R. MORANDINI
Professor, Stazione Sperimentale di Selvicoltura, Florence, Italy
I. SEED PRODUCTION, COLLECTION AND PRODUCTION
This paper, which is being published serially, is based on experience gained in several countries and describes the procedures, equipment and machinery used for several kinds of forest seed. It should be read in conjunction with Handling forest tree seed, FAO Forestry Development Paper No. 4, published in 1955. Comments, amendments and additions will be welcomed in order to make this document more complete and provide an efficient guide to seed handling techniques. A list of contributors and a bibliography are available on request from the forestry and Forest Products Division, FAO Rome.
IN recent years programs of afforestation and reforestation have been planned and carried out at an increasing rate in many areas of all continents. This has resulted in a remarkable increase in the demand for forest tree seeds, which are often quite difficult to find, prepare and handle.
When artificial regeneration was first applied in forestry, the only problem was that of getting seed or nursery stock of desirable forest species, regardless of its genetic worth. But substantial failures of many large plantations, as well as several trials and investigations, proved the basic importance of the seed origin, which is today recognized everywhere by forest geneticists and research workers. Some forest administrations have prepared special laws in order to insure effective control of the forest seed provenance; but even nowadays many countries do not attach sufficient importance to this question.
The first step to insure provenance control is to identify, for each important tree species, a number of production areas or stands which, according to certain criteria, may be considered as suitable seed sources. Seed stands should consist mostly of very good trees, preferably middle-aged (generally 35 to 40 years is a minimum for conifers). Good stem form, small branches, rapid growth rate, and soundness are the main characteristics required. Moreover, it should be kept in mind that seed production is influenced by stand density, and that the stand should cover an area large enough to make collection convenient from an economic point of view.
Seed could also be collected from individual trees which combine to the highest degree the desired characteristics ("plus trees" or "elite trees"). But the collection from individual trees is usually carried out only for scientific purposes.
Lately, the development of forest genetics and tree breeding has led to the concept and establishment of seed orchards. Seed orchards are plantations of genetically superior trees, isolated to reduce or prevent pollination from outside sources. They are established by setting out clones (as grafts or cuttings) or seedling progeny of trees selected for desired characteristics. When seed orchards are established while genetic evaluation of the parent material is still under way, components shown by progeny tests to be undesirable should be removed from the orchards.
Seed orchards are expensive to establish, but they can yield a large quantity of superior seed some years after their establishment. Seed collection is less expensive and the use of special harvesting equipment is possible.
Seed-bearing of forest trees is usually rather irregular from year to year. Only a very few species bear good seed crops every year. Generally one year of a heavy crop (seed year) is followed by one or several years of poor seed crop or none at all. Many conifers show intervals between seed years of three to four years. Most spruces, for instance, bear good crops at intervals of five years and occasionally at longer intervals up to seven years, light crops being rather common in the intervening years.
In order to organize seed collection properly it is therefore very important to forecast fruit-setting some months before collecting time. The estimates of seed crops are generally based on counting or weighing cones or fruits on a number of sample trees. The results are applied to the stand and are finally expressed as numerical values, going from 1 (total crop failure) to 10 (maximum crop, exceptional seed year).
The ripeness of seeds and the beginning of natural seed dispersal are the main factors which limit the time suitable for collection.
Seed ripeness is often judged by the color of fruits or cones, but a better indication is provided by their specific gravity. The specific gravity of cones or fruits decreases as the seed approaches maturity. A mixture of various liquids may be adjusted so that ripe cones of a given species Will just float. Cones may also be tested in a set of liquids, with different known specific gravities.
The surest check of seed ripeness is to cut cones or fruits in two lengthwise, and examine the seeds which occur on the cut surfaces. By this method the number of fully developed seeds per fruit or cone can be counted at the same time. A good knife is often sufficient for this purpose, but special knives have been prepared which prove very useful for most cones and for some tougher fruits of hardwoods.
TIME FOR COLLECTION
Seeds or fruits should usually be collected just as they have reached full ripeness, and before natural dispersal begins. This interval varies considerably in the different tree species, and with local climatic conditions. For many conifers, especially spruces, long autumn rains or fogs delay cone opening for a long time, while a few dry windy days are sufficient to give full seed dispersal. A general idea for seed collecting time in northern Europe is given in Table 1 compiled recently by the United Kingdom Forestry Commission.
COLLECTION ON THE GROUND
Seeds and fruits can be picked up from the ground after shedding. This is the case of many large-sized seeds of broadleaved trees, such as oak, chestnut, and beech. If possible, the ground should be cleaned from brush, and a, sheet placed under the tree makes collecting much easier. Seeds should be picked up as soon as possible after they have fallen to avoid losses and damage by insects, rodents or fungi; but it should be kept in mind that the first seeds or fruits which fall naturally are often of poor quality.
For all trees, a very easy and cheap method of collection is to take account of normal fellings occurring during the seed ripening season, and to collect seeds or fruits from felled trees. This method is largely used in northern Europe.
In North America seeds are sometimes collected in the hoards of small rodents, which often gather large quantities of different seeds, especially nuts. for their winter reserve.
