Forestry & Forest Sciences · Forest Ecology & Biology · also Seed Technology, Forest Seed Science, Tree Seed Handling
Collecting, processing, storing, testing and certifying tree seed so that nurseries get the right seed of known quality and origin.
In the exams: ICAR AICE-JRF 59 Unit 2 (seed collection, dormancy, testing, ISTA rules, certification); IFoS Forestry Paper I Section B; ICAR AIEEA-PG Group 09 (tree seed technology)
Every planted forest begins with seed, and the quality of that seed sets limits that no nursery or silviculture can later remove. Seed of the wrong origin can produce trees that grow poorly, break in snow or die in drought decades after planting; seed of low viability wastes nursery space and money; seed carrying pests or pathogens spreads them to new sites. Tree seed technology is the body of practice that delivers the right seed, of known identity, origin and quality, to the nursery at the right time.
The subject follows the seed from the tree to the nursery: how seeds develop and how the forester knows they are mature; when and how they are collected; how they are extracted from fruits or cones, cleaned and dried; how different kinds of seed behave in storage; why many seeds do not germinate at once and how dormancy is broken; how seed lots are tested for purity, moisture, viability and germination under the international rules of the International Seed Testing Association (ISTA); how forest reproductive material is certified under the OECD Forest Seed and Plant Scheme and similar national systems; how seed zones and documentation of origin protect adaptation; and how seed pests and diseases are recognised and controlled. Nursery sowing and raising of seedlings are covered in forest nursery and plantation establishment, and the choice between natural and artificial regeneration in silviculture principles.
The subject is examined in ICAR's AICE-JRF forestry paper, which names seed collection, dormancy, testing, ISTA rules and certification; in the UPSC Indian Forest Service Forestry optional, which asks about seed technology with tree improvement; in ICAR's AIEEA-PG; and in state forest service papers. Professional work in seed centres and tree improvement programmes worldwide relies on the same principles. Values such as storage temperatures, moisture contents and pre-treatment times vary by species and seed lot; the manual gives principles and common practice and says where values differ.
A seed develops from a fertilised ovule. After pollination and fertilisation, the embryo grows, food reserves accumulate in the endosperm or in the cotyledons, and the seed coat hardens. During development the seed's dry weight rises to a maximum, the point usually called physiological maturity, while its moisture content falls. In orthodox seeds, those that tolerate drying, the final stage is maturation drying, during which the seed loses water and becomes tolerant of desiccation; in recalcitrant seeds this drying phase does not occur and the seed is shed moist.
The time from flowering to maturity varies widely. Many tropical species mature their fruits within a few months of flowering. In most pines the cones take about two years to mature after pollination, so the forester can see the next year's crop as small first-year conelets and forecast it. Collecting seed before it is mature gives seed of low viability and poor storability; collecting too late loses the crop to dispersal, animals or fungi. Judging maturity is therefore one of the first skills of seed work.
Maturity indices are observable signs that seed is ready to collect. Colour change of fruits or cones is the commonest: cones turning from green to brown, fruits turning from green to yellow, red, purple or black, or pods turning brown and brittle. Specific gravity of cones is used for some conifers: as cones mature they lose water and become lighter, and a sample is floated in a liquid of known density; cones that float in it are considered mature, with the critical value set for each species. Moisture content of fruits or seeds can be measured directly. The cutting test examines seeds cut lengthwise: in mature seed the embryo is fully formed and fills most of the embryo cavity, and the endosperm or cotyledons are firm rather than milky. The ease with which fruits come away from the branch and the first natural fall of fruits also indicate maturity. Because no single index is reliable for every species and year, collectors check several on sample trees before starting a large collection.
Trees do not produce equal seed crops every year. Many species have irregular heavy crops (good seed years, or mast years) separated by years of light or no crop. Masting is often synchronised over large areas and is linked to weather in the years of flower initiation and pollination, to the build-up of food reserves in the trees, and possibly to selection for overwhelming seed predators. Some species, such as teak, seed more or less every year; sal has irregular good years; many bamboos flower gregariously at long intervals, sometimes decades, and then die.
