The general objective of a genetic improvement programme should be the sustainable management of genetic variation to generate, identify and multiply for operational planting high-yielding and well adapted genotypes. For an outcrossing species, breeding typically incorporates:
Establishment of an initial base population.
In its broadest sense, for forest species this includes species and provenance testing, and the development of breeding and gene conservation populations.
For forest species this typically includes recurrent cycles of selection and recombination.
The derivation and multiplication of strains (e.g. families or
clones) to be used operationally.
This is the step in which breeding gains are captured and transferred to production populations.
The above are incorporated into a breeding strategy. A breeding strategy should permit continued improvement, and must allow for future alterations in breeding objectives. A sound breeding strategy also incorporates the early collection of appropriate biological and genetic data concerning the species, and should include measures to maintain a broad genetic base and to minimize inbreeding.
There exists a virtually unlimited number of alternative breeding strategies, varying with respect to, for example:
Timing of the different phases is an important component of a breeding strategy. Thorough species and provenance testing and biological studies are important early objectives. The desirability of early availability of improved genotypes for planting dictates, however, that some selection work will precede completion of the above. Some plantation programmes for certain species may not warrant the expense of an intensive breeding programme, and a genetic improvement programme may include little more than species testing and development of appropriate propagation methods.
It has been estimated recently that, of a total of at least 50 000 tree species, some 400 have been the subject of formal breeding or testing activities of some description, 140 with at least one generation of selection and mating, and 35 of these involving at least 20 seed parents (Committee on Managing Global Genetic Resources 1991). The following is a brief survey of the current status of improvement for a representative sample of these species.
For convenience, the term “industrial” is applied here to species used predominantly in large scale plantations aimed mainly at the commercial production of timber. There are currently about 100 million hectares of industrial plantations worldwide (Gauthier 1991), and it has been estimated that projected demands for industrial timber early next century might be met by a total estate of 100–200 million hectares (Mather 1990). For convenience, industrial plantation species are grouped into broad classes in the following survey. It is admitted though that some groups are quite heterogeneous, and programmes for many species straddle climatic and rotation length classes.
These currently comprise less than 5% of established industrial plantings, but this is a group of rapidly increasing importance due to the potential which exists for expanded plantings in many tropical countries. The most important taxa are E. grandis, E. tereticornis and E. camaldulensis, and notably hybrids between E. grandis and E. urophylla and the other two species. E. grandis is widely planted in moister areas, particularly of South America and Africa, due to its fast growth, good silvicultural characteristics, and good pulping qualities. E. camaldulensis and E. tereticornis, more drought hardy than grandis but without the same ability to capture the site, are very widely planted, particularly in India and Africa (Vivekanandan 1985, Koyo 1989). Other species with potential in tropical Africa, Asia and the Pacific, but much less widely used, include E. microcorys, E. pilularis, E. cloeziana, E. citriodora, E. pellita, and possibly E. deglupta (Burley & Barnes 1989). Private companies have featured prominently in the more well known plantings, but government enterprises are also important.
By the standards of most industrial forest plantation species, the tropical eucalypts are relatively easy to breed:
A few very advanced breeding programmes exist, undertaken by both government institutes and private growers. Four generations of selection and open-pollinated recombination have yielded large gains in the Florida (USA) programme (Squillace 1989). Spectacular and well-publicized successes have been recorded with clonal propagation of hybrids in Brazil and the Congo, although the breeding programmes underlying these clonal selection programmes are not as advanced (Campinhos & Ikemori 1989, Vigneron 1989). Clonal programmes are underway also for E. grandis in other tropical countries, but are perhaps unlikely to result in the large gains reported for the hybrids. These advanced programmes represent a very small proportion of the world plantation estate in tropical eucalypts. Provenance and progeny trials have been planted in many areas, some selections made, and seedling seed orchards established, but much of the planting is still conducted with material subjected to little if any improvement. Vigour, form and wood quality are typically the most important selection criteria. Cold tolerance has been an important criterion in the E. grandis programme in Florida (Meskimen 1983, Rockwood et al. 1989), although some freezes probably exceed the levels which can be tolerated by naturally available variants (Meskimen 1983). Most programmes are characterized by open pollinated approaches to recombination, testing and capture of genetic gain (e.g. Geary et al. 1983). Genetic variation of the tropical eucalypts is fairly well preserved in ex situ and in situ stands.
To summarize, genetic improvement programmes offer clear benefits for industrial plantation forestry with tropical eucalypts, and no major biological impediments to the breeding of these species exist. In terms of practical priorities, there is a lot to be gained by the extension of existing technologies to additional programmes. For the pure species, open-pollinated approaches are relatively simple and give good gains. Insufficient understanding and application of sound breeding strategies is a limitation in some programmes.
For the more advanced programmes, the availability of greater frost tolerance would be advantageous in some areas. The availability of better selection methods would circumvent the problem of the expense of clonal testing, and permit the imposition of higher selection intensities in the clonal programmes. Although generation intervals in these species are relatively short, large gains would be available through even earlier selection and flowering.
Although comprising no more than 2–3% of currently existing industrial plantations, this group is of expanding importance, particularly in plantation programmes by private companies in Chile, Africa and Europe. Around 50 000 ha of Eucalyptus nitens plantations have been established worldwide, while E. globulus plantations are much more extensive - over 1 000 000 ha in Portugal alone (Eldridge & Griffin 1990). Species for which plantings are more limited include E. dunnii, E. smithii, E. radiata, E. fraxinoides, E. McArthurii and E. fastigata.
