In any population, there are always some individuals that are bigger, better, and stronger. Farmers recognized this long ago and made special efforts to gather seed from the very best plants to save for the next planting season. In modern times, hybridization of both plants and animals has helped farmers be more productive.
The same techniques that are used to produce improved agricultural plants are also being applied to forest trees. It is a much slower process, however, since trees take such a long time to mature.
An individual tree is the product of two important factors: its environment and its genetic makeup. Through silviculture, forest managers can exercise some control over the environment. Tree improvement concentrates on improving the genetic quality of trees.
The cells of each tree have bits of information that determine the exact characteristics of that individual. These bits of information are called genes, and are inherited from the parents. Some genes govern growth rate and mature size, some control whether a tree will be straight or crooked, some determine whether or not a tree can produce an abundance of seed, etc. It is the genetic makeup of a tree (called genotype) that tree improvement programs seek to understand and modify.
Unfortunately, genes are too small to see and identify, so tree improvement specialists have to work with observable characteristics called traits. Since traits are the outward manifestations of genes, it can be assumed that a tree has genes which correspond to these observed characteristics.
Most of the tree improvement work in South Carolina has been done with various species of pines. The general concept is the same for most trees, but this discussion will follow the tree improvement process in pines.
Pines have 24 chromosomes and an estimated 13 million genes. Breeding for a single trait may involve thirty or forty genes that exert some influence on the desired trait.
The first step in tree improvement is to identify the best individual trees that can be found growing in the wild. This is done by visual evaluation of traits that are important to man's intended use of the species being studied. Straightness, height and diameter growth rate, absence of disease, branch angle and the overall appearance of the tree crown are among the traits prized in forest tree improvement programs.
When a tree is found that exhibits these desirable traits to a greater degree than others of the same age, it is said to be a superior phenotype. That simply means that its external, observable traits are better than its neighbors. Once a tree has passed this test, wood samples are tested to determine if the density of the wood and length of the wood fibers are acceptable. Only then may a tree be designated as a true superior tree.
The location of each superior tree in South Carolina is recorded. Trees are plainly marked with signs and painted bands so they will not be accidentally cut or damaged.
Some tree improvement could probably be gained by simply using the seed from a superior tree, but a single tree can only produce so much seed each year. In addition, it is very difficult for scientists to study individual trees scattered all over the countryside. To solve this problem, tree improvement specialists have devised a way to bring the superior tree from the forest to the laboratory. They take scions, or cuttings, from the superior tree and use them to produce exact genetic copies of the original one. This may be done by rooting the cutting, by tissue culture in a laboratory, or by grafting.
The most common method is grafting. In this method, the tops are cut from rooted seedlings and the scions are grafted in their place. Grafted seedlings, called ramets, are then planted in a seed orchard. (All the ramets produced from the same superior tree are called a clone.) Each tree in the orchard is carefully labeled to distinguish which superior tree clone it represents.
Since the scion material is from a mature tree, grafted trees are capable of producing seed right away. It only takes a few years for the trees to begin producing enough cones and pollen to make a full-scale breeding program worthwhile.
Pines produce both male flowers (staminate strobilli) and female flowers (ovulate strobilli) on the same tree. (These correspond to the stamen and pistil in true flowering plants.) The mature male flowers look like clusters of blue-purple worms; the female flowers look like tiny cones. Both types of flowers are usually produced near the tips of branches.
Have the students look for male and female flowers on pines in the early spring. You can tell when they are ripe by the greenish-yellow pine pollen that covers practically everything during the flowering season. Collect some pollen and look at it under a microscope.
As the female flowers mature, reinforced plastic bags are tied over them to prevent accidental pollination. When the male flowers are almost ripe, they are collected, labeled by parent tree (clone), and their pollen is extracted under controlled conditions. The pollen is then injected through the plastic bags, directly onto selected female flowers. This process is called controlled pollination.
The bags are removed after the pollination season, and the female flowers are labeled so the manager can keep track of both parents of the seed produced by that flower. Since a single orchard may contain thirty to forty clones, a large number of different crosses can be made in a single season.
Fertilization does not occur immediately after pollination. The flowers fertilized in the spring of one year will not become ripe seed-bearing cones until the fall of the next year. When the cones are ripe, they are collected and the seed is extracted. This seed is used to grow new trees for reforestation.
Not all crosses produce good offspring. The only way to know which are good and which are not is to plant the seed and evaluate the trees. The evaluation includes monitoring the growth rate, disease susceptibility, and general tree form. This process is called progeny testing, and usually takes from six to twelve years. The best crosses from the progeny test are used for continued breeding; those that don't perform well are eliminated from the program.
After one generation (or cycle) of breeding, tree improvement has resulted in trees that are 7-12% better overall. With increasing population and high demand for wood products, tree improvement is another way forest science serves the needs of people.
Depending on your location, a tree improvement specialist may be available to demonstrate grafting and controlled pollination for your class. If you are near a seed orchard, a tour could probably be arranged as well. To explore these possibilities, contact the Forestry Commission or one of the large timber companies near you.