Seed years matter in planning. Collection programmes must be ready to take advantage of good years, collecting enough to store for the lean years that follow where the species' seed can be stored. Crop forecasts are made from counts of flowers, conelets or young fruits on sample trees some months ahead. Natural regeneration fellings under shelterwood systems are timed to good seed years, as described in silvicultural systems.
Seed is collected after it is mature but before it is dispersed or eaten. For species whose fruits stay on the tree after maturity, such as many pines with closed cones and some legumes with indehiscent pods, the collection window is long. For species that shed seed soon after maturity, such as many light-seeded species dispersed by wind, the window may be only a few days or weeks. Recalcitrant seeds, such as those of sal and many dipterocarps, are shed moist and lose viability within days or weeks, so they must be collected promptly as they fall and sent to the nursery immediately. Collection calendars for each species and region are kept by seed centres and adjusted each year by observation.
Seed can be collected from standing trees, from felled trees, or from the ground. From standing trees, collectors climb with ladders, climbing spurs (where they do not damage valuable trees), ropes and harnesses, and use pole pruners, hooks and saws to cut fruit-bearing branches or pick fruits; on accessible stands, mechanical tree shakers or hydraulic lifts are used, and in some programmes branches are shot down. Climbing is dangerous and requires training, safety equipment and supervision. From felled trees, cones and fruits are collected during logging operations timed to the seed season, which is cheap but gives little choice of parent trees. From the ground, large fruits and seeds that fall naturally, such as those of oaks, sal, teak, walnuts and many tropical species, are gathered by hand, sometimes after spreading nets or sheets under the tree to catch them and keep them clean. Ground collection is easy but risks collecting immature, insect-damaged, fungus-infected or old seed, and seed from unknown parents if several trees overlap.
The genetic quality of a collection depends on where and from which trees seed is taken. Seed should come from identified, good-quality stands or seed orchards; within a stand, from many healthy, well-formed trees, spaced apart to reduce the chance of collecting from close relatives. Guidance commonly recommends collecting from at least 25 to 50 trees per seed lot, spaced well apart, though figures differ between programmes. Collecting from a few isolated or poorly formed trees narrows the genetic base and risks inbreeding.
Collected fruits and cones are labelled at once with species, location, date, collector and parent trees, and are kept cool, shaded and ventilated during transport. Fleshy fruits ferment and heat in closed bags, killing the seed, so they are transported in open containers and processed quickly. Cones and dry fruits are spread out to avoid moulding. Recalcitrant seeds are kept moist and cool, in ventilated containers, and never allowed to dry. Delays and poor handling between tree and seed centre are a common cause of low seed quality.
A collection is planned weeks ahead. The seed centre estimates the quantity needed from the planting programme, the expected seed yield per unit of fruit, test results of previous lots and the nursery factor, and adds a reserve for storage where the species can be stored. Crops on candidate stands are assessed from flower and fruit counts, and the best stands and trees are chosen. Crews are trained in climbing safety, identification of the species and its maturity signs, correct labelling and handling, and are equipped with harnesses, helmets, ropes, pole pruners, sheets, bags and labels. Permission is obtained where collection is on land owned or managed by others, and local collectors, who often know the trees and seasons best, are paid fairly for their work and knowledge. After collection, quantities and sources are entered in the seed register so that each lot can be traced.
Extraction separates seed from the fruit. Cones of many conifers open when dried; they are dried in the sun on trays or in kilns at controlled temperatures, and the released seed is shaken out by tumbling the cones in a drum. Serotinous cones, sealed with resin, need higher temperatures or brief exposure to heat to open. Kiln temperatures and drying times are set by species, because overheating kills the seed. Dry, dehiscent fruits such as capsules and pods of many broadleaves and legumes are dried until they split and are then threshed or beaten; indehiscent pods are crushed or broken by hand or machine. Winged seeds of conifers are dewinged by rubbing or tumbling, either dry or after light moistening.
