E.J. SCHREINERE.J. SCHREINER is principal geneticist, U.S. Forest Service, Northeastern Forest Experiment Station and is stationed at Durham, N. H., working in cooperation with the University of New Hampshire.
THIS REPORT of tree breeding research and its application to practice in the United States concerns the period since 1936; the status of forest tree improvement as of that year was reviewed in 1937 (Schreiner 1937). For the preparation of this paper a questionnaire was sent to approximately 190 individuals and organizations engaged on some aspect of genetic improvement of forest trees. Information was requested on the year improvement work was initiated, personnel employed at the start and at the present time, a brief summary of major results and contributions with reference to both research and application to practice by species, kind of improvement work, and objectives.
This summary can only highlight the picture of tree breeding in. the United States. The questionnaire undoubtedly failed to reach all individuals and organizations working on tree improvement, and the mass of information received from 107 correspondents would require several hundred pages for even reasonably complete coverage.
There were 7 reporting units ¹ with 9 professional man-years engaged on genetic research with forest trees in 1936. In addition, ten United States Forest Service experiment stations, two state forestry organizations, and one forestry college reported seed origin studies. There is no record of the personnel employed on these studies; they were a part of the applied research in silviculture. Since 1936 the number of units reported engaged on tree improvement has increased from 7 to 135 and the reported professional personnel from 9 to at least 231 man-years.
¹ The term unit will be used in this report to indicate a working unit that may or may not be part of a larger organization; reporting units may have more than one " project " in research and/or application to practice.
From a wish to stress application to practice, the reporting units are classified as primarily research or primarily practice. These are admittedly arbitrary primary objective classifications. Most research units are also involved in some aspect of application, and many units in the practice category are carrying on some research. Genetic improvement work in 1936 was devoted entirely to research. In 1968 there were 62 units and 145 professional man-years (exclusive of graduate students) classified as primarily research and 73 units with at least 86 professional man-years in the primarily application to practice category (Figure 32).
The expansion of activity on tree improvement started slowly after 1936, accelerated during 1950-59, and began to level off in the present decade. In 1950 it was still possible to challenge the forestry schools (and the profession) as follows (Schreiner 1950): " Interest and activity in forest genetics is still limited largely to the research field; the leavening is present but it has hardly begun to make appreciable inroads into the rank and file of the profession. Appreciable progress. will not be possible until the majority of professional foresters are aware of the importance of inheritance and variation for forest management, are alert to genetical differences and are looking for inherent variation among their timber trees. To my knowledge forest genetics is not an undergraduate requirement in any forestry course. elementary forest genetics is an important complement to the technical education of the professional forester, and as such it should be an undergraduate requirement. "
The expansion of tree breeding and application to practice for the periods 1924-1936, 1937-1949, and for 5-year periods since 1950, is shown in Figure 33. Cooperative university/industry tree improvement programmes for application to practice in the south, the southeast, and the Pacific northwest were initiated between 1950 and 1959. This resulted in rapid expansion starting in the period 1950-1954, and the boom c continued into 1955-1959. The past decade has witnessed the gradual and inevitable deflation of this fortunate boom. However, there is ample evidence that forest tree improvement research and particularly the application to practice will continue to increase at an economically rational pace.
In 1968, the number of professional man-years applied, primarily to research and application to practice, respectively by the Federal Forest Service, forestry schools, agricultural experiment stations (including federal research units other than the United States Forest Service), state forestry services, and privately financed organizations, is shown in Figure 34.
ROLE OF THE UNITED STATES FOREST SERVICE
The Forest Service has continued to play an important role in the expansion of tree improvement since 1936 when its two research projects represented more than 75 percent of the. then active forest genetics research. Since 1956 state and private forestry of the Forest Service and the national forests have been active in promoting the application of tree improvement to practice.
Research
The objective of Forest Service research is to learn the basic principles and methods needed to produce improved varieties. When research has developed methods for producing improved varieties or foundation stocks, then the mass production of such varieties becomes the responsibility of production organizations.
Research is conducted at three institutes of forest genetics and a number of breeding projects. Each project is responsible for the basic and applied research needed for the production of improved varieties adapted to its locality. Basic research problems having broad regional or national importance are assigned to an institute or sometimes to a breeding project where the problem can best be investigated. Silviculture, insect, disease and forest products research projects define and perfect ways of assessing desirable traits. When projects are not available to investigate these problems, specialists in these fields may be added to genetics projects.
State and private forestry
The state and private forestry sector of the Forest Service became involved in tree improvement work because numerous allied fields of endeavour which were already responsibilities supplemented developing tree improvement programmes, and because most state programmes were in part federally financed.