TABLE 1. - FREQUENCY OF CONE CROPS AND MONTHS OF CONE COLLECTION
|
Species |
Average frequency of good cone crops (years) |
Months of cone collection |
||
|
Earliest |
Normal |
Latest |
||
|
Scots pine (Pinus sylvestris) |
2-3 |
-Nov. |
Jan. |
Feb. |
|
Corsican pine (Pinus nigra var. calabrica) |
3-5 |
Dec. |
Jan. |
Feb. |
|
Lodgepole pine (Pinus contorta) |
2-3 |
Dec. |
Jan. |
Feb. |
|
European larch (Larix decidua) |
3-5 |
Oct. |
Nov. |
Dec. |
|
Japanese larch (Larix leptolepis) |
3-5 |
Sept. |
Sept. |
Oct. |
|
Hybrid larch (Larix eurolepis) |
3-5 |
Sept. |
Sept. |
Oct. |
|
Norway spruce (Picea abies) |
3-5 |
Oct. |
Oct. |
Nov. |
|
Sitka spruce (Picea aitchensis) |
3-5 |
Sept. |
Sept. |
Oct. |
|
Serbian spruce (Picea omorica) |
2-4 |
Oct. |
Oct. |
Nov. |
|
Douglas fir (Pseudotsuga menziesii) |
4-6 |
Sept. |
Sept. |
Oct. |
|
Noble fir (Abies nobilis) |
4-5 |
Aug. |
Aug./ Sept. |
Sept. |
|
Grand fir (Abies grandis) |
3-5 |
Aug. |
Aug./ Sept. |
Sept. |
|
Western red cedar (Thuja plicata) |
2-3 |
Aug. |
Sept. |
Sept. |
|
Western hemlock (Tsuga heterophylla) |
3 |
Aug. |
Sept. |
Sept. |
|
Lawson cypress (Chamaecyparis lawsoniana) |
3 |
Aug. |
Sept. |
Sept. |
COLLECTION FROM STANDING TREES
The greatest quantity of tree seeds is collected from standing trees. This method is clearly often difficult and expensive, but it ensures that seeds come from selected trees and from the best part of their crown. In the case of shrubby or very small trees, seeds can be picked directly from the branches by the collector while standing on the ground. For small or medium-sized trees, poles, pole saws or trimmers are used for collection, the operator standing on the ground or on the lower branches of the tree.
Hooks of different shapes, cutting edges or little saws are attached to a long pole. By moving the pole, fruits or cones can either be detached from the branches, or the twig (in the case of small fruits) can be cut down completely. Sometimes a sack or a basket with a wire end can be used for fruits which are easy to detach.
The length of the pole (that is the height which can be reached) depends on the weight and rigidity of the pole itself. Different materials are used:
(a) bamboo is very light but not sufficiently rigid;
© aluminium or plastic poles often give good results.
For large-sized trees, the normal method of collecting seeds is to climb the tree. Some forest workers are very good tree climbers, and climb tall trees without, any equipment. But this method, besides being dangerous, tends to make the operator choose trees for their climbability and not from the point of view of seed quality. If possible, good seed trees should be marked by a skilled technician, and workers should not be allowed to climb trees other than the marked ones.
Low branched trees are easily climbed, and a normal climber is able to climb a trunk, even without branches, for a few meters, if the tree diameter is not too big (not over 40 centimeters). Otherwise, some equipment, such as irons or ladders, is required to reach the lowest branches.
Climbing equipment
Foot-clamps, loot-hooks or climbers. These are some of the usual types of climbing equipment: a forged iron is fastened by a belt to the climber's shoe (Figure 1) (sometimes to the leg too), and finishes in a pointed spur, the length varying according to the climbing method. One of the best types is a German one, where the hook is quite short and does not extend beyond the shoe sole, permitting the climber to walk on the soil without difficulty. The irons always cause some damage, especially to very thin-barked trees: round points seem to be preferable from this point of view. Irons should not be used when the bark is frozen.
FIGURE 1. - Tree climbing irons
Courtesy, Morandini.
Ladders. Many types of ladder are used in tree climbing. Type, form, length and material vary largely according to tree species and local conditions. In level country and where forests have an extensive road network special ladders, which are sometimes heavy, can be transported by truck. In mountain forests transport difficulties arise and only small, light ladders may be employed.
For smaller trees, a light wooden ladder is perfectly adequate; types used by fire services, 6 to 8 meters long, do not weigh more than 2 kilograms, and can also be transported in mountain regions: they have to be used in a vertical position, i.e., parallel to the trunk.
Naval rope ladders are sometimes used: a finer cord is thrown to a strong branch, and the rope ladder or cord is then fixed to the branch.
Greater heights require scaling ladders which can be assembled and dismantled. They may be of different types and materials: wood, aluminium, steel, and magnesium alloy are used. The elements of the ladder have a variable length (2 to 4 meters) according to their weight, which should not exceed 3 to 4 kilograms per element, to ensure easy transport during climbing. A small, adjustable platform is placed under the base of each leg, to ensure stability on the ground. Each element is fitted easily and safely into the lower one and, when in place, it is fastened to the trunk by a chain or a belt which the operator fixes while ascending.
Assembled scaling ladders may have one or two legs: two-legged ladders are more common and more easily purchased or constructed.