Fleshy fruits, such as drupes and berries, are depulped soon after collection to prevent fermentation. Fruits are soaked in water to soften the flesh and then macerated by hand, rubbed against a screen or processed in a depulping machine; the pulp is washed away and the seeds are dried in the shade. Teak fruits are drupes enclosed in a felty, spongy covering; the whole fruit, which may contain up to four seeds, is used for sowing, and the felty layer is usually rubbed off. For some species, such as neem, the fruit pulp is removed and the seeds are sown fresh or dried carefully because of their limited storability.
After extraction, seed lots contain debris, empty and damaged seeds, wings, and seeds of other species. Cleaning removes these. Screens and sieves separate by size; air-screen cleaners combine sieves with an air stream that blows away light material; aspirators separate by weight in a rising air stream; gravity tables separate by density on an inclined vibrating deck; flotation in water separates empty and insect-damaged seeds, which float, from filled seeds, which sink, for species that tolerate brief wetting. For some conifers, the incubation-drying-separation (IDS) method upgrades seed lots: seeds are moistened and incubated, then dried briefly, so that live seeds lose water more slowly than dead ones and can be separated by flotation. Clean seed lots store better, give more reliable germination tests and allow accurate sowing rates.
The output of processing is often measured as seed yield: the weight or number of clean seeds obtained from a given weight or volume of fruits or cones. Yields vary greatly by species, year and fruit maturity, and are recorded so that collection quantities can be planned.
Orthodox seed is dried to a safe moisture content before storage. Drying is done in the shade, in ventilated rooms, in low-humidity drying rooms or in driers with controlled air temperature and humidity; drying too fast or too hot cracks seed coats and kills embryos. The weight of seed after drying can be calculated from its moisture contents before and after: final weight equals initial weight times (100 minus initial moisture) divided by (100 minus final moisture), using moisture on a fresh-weight basis. A 50-kilogram lot at 20 per cent moisture dried to 8 per cent weighs 50 × 80 ÷ 92 = 43.5 kilograms.
A few species illustrate how handling follows seed biology. Teak fruits ripen in the cool, dry season and fall over several months; they are collected from the ground or shaken from the tree, dried, rubbed to remove the felty covering, and stored dry, because teak seed is orthodox and keeps for years when dry. Sal seeds ripen at the start of the monsoon in northern India and germinate almost as soon as they fall; they are collected from the ground within days of falling, the wings removed, and sown at once, because they are recalcitrant and lose viability within weeks. Eucalypt capsules are collected when they turn brown, before they open, and dried so that the tiny seeds fall out; eucalypt seed lots contain much chaff (unfertilised ovules), which is why their quality is often given as the number of viable seeds per unit weight rather than as purity. Acacia and other legume pods are collected when brown and dry, threshed, and stored dry; the seed is hard and needs scarification before sowing. Neem fruits are depulped and the seeds sown fresh or dried carefully, because neem seed loses viability within months under ordinary conditions. Pines are collected as closed, mature cones, kiln-dried and tumbled, and the seed dewinged and dried for storage.
Seeds differ fundamentally in how they respond to drying and cold. Orthodox seeds, a term introduced by E. H. Roberts in 1973, tolerate drying to low moisture contents, around 5 per cent or lower, and storage at low and sub-zero temperatures; under such conditions they live for years or decades. Most temperate conifers, many legumes, eucalypts and teak have orthodox seed. Recalcitrant seeds are shed with high moisture content and are killed by drying below a fairly high moisture level; many are also damaged by chilling. They cannot be stored for long by conventional means. Sal and many other dipterocarps, oaks, many species of moist tropical forest, and fruits such as mango and jamun have recalcitrant seed. Intermediate seeds, recognised as a separate group by Ellis and colleagues around 1990, tolerate drying to moderately low moisture contents but are injured by further drying or by low temperatures; neem and coffee are commonly cited examples.
The category of a species determines how its seed is handled and whether it can be stored for future seed years. It is determined experimentally by drying seeds to different moisture levels and testing their germination; for many tropical species the behaviour is still unknown or uncertain.
The life of orthodox seed in storage depends mainly on its moisture content and temperature. Harrington's rules of thumb state that, within the usual range, each 1 per cent reduction in seed moisture content (between about 5 and 14 per cent) and each 5 °C reduction in storage temperature (between about 0 and 50 °C) roughly doubles the storage life; they are approximations that do not apply at the extremes. Reducing moisture from 10 to 7 per cent would, by this rule, extend life about eightfold.
