Production of Seeds of Forest Trees

P. E. HOEKSTRA, E. P. MERKEL, AND H. R. POWERS, JR.

Two MILLION acres of forest land were planted with tree seedlings, for which about a thousand tons of seeds from a multitude of species had been collected, in the winter of 1959.

In 1985 or thereabouts we shall know the results, but until then we cannot be sure whether the trees will be tall and straight or crooked and runty. We are not sure because the genetic quality of most of the seed was not known. In fact, the pine species probably were of a low grade because the seed collectors got much of it from low and bushy trees that they could climb more easily than taller, better trees.

It is too bad that only a few people were concerned' about tree seed, that forests were left to regenerate themselves, that harvested trees often were replaced by undesirable species, that new stands were sparse or failed to come up. Many people considered it farfetched even to think about controlling the quality of the seeds.

The situation has changed quickly and radically. Forestry practices have been intensified in a hundred ways. The planting of forest seedlings has mushroomed, and millions of acres are being planted. Foresters see the need and the opportunity for improving the planting stock.

It is not easy to do so. To provide superior tree seed in the needed amounts takes time. The task is to achieve large-scale production of the best possible seed as quickly as possible.

Selection of good trees as parents,breeding stock from them, and testing the progeny will produce forests of better trees that grow much faster than they do today. In the meantime, land- owners have had to settle for seed that is only a little better.

The most efficient way yet devised to produce large amounts of genetically superior seed is to make a seed orchard, a plantation of genetically improved trees that is intensively managed to produce large seed crops.

We have no production figures of seed orchards in the United States, because they are too young to produce seeds in quantity. Two American natives, loblolly pine (Pinus taeda) and slash pine (P. elliottii), produced 43 and 25 pounds of viable seeds per acre at 20 years of age in Australia. From that, we assume that an acre of seed orchard should produce enough seeds each year to plant at least 200 acres in the United States.

TO ACHIEVE genetic improvement, a number of steps must be taken before the seed orchard can be planted.

The heritable characteristics of the parent trees must be defined. Improvement of growth rate, tree form, and wood properties generally is the aim.

Sometimes special features are emphasized, such as resistance to blister rust in white pine (Pinus monticola, P. strobus), high yield of oleoresin in slash pine, or drought resistance in loblolly pine that is to be grown in some parts of Texas.

The next step is rigorous and careful selection of parent trees within the range of the species that is to be propagated. Selections are grouped by geographic races of the species if the existence of such races is known or suspected. Geographic races have been indicated in loblolly pine, Scotch pine (P. sylvestris), white ash (Fraxinus americana), green ash (F. pennsylvanica), black cottonwood (Populus trichocarpa), slash pine, shortleaf pine (P. echinata), and longleaf pine (P. polustris). Most species with a wide physiographic range may contain two or more races.

The next step is tests to determine how much genetic improvement the selected parents provide. The tests involve breeding the selections and observing the growth rate, form, and wood properties of the resulting progeny. If the young trees do well, the parents are accepted as breeding stock.

To save time, this step may be postponed until the seed orchard is established. Progeny tests can then be made while the orchard is growing up, and unsatisfactory parents are removed as the results of the tests become available.

The final step is vegetative propagation of the parents and planting in the seed orchard. Grafting is the most widely adopted propagation method for establishing a seed orchard, but air layering or rooting of cuttings can be used.

We know little about the relationships between rootstock and scion in the coniferous species. Genetic and physiological incompatibility between the two sometimes cause abnormal growth, imperfect union at the point of grafting, and death. Until the causes of incompatibility are better known, most workers use random nursery seedlings for grafting stock.

The seed orchard should be in a place where there will be no damage from wind, snow, and ice. Soil fertility and internal drainage should be excellent. The area should be easily-accessible to facilitate intensive management, which requires expensive equipment and skilled labor.

The orchard should be isolated from contaminating pollen sources. For southern pines, for example, an isolation strip at least 400 feet wide is recommended. The isolation strips could serve as pasture or cropland or they could be planted to trees of a commercial species that does not hybridize with the orchard trees.

The size of the seed orchard has a bearing on its efficiency even though the strip can be made productive.