The Agricultural Act of 1956, Title IV, paved the way for some of the first efforts on tree improvement by state agencies under cooperative state/federal financial assistance. Twenty-eight states have now established cooperative programmes for tree improvement under Title IV. It is hoped to expand these cooperative efforts because tree improvement work in numerous midwestern, Great Plains, and western states must be accelerated if the forest resources of those states are to keep pace with and contribute their share to the future raw wood resource base of the United States.
National forests
The primary objective of the tree improvement programme in the national forests is to produce genetically improved seed and seedlings required for reforestation of national forest lands. As of 30 June 1968, the national forests had established 478 hectares in 24 seed orchards and 2170 hectares in 165 seed production areas.²
² Natural stands or plantations heavily rogued to preserve only the best phenotypes for seed production.
A second objective is to develop and demonstrate methods and procedures for producing such genetically improved forest tree seed, for establishing seed orchards and seed production areas, and for producing hybrid seed. To encourage other timberland managers to carry on tree improvement programmes, the Forest Service will make propagating material available to them from trees in the national forests to the extent that such use does not conflict with other national forest needs and regulations.
ROLE OF COOPERATIVES
There are now at least 16 region- or state-wide cooperative groups comprised of a few to as many as 27 member organizations; the importance of such cooperation can hardly be exaggerated. Units that are primarily engaged in research, such as Forest Service research projects, genetics projects at forestry schools and agricultural experiment stations, and forestry research foundations, usually provide the scientific guidance; cooperators may or may not help to finance the organization that provides such technical direction. Cooperators usually supply the land, labour and equipment for locating and propagating plus trees, and for the establishment, maintenance and protection of seed orchards and experimental plantings.
Industry cooperatives provided some of the earliest large scale cooperation and have been the prime movers in the expansion of application to practice. For example, the North Carolina State University Industry Cooperative is supported by 23 member companies in 13 states and 3 state forest services. Landholdings of its members total 8 million hectares; the objective is to provide improved stock to plant 120000 hectares per year.
Cooperation on both applied and basic research is also an accomplished fact in many areas. For example, the cooperative blister rust resistance programme for eastern white pine involves eight agencies in three states. This extensive programme could not be carried out without such cooperation.
Regional forest tree improvement conferences have greatly facilitated the exchange and dissemination of information, and personal contacts between tree improvement workers and practicing foresters and forest managers. There are five such conferences representing the central states, lake states, northeastern, southern, and western regions, respectively.
The accomplishments, in both application to practice and in research, have fully justified the expansion of work on genetic improvement during the past two decades; the reported work is listed by species and fields of activity in Tables 11, 12 and 13.
SEED ORCHARDS AND SEED PRODUCTION AREAS
The largest areas are in the genus Pinus (Table 11); correspondents reported a total of 2181 hectares of clonal seed orchards, 152 hectares of seedling seed orchards and 3372 hectares of seed production areas. Pinus taeda and P. elliottii occupy the largest areas of both seed orchards and seed production areas. P. echinata ranks third.
A total of 114 hectares of clonal seed orchards, 1 hectare of seedling seed orchards and 178 hectares of seed production areas were reported for Pseudotsuga menziesii (Table 11). Nine additional conifers account for 44 hectares of clonal seed orchards, 3 hectares of seedling seed orchards and 55 hectares of seed production areas (Table 12).
The selection of hardwood (angiosperm) species and the establishment of seed orchards and seed production areas have just started; 47 hectares of clonal seed orchards, 8 hectares of seedling seed orchards and 28 hectares of seed production areas were reported (Table 13).
PRODUCTION OF IMPROVED SEED AND PLANTING STOCK
Seed orchard and seed production areas
Pinus elliottii and P. taeda clonal seed orchards and seed production areas have already come into commercial production. Fragmentary reports indicate that P. taeda seed production on an annual basis has reached 2087 kilogrammes from clonal seed orchards and 3232 kilogrammes from seed production areas. For P. elliottii the reported figures are 1950 kilogrammes from grafted seed orchards and 209 kilogrammes from seed production areas. P. virginiana, P. echinata, and P. serotina orchards are also beginning to produce seed.
Production of planting stock from clonal orchard seed by the Georgia Forestry Commission since 1965 has been 23.4 million seedlings of P. taeda and 13.3 million of P. elliottii. Members of the North Carolina State University Industry Cooperative produced 30 million pine seedlings from clonal orchard seed in 1968 and they estimate such production in 1970 at 100 million. The Florida University Industry Cooperators reported the production of 20 million seedlings from P. elliottii clonal orchard seed in 1968 and estimate production of 30 million seedlings in 1969.