Orthodox seed for medium-term storage is commonly dried to around 5 to 8 per cent moisture, sealed in moisture-proof containers so that it does not take up water from the air, and kept in cool stores, often a few degrees above freezing; for long-term conservation in gene banks it is dried further and kept at about −18 °C or colder. Seed lots are retested for germination at intervals during storage. Unsealed seed in a humid tropical room takes up moisture from the air and deteriorates rapidly, which is why simple sealed containers and drying are the most important improvements for small seed stores.
Recalcitrant seed cannot be dried, so it must be stored moist, in conditions that keep it alive but stop germination and limit fungal growth: in ventilated containers or perforated bags, often mixed with moist sawdust, sand or charcoal, at the coolest temperature the species tolerates without chilling injury. Even then, storage life is usually weeks to months. Sal seed, for example, loses viability within a few weeks under ambient conditions, so it is sown as soon as possible after collection. Alternatives include raising seedlings and holding them in the nursery as a living store, storing excised embryos or embryonic axes in liquid nitrogen (cryopreservation) in research gene banks, and conserving the trees themselves in situ or in field gene banks. Intermediate seed is dried to a moderate moisture content and stored cool but above the temperature that injures it.
Seed centres range from national facilities with drying rooms, cold rooms at a few degrees above freezing, freezers and controlled-humidity laboratories, to a single room in a range office. The principles are the same. Seed is dried to the target moisture content before it is sealed, because sealing moist seed traps water and speeds deterioration. Containers, such as laminated aluminium foil bags, glass jars with rubber seals or metal tins, must be truly moisture-proof. Each container carries a label inside and outside with the lot number. Stores are kept as cool as possible, protected from rodents and insects, and organised so that the oldest lots are used first. A register records each lot's quantity, moisture content, test results and retest dates. Even without refrigeration, drying orthodox seed well and keeping it sealed in a cool, shaded room extends its life greatly compared with open sacks in a humid store.
A dormant seed is a viable seed that does not germinate under conditions of water, temperature and oxygen that would normally allow germination. Dormancy spreads germination over time and space and helps seedlings appear in a favourable season, but in the nursery it causes slow, uneven germination. Seed that fails to germinate because it is dead, or because conditions are unsuitable, is not dormant; quiescent seed germinates as soon as it receives water.
The classification now most used, by Baskin and Baskin, recognises five classes. In physiological dormancy, the commonest class, the embryo has a physiological block to germination that is released by moist chilling, warm conditions, after-ripening in dry storage or gibberellins. In morphological dormancy, the embryo is underdeveloped at dispersal and must grow inside the seed before germination. Morphophysiological dormancy combines an underdeveloped embryo with a physiological block. In physical dormancy, the seed or fruit coat is impermeable to water, as in the hard seeds of many legumes. Combinational dormancy is physical plus physiological dormancy. Older texts use terms such as seed-coat (exogenous) dormancy, mechanical dormancy for very hard fruit walls, and embryo (endogenous) dormancy.
Physical dormancy is broken by making the seed coat permeable, a process called scarification. In hot water treatment, seeds are placed in water that has just boiled, removed from the heat, and left to soak as the water cools, often overnight; the swollen seeds are then sown. Mechanical scarification nicks, files or abrades the seed coat with a knife, sandpaper or a scarifying drum, taking care not to damage the embryo; it is reliable for small lots of large seeds. Acid scarification soaks seeds in concentrated sulphuric acid for a period set by species and seed lot, followed by thorough washing; it is effective but hazardous and needs protective equipment and careful timing. Many acacias, Prosopis, Albizia, Leucaena and other legumes respond to these treatments. Untreated hard seeds may germinate over months or years.
Physiological dormancy in many temperate trees is broken by cold moist stratification: seeds are mixed with a moist medium or kept moist in bags at low temperatures above freezing, typically a few degrees Celsius, for weeks to months depending on the species. Some species need warm moist conditions first and then cold, imitating summer then winter. Soaking in water for a day or two, or in gibberellic acid solution, helps some species. After-ripening during dry storage releases dormancy in others. Morphological dormancy requires conditions that allow the embryo to grow, usually warm and moist.