The type shown in Figure 2 is a duraluminium ladder, built in 2- to 2.5- to 3-meter sections; supports an drungs are of tubing, 3.5 centimeters and 2.5 centimeters in diameter respectively. The ladder weighs 1.3 kilograms per meter, including the fastening chain. Elements may be connected without limit: this type has been used up to a height of 38 meters. However, the rungs could be more widely spaced, and the connection improved. (This ladder was designed by the Ecole nationale des eaux et forêts, Nancy, France.)
One-legged ladders are useful on steep or rocky ground where it is difficult to lodge a two-legged ladder evenly, and also in the case of a tree with many branches where it is hard to place a two-legged ladder along the trunk.
Small bars are fixed as rungs alternately on either side of the central support. The form of the rungs varies with the type of ladder. One-legged ladders can be attached by chains very close to the trunk against which they are held by a special spacer. These ladders can be attached to trunks which are not quite straight and continued up into the crown. They are easier to place on the ground than two-legged ladders, although transport during climbing is not so easy; but no special difficulties arise. The weight and length of the elements do not differ much from those of two-legged ladders, because the central support is generally in steel tubing. One-legged ladders give very good service, and are widely used especially in Germany (Figure 3).
FIGURE 3. - Type of one-legged ladder.
FIGURE 4. - Transportable extension platform.
FIGURE 5. - The "Baumvelo" apparatus.
Extension telescopic ladders may be placed and transported by special trucks. Fire-service extensible ladders are used to enable the worker to reach heights of from 20 to 25 meters. They may only be used in forests in the plains or for collecting seeds on trees growing at roadsides.
An extension ladder in light alloy, which may reach a length of 14 meters, is mounted on a tractor, and can therefore be used also in forests without roads. A special model has been developed by the United States Forest Service. The truck is 0.5 ton pick-up. on which a rectangular framework is mounted, supporting a heavy-duty aluminium extension ladder, 12 to 15 meters long.
Telescopic platforms rolling on wheels sliding under carriage or mounted on special trucks are also used (Figure 4).
A special apparatus for tree climbing is the Swiss
Baumvelo "(Figure 5). It is a device for climbing straight branch-free trees, without damaging them, and is preferable to a ladder because it is much easier to transport and to use in dense forest stands. It consists of two similar parts, one for each leg. A vertical arm supports a pedal in which the operator's shoe is fastened: at its upper end it is connected to a steel belt, forming a circle of adjustable diameter around the stem; a supporting block is fixed in the middle of the arm. After reaching the crown, the climber can easily leave the Baumvelo," attached to a branch, and climb freely into the crown, special safety belts being provided. This device has been developed by the Eidg. Forstliche Versuchsanstalt, Zurich, and has been favorably received in various countries, especially in the United Kingdom, where the Forestry Commission has improved the related safety equipment.
Courtesy, United Kingdom Forestry Commission.
Another tree climbing device is the tree net, developed in the United Kingdom by the Forestry Commission (Figure 6). A triangular net is suspended by special ropes and snatchblocks from the top of the tree, and covers a large part of the crown; the collector can thus easily reach all parts of the crown. This equipment seems to give good results for large branched trees bearing small fruits which take a long time to collect. Special equipment is available for rigging the net, but the operation is time-consuming and is very difficult, or quite impossible, in dense stands.
Sometimes permanent or semipermanent scaffolding, in iron pipes or wooden poles, is built around the trees; this is done mainly for scientific purposes, to collect seeds or scions or to control the pollination of plus trees.
Collected cones or fruits are put into sacks generally with a capacity of 1 hectoliter (3 bushels), corresponding to about 50 kilograms (100 pounds) in weight. Sacks or other containers should be filled and closed under proper supervision, and their number or weight checked, to ensure provenance control.
If cones or fruits cannot be transported at once to the storage point or factory, provisional storage should be arranged locally, in huts or under some kind of shelter; in any case sacks or containers should not be stored in large amounts over a long period. Various kinds of transport can be employed. Trucks are generally used, but railway transportation is more economical over very long distances. While loading trucks or cars, care should be taken to ensure proper air circulation among sacks. Over short distances, cones could be loaded onto the truck without containers: in this case, each truck should carry cones or fruits of one single provenance and species.
The word "extraction," in the wider sense, includes all the different operations or processes necessary after collection to prepare seed for sowing. This preparation usually consists in separating the seed from all or some parts of the fruit. It is easy to understand that extraction technique varies greatly according to the species: some seeds are quite ready for sowing as collected from the tree; others are obtained after a long series of difficult processes, i.e., opening of cones, separating seed from wings, etc.
Extraction usually consists mainly of one or more of the following processes: washing, maceration, threshing, drying, separation, de-winging, cleaning.
Some of these operations could be carried out in a very simple manner, and do not require any special equipment; but often, and in any case when large amounts of seeds are to be prepared, a special extractory must be established, with stores, machines and any other equipment required for the various operations.
Hitherto seed separation has been carried out by very simple means in most countries, so that machines specially made for forest seed handling are difficult to obtain, and often agricultural machines have had to be adapted for this purpose. During recent years, however, some special machinery has been developed and constructed.
All operations, whether for extraction of small or large amounts of seed, must follow a certain sequence. It is clear that certain processes could sometimes be delayed, or left out altogether in the case of occasional operations, while in the case of a permanent seed plant with large output a careful organization of the working cycle is absolutely necessary. In any case extraction should follow a general line, which is briefly described below.