Certainly these are impressive and very encouraging figures. But the total present and eventual requirements for planting stock is indicated by the reported 594340 hectares of forest and wind-barrier planting and seeding in the United States during the autumn of 1967 and spring of 1968, and the peak-year planting of 870790 hectares in 1959. On the basis of planting genetically improved stock at 2.4x3.0-metre spacing, the 1968 requirement would be approximately 799 million trees; the 1959 acreage would require more than 1170 million trees.
TABLE 11. - SUMMARY OF IMPROVEMENT WORE WITH Pinus AND Pseudotsuga menziesii
Mass production of species hybrids
Two California units have produced more than 113000 hybrid seedlings of Pinus attenuata x P. radiata for reforesting difficult sites and recreation areas where fast growth is needed. They have also produced more than 200000 Pinus jeffreyi x (P. jeffreyi x P. coulteri) hybrids for sites where the reproduction weevil was a problem. This programme was reduced in 1965 because better plantation care has eliminated losses from the weevil, but breeding is being continued on a small scale for seedlings to be planted on difficult sites and recreation areas. They also report that the F1 hybrids are outperforming the backcrosses.
EVIDENCE OF GAIN FROM CLONAL SEED ORCHARDS
Early progeny tests of southern pines that have already been thinned have shown an improvement of about 15 percent in yield. First-year height of 39 P. elliottii progenies have shown 12 percent superiority for orchard-pollinated seed and progenies of the top 10 clones were 23 percent taller than the checks. Other progeny tests with this species, 5 years old or less, indicate approximately 20 percent gain in growth rate when orchards are rogued, and comparable improvement in quality traits.
The profit to be gained from the clonal seed orchard programmes has been estimated at 14 percent for P. palustris, 18 percent for P. taeda, and 19 percent for P. elliottii. These estimates are based on the costs (in excess of normal stand management) of tree selection, orchard establishment, orchard management and progeny testing. Benefits include reduction in rotation age, quality gain and volume gain.
FOREST TREE SEED CERTIFICATION
There is wide interest in the federal certification of forest tree seeds and planting stock, but the enactment of seed certification laws or legal efforts in this direction has been limited to action in about 20 states. The 38-page South Carolina handbook of tree seed certification standards and standards for forest trees progeny testing published in January 1969 is an example of the scope of state certification standards. The handbook covers in detail general standards, field standards, and progeny testing standards.
The South Carolina Crop Improvement Association has been designated by legislation and chartered by the State of South Carolina as the official seed certification agency in that state; tree seed certification standards were included in the programme in 1960. Three classes of seed are recognized.
1. Certified tree seed. Seed of known genetic identity obtained from trees of proved genetic superiority.2. Selected tree seed. Seed from rigidly selected trees or stands that have promise of genetic superiority but that have not been progeny tested.
3. Source-identified seed. Seed from natural stands including seed production areas with the geographic origin known, and from plantations of known local provenance.
The United States is also interested in the Organization for Economic Cooperation and Development (OECD) scheme for the control of forest reproductive material moving in international trade. The Federal Government has agreed to participate in this scheme and has named the United States Forest Service as the designated implementing agency. The Forest Service is now preparing to offer participation to the states on a voluntary basis.
PLUS TREE SELECTION
The reported selections of plus trees totalled more than 8947 trees in 21 pine species, and 3467 trees of Pseudotsuga menziesii - 1567 for clonal seed orchards or breeding and more than 1900 for half-sib progeny tests (Table 11). More than 1044 selections were reported in 10 other conifer species (Table 12). In excess of 1036 selections have been made in 24 hardwood species and hybrid poplars of known parentage (Table 13).
Selections include plus trees for use in seed orchards, progeny tests, gene pools, breeding arboreta and breeding programmes for specific traits. There have probably been several hundred additional (unreported) selections of Pinus sylvestris for traits desired in Christmas trees. Two of the early single-trait selection projects were for blister rust resistance in P. strobus and in P. monticola. The P. strobus selections now include more than - 500 rust-free individuals for use in a large-scale resistance breeding programme. Selections in P. monticola include about 100 trees reselected for general combining ability in transmitting above-average levels of rust resistance.
Another early project to improve a specific character was the selection, controlled breeding, progeny testing, and the establishment of seed orchards to develop high oleoresin-yielding strains of P. elliottii. Intensive search over six of the northeastern states has resulted in the selection of 27 high sap-sugar producing trees of Acer saccharum and 27 additional high producers as assurance against loss of the first-line selections.
PROGENY TESTING
Full-sib progeny tests
Full-sib tests were reported with 11 species of conifers (Tables 11 and 12) and with 4 hardwood species (Table 13). Progeny testing of Pinus strobus and P. monticola for resistance to Cronartium ribicola Fischer started in the early 1940s and early 1950s respectively; this is summarized under improvement of disease resistance. At present the most extensive full-sib testing is with southern pines, to determine the general combining ability of clones in the seed orchards.