Although generally grown on short rotations and selectable at a similar age to the tropical eucalypts, these species are generally more difficult subjects for breeding:
E. globulus is characterized by sophisticated and well-planned breeding programmes which have been established in recent years, e.g. the “nucleus breeding” approach in Portugal, and the “multiple population” programmes in South Africa and Chile. Clonal and seedling seed orchards have been established for these species, but are yet to produce seed in required quantities. Selection criteria are principally vigour, stem form, and wood quality. Resistance to winter freezing is of some interest in relation to possible wider planting of E. globulus (Tibbets et al. 1991). Genetic resources of these species are relatively secure.
Addressing the flowering and rooting problems should be major priorities for more effective breeding of these species. Breeding programmes for these species would also benefit from the availability of greater frost tolerance, better selection methods, and even shorter generations, as described above.
Although currently comprising less than 5% of established industrial plantings, this heterogeneous group is of rapidly increasing importance with considerable potential in many tropical countries. Some species, in particular Tectona grandis, Gmelina and the tropical acacias, are already popular species in both private and public plantation operations.
Total plantings of Gmelina arborea probably stand at over 200 000 ha, with Brazil, West Africa, the Philippines and Malaysia the major centres. Potential exists in many other areas of the tropics, although site sensitivity is a limitation. The tree provides a very useful general purpose hardwood, and high quality pulp. With short rotations (7–12 years in some programmes), early flowering (at 3–5 years), reliable seed storage (under the right conditions), ready coppicing, and good rooting of cuttings, the species lends itself to rapid breeding, although only limited improvement has been undertaken. Controlled pollination is considerably more difficult than for the tropical eucalypts. Provenances have been quite widely tested, and seed orchards have been established in a few locations, e.g. Brazil, Malaysia, and the Philippines. Vegetative propagation of phenotypic selections is practised in Malaysia. Selection criteria are mainly stem straightness, cylindricity and volume.
Tectona grandis is widely planted in Indonesia (over 1 000 000 ha), Thailand, India, several other Asian countries, South and Central America and in many parts of Africa (where it is the most widely planted exotic) (White 1991). The species is moderately amenable to improvement - seedlings flower at age six to nine (age of first flowering is positively correlated with bole length to forking), yield of viable seed is often poor, seed can be stored, trees coppice, and juvenile cuttings root readily. Control pollination is feasible, but not easy. Long rotations, and a consequent long selection interval, is a limitation on rapid breeding. Trials in several places demonstrate wide provenance variation. Breeding programmes are in existence in Thailand, India and elsewhere, and many clonal seed orchards have been established (Burley and Barnes 1989, Kaosa-ard 1989, White 1991).
Acacia mangium has been planted extensively in Malaysia, and large plantings are continuing also in Indonesia. Other species such as A. auriculiformis, A. crassicarpa and A. aulacocarpa are also of interest. Seed production commences relatively early, seed can be stored satisfactorily, controlled pollination is feasible although not easy (Sedgley et al. 1992), and juvenile cuttings root well (Wong & Haines 1992). Broad provenance collections have been made and trials planted, and seedling seed orchards established at several centres. Gene pools are relatively secure.
Cordia alliodora is planted in Central America, Colombia, Venezuela and Ecuador, in plantations as well as an agroforestry crop (Newton et al. 1991). Rotations are about 20 years, and timber is of high quality. Heavy flowering commencing as early as two years and reliable seed storage (Greaves & McCarter 1990) are conducive to rapid breeding. Seed collections have been made and international provenance/progeny trials established. Seedling seed orchards have been established in Central America. The species is quite variable (Greaves and McCarter 1990), and some gene pools are endangered (Newton et al. 1991).
Triplochiton scleroxylon and Terminalia superba have been the subject of provenance/progeny tests and clonal trials in Africa, and plantations are being established. Selections are propagated by cuttings from stump coppice in the case of Triplochiton, and shoots from mature scions grafted onto young seedlings in the case of Terminalia (Leakey 1987). Rotation lengths and selection intervals are longer than for tropical eucalypts. Paraserianthes falcataria is planted in the Philippines and Malaysia, provenance/progeny trials have been established, and seedling seed orchards established. Gene conservation collections are being undertaken for Bombacopsis quinata, Sterculia apetala, Alnus acuminata, and Vochysia hondurensis. All have populations in danger of extinction or genetic erosion (Newton et al. 1991). Seedling seed orchards of Bombacopsis quinata have been established several countries in Central and South America including Honduras and Venezuela. Several members of the Meliaceae are potentially very valuable plantation species, e.g. Swietenia macrophylla, S. humilis, S. mahogani and Cedrela odorata. Damage by shoot borers, however, severely limits the extent to which plantations can be established. International provenance and progeny trials have been established of some of these species. Intraspecific variation in levels of resistance is evident, but the extent to which effective levels are available not determined (Newton et al. 1991, Newton et al. 1993). Some of these species have suffered severe genetic erosion. The use of valuable species of the Dipterocarpaceae (Dipterocarpus elatus, Hopea and Shorea) in plantations has been limited by flowering problems, seed recalcitrance, and silvicultural difficulties. Some of these species are also threatened by erosion of gene pools. A large number of other species, as yet untested or poorly known, are of potential use, including species of Octomeles, Cleistopholis, Pterocarpus and Canarium.