Cones or fruits coming from collecting points go to pre-cleaning, and pass at once, or after storage, to extraction. Fleshy fruits are submitted to maceration, cones are dried (naturally or artificially) and tumbled, other types of fruits are threshed. After extraction, seeds are de-winged (if necessary), cleaned and put in storage.
A seed plant should therefore include the following rooms and equipment:
1. reception room, and precleaning room and equipment (preferably on the ground floor)
2. stores for fruits or cones
3. a local store (placed above the. kilns or extractors) supplying machines
4. kilns or yards to dry cones or fruits, respectively artificially or naturally
5. extraction room and machines (including de wingers and cleaners)
6. seed stores.
Rooms and equipment should be placed so that fruits and seeds pass from one operation to the next in the easiest and safest way, providing as far as possible mechanical transport between machines. A two-storey building is often recommended. The working material comes to the upper floor from the precleaning plant or from the cone or fruit stores. It is placed here, in a local store, directly supplying kilns which are placed on the first floor. From the kilns seed falls down to the ground floor where separation and cleaning equipment is located. Stores for fruits or cones and for seed are generally placed in one or more separate buildings.
The size of any equipment, installation or plant should be carefully studied and planned in relation to the amount of seed required per year. It should be borne in mind that many species do not give a good seed crop every year, so that in good seed years it is necessary to collect and process a supply of seed for two or three years. Therefore, the equipment must be proportionate to the amount of fruits or cones collected in such years.
Even if good storage facilities for fruit or cones are available, it is preferable to keep the working season as short as possible, to avoid storage difficulties, deterioration, and loss of seed quality. The interval between fruit collection and seed preparation is strictly related to the local climatic conditions.
The choice of machinery should be made after considering the output per hour required, which may be calculated by the formula:
where:
|
W |
is the working capacity of the machinery (in hectoliters of cones per hour; |
|
S |
amount of seed required each year (in kilograms) |
|
F |
average. frequency of seed crops (in years) |
|
Y |
seed yield (in kilograms) per hectoliters of cones or fruits |
|
P |
working period (in days) |
|
T |
daily working time (in hours). |
On the basis of this formula and given the following data, the resulting output per hour would be 1 hectoliter.
|
Amount of seed required each year |
= |
1,000 |
kilograms |
|
Average frequency of crop |
= |
3 |
years |
|
Total amount of seed to be prepared in seed years |
= |
3,000 |
kilograms |
|
Seed yield per hectoliter of cones or fruits |
= |
2.5 |
kilograms |
|
Corresponding amount of cones or fruits to be processed |
= |
1,200 |
hectoliters |
|
Working period |
= |
100 |
days |
|
Output per day |
= |
12 |
hectoliters |
|
Daily working time |
= |
12 |
hours |
For small amounts, it is often more economical to prepare seed by hand or with very simple equipment. But for very large seed production, all modern machinery and auxiliary equipment will greatly reduce costs: mechanical elevators, transport bands or blowing pipes give positive results and often allow a better handling of the seed. Sometimes, a choice must be made between a certain number of small plants placed in the forest and dealing with a limited amount of seed from the local provenances or a large central plant, handling seeds from a wider area.
Such small plants, where work is usually done by hand, could be equipped with relatively simple machines which reduce time and transport costs; they are preferable for species requiring quite simple handling.
Large central plants are run more economically if very large amounts of seed are to be produced, especially for species which require a long series of difficult processes. In a large plant it is possible to establish all types of equipment, even if very expensive. Full mechanization and automatic control ensure good organization of the work and eventually a continuos working cycle with an increase in the daily output of the plant.
Of course, such plants demand a large capital investment, and costs should be carefully studied. It is probable that they are profitable only if the yearly production can be estimated at several thousand kilograms of seed.
Cones or fruits, after transportation from the collecting place to the working plant, should be cleaned before going to the kilns or extractors, or to the stores, since they are often mixed with pieces of bark, wood, and twigs or other impurities.
This rough cleaning may be done by flat (Figure 7) or by rotating screens.
Another method of precleaning is by flotation: damaged fruits and impurities come to the water surface and can easily be skimmed off. But the commonest method is the use of oscillating screens or vibrators.
For seeds of some trees, as Carya, Juglans, Quercus, Ulmus, cleaning is the only handling required.
FIGURE 7. - Plat screen for cone cleaning before storage.
Courtesy, Morandini.
Storage of fruits or cones should be planned with care. Many fruits, especially conifer cones, need a period of storage to become completely ripe. If a large amount of any fruit is collected in a very short time (for some species the collecting season is not more than two weeks since seeds fall quite early when ripe) it may be necessary to store it for some months, depending on the working capacity of the plant.
Storage accommodation should always ensure proper air circulation, and strictly avoid any stagnation or humidity, or very sudden variation in air temperature. Cones or fruits should never be placed directly on concrete floors: wooden or brick floors are recommended. Windows should be without glass but protected by shutters. Nonmetallic roof covering is preferable.
A many-storied building is usually better as elevators or other equipment allow easy transportation between different floors. Floor surface should be divided into compartments (Figure 8) each containing fruits or cones of a single provenance; to assure an easy separation of provenances, the area of each compartment should not he more than 8 to 10 square meters. Cones or fruits should be stored in thin layers (no more than three or four times their length) and stirred very often. Sometimes a part of the compartment floor is easily moved (or opened), so that fruits may fall down to the floor below.