Teak illustrates mechanical or fruit-coat effects: its hard, thick endocarp and felty mesocarp slow germination, and traditional pre-treatments include alternate soaking in water and drying in the sun over a period of days, which cracks and weathers the fruit and improves germination; germination of teak is nevertheless slow and spread out. Pre-treatment prescriptions for each species are given in seed manuals and should be confirmed with tests on the actual seed lot, because dormancy varies between lots and provenances.
Seed testing measures the quality of a seed lot so that buyers and nurseries know what they are getting and can calculate sowing rates. Testing begins with sampling: primary samples are drawn from many points in the lot, combined into a composite sample, and reduced to a submitted sample sent to the laboratory, from which working samples are taken for each test. A test is only as good as the sample, so sampling follows set procedures.
The purity test separates a working sample by weight into pure seed of the species, other seeds, and inert matter such as wings, debris, broken seeds below a set size and soil. Purity is the percentage by weight of pure seed. The weight of 1,000 pure seeds (the thousand-seed weight) is determined by counting and weighing replicates; the number of seeds per kilogram is one million divided by the thousand-seed weight in grams, so a thousand-seed weight of 25 g means 40,000 seeds per kilogram.
Moisture content is determined by weighing a sample, drying it in an oven under prescribed conditions, and weighing it again. In ISTA rules, moisture content is expressed on a fresh-weight (wet) basis: the loss in weight divided by the original weight, times 100. The low constant temperature oven method dries the sample at about 103 °C for about 17 hours; a high constant temperature method at about 130 to 133 °C for a shorter time is prescribed for some species. If a sample weighing 10.0 g weighs 9.1 g after drying, its moisture content is 0.9 ÷ 10.0 × 100 = 9.0 per cent on a fresh-weight basis; on a dry-weight basis it would be 0.9 ÷ 9.1 × 100 = 9.9 per cent, which is why the basis must always be stated. Rapid electronic moisture meters are used in seed centres but must be calibrated against the oven method.
The germination test measures the proportion of seeds that produce normal seedlings under favourable conditions. ISTA rules prescribe, for each species, the substrate (paper, sand or soil), temperature regime, light, any pre-treatment, the test duration and the days of first and final counts. Four replicates of 100 pure seeds are usual. Seedlings are classified as normal (with all essential structures able to develop into a plant), abnormal, or not germinated (hard, fresh ungerminated, or dead). Germination capacity is the percentage of normal seedlings at the end of the test. The speed of germination matters too: germination energy is the percentage germinated by a set early count, and Czabator's germination value combines speed and completeness as the product of peak value (the highest value of cumulative germination per cent divided by days) and mean daily germination (final germination per cent divided by test days). A lot with 80 per cent final germination over 20 days has a mean daily germination of 4; if its peak value is 6 (48 per cent by day 8), its germination value is 6 × 4 = 24.
Purity and germination combine into pure live seed: purity times germination, so a lot of 95 per cent purity and 80 per cent germination has 76 per cent pure live seed. Sowing calculations use this figure with the thousand-seed weight and a nursery survival factor.
Germination tests can take weeks or months for slow or dormant species, so quicker viability tests are used. The tetrazolium (TZ) test soaks seeds, after cutting or preparing them, in a solution of 2,3,5-triphenyl tetrazolium chloride; living tissues, through the activity of dehydrogenase enzymes, reduce the colourless salt to a red, insoluble formazan, so live embryos stain red and dead tissues stay unstained. The pattern and intensity of staining are interpreted by trained analysts according to the rules, because partly stained embryos may or may not be viable. The test gives results in a day or two and shows viability regardless of dormancy, but it does not show vigour or dormancy themselves, and results can exceed germination in dormant lots. Other quick tests include the cutting test (cutting seeds to see whether the embryo and endosperm are sound), X-radiography (showing filled, empty and insect-damaged seeds without destroying them) and the excised embryo test, in which embryos are removed and germinated on moist paper.