TABLE 12. - SUMMARY OF IMPROVEMENT WORK WITH OTHER CONIFERS
Fifteen-year-old progeny tests of Pinus taeda in Texas are being evaluated for growth rate, drought resistance, and other traits. The total area in the Texas tests now approaches 100 hectares and the total number of parents is well over 300.
Members of the North Carolina State University Industry Cooperative have established nearly 200 hectares of control-pollinated progeny tests. The mating design involves crossing each clone to a number of predetermined tester clones (the minimum is set at four clones) chosen at random from the same orchard. In addition to crossing the tester clones with all clones in the orchard, each tester is mated to all other testers. Some cooperators are now starting Phase II orchards from progeny-tested first orchards
The University of Florida Industry Cooperative reported that progeny tests of about 75 percent of the Pinus elliottii clones have been established and that P. taeda progeny testing has been started. Most of these tests are 5 years old or less.
Techniques have been developed for satisfactory evaluation of gum-yielding ability in 2½-year-old progenies of Pinus elliottii and progenies of this species are also being screened for response to fertilizer and fusiform rust resistance.
Half-sib progeny tests
Tables 11, 12 and 13 list 30 species for which half-sib progeny tests were reported. Large-scale tests with approximately 35000 progeny trees of selected Pinus taeda, P. elliottii, P. palustris, and P. echinata parents were established in 58 test plantations in Georgia between 1951 and 1960. More than 46000 half-sib progeny trees of 74 P. taeda and P. echinata plus trees are being tested in Arkansas.
Probably the most extensive half-sib testing is with Pseudotsuga menziesii in the Pacific northwest. More than 1900 parent trees have been selected for a progressive improvement programme that emphasizes half-sib progeny tests rather than intensive initial selection (Siren 1966). The proposal is based on the results of a 50-year study of Douglas-fir families of known parentage; to a large extent, estimates of gain and problems of implementation are known.
For a forest unit of 30000 to 60000 hectares about 300 parent trees, divided between a seed-source group (75 to 100 trees) and a reserve group (200 to 225 trees), are selected along forest roads. The degree of selection may range from random to rigorous as decided by the landowner. Wind-pollinated seed is collected from all 300 trees in sufficient quantity for the progeny test. Cones would be collected from the trees of the seed source group in good seed years to supply the forest unit's planting programme. As progeny test results accumulate, opportunities will be provided to initiate various seed orchard and breeding programmes based on tested parents or families; all 300 selected trees would be saved for perhaps 50 years.
Half-sib tests are under way with Juglans nigra for resistance to drought and to Marssonina juglandis (Lib.) Magn. (walnut anthracnose), and with Acer saccharum for high sap-sugar production. Combination progeny test/seedling seed orchards have been established with various species including Pinus resinosa, P. sylvestris, Platanus occidentalis, and Liquidambar styraciflua.
A summary of research accomplishments is essential to complete the picture of tree breeding in United States forestry practice. Forest research must keep well ahead of practice to provide materials and/or methods that can be applied to practice with a minimum of calculated risk. The gestation period will vary widely, but recent research achievements and research trends point to probable areas of application to practice within the next ten years.
SEED ORCHARD TECHNOLOGY
Research to provide the technology essential for successful establishment, maintenance, protection, and profitable operation of seed orchards has involved a diversity of studies. These include research on stock-scion incompatibility, shearing methods to slow height growth of grafted trees, cultural treatments to stimulate flowering and cone production, methods for control of insects, diseases, and competing vegetation, evaluation of foreign pollen contamination at orchard sites, and mechanical cone harvesting. Research results - such as those that have shown the importance of nitrogen fertilization for pine cone production, and the fertilizer x clone flowering responses that have suggested fertilization of clones according to their individual needs - are being applied to practice.
PROVENANCE TESTS
The objectives of provenance tests are both practical -selection of the best provenances for reforestation - and experimental, including such aspects as evolutionary trends, patterns of genetic variation, amount of genetic differentiation, and genotype-environment interaction. Traits studied include growth rate, adaptability, disease and insect resistance, timber and wood qualities, oleoresin chemistry, and foliar mineral element content.
Provenance trials have been reported for 29 conifer species (Tables 11 and 12) and for 14 hardwood species (Table 13). Some of these tests have been designed for conversion to seedling seed orchards of the best individuals in the best provenances.
