During the past decade there has been a welcome increase in the number of publications on forest seed. Some of these have dealt with seeds of single important genera e.g. Eucalyptus (Boland et al. 1980), Acacia (Doran et al. 1983), Prosopis (Ffolliot and Thames 1983). Some have dealt with a single operation e.g. seed collection (Yeatman and Nieman 1978, Mittak 1978, Robbins et al. 1981). Others have dealt more comprehensively with the full range of seed handling operations in a particular country e.g. USA (Schopmeyer 1974), France (CEMAGREF 1982), UK (Gordon and Rowe 1982) and Colombia (Trujillo Navarrete). The lecture notes prepared for the FAO/DANIDA Training Course on Forest Seed Collection and Handling (FAO 1975b) covered all aspects of seed handling and included examples of several tropical species, while valuable papers from individual authors working in the tropics have been submitted to several meetings of IUFRO's Working Party S2.01.06 on Seed Problems, in particular the IUFRO/ISTA/INIF workshop on Tropical Seed Problems which took place in Mexico in October 1980 and the IUFRO Symposium on Seed Qaulity of Tropical and Subtropical Forest Tree Species held in Bangkok in May 1984.
Nevertheless, there is still a need for a systematic and up-to-date account of the principles of forest tree seed handling and their application to the special problems of tropical forest seeds. The present Guide is an attempt to fill the gap. It is addressed to foresters, seed collectors, researchers and suppliers, nurserymen, horticulturists and anyone concerned with the handling and use of forest seeds, amateur and professional alike.
Most comprehensive manuals on seed handling consist of one part which describes the principles of seed handling in general and a second part summarising specific characteristics and recommended handling methods for each species (see e.g. Schopmeyer 1974, Gordon and Rowe 1982). The present Guide covers only the general principles. The DANIDA Forest Seed Centre intends to complement it by publishing a) A series of species seed leaflets giving detailed information and prescriptions for individual species and b) A series of technical notes describing particular seed handling techniques and items of equipment. It may be argued that, if a clear guide to practice is available, there is little need to describe principles. To the curious and enquiring seed handler, however, there is an obvious advantage in knowing not simply what to do but why, to follow a rational explanation rather than a rigid “cookbook”. For many tropical species, too, there is still inadequate knowledge to justify the setting of standardised prescriptions for handling; in such cases the seed handler may have to carry out his own research to determine the best methods under local conditions and will benefit from access to literature drawing on a wide range of experience with other species. It is hoped therefore that this Guide will provide a useful framework for more specific information on individual species and techniques. Although examples are drawn from tropical species whenever possible, temperate experience is also quoted where tropical experience is lacking.
In this Guide, the present chapter (Chapter 1) is followed by a simplified and concise account of seed biology (Chapter 2). The remaining chapters describe the planning and operation of seed collection (Chapters 3 and 4), seed handling between collection and processing (Chapter 5), seed processing (Chapter 6), storage (Chapter 7), pretreatment (Chapter 8) and testing (Chapter 9). The appendices include examples of seed documentation and of long-term storage design, a bibliography, and examples of the logistics and equipment involved in seed collection. A glossary appears as Appendix 7 and may be found helpful, particularly for the botanical terms in Chapter 2.
The Guide has been compiled from a survey of forest literature and has drawn heavily on the experience recorded in the references quoted on page 1, as well as earlier publications such as those of Baldwin (1955), Holmes and Buszewicz (1958) and Morandini and Magini (1962). Little mention is made of the basic cytology and biochemistry of seeds; existing knowledge is based mainly on research done on agricultural seeds and is still incomplete. Readers requiring further information from fundamental studies on these aspects should refer to texts such as those of Kozlowski (1972), Roberts (1972), Heydecker (1973), Mayer-Poljakoff-Mayber (1975) and Bewley and Black (1983). Periodicals which publish new information are “Seed Science and Technology” (the journal of ISTA), “Advances in Research and Technology of Seeds” (Wageningen, Netherlands), “Seed Abstracts” (Commonwealth Agricultural Bureaux, UK) and “Journal of Seed Technology” (the journal of AOSA).