To summarize then, some tropical hardwoods compare well with tropical eucalypts as subjects for breeding, while some are a little more difficult. In general though, breeding is not yet as advanced. Broader implementation of good improvement programmes is an important priority for these species. Some other species pose major problems: e.g. insect susceptibility in the Meliaceae, flowering and seed problems in Dipterocarpaceae. Many other potentially valuable species are simply not well known, in terms of their adaptation and biological and genetic features, and gene pools are probably under threat in parts of the species ranges. The testing of potentially useful species, characterization of mating systems, provenance collection, establishment of trials, implementation of gene conservation measures and commencement of other breeding work poses a very large and urgent task.
Comprising perhaps 5–10% of established industrial plantations, poplars and willows are important plantation trees in many countries, including in Europe, North America, China, Argentina, India, Africa, and Australia (Pryor 1992). Small private plantings feature strongly. The group includes a large number of species and subspecies.
Relatively short rotations, early and prolific flowering and ease of vegetative propagation are features which render the poplars amenable to rapid breeding and good capture of genetic gain, although seed storage is a problem. Apart from the long history of improvement, two features of poplar breeding programmes distinguish them from those for most other forest tree species (and present similarities to some fruit tree breeding programmes):
The emphasis on between and within species hybridization, frequently involving well characterized clones, followed by clonal testing (Ostry & McNabb 1986, Carter et al. 1988, Herpka 1987). Most commercial plantings comprise tested clones, frequently with cultivar names as for ornamental horticulture.
The emphasis on disease resistance (e.g. to Melampsora, Marssonina and Septoria) as the major selection criterion (Herpka 1987, Ostry & McNabb 1986, McNabb et al. 1990, Pryor 1992), with maintenance of growth characteristics a secondary objective. Disease resistance breeding has emerged in response to several clone-specific calamities.
The importance of the disease problem in poplars, and the long history of wide deployment of individual clones, may not be coincidental, and may provide a lesson for more considered and cautious deployment in clonal programmes with other species.
An important factor limiting improvement in many poplar programmes is the availability of required levels of disease resistance. As above, large benefits would accrue through the development of better selection methods and shorter generation intervals.
Plantations of species of this heterogeneous group, located in particular in Europe, North America and China, comprise perhaps 5–10% of the current industrial forest plantation estate.
Although plantings of long rotation, high value species such as Juglans and Prunus are currently insignificant in total extent, it has been suggested that small private plantings of these could assume greater importance in the future on disused agricultural land (Von Althen 1991). For these species, small breeding programmes, by European and North American government institutes and cooperatives, are in existence. The long generation interval (related to both the long selection interval and the long delay to flowering), difficulty in controlled pollination, intermittent and light seed crops, and seed recalcitrance (Rink & Stelzer 1982, Beineke 1982, Robinson & Overton 1989) are limitations to the breeding of walnut. Orchards established many years ago are only now producing seed. Vigour and stem form are important selection criteria, but wood quality is also of particular importance. High values of the product have led to some use of grafted plantations, and patented clones are available (Beineke 1982, Beineke 1989, Robinson & Overton 1989). Unmanaged exploitation has led to scarcity of the species in many areas of its natural range (Robinson & Overton 1989). Breeding programmes incorporating clonal testing are underway for Prunus in Europe. With restricted current plantings and long generation intervals, breeding of these species beyond selection and either seed orchard establishment or propagation of superior genotypes is not readily justified. This is a group for which effective early selection would be particularly valuable.
Robinia pseudoacacia is a leguminous tree of considerable value in fast growing biomass plantations. Plantings stand at over 3 250 000 ha, with large areas in China, South Korea, and Hungary (Keresztesi 1983, Hanover et al. 1991). A precocious flowerer, regular and abundant seeder, with seed readily stored, and easily propagated from shoot or root cuttings, the species is very amenable to breeding. Hungarian breeding programmes commenced several decades ago have been based on crossing (including with related species) and clonal selection, and many registered cultivars are available (Keresztesi 1983). Selection criteria are wood quality (in particular), frost tolerance, growth and form. The origin and genetic base of many plantings, however, are unknown. Breeding programmes incorporating provenance and progeny testing have been commenced in North America (Mebrahtu and Hanover 1989, Hanover et al. 1991). Greater frost resistance in the species would extend its range.
Acacia mearnsii is widely planted for bark (tannin), mining timber, poles and fuelwood especially in southern and eastern Africa, and China. Difficult to control pollinate and to propagate vegetatively, breeding programmes are based on the open-pollinated progeny trial/seeding seed orchard approach (Raymond 1987, Hillis 1989). Alnus glutinosa is a very precocious (as early as two to three years) and heavy seed producer. Provenance/progeny tests have been established and seed orchards have been planned (Carter et al. 1988). Betula species similarly flowerer precociously and abundantly, and greenhouse seed orchards in Finland are now producing seed (Carter et al. 1988). Species involved in these programmes are B. verrucosa and B. pubescens. A plus tree selection programme for Betula verrucosa commenced in Sweden in 1988, and progeny trials and clonal tests have been established (Danell 1991). Small breeding programmes are in existence, and orchards have been established, for a number of other North American hardwoods, e.g. Platanus occidentalis, Fraxinus pennsylvanica, Liquidambar styraciflua, Quercus rubra and Liriodendron tulipifera (Squillace 1989).