The total area of the store depends on the amount of fruits to be handled, on the duration of storage, and on the working capacity of the seed extractory. If many species are worked in the same plant, one should bear in mind that their collection times are often different: the storage surface could then be reduced. In central Europe, for example, spruce cones are collected in September-October, while those of larch and pines are collected much later, sometimes even in winter. By the beginning of the collection time for larch, a large proportion of the spruce cones will have been already handled, and the space in the store emptied.
FIGURE: 8. - Compartments in a cone store.
Courtesy, Morandini.
The separation of the seed may be obtained by purely mechanical means, such as threshing; by maceration (fleshy fruits); or by drying the fruit. The last is the more frequent case, almost general for conifers, whose cones should be dried to get their scales open and release the wed.
DRYING FRUITS AND CONES
All fruits (including cones), have at their full ripeness a variable but always high moisture content. For example, in cones of some pines this varies between 35 and 65 percent. In natural conditions, fruits on the trees are largely exposed to heating by the sun, and to open air movement; both, but especially the second agency, provide a progressive drying of the fruit, which eventually causes the splitting of the fruit envelopes (i.e., beech or chestnut), or the opening of the cone scales (i.e., Pinus spp., Picea spp.), or the disintegration of the whole cone (i.e., Abies spp., Cedrus spp.). The speed of this process depends upon the climatic conditions and the local weather, and its duration varies greatly according to the species. Cones of Abies disintegrate just as seeds are ripe, while the same process occurs in Cedrus spp. only some months later. Spruces open their cones and seed falls early in autumn, while cones of larch and of many pines remain closed till late winter. Of course any variation of the local weather can change the atmospheric conditions and induce cones to open. In foggy, rainy autumns spruce cones too remain closed till very late, but a few dry, windy days make them open quickly.
Artificial drying should reproduce as much as possible the natural process. Fruits should be submitted to progressive drying by a continuous release of moisture from the fruits. Air coming in contact with fruits should be always drier than the fruits. This can be obtained by circulation of air, either artificially heated or unheated.
Drying in the air
The circulation of air at room temperature is sufficient to open and disintegrate cones of Abies and Cedrus spp., and separate seeds of some hardwoods, such as beech or oak, from their hulls or coverings. Fruits, which are placed in thin layers on the floor (which should be of wood or bricks, not of concrete) or on canvas, in a well-ventilated room, should be stirred frequently to avoid any moisture stagnation. The stores described for keeping cones or fruits could very well be used for air-drying too.
This method is used for very small amounts of fruits, which do not warrant any special equipment, and for species which could be damaged by any heating, including sun heating. It is commonly used for Abies spp. Depending upon the atmospheric conditions, its duration is often very long, and in a wet season a good deal of work is called for in stirring cones frequently.
Drying in the sun
This method of drying is still largely used: it is convenient for occasional operations, but for some species it is commonly used for regular operations too. This was of course the first method followed in fruit drying. Fruits are placed in layers on platforms or on screens, and exposed to the sun. Mediterranean pines, such as P. pinea, and P. halepensis, are always handled by this method. Medium-sized cones, such as those of P. pinaster or P. halepensis, are placed on a platform (of pressed soil, or bricks) in a uniform layer 10 to 15 centimeters deep, stirred frequently, and exposed to the sun until scales open; a cover is spread over the cones in bad weather. As cones open, they are shaken on the platform, and seed is collected on it. Cones of P. pinea are placed in long rows, piled about 50 centimeters high. Open cones are gradually collected from the upper layer of the row and are ready for tumbling.
Other species are exposed to the sun on wire screens with meshes of suitable size to let seeds drop through onto a canvas spread under the screen. If local climate is quite cold and damp at night, screens should be removed and put under cover. A glass is sometimes placed over the screen, to protect cones against rain and to avoid the dispersal of sun heat; but this method is rarely used today since it requires too much labor, and glass may cause overheating of the cones with damage to the seed.
Cones of most species generally open in the sun in a relatively short time: white and yellow pine and Douglas fir require from three to eight days; but other species could require more than one month. Some species such as longepole pine, open so irregularly in the sun that artificial drying is recommended.
Drying is easy during spring and summer, but during autumn and winter when most cones and fruits ripen, it can be carried out only in hot dry regions. There. it is often very convenient, since it does not require expensive installations; but where the weather is changeable, artificial heating is more convenient, as it permits normal work irrespective of weather conditions.
Drying in heated kilns
Drying in heated kilns is used mostly for cones of conifers, and therefore only cones will be dealt with here, although some other fruits can be handled in this way.
The main difficulty in drying cones by natural methods is the absolute impossibility of controlling air humidity and temperature. An increase in the humidity of the air may cause a reclosing of the cones and sometimes a consequent "case-hardening," i.e., cones do not react to further drying but remain closed. Artificial heating, on the contrary, permits control of air moisture and temperature, a much shorter handling time is required, and the process can be continuous so that work organization becomes more effective. Of course, artificial heating requires installations and equipment which are often very expensive. Permanent extraction plants with such equipment should therefore be established only if enough seed is obtained to make the investment pay.
