POPLARS CAN BE BRED TO ORDER

ERNST J. SCHREINER.

Scientific breeding has given us the hybrid poplars that grow faster than our native species and resist better the inroads of insects and disease. New hybrids now can be practically bred to order.

Poplars offer outstanding possibilities for rapid improvement through scientific breeding for several reasons. First of all, nature has provided a wide diversity of germ plasm, the stuff that controls inheritance; there are a large number of relatively rapid-growing species and varieties that extend over a wide climatic range. Because poplar species hybrids are reasonably fertile, one can create new types that combine the best characteristics of many species and varieties. Breeding is simplified because branches cut from mature trees can be made to flower and fruit in the greenhouse. Most poplars can be reproduced easily from stem cuttings. Thus inherently excellent trees can be utilized almost at once without continued breeding for many generations to get the type true from seed; a new and improved hybrid can be multiplied by cuttings with the assurance that every tree will be exactly like the selected individual.

The painstaking, patient work of improving forest trees began in Germany in 1845, when Johann Klotzsch crossed two species each of pine, oak, elm, and alder, and observed that after 8 years his hybrids averaged one-third taller than the parent species. In the following 60 years scientists accumulated additional evidence on the occurrence of hybrid vigor in crosses between tree species and varieties, but there was no effort to create better forest trees by scientific breeding. Augustine Henry, professor of forestry in the Royal College of Science, Dublin, was the first forester to do something about it and to urge strongly that the artificial production of trees by crossing was a new and important field of forest research.

"In countries like our own," he said in 1910, "the only hope of salvation for forestry is in growing timber rapidly. . . . We are ourselves making some experiments in cross-fertilization this year; but more workers are required in this field. Hitherto, nothing whatever has been done to improve the breeds of forest trees; and foresters have never even thought of the possibilities in this direction, though gardeners and farmers have shown the way for centuries."

Four years later he described several hybrids he had made, including a vigorous poplar hybrid ( X Populus generosa), and again directed attention to certain first-generation hybrid trees that, as in other plants, "are remarkable for their size, rapid growth, early and free flowering, longer period of life, the ease with which they can be multiplied, and in all probability their comparative immunity from disease."

IN THE UNITED STATES, suggestions on breeding forest trees began to appear in the reports of the American Breeders Association in the early 1900's. But hybridizing poplars so as to produce fast-growing trees for reforesting cut-over pulpwood lands was first advocated in 1916 by Ralph H. McKee, then head of the Paper and Pulp School in the University of Maine.

Dr. McKee, convinced of the practical possibilities by Augustine Henry's hybridization results, turned for help and advice on a breeding program to A. B. Stout, plant breeder and director of laboratories at the New York Botanical Garden. The Oxford Paper Co., in Maine, agreed to finance the program, and in April 1924 the work was started in cooperation with the New York Botanical Garden. Botanists and foresters had laid the ground work; the persistence of Dr. McKee, a research chemist, brought the financial support for large-scale hybridization.

The project had headquarters at the New York Botanical Garden under the supervision of Dr. Stout, who was directly and solely responsible for the planning and direction of the poplar breeding. The most complete collection in the country of poplar species and varieties of blooming age was in Highland Park, Rochester, N. Y. Within 2 years the breeding work at the New York Botanical Garden and Highland Park had produced thousands of new hybrids; the hybrid seedlings in pots filled two entire greenhouses. Late in 1926 the Oxford Paper Co. established near Rumford Falls, Maine, a nursery devoted entirely to the propagation of the best of those hybrids. There, more than 13,000 seedlings, which represent 99 cross-combinations among 34 types of poplars, were set out in forest plantations in 1927 and 1928. Those seedlings represent thousands of new combinations of germ plasm.

