic advances that have been possible with many agricultural crops take much longer with trees it might take decades to learn if a particular tree or cross has any special value or not (Oregon State University, 2007). The presence of wide variation in traits creates the potential to produce a large volume of better-quality wood by providing tree breeders with the opportunity to develop improved varieties than that can be achieved from wild materials in its natural state.

Plus tree selection: Plus trees are those superior trees that are selected from the best forest stands as breeding stock for improved varieties. Plus trees are selected on the basis of their appearance i.e. Display superior characteristics to the surrounding trees in the stand. The characteristics are like straighter stem, larger volume, etc. Shoots extracted from these trees are grafted on seedling rootstock in the nursery. Those clonal, genetically identical plus trees help to preserve the genes and used as breeding material. Only outer appearance alone cannot reliable to breeding value. The breeding value must, therefore, be determined by testing their offspring or progeny.

Controlled crossing: The purpose of controlled crossing is to produce seed from selected parents. And/or to combine the best traits of the selected trees.

Progeny testing: Progeny produced from plus trees should be examined so, that it helps in developing improved varieties. Superior progeny is used as breeding stock for the next generation. The performance of the progeny is determined in progeny tests established on forest sites, especially for the species. Those progenies are planted in replicated plots and their growth and development assessed at periodic intervals. It may take a longer time before realizing information on the performance of the progeny is known. The results of progeny tests are used to select the plus trees with the best breeding value. These can be either the original plus tree or superior individuals selected within the progeny test itself. Materials (graft or seed) from these trees are then used to create seed orchards for the production of improved planting stock.

Vegetative Propagation: Vegetative propagation of the plant is possible using vegetative tissues which result in genetically identical to the original “donor” parent plant. It is an important tool widely used in tree breeding to manage breeding populations more effectively. The major advantage is the mass production of new plants possible over sexual reproduction and all the genetic components of parent plants preserved. Vegetative propagation may occur both naturally and artificially. Vegetative propagation occurs naturally through rhizomes, corms, bulbs, tubers, and runners. Artificial vegetative propagation includes grafting, cutting and tissue culture in addition. 

Some modern tree breeding techniques are listed below:

Seed orchard: It is intensively managed plantation of specifically arrange trees for the mass production of genetically improved seeds to create plants or seeds for the establishment of a new forest. It is a common method of mass production of genetically improved forest trees from breeding populations. Genetically superior trees separated from the rest of others and intensively managed to produce frequent abundant, and easily harvested seeds; and designed and managed to produce seeds of superior genetic quality compared to those obtained from seed production areas, seed stands.

Clonal propagation: Within the population sometimes some individuals often found with outstanding far superior characteristics to the rest. Similar to Vegetative propagate parts of those individuals are used to produce superior clonal varieties of the same traits as the parent trees. While developing these varieties it is necessary to test the performance of the clones to ensure that they are indeed superior and will perform consistently over a range of site types. Clonal testing is similar to progeny testing having the objective of screening the best material for the production of improved planting stock.

Somatic Embryogenesis: In conifers, somatic embryogenesis is a recently developed cloning technique, an unlimited number of genetically identical copies of trees can be produced from a single seed which is genetically superior and high-value trees by deploying the best available genetic stock to the commercial site (FAO).

Micro-propagation or In- Vitro propagation: Rapidly multiplying stock plant materials to produce a large number of progeny plants to produce sufficient plantlets for plating from a stock plant which do not produce seed and does not respond to vegetative propagation. Micro-propagation techniques are widely used in Fagus, Eucalyptus, Acacia Aegle, etc. (FAO).

Biotechnology: Any technique that used living organisms to make or modify a product, to improve plants or animals or develop micro-organisms for a specific use. Biotechnology today refers to the commercial application of living organism or their products which involve deliberate manipulation of their DNA molecules. Few transgenic plants are likely to produce for large-scale through some type of low-cost cloning, which allows replication relatively easy in some tree species.

Tree Genetics:

Breeding between the best species within the best population is a quick and inexpensive way of obtaining best genetic property in the offspring.
Genetic gain can no longer greater than the quality of parents used unless through intensive selection. When genetic gain tends to increases, genetic variation must be reduced. It’s tradeoff between genetic gain and diversity. Trees require a lot of land and time to breed and test, and desirable traits are not always easily measured.