Small drying units mounted on trucks have been tried in the past. The first one was constructed at, Eberswalde (Germany) in about 1930; this unit could be moved from one forest to another, and could extract separately the seed of each local provenance, thus bringing about some reduction in the transportation cost. Recently, infrared equipment has been tested to dry cones on the truck. But permanent units are on the whole more effective and seem to be more economical.
Cone drying by artificial heat should be carried out in such a way as to obtain drying in the shortest time. without damaging the viability of the seed. For this purpose, the following recommendations should be carefully observed.
1. Cones should be properly cured before they are put in the kiln.
2. Air temperature should be controlled and kept to the minimum level sufficient to dry cones.
3. Cones should not be heated longer than necessary.
4. Air in the kiln should be kept as dry as possible.
Moisture and temperature controls are of the utmost importance. The ideal handling would be to expose the cones to a flow of air, whose moisture content should decrease gradually, thus getting a regular, progressive drying and opening of the cone scales.
Temperature and moisture control
Heating increases the saturation deficit of the air, thus obtaining a further release of moisture from cones. But air temperature must not rise above a certain limit, beyond which seed viability in seriously damaged. Generally, temperatures of 40°C to 50°C may be reached. Such temperatures, however, are only allowed if the air humidity and the cone moisture are very low, a combination occurring only at the end of the drying process. At the beginning, hot air acting on wet cones would cause scalding of the seed.
In the past, decreasing temperatures have occasionally been used: but in modern extraction plants either constant or increasing temperatures are employed. Moisture control could be obtained by chemical means, forcing air through filters containing drying materials, but heating is generally much easier and cheaper and it is almost always preferred.
Air heating. This can be obtained by different methods, any source of heat being suitable. In large extraction plants any kind of fuel can be used, including emptied cones or other wastes of extraction. Modern plants often use oil, since it insures a more regular combustion and a completely automatic temperature control. Sometimes electric heating is employed, but it is very expensive and is therefore used only for very small dryers, e.g., for laboratory units.
The control of air temperature is relatively simple in modern plants. A thermostat located where the air enters the drying room can stop or slow down the heating, or permit a flow of cold air to enter and reduce temperature. In old plants temperature is controlled by the operator. Thermometers and thermographs allow the operator to follow the variation of temperature inside the kiln; they should be connected to an alarm bell which rings when a critical temperature is reached.
Air humidity. The control of air humidity is much more difficult: hygrometers and hygrographs can record the humidity variation, but it can be regulated inside the kiln only by forced ventilation. As already mentioned, seed may also be damaged by relatively low temperatures when humidity is high, so that every possible care should be taken to avoid any stagnation of moist air in the kiln. Modern extraction machinery is generally based on a continuous forced ventilation, but in the case of old types of extracting machinery the air change is only periodic or sometimes completely absent.
According to Toumey and Korstian (1954), the relative humidity in the drying room should be kept lower than 50 percent during the early period of drying and below 10 percent at the end of the handling.
Precuring
Cones or fruits are collected as they appear to be ripe, but since natural seed dispersal often immediately follows full ripeness, collection starts before ripeness, particularly if a large amount is required.
Fruits do not all ripen at the same time, however, even for the same species and in the same forest. Consequently, many fruits or cones are not fully ripe when they reach the extractory. They can be ripened by storing in thin layers and in well-ventilated rooms. The longer this precuring period lasts, the more complete is the cone ripeness and the better is the seed quality. It has been found that the shortest time to get good results is two weeks, but many species demand more than six or eight weeks.
Precuring avoids the case-hardening of cones which lose from 10 to 50 percent moisture according to the length of the precuring period, thus facilitating and shortening the drying process in the kilns. This saves fuel and considerably reduces handling costs. Just before the cones are passed into the kiln precuring can be carried out effectively by setting the cones in a local store situated near the top of the kiln, where air from the exhaust, still warm, passes through.
Types of kiln
In the past many types of kiln of local design and construction have been used but they were often only suitable for local conditions.
Modern installations present a large variety of types, which fall broadly into two major categories: progressive kilns and rotating drum kilns.
Progressive kilns. In this type of kiln, cones are exposed to an air flow getting hotter and hotter, and drier and drier, from the beginning to the end of the drying period. Schematically, the cones are placed in the kiln on a set of trays, one above the other, in a sort of tower; trays descend during cone treatment. From the bottom of the kiln the hot dry air hits the lowest tray, containing fairly dry cones; passing through the cones, the air loses some heat and takes some moisture from the cones. In the second tray, from the bottom, cones have somewhat higher moisture content and the air is less hot and less dry. In the last tray, the cones, still having the original moisture content, are engulfed by warm, humid air.
At the right moment, a tray is taken away at the bottom, and the whole tray set descends one step, while a new tray with fresh cones is put on the top of the pile.
Hot air can circulate simply by convection, but forced ventilation makes the operation quicker and more regular. The distance between two successive trays should be sufficient to contain open cones which have a volume 2 to 3 times that of closed cones. The time required for complete drying varies according to the initial moisture content of the cones, the volume of circulating air, and the type of air circulation (convection or forced ventilation). For some species (i.e., Abies spp.) heating is harmful, and drying is obtained by forced circulation of air at room temperature.