Species of this group make up 35–40% of the world plantation estate, and potential exists for further plantings in many developing countries. Included are the important Pinus species P. taeda, P. elliottii, P. caribaea, P. radiata and P. pinaster. P. taeda comprises about 80% of some 10 000 000 ha of southern pine plantations in south-eastern USA (Lantz & Kraus 1987), and substantial areas exist also in China (220 000 ha) and Brazil. P. elliottii makes up the major part of the remainder of the USA plantations, and is important also in China (880 000 ha), Brazil (several hundred thousand hectares at least) and South Africa (some 150 000 ha). Substantial plantations of P. radiata exist in Chile, New Zealand and Australia (each more or less 1 000 000 ha). P. pinaster is an important plantation species in France (1 200 000 ha) and Portugal (900 000 ha), and is planted also in Chile and Australia (Destremau et al. 1982). World plantings of P. caribaea are over 500 000 ha, mostly in Venezuela although the species is important also in Australia, Fiji and Brazil, and potentially so in many other tropical countries. Several other pine species are important regionally, e.g. P. patula, P. rigida, P. palustris, P. echinata, P. virginiana, and P. kesiya. Some other pine species, e.g. P. oocarpa and P. tecunumanii, have potential in tropical regions.
Although the seed development cycle is longer than for many hardwoods, most of these pine species have some features which facilitate breeding:
Cuttings can be propagated from young seedlings of many species, but propagation of cuttings from older trees generally is difficult.
Well established breeding programmes are in existence, at or near the third generation of selections in the USA P. taeda, New Zealand P. radiata, and Queensland P. caribaea programmes. (NCSU- I.CTIP 1990, Shelbourne 1986, Kanowski & Nikles 1989). A substantial proportion of the world's pine plantings are conducted with genetically improved material derived from the breeding programmes. The ease of controlled pollination has led to its frequent use in mating programmes. The large USA breeding programmes are typified by sophisticated mating designs for recombination and testing, e.g. diallels and factorials. Pair mating, polycrossing, or open pollination is more common elsewhere. Selection criteria are mainly vigour, stem form and wood quality, with disease resistance of some interest also, particularly for P. taeda, P. elliottii and P. radiata. The most advanced programmes employ controlled pollination also for the capture of genetic gain - to produce superior full-sib families, sometimes with multiplication by cuttings. The latter applies in particular to P. radiata in New Zealand and Australia, and to the hybrid between P. elliottii and P. caribaea in Queensland. Open pollination, however, remains the most common approach to the capture of genetic gain, and pollen contamination in seed orchards (50% even in well isolated orchards according to Hodge et al. 1991), is undoubtedly a cause of substantial leakage of this genetic gain in many programmes.
Major limitations to improvement are the long intervals to flowering and selection (resulting in a long generation interval), inaccurate selection, and the poor rooting of cuttings from older trees. Gene pools for most species are reasonably well preserved in ex situ plantings, although some provenances of the Central American pines are endangered.
Also included in this group are Cryptomeria japonica and Chamaecyparis obtusa, which make up the greater part of the Japanese plantations of some 10 000 000 ha. Historically, the Japanese programmes have been based on the vegetative propagation of selected varieties, each comprising a small number of phenotypically similar clones. Provenance and progeny tests have been established and crossing programmes implemented in more recent times.
Araucaria cunninghamii is a valuable timber tree represented by 50 000 ha in Australia but with considerable potential elsewhere in the tropics. Mature trees coppice readily, and cuttings root well, but a rigid orthotropic and plagiotropic branching system places restrictions on vegetative multiplication rates with cuttings. Irregular flowering, and long delays to the production of male strobili, place further constraints on the breeding of the species, although an active breeding programme tailored to accommodate these characteristics has resulted in substantial gains (Nikles et al. 1988). Another fairly widely planted species of this genus is A. angustifolia, in South America. For this species, however, little improvement has been undertaken, and the species is endangered in parts of its natural range.
Genetic improvement offers clear benefits for planting programmes with species of this group. Broader application of good improvement programmes is an important priority, particularly in tropical countries. The broader development of technologies for the mass production of superior full-sib families would also be highly beneficial. In terms of research objectives in advanced breeding programmes, major benefits are likely to accrue from more effective control of the maturation state (particularly in relation to the rooting of cuttings), more effective selection, and shorter generation intervals.
This group, comprises 35–40% of global industrial plantations. Important species included here are Pinus sylvestris (annual plantings of 90 000 ha, 90 000 ha, 100 000 ha, 40 000–50 000 ha in Finland, Sweden, China and Poland respectively, and with additional large programmes in countries of the former USSR), Picea abies (a major plantation species in Europe and North America), other Picea species, e.g. P. sitchensis, P. mariana P. glauca and P. engelmannii (important in U.K., Canada and north-eastern USA), Pinus contorta (at least 350 000 ha established in Sweden (Fries 1987), and large areas also in Canada (Ying et al. 1985, Konishi 1986)), Pseudotsuga menziesii (a very important plantation species in north-western USA and Canada, but represented by some 350 000 ha also in Europe), Pinus banksiana (important in eastern Canada), P. strobus (USA), Larix species (extensive plantings in South Korea and grown also in north-eastern USA and Europe). Some large planting programmes are undertaken by government bodies, e.g. in the UK and Canada. A prominent feature of the industry in several European countries is the large number of small growers.
