The Use of Hybrids

The use of hybrids offers exceptional opportunities for increasing crop production, and timber trees may here be considered simply as a forest crop. Tree strains that cross readily do not occur mixed together in the forest, else they would eventually "blend" into one form by hybridizing. We therefore find, except in a small number of specialized cases, that crossable species, varieties, or other categories of plants Occupy different localities. Because hybrid trees are usually intermediate between the parents in growth rate and other quantitative characters, the hybrid can be used to advantage in the region where the slower growing parent occurs.

A good example of this type of hybrid is that produced by using the pollen of a hybrid .( Jeffrey pine X Coulter pine) on one of the parent forms ( Jeffrey pine), the resultant hybrid being known as a backcross. Seeds of the backcross hybrid and of wind-pollinated Jeffrey pine were collected from the same parent tree and planted together in the seedbed. At 3 years of age the hybrid was 184 percent taller than the pure Jeffrey pine. Seeds from the pollen parent were not available, but pure Coulter pine seedlings grow at about the same rate as the backcross hybrid. The hybrid, therefore, will be planted in the range of the slower growing parent, the Jeffrey pine.

Another intermediate type of hybrid is that obtained by crossing the poorly formed, and not very desirable jack pine of the Lake States with the straight-growing lodgepole pine of the Sierra Nevada. At 3 years of age the hybrid trees approximate or slightly exceed the height of pure jack pines and are 179 percent of the height of lodgepole pine. The logical locality in which to use this hybrid in forest planting is where jack pine is now planted, because the hybrid has the straight, erect growth habit of the lodgepole pine.

While the intermediate type of hybrid has a definite field of usefulness, there is another type of hybrid with even greater potential value. This is the type that shows hybrid vigor or, to the geneticist, heterosis. This phenomenon, although not yet fully understood, is of tremendous value to the agricultural world. It is hybrid vigor that makes some of the poplar and pine hybrids grow at twice or more the rate of the parental forms. This means that a timber tree adaptable to the location and having hybrid vigor may grow to harvesting size in one-half or one-third the time required for a good, nonhybrid forest tree to reach the same size.

The hybrid between the eastern and the western white pines, as an example, shows greater growth capacity than either parent. At Placerville the two parent forms grow at approximately the same rate, but the average hybrid at 3 years of age was 232 percent of the height of the seed parent, the western white pine. The difference in volume, or weight, was even greater. At 4 1/2 years the cut-off top of the largest eastern white pine seedling weighed 64 grams, that of the largest western white pine 72 grams, and that of a large (but not the largest) hybrid seedling 232 grams, or 322 percent of the weight of the better parent. This hybrid warrants trial wherever white pines are planted for timber production. It is being tested for resistance to blister rust by Dr. Willis Wagner, of the Department of Agriculture.

With hybrid trees, as with other new things, costs and quantity production must be considered. A record has been made of the cost, in man-days, of hybrid seeds and of the quantity of seeds it is possible to produce. In pine hybridizing, bags are placed over the young conelets that are to be pollinated. This precaution prevents contamination by air-borne pollen of the forest. Because this technique is always used, the bag is a convenient unit of measurement of effort. What is termed a 1,000-bag program carried out by skilled and experienced workers on large trees in the forest would require from 30 to 45 man-days. (The requirements and yields of the 1,000-bag program are estimates based on the use of only a few dozen bags, as manpower shortage has prevented carrying out a 1,000-bag program for any single cross.) The yield per 1,000 bags has varied from 6,500 hybrid seeds for the poorest to a maximum of 432,000 hybrid seeds for the best yield.

By applying the interplanting method, that is, using hybrid seeds for every sixth row, for example, at a spacing of 6 feet by 6 feet, and deducting 40 percent from total seed number for possible losses, the acreage that could be planted per 1,000-bag breeding program would range from a minimum of 19.5 acres to a maximum of 1,296 acres. As more experience is gained, costs no doubt can be reduced.

In some genera of trees hybrid vigor is rare. In others, such as the poplars and the pines, hybrid vigor is more frequent. To date, there is no predicting which crosses will exhibit hybrid vigor and which will not. Of the 12 hybrids between pine species produced at the Institute since 1940, 4 show definite hybrid vigor at 3 years and I or 2 others, as yet too young to evaluate, look promising. The remainder are intermediate. Trees that show hybrid vigor will probably become the most used planting stock of the future. Standard strains or intermediate hybrids will have little appeal for the forest planter, except in special cases where resistance to insects, disease, or drought may influence the decision.

Thus far, few hybrid trees have bee' used in forest planting. R. H. Richens of Great Britain, in the most complete review to date of the literature of tree breeding, lists 405 forest-tree hybrids. Of these, only 54 are coniferous trees, which yield the greater part of the world's construction lumber, while 351 are hybrids of nonconiferous species that are used for paper pulp, cabinet work, match stock, et cetera. The world, facing a rapidly shrinking supply of timber, is looking for some means of relieving the situation. There is a rapidly increasing pressure to try out hybrid trees in forest planting. The indication that fast-growing hybrids can be produced in abundance, together with the possibility that financial risk can be reduced during the necessary trial period by interplanting the hybrids with known varieties, tends to increase interest.

This trend is international in scope. There is hardly a week that does not bring inquiries from foreign lands to the Institute of Forest Genetics. Some of these inquirers wish to send young men for training in hybrid-seed production. Others want information on methods of hybrid production and still others wish to have hybrid seed sent to them.

As for plans in this country—there will be a large increase by many agencies in hybrid planting, both of the hardwoods, such as poplar and chestnut, and the coniferous species, such as the pines. As many hybrids are being produced at the Institute of Forest Genetics as the available manpower permits. Two or three of the most promising new hybrids are being used in this effort. The California Region of the United States Forest Service has started to use the hybrids in forest plantings throughout the State, giving them an actual field test under a wide variety of conditions.

THE AUTHORS

Palmer Stockwell is a geneticist in charge of the Institute of Forest Genetics, a branch of the California Forest and Range Experiment Station, maintained by the Forest Service in cooperation with the University of California at Berkeley, Calif. He has served with the Boyce Thompson Southwestern Arboretum in Arizona, the Carnegie Institute of Washington at Palo Alto, Calif., and the Soil Conservation Service in New Mexico. He is a graduate of the University of Arizona and holds the doctor's degree from Stanford University.

F. I. Righter, geneticist, was with the Eddy Tree Breeding Station before it became the Institute of Forest Genetics. A graduate of Cornell University, where he subsequently taught forest management, he worked on cane plantations in Cuba and Hawaii, and for the Forest Service in the Southern States.