Test results are used directly in the nursery. The number of plantable seedlings expected from a kilogram of seed equals the number of seeds per kilogram times purity times germination times a nursery factor (the proportion of germinated seeds that become plantable seedlings, from local experience). If a lot has a thousand-seed weight of 25 g (40,000 seeds per kilogram), 95 per cent purity, 80 per cent germination and a nursery factor of 0.7, each kilogram yields about 40,000 × 0.95 × 0.8 × 0.7 = 21,280 plantable seedlings, so 50,000 seedlings need about 2.35 kilograms. Using the germination of a fresh test of the lot, rather than a figure from the literature or from an old test, avoids both waste and shortfalls. Where the germination test of a dormant species would take too long, a tetrazolium result can be used for planning, with the caution that it may overestimate field germination.
Laboratory germination tests are run under near-ideal conditions, so they can overestimate how seed will perform in the nursery or field, where temperature, moisture and pathogens are less favourable. Seed vigour describes the properties that determine how quickly and uniformly seed germinates and how well seedlings grow under a range of conditions. Two lots with the same germination capacity can differ in vigour; the more aged lot germinates more slowly and is more sensitive to stress. Speed-of-germination indices such as germination value, accelerated ageing tests (holding seed at high temperature and humidity before a germination test) and electrical conductivity of seed leachates are used to compare vigour, mostly in research and in large programmes. In everyday practice, a lot that germinates slowly and unevenly in the laboratory should be expected to perform worse in the nursery than its final germination percentage suggests.
The International Seed Testing Association (ISTA), founded in 1924 and based in Switzerland, develops and publishes the International Rules for Seed Testing, accredits seed testing laboratories, and issues international certificates. Its aim is uniformity: a seed lot tested in one country should give results comparable with a test of the same lot elsewhere, which is essential for international trade in agricultural and tree seed. The Rules are revised regularly and cover sampling, purity, other seed determination, germination, tetrazolium testing, moisture content, weight determination, seed health, variety verification and other tests, with species-specific prescriptions. Many forest tree species have prescriptions in the Rules, though for many tropical species the methods are still being developed.
ISTA issues two main kinds of international certificate. The Orange International Seed Lot Certificate is issued when the sampling of the seed lot and the testing are both done under ISTA rules by an accredited laboratory, so the results apply to the whole lot. The Blue International Seed Sample Certificate is issued when the laboratory tests only a sample submitted to it, without responsibility for how it was drawn, so the results apply only to that sample. Accredited laboratories must meet ISTA's quality standards and take part in proficiency tests. ISTA rules also set maximum seed lot sizes for each species, so that a lot is uniform enough for a sample to represent it, and tolerance tables, which state how far results of repeated tests may differ before they are considered to disagree.
National seed laws and standards for tree seed often adopt ISTA methods, and many national forest seed centres follow them even where they are not accredited. In India, forest seed testing and handling are carried out by forest department seed centres and institutes under ICFRE, which have published seed handling and testing guidelines for important species. Results reported to buyers should state the test methods used, especially the moisture basis and the germination conditions, because results obtained by different methods cannot be compared directly.
Seed testing tells the buyer how much of a lot is live seed of the species; it does not tell where the seed came from or how good its parents were. Certification of forest reproductive material provides that assurance. Under a certification scheme, an official authority registers the basic material (the stands, seed orchards, parents or clones from which seed or plants are produced), supervises collection and processing, and issues certificates and labels that travel with the material. This matters because the genetic origin of seed affects adaptation and growth for the whole life of a plantation, and errors are discovered only years later.
The OECD Scheme for the Certification of Forest Reproductive Material Moving in International Trade, usually called the OECD Forest Seed and Plant Scheme, was established in 1967 and has been revised several times. Participating countries apply common rules for approving basic material and certifying reproductive material. The scheme defines four categories. Source-identified material comes from basic material within a defined region of provenance, without selection for quality; its label is yellow. Selected material comes from stands selected for superior characteristics at population level; its label is green. Qualified material comes from seed orchards, parents of families, clones or clonal mixtures whose components have been selected individually but not yet tested; its label is pink. Tested material comes from basic material whose superiority has been demonstrated by comparative tests or genetic evaluation; its label is blue. The European Union's directive on the marketing of forest reproductive material (Council Directive 1999/105/EC) uses the same four categories.