In many countries of the world the number of trees planted increases each year. A recent survey (Lanly 1982) has estimated that the area of forest plantations in tropical countries will increase from 11.5 to 17.0 million ha between 1980 and 1985, an increase of 48 % in five years. Of the 11.5 million hectares at the end of 1980, 40 % had been planted in the previous five years. The average annual planting rate is expected to increase from 0.92 million ha in the period 1976 – 80 to 1.10 million ha in the period 1981–1985, an increase of 20 %. Large planting programmes are also in operation in many temperate countries. In addition to new planting, replanting of harvested plantations of non-coppicing species must be carried out on a considerable scale each year.
Large as these areas may appear, they amount to only about a tenth of the area of natural forest being destroyed in the tropics over the same period. A further increase in planting rates beyond 1985 will certainly be necessary.
Forest plantations are a powerful tool in the continuing efforts of foresters to increase productivity per unit area - the only means of reconciling the increasing demands for forest products and services on the one hand with a decreasing area of land available for forestry on the other. A combination of intensive site preparation with the use of uniform, well-grown nursery stock, planted at uniform spacing, increases growth and yield, reduces rotation length, facilitates tending and harvesting operations and improves the quality and uniformity of wood, as compared with natural forest. Plantations also offer the means of using on a large scale the genetically improved material developed by tree breeders. Although there is no case for the indiscriminate replacement of all natural forest by plantations, their judicious use, by providing an alternative source of forest products, can itself reduce pressure on the remaining natural forest and so help to conserve it as a habitat and a source of genetic diversity.
Not only do plantations have a major role as producers of timber, pulpwood and wood-based panels for forest industries, but fuelwood and pole plantations and farm woodlots are locally important in many countries. Tree-planting is not confined to block plantations. Shelterbelts and dispersed planting for soil stabilization, habitat improvement, urban and rural amenity or as part of an agrisilvicultural system all benefit the human environment. With such a variety in planting purposes, it is not surprising that the scale of tree-planting and the variety of species planted continue to grow in so many countries. The current greatly increased interest in agroforestry opens up a whole new range of species for trial. Ability to grow in symbiotic relationship with agricultural crops will be the essential characteristic and will involve criteria such as rooting habit, ability to fix nitrogen and multipurpose uses (food, wood, shelter). Low stature may be beneficial and shrubs may become as important as trees. These new developments will introduce new opportunities and new problems in seed collection and handling.
With a few exceptions, notably among the poplars and willows and in some tropical species of Casuarina, trees are propagated from seed, and the suitability and quality of the seeds have a big effect on the success of the plantations raised from them. The use of sound seed from stands of high inherent quality is widely recognized as the best means of ensuring fast-grown and healthy plantations capable of yielding high quality wood (Aldhous 1972). Seed quality comprises both genetic and physiological quality. The present Guide is concerned with physiological quality. Genetic quality is touched on briefly, for example in Chapter 3 on planning seed operations, but for a full discussion of the subject readers are referred to detailed accounts of tree improvement such as those of Wright (1976), Faulkner (1975), Burley and Wood (1976), FAO (1974, 1980), Nienstaedt and Snyder (1974), Rudolf et al. (1974), Barber (1969), van Buijtenen et al. (1971), Zobel and Talbert (1984).
It must be stressed that “good seed” implies seed which is both of high viability and vigour and is genetically well suited to the site and to the purpose for which it is planted. Physiologically good seed may lead to successful establishment of a plantation but this is of little value if it is slow-growing, ill adapted to the site or produces the wrong kind of wood because the provenance or genotype was incorrectly chosen. On the other hand, there is little point in producing genetically improved seed at an increased cost if it is killed by poor handling techniques and has to be replaced or supplemented by inferior seed in order to achieve planting targets. Good seed handling is an essential complement to genetic improvement.
Quantity, as well as quality, of seed production is important. In natural stands variation in the quantity of seed produced affects the forester's decision as to the years in which to collect seed and the trees from which to collect it, as described in Chapter 3. More intensive management affords him the opportunity of stimulating heavy seed production in genetically superior crops through deliberate treatment, e.g. thinning in seed stands or a combination of initial spacing, irrigation, fertilization and thinning in seed orchards. Description of these methods is outside the scope of this book, since they are properly a part of seed stand and seed orchard management, but programmes of seed supply should include measures to improve production quantitatively, as well as improving the genetic quality of the seed and the efficiency of collection and handling.
|1.1 Estimated total areas of forest plantations in Tropical Africa (37 countries), Tropical America (23 countries) and Tropical Asia (16 countries) 1975–1985. (source Lanly 1982)||1.2 Estimated annual planting rates (industrial and non-industrial forest plantations). Recent trends in three major tropical planting areas - Brazil, India and Tropical Africa. (source Lanly 1981 a, 1981 b, 1981 c)|
1.3 Effect of species choice on quantity of seed required per unit area. The total area of the square enclosed for each species is proportional to the weight of seed required in gm/ha, the figure stated against each species. Except in the one example of aerial sowing in E. delegatensis, the method of establishment is by planting stock raised in nurseries.
