Making Better Forest Trees Available

H. A. FOWELLS.

FOREST geneticists create or find trees that are new or better in resistance to diseases or insects, quality of wood, branch form, and growth.

Selection is one way of obtaining superior trees. In the forests there are trees that are better than average and can transmit some of this superiority to their progeny through their seeds.

Crossing two trees with the aim of combining in the progeny desirable traits of the parents is another method. Usually the progeny are intermediate between the two parents with respect to their characteristics. Sometimes they inherit the trait of one and not the other. Occasionally the offspring exhibits the phenomenon called hybrid vigor and surpasses its parents.

Hand; or controlled, pollination is the usual method of creating hybrid forest trees. Pollen from one parent tree is placed on the flower of another parent tree. The forest geneticist covers the female flower (or conelet, in the conifers) of the seed-bearing parent with a bag of cloth or plastic material before the flower is ready to receive the pollen. Then, at the right time, he squirts some pollen into the bag with a syringe so that the flower is thoroughly dusted with it. He leaves the bag over the flower at least until the danger of contamination by other pollen has passed.

In the pines, which take 2 or 3 years to mature cones, fertilization takes place about a year after pollination, and he may have to protect the developing cone and its hybrid seed from insects, fungi, and squirrels.

Producing seeds in this way is a tedious job, but in Korea the hybrid between pitch pine and loblolly pine is being produced on a large scale by hand pollination. The Koreans put on about 23,500 pollination bags in 1956 and estimated that 2-year-old trees from the hybrid seeds would cost about 1.50 dollars per thousand more than 2-year-old trees from ordinary seeds.

The production of hybrid poplar seed is a special case of controlled pollination. One poplar has flowers that are all or mostly male. Another has predominately female flowers. Therefore one can bring into the greenhouse branches of the male parent and the female parent. The flowers mature if the ends of the branches are placed in water. Intermingling of the branches promotes pollination, or the pollen may be collected and placed on the female flowers. The seeds mature in 4 to 6 weeks.

MASS or semi-controlled pollination is a proposed method for distributing pollen to the flower with less work than is required in pollination by hand.

By this method, pollen of one parent tree or trees is blown onto the crown or flower-bearing branch of the tree selected to bear the seed, when the flowers are receptive.

Obviously much pollen, which is expensive, would be wasted. Pine pollen cost as much as 95 dollars a pound in 1959. But J. W. Duffield and R. Z. Callaham, two forest geneticists who worked at the Western Institute of Forest Genetics in California, demonstrated that pollen can be diluted with old pollen or incompatible pollen without seriously reducing the amount of good seeds produced. They discovered that a mixture containing 50 percent of old, nonviable pollen reduced the yield of seed only slightly.

Because pollination by other trees is possible, the seed parent tree must be isolated from contaminating sources of pollen. The seed parent must be self-incompatible or produce female flowers at an earlier age than it produces male flowers. J. W. Wright, of Michigan State University, found that white pine (Pinus strobus) and Norway spruce (Picea abies) produced many female flowers for at least 10 years before they began to produce male flowers.

SEED ORCHARDS are comparable to fruit orchards, except that the product is tree seed instead of edible fruit.

In seed orchards, the superior parent trees are planted at regular intervals to insure that cross-pollination takes place. If the trees are of the same species, cross-pollination takes place readily. The production of F, inter-specific hybrids, or first generation hybrids between two different species, in seed orchards is complicated by the fact that in only a few species do the male and female flowers of the two species mature at the same time.

The difference in time of flowering may be so great that, as Dr. Wright estimated, at least 90 percent of the reported crosses in pine, spruce, poplar, and maple would have to be made by hand pollination with pollen collected and stored until the time of pollination.

However, F₂ hybrids, the second generation hybrids, can be produced in seed orchards if the F, hybrids are fertile. The F₂ hybrids will be less uniform than the F₁, but uniformity is not always necessary in forestry.

Not all of the planted trees grow to maturity. The rest are harvested in intermediate cuts and are useful in keeping the stand closed, thereby utilizing the capacity of the site and making for trees of better form.

NEW KINDS of forest trees also can be multiplied or increased by vegetative propagation, but for most species doing that is less feasible than by seed. Because forest trees do not breed true, however, vegetative propagation is a means of preserving a particular genotype. This is particularly important for trees that have value as ornamentals because of form or color of foliage.

Two main methods of vegetative propagation are the rooting of cuttings and the grafting of scion wood of the selected tree upon a root stock.

Cuttings of many species of forest trees will root, but a number can be rooted only with such difficulty that this process is not used widely for extensive planting of new trees.

The exceptions are willow and poplar. Some poplar hybrids and selected cottonwoods have grown extremely fast on sites that suit them. With more intensive forestry practice in this country, the planting of poplars and cottonwoods, particularly on overflow and bottom land, will become more common.

One procedure for planting poplars and cottonwoods is to root the cuttings in nurseries and then to plant the rooted cutting. Another is to plant the cuttings directly in the field or forest. Poplar cuttings are made from dormant year-old wood and generally are 12 to 18 inches long and 1/2 to 1 inch in diameter.

Cuttings from selected Monterey pine trees (Pinus radiata) were produced in Australia on a large scale for forest planting. The cuttings were rooted in a nursery in metal tubes, which could be collapsed and removed when the new trees were planted out. Cuttings of 1-, 2-, 3-, and 4-year-old wood of Monterey pine, which is widely planted in Australia and New Zealand, were rooted.