Local seed sources may involve minimum risk, but on the basis of early results they are not always superior or even equal to distant provenances. For example, ten-year results of a Pinus taeda study in Arkansas have shown that 28 of the 36 seed sources were superior in volume to the four local sources included in the test; the Atlantic and Gulf coast sources were generally superior to the inland sources. Ten-year data from the Southwide Cooperative seed source study indicate that Pinus taeda and P. palustris from southern coastal sources can be safely planted from 160 to 240 kilometres north of their latitude of origin with a marked genetically conditioned growth improvement relative to local sources. Northern and western provenances of P. taeda have also exhibited a high degree of fusiform rust resistance wherever planted.
There has been increasing interest in testing exotics; this is apparent from the many provenance tests of nonendemic species. Exotics are also being included in an increasing number of gene pools and breeding arboreta.
HERITABILITY STUDIES
There has been rapid increase in heritability studies on many economically important traits including growth rate, pest resistance, stem form, branching traits, morphological traits, seed and cones, needle length and colour (in Scotch pine for Christmas trees), time of flowering, leaf flush and abscission, fiber characteristics, oleoresins, and terpenes. Heritability studies were reported for 24 species (Tables 11, 12 and 13).
GENE POOLS AND BREEDING ARBORETA
Seed orchards, progeny tests of plus trees from widely separated localities, and provenance test plantations will provide a large number of different, broad-base gene pools for future breeding. Respondents have also re ported the establishment of breeding arboreta designed for both intraspecific (racial) and species hybridization.
TABLE 13. - SUMMARY OF IMPROVEMENT WORK WITH HARDWOOD (ANGIOSPERM) SPECIES
Interspecific hybridization will produce the greatest diversity of gene combinations. With reasonable provision for maximum panmixes such gene pools, explosive evolution to provide genotypes for an extensive range of selection may be expected; and from year to year such panmixes can continue to provide new genotypes and variations of previously produced types.
BREEDING FOR RESISTANCE
The reduction of losses from forest pests is an essential requisite for increased forest production. Our national loss to diseases and insects resulting from killed timber has been estimated at 20 percent of the annual cut and loss due to retarded growth as another 20 percent of the annual cut. Thirty units reported active work directed toward genetic improvement in resistance to one or more of the following:
1. Cronartium ribicola Fisch. (white pine blister rust) on Pinus strobus, P. monticola, and P. lambertiana.2. Cronartium fusiforme Hedge. and Hunt (fusiform rust) on southern pines
3. Scirrhia acicola (Dearn.) Siggers (brown spot) on Pinus palustris.
4. Various poplar diseases on species and hybrids of poplars.
5. Endothia parasitica (Murr.) A. & A. (chestnut blight) on Castanea spp. and hybrids.
6. Ceratocystis ulmi (Buism.) C. Moreau (Dutch elm disease) on Ulmus spp.
7. Cronartium rusts on Pinus banksiana.
8. Pissodes strobi Peck (white pine weevil) on Pinus strobus and P. banksiana.
9. Megacyllene robiniae Forst. (locust borer) on Robinia pseudoacacia.
10. Cone insects on conifers.
11. Air pollution damage on conifers.
Research on resistance to some of these diseases and insects and to air pollution has already provided the basis for application to practice, and it is a safe prediction that application to practice for all of these pests may be started within the next decade. Research on forest nematodes has barely begun but the successful work in agriculture and horticulture indicates that a reasonable expansion of forest nematology research also could begin to provide resistant clones within the next ten years.
White pine blister rust
Research on blister rust resistance includes basic research on the nature of the mechanisms and expressions of resistance, evaluation of heritability and studies to determine whether there are races of the pathogen that vary in pathogenicity on the pine host. Efficient resistance-screening methods are available, including a screening method for bark resistance by grafting a ring of infected bark to a healthy stem.
Pinus strobus. The most extensive resistance breeding programme with this species involves the cooperation of eight agencies in three states. Initial selection of rust-free parents has been completed and grafting of each selection is almost completed (537 clones, 4892 grafts). Control pollinations have been made on more than 250 parents using 2 bulked tester pollens from resistance-tested selections.
A broad-base gene pool is being established in Minnesota with progenies of 30 to 40 trees selected in each of many ecotypes in high-hazard zones for rust infection throughout the white pine area of the state. Seedlings will be grown under conditions favourable for natural inoculation and half-sibs from the surviving trees will be used as Phase II mother trees.
Pinus monticola. Research with western white pine has reached the application stage. Narrow-sense heritability of resistance has been estimated to vary from 65 to nearly 100 percent depending on the progeny test. There seem to be several seats of resistance in foliage and bark, and perhaps as many resistance genes active at these sites.