Pinus contorta (Fries 1987) and P. banksiana (Park et al. 1989) are reasonably precocious and reliable flowerers, but several of the other species mentioned are characterized by light and/or irregular flowering, e.g. P. sylvestris (Matyas 1991, Koski 1991), Picea abies (Wellendorf 1989, Dietrichson 1989, Ruotsalainen and Nikkanen 1989) and P. engelmannii (Konishi 1986). The major obstacle to improvement, however, and the main difference from the medium rotation conifers, is the longer selection interval, rendering traditional breeding very slow.
Well established breeding programmes, largely the responsibility of government institutes, exist for these species. Most testing and recombination programmes are based on open-pollination (Lindgren 1991. Fries 1987, Morgenstern and Park 1991) although some pair mating is used, and more sophisticated mating designs for Douglas fir. Interspecific hybridization is of significance in Larix (Paques 1989). Some of these species display marked provenance X planting site interaction, particularly in relation to temperature sensitivity, and definition of breeding zones is an important feature of some programmes, e.g. for Pinus sylvestris (Danell 1991), P. banksiana (Miller 1984), and Picea abies (Dietrichson 1989). Major selection criteria are vigour, straightness and wood properties, frost resistance in Pinus sylvestris (Oleksyn 1991) and Picea abies (Van de Sype 1989), disease resistance in P. contorta (Ying et al. 1985), and insect resistance in Picea abies. The open-pollinated orchard, of which large areas have been established, remains the main approach to the capture of genetic gain, but most orchards are not producing enough seed to make much impact on the requirement for planting stock. Approximately 200 ha of clonal tests have been established in Germany (Kleinschmit & Svolba 1989), and over 10 000 clones are under test in Sweden (Danell 1991). Commercial use of clones, however, remains very limited, due partly to a reluctance of growers to pay higher prices for cuttings (P. Monchaux pers. com.) and to doubts concerning the capacity to retain sufficient levels of juvenility through a lengthy sequential propagation programme (Kleinschmit & Svolba 1989). Most gene pools are reasonably well preserved. The broader development of technologies for the mass production of superior full-sib families would be useful for these species. Some programmes would benefit from a greater capacity to manipulate frost tolerance, and some from the ability to manipulate flowering. Breeding of the long rotation species would benefit particularly from reductions of the generation interval through early selection. It should be noted that in some countries, plantations of these species are used for many purposes. Intensive breeding for enhanced timber production in such plantation programmes may not be desirable, particularly where reduction of genetic diversity is involved. Legislation reinforces this view in some countries.
There is a large requirement worldwide, mainly in developing countries, for fuelwood plantations, plantings to arrest and reverse land degradation, and, most importantly, tree plantings integrated into farming systems to provide mulch, fodder, shade, fuelwood, timber, fruit, and soil improvement and protection. Many of the species discussed in this category can be used in industrial plantations (as defined above), and identification of a “non-industrial” group of species is a demarcation which is more convenient than clearly defined.
Large fuelwood plantations, e.g. of tropical eucalypts or casuarinas, located near cities are one approach to the urban fuelwood crisis. Operated on a commercial basis, these can be viewed as industrial species. Current plantings are not widely based on improved material, but simple improvement programmes would be justified in some of these.
Major rehabilitation plantings, e.g. with casuarinas or acacias, are publicly funded. Improvement beyond species and provenance selection may be difficult to justify on purely economic grounds, and may furthermore be of limited desirability in those cases where maintenance of diversity in such plantings is of major importance.
Examples of integration of trees into farming systems include:
Gliricidia is a fast growing tree used widely in the Philippines, India, Sri Lanka, Indonesia, East Africa and its native Central and South America for fodder, fuelwood, live fences, construction wood and mulch. Subjected to agricultural use for centuries, its true natural range is uncertain, and many land races have been created (Hughes 1987). Flowers are produced within the first year or two, seed production is prolific, and cuttings are easily propagated. Large collections have been made and some provenance comparisons carried out, but breeding is not very advanced. A breeding seed orchard of a particular desirable provenance will be established at the ICRAF research station in Zambia (Simons 1992b). Cuttings of very limited numbers of locally selected clones are the planting stock in some areas. Genetic variation with respect to a range of characters is substantial (Simons 1992b). Selection criteria vary with use, but timing and reliability of leaf production are important parameters for fodder production.
Leucaena is a long cultivated tree of importance in South East Asia, Nepal, India, Africa and Latin America. It is nitrogen fixer which provides good forage, green manure, firewood and small timber. Frequently used with other crops, it is also the major component of energy plantations established in the Philippines (Durst 1987). L. leucocephala seeds very precociously and prolifically, but is not easily propagated by cuttings. Unlike other tree species reviewed here, L. leucocephala is frequently self-pollinating, although this is not a feature of other species of the genus. With respect to genetic improvement, Leucaena has received more attention than most other non-industrial trees. Two directions have been followed in the Hawaiian programme of Dr Brewbaker:
testing of self-pollinated lines
In most areas, the psyllid Heteropsylla cubana causes serious damage (McDicken & Taylor 1988, Durst 1987), and resistance to this pest is an important selection criterion. Others include biomass, cold tolerance, seedlessness, fodder quality, and dry season leaf retention. Although yield increases of well over 100% have been achieved through simple selection (MacDicken & Mehl 1990), breeding is yet to have a major impact in planting programmes, many of which have suffered from off-site planting and a very narrow genetic base. For L. leucocephala, locally produced seed is what is generally used. Some Leucaena species are under threat, and collections are underway.