B. J. Zobel, at North Carolina State College, calculated that a square seed orchard of 4 acres surrounded by a 500-foot isolation strip occupies 40 acres in all. He recommended that seed orchards. be made as large as circumstances permit. The Georgia Forestry Commission, following this reasoning, started a 325-acre orchard for slash pine and loblolly pine.

The orchards contain a number of plants propagated from each parent tree, which constitutes a clone a group of plants derived from a single individual by grafting, for example.

We have no firm experimental evidence on the minimum number of clones necessary for adequate cross-pollination. Most European workers give a range of 9 to 50 clones. Dr. Zobel reported that most seed orchards in the Southeastern States contain 15 to 25 different clones on each acre.

Clones should be arranged in a random pattern to permit statistical analysis of clonal differences. Two plants from the same clone should have at least two other plants in between to reduce pollination within the same clone.

Spacing should be wide enough to allow good development of tree crowns and rate of growth. Final spacings of 20 X 20 feet to 30 x 30 feet are recommended. The wider spacing is for the large-crowned species. For slow-growing conifers with narrow crowns, initial spacing may be 15 x 15 feet, to make sure that the pollen production, which is low at the start, will be used more efficiently.

Spacing can be increased by removing every other tree when the trees become crowded.

A permanent sod cover in the seed orchard prevents erosion, but it competes with the trees for moisture and nutrients. It probably is best to clean-cultivate the whole area or around the base of the trees during the years that they grow rapidly in order to conserve moisture.

Irrigation may be feasible when moisture is critical.

Nutrient levels in the soil may be improved by applications of fertilizer and the use of leguminous cover crops.

The best system of orchard management must be worked out for each species and soil region.

Seed collection would be simplified if the orchard trees could be kept at a maximum height of 25 to 30 feet. Pruning to achieve that and to develop rounded, bushy crowns would be simple in hardwoods but does not offer much promise for conifers. Severe pruning to limit the height may reduce seed production of coniferous species because most of the cones are found in their upper parts.

TREE SEED ORCHARDS in the United States produced a small amount of seed in 1961. It will be at least 1970 before the orchards will add much to the supplies of seed of some species. Fewer than 500 acres of tree seed orchard had been established in 1961, but the area was being expanded rapidly, notably in the Southeast.

T. O. Perry and Wang Chi Wu, at the University of Florida, estimated that 6 thousand acres of orchard will supply all of the slash and loblolly pine seeds needed. We have no estimates of this kind for other species.

Seed orchards in North America in 1961 included such species as slash pine, loblolly pine, shortleaf pine, eastern white pine, western white pine, ponderosa pine (P. ponderosa), red pine (P. resinosa), Douglas-fir (Pseudotsuga menziessi), noble fir (Abies nobilis), and hemlock (Tsuga heterophylla).

The parents in only two of the seed orchards had been progeny tested; a small experimental slash pine seed orchard near Lake City, Fla., for high yield of oleoresin, and a shortleaf seed orchard near Union, S.C., for resistance to the littleleaf disease.

Seed orchards will produce large quantities of seed efficiently, although genetic improvement may be slight at the start. They have practical advantages over random collection of seed from natural stands. Large amounts of seed can be collected with less effort and at the proper time. Seed yields per bushel of cones will be greater because of better cross-pollination. Seed orchards, above all, afford good opportunities for stimulating and protecting abundant cone crops through intensive management practices.

SEED-PRODUCTION areas can serve until seed orchards come into production. They are good natural or planted stands of seed-bearing age, which have been thinned to provide the best possible growing space for the remaining trees.

As a rule, not more than 30 to 40 of the best formed trees are left after one or more cuts. Genetic improvement by this practice is uncertain, but since it insures that seed comes from the best trees, it is an improvement over random seed collection without any control over the source of seed. This type of control is important because poor form can be inherited. One southern pine tree, whose poor form can be passed on to the next generation, may produce enough seeds to plant 30 to 40 acres.

Several thousand acres have been converted to seed-production areas for slash pine, loblolly pine, red pine, eastern white pine, western white pine, ponderosa pine, sugar pine (P. lambertiana), and Douglas-fir. The total area in seed-production tracts is being extended rapidly over the whole Nation. The most rapid expansion is taking place in the pine belt of the Southeast.