Types of basic material include seed sources (trees within an area), stands, seed orchards, parents of families, clones and clonal mixtures. The certificate records the category, type of basic material, region of provenance, whether the material is autochthonous (native to the site) or not, and the quantity.
National programmes develop a hierarchy of seed sources. Seed collection areas or seed stands are natural or planted stands of good phenotypic quality designated for collection. Seed production areas are good stands that have been improved by removing inferior trees and managed for seed production. Seed orchards are plantations of selected clones (clonal seed orchards, established by grafting or rooting cuttings of plus trees) or of seedlings from selected parents (seedling seed orchards), isolated from outside pollen and managed for abundant, easily collected seed of improved genetic quality. Progeny and provenance tests evaluate the material and allow orchards to be rogued and the material moved up to the tested category. Tree improvement and orchard design are covered in forest genetics and tree improvement.
In India, forest departments and ICFRE institutes have designated seed stands and seed production areas for important species such as teak, sal, eucalypts, Casuarina, Dalbergia sissoo, Gmelina arborea and many pines, and have established clonal and seedling seed orchards of teak, eucalypts, Casuarina and other species in tree improvement programmes. Plus trees, individual trees of outstanding phenotype selected in natural stands or plantations by comparison with their neighbours, provide the material for clonal orchards and progeny tests. State forest seed centres collect, process, test and supply seed, and keep registers of seed sources. A formal national certification system for forest reproductive material comparable to the OECD scheme has not been fully implemented across all species and states, so buyers should ask for documentation of source and test results with every seed lot.
A provenance is the geographic place of origin of a seed lot or of the parent trees. Populations of a widespread species are adapted to their local climates, soils and day lengths, so seed from different provenances can differ markedly in growth, form, frost and drought hardiness, timing of bud flush and set, and pest resistance. Provenance trials, in which seed from many origins is grown together at several sites, reveal these differences and have shown both cases where local seed is best and cases where certain non-local provenances grow better. Moving seed too far, especially from warmer or lower sites to colder or higher ones, risks frost damage and failure; the reverse may lose growth potential.
A seed zone is an area with fairly uniform ecological conditions within which seed can be moved with little risk of maladaptation. Seed zones are delineated using climate, physiography, vegetation and, where available, genetic data from provenance trials, and are often subdivided by elevation bands. Seed transfer guidelines state how far, in distance, elevation or climate, seed may be moved from its origin. Regions of provenance under the OECD and EU schemes serve a similar purpose for certified material.
Climate change complicates seed transfer. Seed adapted to the present climate of a site may be poorly adapted to its climate in fifty years. Climate-based seed transfer systems, used in some Canadian provinces and elsewhere, match seed sources to the expected future climate of the planting site, and assisted migration, moving seed somewhat towards warmer origins than the planting site, is debated and increasingly applied within limits.
Every seed lot must carry documentation that allows its origin and history to be traced. A seed lot record typically includes a unique lot number, species, provenance (country, region, locality, coordinates and elevation), type of seed source (stand, seed production area, orchard), number of parent trees, collection dates, collector, processing details, quantity, test results and storage location. The record is kept in the seed centre's register and follows the lot through the nursery to the plantation, so that the origin of a planted stand can be identified decades later. Mixing seed lots of different origin without records is one of the most damaging errors in seed work, because it destroys the information needed to judge performance and to choose future seed.
Insects destroy large parts of many tree seed crops before and after collection. Cone and seed insects of conifers include cone moths and cone beetles that feed inside developing cones, and seed chalcids (Megastigmus species) whose larvae develop inside single seeds, leaving them looking normal from outside. Bruchid beetles attack the seeds of many legumes, including acacias and Prosopis, in the pod and continue to breed in stored seed. Weevils, such as Curculio species in acorns and chestnuts, lay eggs in developing seeds. In the tropics, seed borers and fruit flies damage many species. Insect damage is detected by cutting tests, X-radiography and flotation, and losses are reduced by timely collection, rapid processing, cleaning out damaged seed, cold or freezing treatment of dry orthodox seed, and fumigation or insecticide treatment where permitted by law.