Sixty-nine of the most promising hybrids were being propagated for large-scale reforestation planting in 1932, when a change in pulpwood utilization practically eliminated the Oxford Paper Co.'s need for poplar pulpwood; research chemists had adapted the soda process to the utilization of birch, beech, and maple. Previously, only aspen wood, the native poplar, was used for soda pulp to provide the short-fibered stock necessary for the manufacture of most high-grade papers. Birch, beech, and maple grow abundantly near Rumford, and good business dictated their use. Nevertheless, the company continued to maintain the hybrids because of their potential value for reforestation. During the depression years the project was on a bare maintenance basis and it was not possible to establish test plantations of the best hybrids.

Congress appropriated funds for research in tree breeding in the Northeast in 1936, and the Oxford Paper Co. transferred the new hybrids and its breeding records to the Northeastern Forest Experiment Station. It took several years to propagate stocks of the selected hybrids, and then, just when planting stock in sufficient quantity for large-scale field tests was available, the war stopped the work, and it went back again to a maintenance basis.

In 1947 we started once more to build up our growing stock of 200 selected hybrids for comprehensive forestation tests throughout the Northeast. We hope this time to be able to complete the job. Research with living trees cannot be slowed down, stopped, and started again, without loss of results out of all proportion to the length of the inactive period. Nature keeps her steady pace without regard for depressions and wars; trees that can be transplanted this spring will be too large next year; trees that are dying this month can provide symptoms of their malady if they are examined in time, but next month may be too late; a thinning in hybrid poplar delayed a year can result in more than merely retarded growth it can start the entire stand on the road to degeneration.

DURING OUR YEARS of work with it, we have thought of the hybrid poplar as the farmer's tree a tree that can provide a forest income for many farmers, with marginal land and depleted wood lots, earlier than the slow-growing species can.

The rapid growth of the hybrids has been demonstrated in the original plantations in western Maine, where the growing season is relatively short. They were planted 6 by 6 feet apart on farm land abandoned about 60 years ago. They were never thinned (contrary to good practice) because we wanted to let natural selection eliminate the weak.

Judging from the growth of the selected hybrids in the unthinned plantations, we can predict that with good forestry methods the poplar hybrids will produce at least 40 cords of wood an acre in 15 years. The prediction is based on the actual growth of individual hybrids whose equally vigorous neighbors gave them far greater competition than would be permitted under good forestry. At the prewar roadside price of $8 a cord, we figure a gross income of $320 for 40 cords an acre over 15 years-10 cords an acre from thinnings between the eighth and tenth years, and 30 cords an acre at 15 years. Properly thinned stands need not be cut in 15 years; in 20 to 25 years they would produce logs for veneer or lumber.

As for net income, a farmer would calculate the amount of work he would have to do on his plantation. Each newly planted acre will require approximately 10 days of work from planting to harvest, of which all but the planting and cultivation in the first year can be done during slack seasons. The work of the first year ( preparation of land, planting, and cultivation, which would take 4 man-days an acre) would be required only once when the timber is harvested the hybrid poplars will regenerate themselves from root suckers. In the third or fourth year, thinning the trees would take 1 man-day an acre. Between the eighth and tenth year, thinning would require approximately 5 man-days an acre. Besides work required to grow the crop, there will be the harvest labor, which can also be handled as a winter or off-season job. If the farmer does the work himself, he simply transfers the difference between gross and net income from one pocket to the other; if the work is done by hired labor, the net income will compare favorably with that from many other farm crops.

These estimates are based on growth in Maine, where the growing season is short. With a longer growing season, the hybrids will grow faster. Dormant cuttings (12-inch lengths of 1-year-old stems without roots) of 102 different hybrids were planted at the Agricultural Research Center in Maryland in the spring of 1947. Fifty of them grew to an average height of 6 to 8 feet in a year. In 1 year from cuttings, the same hybrids would grow only 2 1/2 to 3 feet in Maine and 4 to 5 feet in western Massachusetts.

OTHER CHARACTERISTICS than rapid growth have been considered in making the selections. The hybrids were selected for resistance to poplar diseases present in the Maine plantations, and they now are being subjected to direct inoculation tests. The variation among the new hybrids in susceptibility to damage by Japanese beetles has been observed for several years in Connecticut, Pennsylvania, and Maryland. Only a few have so far been found susceptible to the insect.