Breeding Forest Trees

To make certain of the uniformity and adaptability of new farm crops, long, carefully conducted field trials by the breeder are necessary. The average farmer cannot afford to risk the loss of his land and labor for a year to try out a new strain of unknown value. It may take as long as 20 years to produce and test a new form of an annual crop such as wheat before it can be safely recommended for use. In this respect the forester planter has a distinct advantage over the farmer.

By interplanting a hybrid or other new form with the standard strain or stock that would have been used throughout if the new form were not available, promising new trees may be put to use as soon as they are developed or discovered, without subjecting the forest owner to too much risk. The new trees can be planted at suitable intervals among a natural stand of young trees, or in every fifth row, or other selected interval, where an entire area is to be planted. Several advantages accrue. The new trees can be made to cover a greater acreage by inter-planting, thus reducing the cost of using a new and relatively expensive form. If it proves to be superior, it will occupy the site at maturity, crowding out most of the slower-growing natural stand or standard strain.

A common practice is to plant 1,200 trees to the acre where from 100 to 200 trees are desired in the final stand of mature trees. This great excess of planting stock over expected mature trees represents another way in which forestry differs from agriculture. If, on the other hand, the new strain proves to be inferior to the natural stand or the planted standard strain, it will be suppressed and crowded out before the trees reach harvesting size. There would be no reduction of yield because the new form was a failure.

An objection, frequently voiced, to the use of hybrid trees is that, although the hybrids themselves may be superior to standard strain trees, their offspring, the second generation hybrids, will be worthless, and in consequence the entire area would have to be replanted; whereas, if the standard strain were used, the area would be reseeded naturally from these trees and no subsequent replanting would be necessary.

This line of reasoning is applicable to some farm crop plants but it does not apply to forest trees. For example, Pinus attenuradiata is a hybrid between the Monterey and knobcone pines, produced at the Institute of Forest Genetics. The first generation hybrids, or F, plants, are reasonably uniform in growth rate and other characters. Seedlings from these trees representing the second generation of hybrids, or F2 plants, show great variability. However, a considerable number of the trees ( approaching 50 percent) are as good as the F, hybrid, and a few are even better. In a stand composed of this F2 hybrid population there would be enough good trees of the second generation to repopulate the area and crowd out inferior trees before the stand reaches maturity. Another point is that although many hybrids of crop plants are sterile or practically so, pines are not; in fact, no sterile pine hybrid has yet been reported by anyone.

Besides an acceptable theoretical background for a hybridizing program, numerous techniques to assure the success of the program must be worked out. They will vary with the location of the breeding station, the kinds of trees used, and the individuals doing the work. Because it is impossible to cover all the new methods and techniques in use by tree breeders, a few typical examples based on experience at the Institute are selected.

Techniques that assure control of pollination are essential in plant-breeding work. Most of the genera of timber trees are wind-pollinated, and wind-borne pollen is usually a fine, dry, powdery dust that penetrates all but the most closely-woven fabrics. To find a material for pollination bags, many fabrics were sprinkled with pine pollen and examined under the microscope. A 10-ounce cotton duck was finally chosen because it is light, keeps out foreign pollen, and yet permits a slow circulation of air through the fabric, thus preventing condensation of water inside. The bags are fastened over the cone-bearing branch tips some time before pollen is shed from neighboring trees. The pollen to be used is collected before it starts to shed from the catkins. It is then extracted in the laboratory under conditions that prevent contamination by undesired pollen. At the time the cones are receptive, pollen is injected into the bags with a syringe.

Another serious problem arises when two species that are to be crossed flower or are ready for pollination at different times—sometimes several weeks apart. Usually, pollen is collected from the early form and used to pollinate the late form. At times, however, it is desirable to reverse the procedure, i. e., make the reciprocal cross. It has been necessary, therefore, to work out methods for storing pollen from the late-flowering trees until the following year, when it can be applied to the early forms.

If pine trees are to be hybridized, the worker must have access to both pollen-bearing catkins and young seed-bearing cones. The poplars and certain other species will continue to develop flowers and mature the seeds when the branches are cut and placed in water; thus it is possible to carry out a hybridizing program indoors. That is not possible with the pines. It is necessary in pine breeding to climb the trees repeatedly. To avoid injuring the bark, a rope is thrown over the lowest branch and the geneticist climbs the rope. This strenuous but direct and effective method has been adopted because of the difficulty of transporting ladders or other heavy equipment on the forested mountainsides where the program of the Institute is carried out.

In experimental work time is of the essence, and research workers are always seeking short cuts. Increased growth rate being a most important objective of forest tree breeding, it is of major interest to learn as soon as possible the growth rate of a new hybrid. Recent work by John T. Buchholz of the University of Illinois gives promise that a pine hybrid of superior growth rate may be recognized by examination of some of the embryos even before the seeds mature. This would enable the breeder to evaluate the growth rate of a hybrid within 15 months, instead of waiting 3 to 5 years for the usual nursery trials to indicate its value before large-scale production is started.

At one time field trials of 10 to 20 years were deemed necessary to determine the growth rate of a tree. It is now known that seed size and time of germination affect the early growth rate of a tree but are not related to inherent vigor or ultimate size. In nursery tests, weighing the seeds and comparing trees from seeds of like weights eliminates the variation that would be introduced by using ungraded seeds. All seeds are stratified or packed in moist sand and moss at 40° F. for 60 days or longer. This treatment causes the seeds to germinate more or less simultaneously, eliminating effects due to difference in time of germination. If such factors are controlled, the growth rate of 2- to 3-year-old nursery-grown trees is a reliable guide to the inherent growth rate of the new form.