Calliandra calothyrsus is a nitrogen fixing, thornless legume producing high quality fuelwood and fodder, with considerable potential in site amelioration, particularly sites too wet for Gliricidia and Leucaena. The species flowers in its second year and coppices and roots well. Provenance and progeny tests have been planned (Burley & Barnes 1989, MacQueen 1992).
Prosopis is an arid zone genus of about 45 species, highly tolerant of drought and of difficult soils, grown widely on marginal land in India and countries in South America and with potential in many other countries. The trees provide fuelwood, construction timber and fodder. Coppicing is prolific (Walker 1988). A range of provenance trials coordinated i a by FAO, involving several species, are in existence (Cossalter et al. 1987, FAO 1988a, Matheson 1990, Dunsdon et al. 1991, Bach 1992, Graudal & Thomsen 1992). Objectives of breeding include growth increases, reduction of thorniness, and improvement of fodder quality.
A mimosoid legume genus of about 400 species, Inga is planted widely in Latin America to provide shade over coffee, wood, fuelwood, nitrogen fixation, green manure and fodder. Seed recalcitrance has limited plantings in other regions. Little collection work has been undertaken.
There are about 135 African acacias, of which A. nilotica, A. tortilis, A. senegal, and Faidherbia alba are particularly important. Very drought resistant, these species provide fodder, timber, charcoal, shade, honey, gum arabic, tannins and dune stabilization in arid areas of Africa and the Indian subcontinent. Some species display salt tolerance. The species tend to be prolific seeders, seeds can be stored, and stumps produce coppice (Fagg & Barnes 1990). Species are characterized by considerable genetic variation, and some collections have been undertaken and provenance trials established under the auspices of FAO (Cossalter et al. 1987, FAO 1988a, Koyo 1989, Bach 1992, Graudal & Thomsen 1992). A. nilotica is regarded as a weed in Indonesia and Australia.
Erythrina poeppigiana and E. fusca are used as pollarded trees providing shade and mulch for coffee in Central America. E. berteroana is used in living fences. The species flower precociously (at three years) and abundantly. Cuttings root well, and direct planting of unrooted stakes is common. Improvement work conducted at CATIE is along poplar lines, involving the clonal testing of individuals from natural and planted stands.
The genus Paulownia is represented by about nine species native to China, where there are reportedly 1 300 000 ha planted in mixed cultivation with crops (Chinese Acad. For. 1986). The species is suited to intercropping due to late leaf emergence and leaf fall, and a deep root system (Chinese Acad. Forestry 1986), and produces timber within five to six years. P.tomentosa and P.kawakamii usually flower in the second year after planting, and P.fortunei and P. catalpifolia in the fifth to sixth year. Seeds can be stored satisfactorily, and cuttings can be propagated from both young seedlings and the roots of mature trees. In China, the approach to breeding envisaged is controlled pollination (in particular interspecific hybridization) and then clonal testing (Chinese Acad. For. 1986). Selection criteria are height (to minimize shading of crops) and freedom from the many diseases and pests. Planting stock currently used comprises selected clones of some good hybrids. The North American breeding programme is at the species and provenance evaluation stage (NCSU-Industry Hardwood Research Co-op. 1990).
Dalbergia sissoo has long been valued in Pakistan, India and Nepal for its high value cabinet timber, fodder, fuelwood, charcoal, honey, medicinal properties, windbreak quality and capacity to fix nitrogen (White 1990). It is used both with crops and as an industrial plantation species (Sheikh 1988). The species flowers abundantly and regularly from age four, and good rooting of cuttings has led to their use as the most common method of propagation (Sheikh 1988, White 1990). The breeding system of the species is not well known (White 1990). Seed orchards were established in Pakistan in 1975 but improvement is not very advanced (White 1990). Selection criteria are wood yield and quality, but with consideration given also to fodder and honey properties.
Casuarina equisetifolia and C. cunninghamii are used widely for windbreak plantings and fuelwood production in areas of the tropics. C. equisetifolia comprises the major component of a shelterbelt covering over 1 000 000 ha in China (National Research Council 1984), Coastal sand dune fixation is one of the most successful applications of this species. Both species are dioecious. For industrial plantations, provenance trial establishment, plus tree selection, and the establishment of Breeding Seed Orchards using the multiple population approach has been recommended (Burley & Barnes 1989). Breeding of Casuarina in Egypt incorporates selection for drought resistance (Burley 1985). A sterile hybrid between C.junghuhniana and C.equisetifolia is propagated vegetatively and widely planted in Thailand (Willan et al. 1990).
Azadirachta indica is a promising species of uncertain natural distribution due to centuries of traditional use (Willan et al. 1990). Poor seed viability is a serious problem. Seed collections have been made and provenances are under test (F/FRED 1988, Nikles 1992). Growth rate, insect resistance and azadirachtin content will be important selection criteria (Nikles 1992).
A large number of other species are of potential value in non-industrial plantings, most remaining poorly studied. These include species of Sesbania, Pithecellobium, Albizia, Caesalpinia and Parkinsonia (MacDicken & Mehl 1990, Dunsdon et al. 1991).