Every tree improvement program includes,
1. Determination of species; geographic sources within a species
2. Determination of amount
3. Packaging of desired qualities into improved
4. Mass producing improved individuals
5. Developing and Maintaining a genetic base population.

Application of tree Improvement

Tree improvement helps in increasing yields and shortened rotations so it has a large potentiality to supply timber and wood demand of the world. It greatly increases the benefits flowing from the managed forest by increasing forest productivity to meet rising worldwide demand for and hence balances supply and demand for forest products. Intensifying management on most productive forest land concentrates wood production on a smaller land base permitting remaining area flexible towards other objectives. Different countries have been developing tree breeding techniques and achieving maximum benefits from it. The major application of tree improvement is to boost growth, quality of the product, pest resistance and adaptability of forest stand and to conserve genetically superior high yielding species from tree stands. Increasing yield and shortened rotation are the major prospects. In Southeast Asia, the wood resource for pulp mills is primarily from plantations of Acaciamangium, A. crassicarpa, Gmelina arborea, and Eucalyptus spp. The plantation growth rate of 25 m3/ha/yr. harvested at age 6 years when applied genetic improvement and tree breeding average growth rates rise to 35 m3/ha/yr. (Kellison, 2005).
The vegetative propagation technique for tree breeding of Teak has given high priority with great success in Thailand. The tissue-culture technique of teak has also been developed on a commercial scale. The seedling produced using this procedure price three times compare to the traditional seedling. However, the combination of tissue culture and subsequent cutting of tissue-culture plantlets proved both technically and economically feasible for large scale production of clonal planting material (Apichart et al., 2000).
Cloning of Poplar in Greece for biomass or wood production is largely successful. More than 100 Intra and interspecific crosses of poplar are included in tree breeding. The best clone of Poplar is P. xeuramericana with an average production of 16.54 tons per hectare per year followed by another clone with average production of 14.40 t/ha/yr. (Aravonopoulos, 2010).
Forest productivity can be increased significantly through clonal forestry. Eucalyptus, Poplar, and Acacias have significantly shortened the harvesting cycles through tree breeding and tree improvement programs. The trees can be harvested every five years without sacrificing the quality of wood. The current finding shows pulp yield independent of growth rate. Increasing yield per hectare and reducing the unit cost of wood may boost up sustained commercial forestry in the near future.

Conclusion and Recommendations

Tree breeding helps in the mass production of seedlings for achieving higher productivity of timber and wood products. Conventional and modern both techniques have been used for tree breeding globally. It reflects the possibility to shape the genetic potential of the production population with lots of silvicultural investment by increasing the quality and quantity of wood. Forester and land users embracing concepts of intensive management and applied genetics that profoundly challenges traditional forestry principle and conventional practice. Natural forest replaces by a vast scale with plantation of the improved tree becomes basically matters of economics. It is recommended that increases in wood production can be done without jeopardizing the long-term future of the basic forest resources or without impairing its ability to supply other needs to society.


References

• Charvin A. (2016): SlideShare https://www.slideshare.net/anandcharvin/tree-improvement-techniques-practices
• Williams C.G.  (2007): Duke University, Forestry encyclopedia https://sites.google.com/site/forestryencyclopedia/Home/Tree%20Improvement
• Daniels, J. D. (1984). Role of tree improvement in intensive forest management. Forest Ecology and Management, 8(3-4), 161–195. doi:10.1016/0378-1127(84)90052-5 
• Oregon State University. (2007): https://today.oregonstate.edu/archives/2007/nov/genetic-techniques-speed-tree-improvement
• Thakur, R. B., & Schmerbeck, J. (2013). Role of Tree Breeding in Timber and Wood Supply in World and India: Status and Outlook. The Initiation, 5, 153-163.
• Kellison, R.C. 2005. Production forestry into the 21 st century: A world view. Champion International Corporation, 1316 Dixie Trail, Raleigh, NC 27607
• Apichart, K., Verapong, S. and Erik, D. K. (2000). Experience from Tree Improvement of Teak (Tectona grandis) in Thailand
• Aravanopoulos, F.A. (2010). Breeding of fast-growing forest tree species for biomass production in Greece. biomass and bioenergy 34 (2010) 1531e1 5 3 7. http://www.elsevier.com/locate/biombioe