FIGURE 10. - Type of rotating kiln
In some kilns, cones are forced to fall down from one tray to the one below by opening the trays bottom, or by inclining the tray. In other types, trays are replaced by slow moving bands: cones move on a band till they fall to the one below.
In the last two types, cones are shaken as trays fall from tray to tray, so that most of the seeds come out: special devices under the bottom tray collect the seed. while cones pass to further handling.
Similar working schemes are applied in some small extraction units, planned chiefly for laboratory extraction (Figure 9). The Forest Experiment Station of Mariabrunn (Austria) has developed a small kiln of this type to extract seed from cones of individual trees, as well as to carry out experimental seed extraction to test the seed yield. This type usually has electric power heating.
Some kilns have just one or two trays, which may be opened: from the lowest, cones fall into a drum, where they are tumbled while still drying. This is the method used in some old and ver N, large extraction plants, still operating satisfactorily. At Wolfgang (Germany) four such "towers- could extract all the conifer seed required in the Federal Republic.
Rotating drum kilns. Cones are put in a cylinder made of perforated steel plate, rotating on a central axis. The cylinder is contained in a box, where forced air circulation is provided by an electric fan. During the extraction the cylinder rotates continuously, shaking the cones. The air temperature increases gradually, from room temperature to the maximum fixed level. The strong ventilation, the progressive heating and the continuous shaking make the cones dry and open in a short time. Seeds released from cones pass through the holes of the plate, and are blown at once out of the kiln. Temperature and ventilation control is generally fully automatic.
This type of kiln offers many advantages, and is usually preferred in modem extraction plants. The time needed for seed extraction is greatly reduced; the shaking of cones during drying causes the cones to open and the seed to fall. Separate tumbling is unnecessary, and labor and time are saved.
Seed is taken out of the hot air as soon as it comes out of the cone in order to avoid seed damage. The capacity of the cylinder is reduced so that extraction can be done separately for cones from different provenances, even if the quantity is small. Kilns of this type may have electrical heating or a separate heating unit (Figure 10) which can use different fuels (cones, coal, oil).
The cylinder, the electric motor, the ventilator and the equipment for air and temperature control are contained in a steel box above which is a predrying box with two floors. From this box, cones fall into the cylinder, where they are exposed to increasing temperatures (40-45-50-55-60°C) for three to four hours. At the end of the extraction, reversing makes the cylinder open: emptied cones fall down, and other cones come into the kiln (Figure 11). This type has a working capacity of 25 to 30 hectoliters of cones per day.
Modern rotating kilns are generally of compact metal construction, moderate in size so that they can be placed in small, inexpensive buildings. The output is relatively low as the capacity of the cylinder is of 3 to 4 hectoliters. These kilns are generally planned for working in a set of two or three, so that they are suitable for plants of different sizes. Their moderate cost is also a factor which makes them suitable for use in small plants.
Occasional drying methods
For small amounts of cones, and especially for occasional operations, drying can be obtained by very simple means. A room with any source of heating is sufficient. If possible, cones should be spread on single trays, and placed near, but not too close to, the source of heat. Doors and windows may be opened carefully or appropriate holes made in them to ensure proper air circulation. Heating should be kept low and cones should be frequently stirred. When such primitive treatment is adopted, any haste may cause damage to the seed, because safe drying can be obtained only by a very limited heating and therefore takes quite a long time.
Kilns constructed for other purposes, such as lumberdrying or hop-curing kilns, can be used for cone drying cheaply and with good results. Of course, care should first be taken to see what facilities they have for the control and regulation of temperature and humidity.
Fire danger
Any artificial heating involves, of course, a fire hazard. This is particularly true in handling cones, which always contain resin, often in large amounts; besides which, the cone scales themselves are, when dry, very inflammable. Great care must therefore be taken. No heating of any kind should be allowed in the cone store. In the extractory plant dust, which consists chiefly of particles of cones and resin, should be removed, especially near the kilns. In modern plants, this can be done by a vacuum equipment. Pneumatic pipes suck the dust from the interior of the machines into a special container.
The building where drying equipment is installed should be of masonry, and wood should never be used in the construction of the heating and kiln compartments. All measures should be taken to prevent and control possible fires, such as the provision of extinguishers and the installment of iron doors.
TUMBLING
When cones open after natural or artificial drying, not all seeds are detached from the scales and fall out; some are usually left inside the cone. Seeds of some species fall easily after a thorough shaking, but cones of others require a severe and prolonged jarring. The method by which cones have been dried makes a great difference. Adequate precuring, regular drying, and no excessive heating ensure a more complete opening of the cone scale and an easy seed release.
FIGURE 11. - Working plan of the rotating kiln shown in Figure 10.
After drying, especially by heating, the seed should be removed from the cone as soon as possible, because open cones exposed to cold, wet air, can reclose in a short time. It would be wise to keep dried cones in a warm room until all seed extraction processes have been carried out.
In order to separate seed, cones are put into a drum or tumbler, which is a barrel or cylindrical or rectangular box, rotating on a central axis; the walls of the barrel or box are generally of wire mesh large enough to let seed pass through. As the drum revolves, the cones are thrown against the walls, more or less violently according to the revolving speed, while the seed comes out of the cone and falls through the wall meshes into a collector.