To summarize for the non-industrial species then, moderately comprehensive data on biological features influencing breeding are available for only a handful of species. Several of these taxa are highly variable and display features which render them particularly amenable to very rapid improvement by traditional means - precocious and abundant seeding, coppicing, and ease of vegetative propagation. Biological features of many potentially valuable non-industrial species remain largely unknown, and collection of data of this type will be an important component of the initial phase of any breeding programmes. Although not well studied, some gene pools are under threat, and some species display unpredictable patterns of variation as a result of human disturbance, rendering sampling strategies more difficult to design.
Considered collectively, non-industrial trees differ markedly from industrial trees in the wide variety of end uses and thus selection criteria - e.g. fuelwood quality, foliage yield, fodder quality, coppicing ability, phenological characteristics, timber quality, nitrogen fixation, root features, tree form and resistance to insects, disease, drought and other stresses. Some of these, e.g. fuelwood quality, are very complex. For some farmers, reliability of production may be more important than mean annual yield (Simons 1992a). In agroforestry systems, the interactive effect of the tree component with the associated crop is important (Willan et al. 1990). Desired features of agroforestry species have sometimes been integrated in terms of “ideotypes” (Glover 1990). Although selection work has been undertaken in a few programmes, and clonal propagation of selected clones is used in Paulownia and Erythrina, most non-industrial species remain at the species testing stage. The work involved in just selecting the most promising of perhaps thousands of other potential valuable but as yet untested species is formidable.
Improvement beyond species and provenance testing may not be warranted in many programmes with non-industrial species:
Major emphases of improvement for non-industrial species are likely to be taxonomic studies of variation, species and provenance testing, the assessment of some features of reproductive biology, and conservation activities. Where regionally consistent selection criteria can be defined, some clonal selection may be important for species easily propagated vegetatively, and perhaps seed tree selection for species propagated by seed. Sophisticated breeding programmes are unlikely to be warranted for most of these species. Interspecific hybridization is of proven value for some taxa, and holds promise also for others, but must be used in the context of a sound improvement programme.
Tree improvement in developed countries mainly concerns industrial species, although agroforestry is becoming more important. Some form of genetic improvement programme is a feature of most plantation programmes for industrial species. Most are based on recurrent cycles of phenotypic selection (frequently followed by testing of progeny) and recombination among selected individuals. Exceptions are the poplars and cryptomeria, for which clonal selection has been the traditional approach. Vigour, stem form and wood quality are the most important selection criteria in most programmes. Resistance to disease or insects in particular, and also to cold and drought, are significant in several programmes. The importance of disease resistance in the poplars is worthy of special note. Most characters of major importance are under polygenic control. Although there are some clonal programmes, and programmes based on the propagation of full-sib families, the open-pollinated orchard is the most common approach to the capture of genetic gain. Gene pools are reasonably well preserved for most of these species, although locally adapted populations may be in danger of depletion or genetic pollution in some cases.
Breeding activities (e.g. crossing programmes, progeny testing, selection) are generally carried out under the guidance of personnel with expertise in quantitative plant breeding, supported by good systems of field experimentation and data base management. The personnel involved are frequently employees of the grower organizations, although independent research institutes are involved in some cases. Breeding cooperatives, sharing genetic material, knowledge and work, are particularly important for Pinus taeda, P. elliottii, P. radiata, and some north American hardwoods (e.g. the North Carolina, Florida and North Central Hardwood cooperatives, and the Southern Tree Breeding Association).
Supportive research (e.g. assessment of reproductive behaviour, mating patterns, genetic parameters) is conducted frequently by the above groups, but also by universities and other research organizations. Strategic research (aimed at the development of better breeding methods) is undertaken primarily by universities and other research organizations, although several of the larger private growers also conduct long term strategic research. As pointed out by Kanowski (1993), research in tree improvement is greatly under-resourced, in both absolute terms and relative to agriculture.
Significant genetic gains are being achieved in these breeding programmes but, in particular for the long rotation species, there has been only a minor impact to date on the genetic quality of plantations.
Major limitations to rapid improvement with most of these species are:
Long generation intervals, related to:
poor juvenile-mature correlations. Although there is some debate about this, in general, for many characters, these correlations are not high enough to permit very early selection. For Pinus elliottii, for example, it has been suggested that growth up to five years involves different physiological mechanisms to subsequent growth (Hodge et al. 1991). It should be noted that selection intervals for many of the breeding programmes discussed above are shorter than those recommended by some authorities. Zobel & Talbert (1984), for example, advocate half rotation age as the appropriate selection interval.
long juvenile phase with respect to flowering.
Low effectiveness of selection for many characters. Most major selection criteria are of low heritability, such that progeny testing is necessary. Some characters, e.g. resistance to frost, drought and insects, are difficult to test in a systematic way.
Through the use of open-pollinated orchards, the exploitation of only a part of the genetic variation available.
Major research priorities for these species should be the broader development of methods for the propagation of full-sib families and clones, and the development of methods for early and more accurate selection and the promotion of precocious flowering.