Approximately 400 rust-free individuals in heavily infected stands have been screened and about 100 of these have been reselected for general combining ability in transmitting above-average resistance to four test cross progenies. These reselected GCA parents have been recrossed by mating pairs from within the same elevation zones. Large progenies thus produced are artificially inoculated and survivors will be planted in three elevational-zone seedling seed orchards to produce F2 seed with a second increment of gain in resistance. Orchard F1 foundation stocks are now being grown for outplanting in 1971 to 1974, and F2 seed production by 1985. Sufficient seed production to plant 6000 hectares per year is anticipated.
Second stage selection involving about 3000 candidates has been started to broaden the resistance base and improve growth rate; more than 2000 new candidates were found in 1967 and 1968.
Improvement in progeny testing and artificial inoculation methods have reduced the cost per candidate from over $600 to under $20O, with further improvements in sight. Progeny tests - up to 120 candidates x 4 test crosses and self, x 10 replicates x 16 seed spots x 1 to 4 seeds per spot are presown in the seed house on paper towelling strips with methylcellulose mucilage. Such a test covering 1000 linear feet of 4 foot wide nursery bed can be sown in one or two days with a four-man crew, avoiding errors of hand sowing and bad sowing weather.
Pinus lambertiana. Crossing sugar pines that exhibited natural resistance to blister rust was started in 1959; through 1968 about 2200 progeny trees representing 200 crosses have been outplanted. Forty-six crosses of 26 parents have been adequately tested, four of these have 45 percent or greater resistance. Plans call for selecting and propagating approximately 80 plus trees for seed orchards and pollination of these selections with tested transmitters of resistance.
Fusiform rust of southern pines
Methods have been developed for large-scale screening for resistance in progenies of P. taeda, P. elliottii, and hybrids with resistant species. The best P. elliottii parents thus far screened yield progenies in which rust susceptibility is reduced by 60 percent. Basic research includes gamma radiation of seed and evaluation of seedlings for rust resistance and desirable mutations.
Controlled species hybridization of southern pines has confirmed natural introgression as the most probable basis for provenance resistance to this rust. Interspecific hybridization has produced three southern pine hybrids (P. echinata X P. elliottii, P. echinata x P. taeda, and P. palustris x P. elliottii) field-tested in Mississippi, that are resistant and grow as rapidly or nearly as rapidly as the faster growing parent on the same site.
Brown spot of Pinus palustris
An effective screening method is available for this disease. Half-sib progenies of more than 600 trees have been screened; approximately 10 percent of these trees have been tentatively classified as resistant and have been established in clone banks for future breeding. Intraspecific crosses between resistant parents have been field-tested and have demonstrated a high level of resistance.
Diseases of poplars
Hybrid poplar clones are available that have been evaluated by inoculation and clonal field tests on upland sites in the northeast for field resistance to Melampsora leaf rust, and Septoria, Dothichiza, and Valsa canker diseases. Some of these clones have also been tested for adaptability, growth, and disease resistance in Europe. Clones of Populus deltoides resistant to Melampsora have been selected in the Great Plains and in the Mississippi river valley.
Chestnut blight
Breeding for resistance to chestnut blight has been in progress for many years. Several resistant timber type Chinese chestnut clones and species hybrids are available but lack of a commercially feasible method of clonal propagation has prevented their commercial utilization.
Dutch elm disease
Work on resistance to Dutch elm disease has involved extensive inoculation tests and some hybridization and backcrossing. One unit reports that from 21000 inoculated seedlings representing 55 collections of Ulmus americana, 16 trees survived after 7 to 1:1 severe stem inoculations in 5 to 8 different years, including 3 consecutive years.
White pine weevil
This insect is the most serious pest of Pinus strobus. The possibility of provenance differences in resistance is being investigated in the northeast. An immediate problem is to develop practical and efficient methods for rapid screening for resistance. This has involved considerable study and evaluation of the physiological, morphological, anatomical and biochemical traits responsible for weevil resistance. Cage testing has produced some promising leads.
Controlled pollinations using 24 clones and 6 pollen parents of Pinus banksiana have been made to determine the nature and mode of inheritance of resistance in this species.
Resistance to air pollution
Research on sensitivity or resistance to air pollution is receiving increasing attention, particularly with conifers. Tree-to-tree variation in Pinus strobus in resistance or susceptibility is striking. It is now possible to use selected clones of this species to detect even low levels of air pollution, and to determine something about the concentration level of pollution by the use of clones of different sensitivity. It is also probable that soon it will be possible to identify some pollutants by the type of symptom reactions of P. strobus clones. A seed orchard has been established to test the feasibility of producing highly resistant stocks.
