Chapter 2 Improved Breeding
Forest tree breeding involves the genetic improvement of forest trees and the development of high-quality varieties. The main goal of this process is to cultivate and propagate superior tree varieties that meet production needs, are genetically stable, and can adapt to various environmental conditions. These improved varieties play a crucial role in maximizing the natural productivity of forests, enhancing the quality and yield of forest products, increasing resistance to pests and diseases, and ensuring the multiple benefits of forestry. In practice, improved varieties refer to those that have been enhanced through selective breeding and trait improvement.
The primary methods for producing seedlings in forestry include grafting and seed orchards. While mother trees can also be used in breeding, their level of improvement is generally lower than that of seed orchards. Genetic testing and selection are essential components of this process, as they help ensure the genetic quality of breeding materials.
Breeding programs are closely linked with various scientific disciplines. To elevate the standard of breeding work, it is necessary to accelerate the genetic improvement of trees by integrating knowledge from other fields and enhance the efficiency of propagating elite species. This will allow improved varieties to play a more significant role in forest production.
Section 1: Establishment and Management of Mother Trees
First, the significance of establishing mother trees
A mother tree stand is a designated area where seeds are collected after the genetic quality has been improved. It is also known as a seed forest and serves as one of the forms of seed breeding for forest trees. Its main purpose is to produce high-quality tree seeds.
In addition to establishing seed gardens and grafting nurseries, the establishment of mother trees is essential for improving the quantity and quality of forest seeds. For larch, although seed gardens can provide high genetic gains, the process is complex, costly, and time-consuming. Establishing mother trees, on the other hand, is simpler, cheaper, and faster. Through proper management, they not only meet immediate seed demands but also improve the genetic quality of seeds. Therefore, mother trees are an effective way to achieve large-scale production of improved forest tree varieties.
For example, the Mengzi Forest Management Bureau in Hebei Province established 5,000 acres of *Larix principis-rupprechtii* mother trees in 1976. Through careful selection, thinning, and maintenance, from 1980 to 2001, the genetic gain of the best seeds reached 10%, with a total of 29,565 kg of high-quality seeds produced. These seeds were not only used locally but also exported to several provinces including Liaoning, Shanxi, Shaanxi, Inner Mongolia, and Beijing.
Second, the conditions for establishing mother trees
(i) Climate Conditions
The annual average temperature should be above 1°C, with a growing season of at least 100 days and an accumulated temperature of over 2,500°C above 10°C. Low temperatures can cause frost damage, and a short growing season may affect seed development and seedling quality.
(ii) Site Conditions
Soil fertility and terrain are key factors. The best soil types are those that are rich in humus and moderately deep. Acidic soils are suitable for larch. Open, flat areas or gentle slopes (no steeper than 20°) with good sunlight exposure and proximity to water sources are ideal. The elevation should be around 1,000 to 1,200 meters.
(iii) Stand Condition
The stand must be of high quality to support the growth of mother trees. The following factors are important:
1. Quality of the mother trees: A higher proportion of good mother trees improves the genetic quality of the seeds. In Longtoushan, over 70% of the selected trees were elite.
2. Canopy density: A canopy closure of 0.4 to 0.6 is ideal. Too dense or too sparse can affect light availability, which in turn impacts seed yield and quality.
3. Forest age: The optimal age for *Larix principis-rupprechtii* is between 7 and 10 years. Younger stands should be thinned to select the best trees.
4. Tree health: Trees should be fast-growing, straight, well-proportioned, and free from pests and diseases.
5. Stand density: A planting density of 222 trees per mu is recommended. Proper spacing helps form a complete canopy and increases the solid area.
6. Additional conditions: Easy access for machinery, irrigation facilities, and isolation zones to prevent unwanted pollen contamination are also important.
Third, division of mother trees and provenance selection
(a) Road layout
To facilitate management and seed collection, roads should be built along the mountain if the area exceeds 10 hectares. Main roads should be 6–8 meters wide, with smaller roads (4–6 meters) between operating areas.
(b) Isolation belts
Coniferous isolation zones should be at least 100 meters wide, while broad-leaved trees require 200 meters. Non-forest areas need 300 meters. If the surrounding stand is of the same species, a 100-meter buffer zone is sufficient.
(c) Provenance selection
Provenance refers to seeds or propagation material from different geographic locations within the same species' range. Provenance tests compare the performance of these materials. Selecting high-yielding and stable provenances ensures better afforestation outcomes. When choosing seeds, it's important to match them to the site conditions, such as climate, soil type, and topography. Reference should also be made to the national seed area divisions for *Larix principis-rupprechtii*.
IV. Management of Mother Trees
(I) Thinning techniques for mother trees
Thinning is a critical step in establishing mother trees. By removing inferior trees, it improves light, water, and nutrient availability, promoting healthy growth and increasing seed yield and quality.
1. Classification of mother trees: Before thinning, trees are divided into three categories:
- V1: Good mother trees – strong growth, straight trunks, and no pests or damage.
- V2: Poor mother trees – weak growth, crooked trunks, or damaged.
- V3: Normal mother trees – in between the two.
2. Principles of thinning: Preserving high-quality trees and removing poor ones is essential. Careful attention should be given to spacing to ensure even distribution.
3. Timing of thinning: Early thinning can delay seed production, while late thinning may lead to dead branches. The first thinning should occur when the trees are about 10 years old.
4. Retention density: Keeping 70–80 trees per acre is optimal for both yield and quality.
Table 2-1 shows the effect of different retention densities on seed yield. The highest yield was achieved with 75 trees per acre, which was 11 times greater than the control.
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