It is of some interest to note that an apparently slow rate of advancement with respect to genetic quality of plantations is not unique to forest tree species. The avocado varieties Fuerte and Hass, for example, were introduced to the industry over 60 years ago, but remain the major commercial varieties. Similarly, the major kiwifruit varieties were selected several decades ago. The advances which have taken place with these crops have been horticultural, not genetic. Silvicultural advances are critical also for forest tree species, but it is unlikely that the intensity of horticulture given to fruit tree crops would be affordable. For some agricultural species, the genetic superiority of modern varieties requires intensive agriculture, including comprehensive pest and disease management, for its expression. Modern high yielding varieties tend to lack much of the natural resistance of old land races or wild relatives (Harms 1992). The lower intensity of silviculture affordable renders desirable, then, the more careful management of genetic improvement for forest tree species.
There are many examples of very successful industrial plantation programmes, including some of the world's best, in developing countries. Mostly these are managed by private forestry companies, and involve biologically well-known plantation species. These operations employ breeders, and in some cases hire consultants. Limitations and priorities for these programmes are much the same as those discussed above.
As noted above, an additional area of up to 100 million hectares of plantations will be required globally in order to meet future predicted demand for industrial timber. It is likely that the bulk of this additional area will be located in tropical countries. To some extent, this will be accomplished with already proven industrial species - conifers, tropical eucalypts, and other established tropical hardwoods. It is likely, though, that the use of other industrial species will be required:
to utilise a broader range of sites
to meet a demand for products currently provided by harvesting in natural forests.
Selection criteria for these “new” industrial species will be similar to those for established species. Some are probably very amenable to improvement, while others present problems, e.g. flowering and seed problems, and insect susceptibility. Many potentially valuable species are simply not well known, in terms of their adaptation and biological and genetic features, and gene pools are probably under threat.
There is a global requirement also for several hundred million hectares at least of non-industrial plantings, mainly located in developing countries. It is noteworthy that while the plantation area under industrial species in the tropics doubled in the 1980s, the area under non-industrial species almost tripled (FAO 1993). Non-industrial species differ markedly from industrial species in the wide variety, and the complexity, of selection criteria. Several of the taxa are highly variable and display features which render them particularly amenable to very rapid improvement by traditional means. Biological features of many potentially valuable non-industrial species, however, remain largely unknown. Although not well studied, some gene pools are under threat, and some species display unpredictable patterns of variation as a result of human disturbance. Although selection work has been undertaken in a few programmes, most non-industrial species remain at the species testing stage. There are some 2 000 tree species that have been planted as “multipurpose” trees (Burley 1985), and many more species would have potential. As pointed out by Wood (1991), the number of tree species of potential interest in developing countries is much larger than the number of food crops in these regions. Undoubtedly, the number of species to be covered limits the progress which can be made with any one. It is therefore necessary to address the question “to what extent is interest in a large number of species necessary?”. While selection of an appropriate site is a critical factor in the establishment of any industrial plantation, the establishment of non-industrial plantations is often less flexible - plantings are frequently required in a particular area, e.g. for rehabilitation. Non-industrial species are thus required for a wide variety of sites and conditions. Furthermore, sites are often marginal, with variable environments and subject to periodic stresses, and testing over several years is desirable. Socio-economic considerations will furthermore strongly influence the choice of species for local use. For these reasons, reliance on a few species is unlikely to be feasible. The work involved in selecting the most promising species is formidable.
Genetic improvement of the new industrial species and non-industrials is generally the responsibility of state, national, and in particular regional and international organizations. Involved are international bodies such as FAO, agencies operating bilaterally, e.g. AIDAB, ACIAR, DANIDA, FINNIDA and GTZ, research organizations such as CAMCORE, CSIRO, CIRAD-Forêt, IBPGR, CIFOR, ICRAF, OFI, CATIE, and F/FRED, and many national institutes. Typically, limited resources are very thinly spread. Growers are unlikely to conduct formal breeding themselves, and frequently may prefer to minimize their expenditure on establishment by producing their own planting stock. General limitations to tree improvement in developing countries are a lack of staff with the relevant skills (Griffin & Nikles 1984, FAO 1988b, Koyo 1989, Mburu 1989, Brouard 1990, Kio 1991, de Freitas 1991, Ng 1991), inadequate facilities (FAO 1988b, Koyo 1989, Mburu 1989), poor communication, particularly with forest managers (Griffin & Nikles 1984, FAO 1988b) and poor language skills (Ng 1991). It is these resource limitations and the variability in user requirements, rather than biological limitations, which constitute the major impediment to the rapid improvement of non-industrial species (almost totally dependent on public funding).
The broader implementation of good improvement programmes will be an important priority for the new industrial species. The testing of potentially useful species, characterization of mating systems, provenance collection, establishment, management and evaluation of trials, implementation of gene conservation measures and commencement of other breeding work, poses a very large task.
As noted above, major emphases of improvement for non-industrial species are likely to be taxonomic studies of variation, species and provenance testing, the assessment of reproductive features, and conservation activities. Interspecific hybridization may be of value for several taxa. Where regionally consistent selection criteria can be defined and the tree crop is considered sufficiently important, clonal selection programmes are likely to be of value for species easily propagated vegetatively, and seed tree selection for species propagated by seed. Sophisticated breeding programmes are unlikely to be warranted for most species. Simons (1992a) makes the point that new genotypes may need to be markedly superior to existing material to be adopted by farmers. This argues for a once only improvement effort, or perhaps for breeding for occasional “quantum leaps” rather than incremental gains. Improvement programmes for non-industrial trees, as for industrial species can only be conducted effectively where planting programmes are based on sound establishment and management practices.