CONTROLLED INTRASPECIFIC AND INTERSPECIFIC BREEDING
Exploratory breeding
Exploratory breeding has laid the foundation for both intraspecific breeding and species hybridization. Research involving selfing, intraspecific diallel crosses and polycrosses, and interspecific hybridization, has provided and will continue to provide the basis for improved breeding methods.
Extensive exploratory hybridization in California has supplied information on species crossabilities in Pinus. This has provided a firm basis for understanding species relationships and their taxonomy, and has contributed to a solid foundation for the mass production of desirable F1 pine hybrids not only in the United States but also in other parts of the world. Exploratory research in the south has shown that all major southern pine species can be crossed.
Between 1937 and 1956 a broad programme of exploratory breeding was carried out in the northeastern region with the results shown in Table 14.
TABLE 14. - SUMMARY OF EXPLORATORY BREEDING IN THE NORTHEASTERN REGION
|
|
Selfing |
Intraspecific |
Interspecific |
|
Acer |
1 2/4 |
1 4/2 |
² 11/22 |
|
Betula |
4/1 |
5/1 |
14/8 |
|
Fraxinus |
0/1 |
3/2 |
4/21 |
|
Picea |
3/4 |
7/1 |
26/17 |
|
Pinus (Haploxylon) |
3/2 |
¾ |
13/33 |
|
Pinus (Diploxylon) |
2/7 |
7/3 |
40/69 |
|
Quercus |
0/8 |
¾ |
2/60 |
¹ (No. of species that produced progenies)/(No. of species that did not produce progenies)² (No. of species combinations that produced progenies)/(No. of species combinations that did not produce progenies)
Selfing
Information on self-compatibility is essential for decisions on the possible use of selfed lines for breeding programmes and its significance in clonal seed orchard practice. Studies with Pinus elliottii and P. taeda have shown that selfing depresses such traits as seed germination and growth rate. Estimates of the degree of natural selfing for these traits are being used to evaluate the extent of inbreeding and pollen contamination in seed orchards. One unit reported that breeding programmes based on inbred lines are not practical for the major southern pine species.
A systematic search in natural populations of Pseudotsuga menziesii has resulted in finding several good selfers, some with conspicuous genetic markers. Selfing studies with this species have also provided an estimate of the genetic load as averaging 9 lethal genes per individual.
Individual variation in self-compatibility has been demonstrated in a number of hardwood species.
Intraspecific breeding
In addition to the full-sib progeny tests for clonal seed orchard evaluation and improvement, intraspecific breeding for studies on heritability of particular traits was reported by 12 units. A major practical objective of selective intraspecific breeding is to develop varieties that combine several desirable traits such as fast growth, environmental adaptability, pest resistance, and other special traits. For example, breeding is in progress to produce a Pinus elliottii variety that combines rapid growth, better timber quality, and fusiform rust resistance with high oleoresin yield. About 100 selections with such qualities are being bred and progeny-tested in cooperation with 15 state and private forestry organizations. This breeding work has provided considerable basic information on variation and inheritance of oleoresin yield, growth rate, stem straightness, crown width, stem-wood specific gravity, turpentine composition, levopimaric acid content in oleoresin and tall oil yield.
Interspecific hybridization
Species hybridization was reported by 31 units in one or more of the following 10 genera: Abies, Acer, Betula, Fraxinus, Larix, Picea, Pinus, Populus, Quercus, and Ulmus.
More than 90 interspecific Pinus hybrids have been produced in California since 1925. In the northeast, 99 successful combinations involving 18 natural Populus species and varieties have been evaluated for adaptability to upland sites; a number of successful species combinations were produced in six additional genera (Table 14).
Crossability barriers in the soft pines, P. strobus x P. cembra, P. strobus x P. kariensis, and P. strobus x P. flexilis, are reported to be the result of embryo inviability rather than gametic incompatibility. Irradiated mentor pollen has been used successfully to accomplish remote hybridization of Populus trichocarpa with P. alba, P. grandidentata, and P. tremuloides. This offers a lead for avoiding embryo inviability in species crosses.
Work on the mass production and testing of Pinus nigra x P. densiflora hybrids was reported from Michigan. In the northeast, two orchards are being established to produce hybrid seed between selected clones of Pinus rigida and P. taeda.
ASEXUAL REPRODUCTION
The production of forest planting stock with maximum and readily manipulated genetic improvement will require practical asexual propagation methods for the use of superior genotypes - without alteration or dilution of their genetic potential - in synthetic multi-clonal hybrid varieties.
There are at least two approaches to this problem: through the search for and use of genes for gametophytic apomixis or adventitious embryony; and through vegetative propagation of cuttings or other propagules. The recent expansion and progress of basic research on vegetative propagation justify the prediction that practical methods will be available for present difficult-to-root species within the next decade.