Seed-borne and storage fungi reduce viability and carry diseases to the nursery. Storage moulds, mainly Aspergillus and Penicillium species, grow on seed stored too moist and warm. Pathogens such as Fusarium and the damping-off organisms can be carried on or in seed. Recalcitrant seeds, stored moist, are especially prone to fungal attack. Ground-collected seed is more likely to be contaminated than seed collected from the tree. Control measures include collecting from the tree where possible, drying orthodox seed promptly, clean processing and storage, surface sterilisation, hot-water treatment for some species, and seed dressing with approved fungicides. Seed health tests, such as incubating seeds on moist blotter paper or agar and identifying the fungi that grow, are covered in the ISTA rules.
Seed moving between countries can carry pests and pathogens into new regions where they have no natural enemies. International trade in seed is therefore subject to phytosanitary regulation under the International Plant Protection Convention (IPPC) and national plant quarantine laws. Exporting countries issue phytosanitary certificates confirming that consignments meet the importing country's requirements, which may include inspection, testing, treatment or prohibition of particular species or origins. Seed centres must know and follow these rules, and researchers exchanging small seed samples are bound by them as well.
Protecting seed is easier before damage than after. In seed orchards and seed production areas, monitoring of flowers, cones and fruits for insect attack allows timely control, and removal of infested cones or fruits reduces the next generation of pests. Collection at the right time limits the period when seed is exposed on the tree or ground. In storage, the main defences are drying orthodox seed to low moisture, which also stops most insects and fungi; cleaning out damaged seed and debris; using clean, sealed containers; keeping stores cool; and inspecting lots regularly. Freezing dry orthodox seed for a period kills many insects without chemicals. Chemical treatments are a last resort, used only with approved products and with care for workers and for the viability of the seed.
A student should be able to name maturity indices and collection methods for common species, classify species as orthodox, intermediate or recalcitrant with examples, match dormancy types with pre-treatments, and carry out purity, moisture, cutting and germination tests in the laboratory. Practise the calculations: moisture content on fresh and dry basis, weight after drying, seeds per kilogram from thousand-seed weight, pure live seed, germination value and seed needed for a sowing target. Learn the ISTA certificate colours and the OECD categories and labels.
A forester in a seed centre or nursery division plans collections for the coming seed year, organises safe climbing crews, supervises extraction and drying, stores seed in sealed containers in cool stores, keeps the seed register, sends samples for testing, calculates sowing rates from test results, and makes sure every lot is documented by origin. When germination is poor, the forester checks whether the seed was immature, damaged, dormant, badly stored or diseased before blaming the nursery.
A senior forester manages a regional or national seed programme: designating and managing seed stands, seed production areas and orchards, setting seed zones and transfer rules, running a testing laboratory to ISTA standards, implementing certification under national law or the OECD scheme, planning seed supply for large planting programmes across good and lean years, and handling phytosanitary requirements for import and export.
A researcher studies seed biology and technology: desiccation tolerance and storage behaviour of tropical species, dormancy physiology, rapid viability and vigour tests, cryopreservation of recalcitrant germplasm, genetic diversity of seed lots and orchards, climate-informed seed transfer, and seed pest ecology. Such work extends ISTA prescriptions to new species and informs seed policy and conservation strategies.
This manual gives principles and common practice; it does not give species-by-species collection calendars, kiln schedules, storage moisture targets, pre-treatment times or ISTA test prescriptions, which should be taken from the current ISTA Rules and from regional seed manuals. Storage behaviour of many tropical species is still uncertain and is not tabulated. Tree improvement, seed orchard design and genetic testing are covered in forest-genetics-tree-improvement; nursery sowing in forest-nursery-plantation. National seed laws and certification procedures in individual countries, including India, are described only in general terms. Fumigants and pesticides are not named because approvals vary by country.
Original practice questions written to the syllabus and pattern of the exams named in each question's examTags. They are not past papers and do not reproduce any board's questions.
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