There is a considerable body of published information on seed handling in the temperate zones but published accounts of tropical experience with forest seeds are scattered and incomplete. While recapitulating the principles of seed handling derived from temperate experience, this guide seeks to illustrate these, as far as possible, by examples taken from tropical species. At the same time it must be emphasized that there is great variation in seed biology and that certain techniques which are commonly practised with good results in the temperate zones, e.g. stratification or prechilling, may have no useful function whatever in the tropics. It is dangerous to extrapolate temperate experience to tropical species, experience in the dry tropics to rain forest species, or experience in tropical agriculture to tropical forest tree seeds, without testing it in each case.
Valuable lists of equipment and suppliers have been published in the last decade. That published by ISTA (1982) covers equipment used in seed testing. The older list compiled by Bonner (1977) covers all operations from seed collection to seed testing and includes the names of equipment users as well as suppliers. Both equipment and techniques must be adapted to local species and conditions. For example hard-coated seeds of high natural longevity which are produced regularly every year require no more for storage than a well-ventilated room with protection against pests and diseases. A domestic refrigerator or “deep freeze” may be perfectly adequate for long-term storage if only small quantities are to be used each year. Large units and mechanized equipment may be essential in some countries, but heavy expenditure on such items should be incurred only after a full evaluation of the alternatives.
The chain of seed handling operations is only as strong as its weakest link. Therefore great care must be taken at all stages if seed viability is to be maintained all the way from the parent tree to the nursery bed. If a seed has lost viability in an early stage of the process, the best storage or pretreatment can never bring it back to life. Perfect extraction and cleaning of seeds are a waste of money if they are killed later by incorrect storage conditions or careless handling in transit. The greatest risks to seeds occur during temporary storage immediately after collection, during transit to the processing depot, and again during transit from the seed store to the nursery. These are the periods when sophisticated equipment is not available and everything depends on common sense and meticulous care to ensure good ventilation and avoid extremes of temperature.
“International exchange of tree seeds is an opportunity to share the world's forest wealth” (Baldwin 1955). The widespread success of species such as Pseudotsuga menziesii, Pinus radiata, Eucalyptus globulus and Tectona grandis is a convincing demonstration of the advantages which many countries can derive from the introduction of exotics. International cooperation in seed procurement is now practised by many countries and has long been supported by international organizations such as FAO and IUFRO. It should be increased. At the same time the movement of seed between countries does involve some additional problems in maintaining both the viability and the identity of the seeds. Phytosanitary regulations, including inspection and treatment, must be complied with in order to reduce to a minimum the risk of importing new and dangerous diseases. But every effort must be made to eliminate all unnecessary delays in transit, including those imposed by customs, plant hygiene and air lines, and to avoid excessive or duplicated fumigation treatments which kill the seeds. Both sender and recipient must be aware of all the current regulations in force in the exporting and importing country and must plan the seed shipment well in advance to ensure smooth and unimpeded movement of the consignment.
Apart from a few well-known species such as Tectona grandis, research on tropical forest seeds has been inadequate in comparison with both the severity of the problems and the large number of species of potential value for plantations. Much has still to be learned. A first step to this is a good understanding of the natural reproductive biology of each species. For species of the dry tropics which survive naturally by means of seedcoat dormancy, storage should present little problem and a modest programme of research should indicate the most appropriate pretreatment to overcome dormancy and induce uniform nursery germination. The problem of preserving viability in recalcitrant seeds of tropical rain forest species, especially those which will not survive temperatures below about 10° C, is a much more intractable one. A variety of possibilities has been suggested (King and Roberts 1979) but so far little progress has been made towards a practical method of storage which would be applicable in large-scale afforestation projects. Much more research is needed to solve this problem. Until a solution is found, the dipterocarps and other recalcitrant species of the tropical rain forests will remain as much non-starters for widespread plantation forestry as they have been in the past.