Tissue culture may or may not provide a direct method for eventual commercial clonal propagation, but it is certain that such in vitro techniques offer particular advantages for basic biochemical and biophysical investigations on vegetative propagation. There has been recent but rapid progress in this area. Complete plant lets of P. tremuloides have been grown from subcultured callus tissue, and work is continuing on callus differentiation and on methods for isolating single cells for possible growth into trees. In vitro culture of diploid callus of all southern pines is also in progress with emphasis on plantlet formation to permit the ' wholesale production " of desirable clones or their own roots.
NATURAL VARIATION, GENECOLOGY AND INTROGRESSION
There is much research on the natural variation of morphological, physiological, and anatomical traits, including wood characteristics. Reports received on genecology and introgression research are far short of actual work in this area. Current research was reported on Populus deltoides, P. tremuloides, P. grandidentata, Betula allegheniensis, B. pumila, B. lenta, B. papyrifera, Picea pungens, Engelm., P. engelmannii and the Abies grandis complex.
GENOTYPE - ENVIRONMENT INTERACTION
Information in this area will be available from clonal, progeny, provenance and other replicated genetics plantations. Respondents have reported specific studies to determine the effect of genotype/environment interaction on localized adaptation, percent summerwood, specific gravity, tracheid length and wall thickness, and pulping and paper-making qualities; and for evaluating the " genetic architecture " of Pinus radiata.
PLOIDY
Much of the research on natural and artificially produced haploids (monoploids) and polyploids is being done with poplars. Three monoploids of P. tremuloides have been obtained from crosses with P. alba pollen, and experimental induction of haploid parthenogenesis has been successful in P. trichocarpa. Triploid poplar hybrids, produced by crossing tetraploid P. tremula on diploid female P. tremuloides, have approximately 30 percent longer fibre lengths and higher specific gravities than comparable diploids. The induction of colchiploidy, with varying success, was reported in Pinus and Abies.
BIOLOGY AND CYTOLOGY OF REPRODUCTION
There is active current research on many aspects of reproduction, i.e., research with Acer saccharum on pollen germination, formation and development of flower primordia, dichogamy, stigma receptivity, self- and cross-compatibility, agamospermy and agamocarpy, and cytological studies of reproduction in Prunus serotina and Ulmus americana. Research was also reported on the induction of normal and hermaphroditic flowers in Pinus, on fertilization in Pinus and Picea, and on sporogenesis and gametogenesis in Picea pungens.
PHYSIOLOGICAL RESEARCH RELATED TO GENETIC IMPROVEMENT
Much of the research in forest tree physiology will have a direct bearing on tree improvement. Research units directly concerned with genetic improvement have reported physiological research under controlled and/or natural environmental conditions on photosynthetic efficiency, identification of the most efficient environmental regimes for particular interspecific and intraspecific populations and production goals, growth relations, flowering, summerwood formation, drought resistance, aging, and biochemical studies related to pest resistance and taxonomy.
There has been much empirical, and recently increasing basic research on vegetative propagation by both cuttings (discussed under asexual reproduction) and grafting with conifers and hardwoods. Grafting studies have demonstrated considerable individual variation in graftability and cone production.
Rapidly expanding research on tissue and embryo culture will provide basic information on the control of growth and development. Tissue culture can supply information needed for clonal propagation of superior genotypes for commercial use or of individual genotypes, such as haploids or polyploids, for use in tree breeding. Embryo culture will have a wide range of usefulness in improvement breeding, particularly for the successful propagation of hybrid crosses that normally result in embryo abortion.
Radiation studies with forest trees have several objectives: to determine tire effect of ionizing radiation on forest tree species; to induce mutations of value for improvement breeding or as markers for genetic research; and to compare the effect of gamma or other types of radiation with other mutagenic agents.
QUANTITATIVE GENETICS
Basic and applied research on quantitative genetics includes design and analysis optimization, breeding theory, allocation of selection intensities in provenance and individual tree selection systems and analysis of introgression. Through studies of quantitative genetics, methods used in plant breeding are being adapted to fit problems encountered with forest trees. Optimum allocation of selection intensities in first and second growth selection situations have been published, as well as estimated rates of gain based on published heritability figures for probable breeding situations which might be employed for southern pines.
SCHREINER, E. J. 1937. Improvement of forest trees. En U.S. Department of Agriculture. Yearbook, p. 1242-1279.
SCHREINER, E. J. 1950. Genetics in relation to forestry. J. For. 48: 33-38.
SILEN, R. R. 1966. A simple, progressive tree improvement program for Douglas-fir. U.S. Forest Service. Note PNW-45. 13 p.