The process is carried out in the same way as in the cylinder of rotating kilns. The drum may be closed at both ends, and filled and emptied at the end of each operation cycle (Figure 12). In modem types, reversing makes the cylinder open; in other types the drum is open at both ends and is quite long, with its axis somewhat inclined so that the cones are fed in at the upper end and, during rotation, roll slowly to the other end, where they fall out (Figure 13). Such a drum gives a continuous operation cycle, but the rotation speed must be slow, with the result that the vibration may be inadequate for some species.
FIGURE 12. - End view of extractory cone drum showing the starter and enclosed chain drive.
FIGURE 13. - Rotating cone tumbler.
The walls of the drum are sometimes of thin bars, placed very close together.
All such drums can be operated by hand or by a motor; the turning speed can vary according to the species. Small types are easily transportable, and can be used for very small amounts of cones in occasional operations. In these cases, some devices similar to those used for agricultural crops may also give satisfactory results.
If no equipment is available, seed extraction can be obtained by flailing open cones on a floor; in the case of very small cones, they are put into sacks before flailing.
For species with very small Seeds, such as Tsuga and Cupressus, a little revolving box is used: light wooden frames support a mosquito wire net. The box rotates on its axis, just as in larger tumblers, but for these species a few rotations suffice to extract all the seed.
THRESHING
Seeds of some species, such as Liriodendron, Catalpa and Cercis, can be extracted easily from the fruit. Their pods are flailed on a platform and generally break without difficulty. For large quantities mechanical threshers are used. Generally, threshers used in agriculture for similar types of fruit can be easily adjusted by altering the distance between the crushers. The other operations follow in the usual way.
The cones of some conifers, especially some pines, do not open completely even after a long drying, and even tumbling is not sufficient, so that further mechanical handling is needed to extract the seed still remaining in the cone. After sun drying, cones of P. pinea are treated in large threshers, so that the scales detach quickly and completely from the cone axis. Larch cones are often difficult to open as they have a very high resin content and the resin melts at a temperature lower than that sometimes required to open the cone. After partial drying, therefore, the cones are crushed in special mills.
In stationary or rotating kilns, the cones, after normal drying, fall into a cylinder equipped with a series of knives: fast rotation provides both tumbling and cutting of the cones at the same time.
For some fruits, hammermills are used, always with the aim of breaking the casing of the fruit. Again, hammermills used in agriculture are quite effective and suitable.
The processes of tumbling, threshing, crushing or cracking can cause damage to the seed, thus inducing an abnormal seedling development or an early loss of the germination power. These mechanical processes should be carried out carefully, the intensity of shaking and flailing being proportionate to the hardness of the seed walls. It is better to leave some wed inside the cones than to risk severe damage to the seed.
DE-PULPING FLESHY FRUITS
The fruits of many species, such as Juniperus spp., Prunus spp., Malus spp. and Rhamnus spp., are fleshy or semifleshy.
Some of these fruits (i.e., Juniperm spp.), may be dried just before. storage, drying being effected by natural or artificial heat. But safe storage usually requires separation of seed from the pulp of fruit: this can be obtained by macerating the fruit in water. Fruits are left in open containers until the pulp becomes soft and decays; they are then placed in water (often some lye is added), stirred and mashed. After a certain time, the seed separates from the pulpy parts, and sinks to the bottom: after proper drying (preferably by unheated air) the seed finally goes to cleaning or to storage.
Some special machines have been developed for de-pulping fleshy fruits and adapted hammermills, cornshellers and macerators have also been used. A small portable type of macerator consists of a plate spinning at the bottom of a seed hopper: pulps are washed out through an appropriate outlet, while seed is retained.
METHODS OF EXTRACTION BY CLEANING
Many fruits are quite ready for sowing as they are collected, or just require the separation of seed from the casing. Beech, oaks, chestnut, and hickory require only a very rough cleaning, which can be done by winnowing, screening, or some other very simple method. These seeds are often extracted by hand, or handled in a separator normally used for agricultural products. Almost all the abovementioned machinery can do an effective job provided the size of the seed is taken into account.
Seed which has been extracted by the various methods already described requires further handling before storage. Wings or some other parts of he fruit may be still. attached to the seed, and many impurities of different types are always present. Cleaning provides a thorough separation of pure seed from inert matter and from shriveled and empty seeds, and prepares it for storage and sowing in the beat condition. The species and the amount of the seed will determine whether cleaning is best carried out by hand, by improvized equipment, or with the aid of special machines. De-winging, fanning, screening, and separation are often needed to obtain seed of good quality. As a general rule, the aim of cleaning is to get completely pure seed, but this is not always possible. On the other hand, there is often no particular advantage in cleaning beyond a certain limit.
Modern machinery has reached a very high degree of efficiency in this field, but in many cases, complete separation would cause a loss of seed. In the case of larch, for instance, the seed should not be cleaned to a purity higher than 65 percent as it has been proved that further cleaning causes a drastic loss of good seed.
DE-WINGING
Many forest seeds, and almost all those of conifers, are winged. In some species, e.g., Pinus pinea, the wing is quite rudimentary or very short, but in others it is long and hard. Sometimes the wing completely covers the seed (Abies spp.).
For some species it would be preferable to store a seed with its wings in order to ensure a better air circulation. On the other hand, sowing winged seeds may cause many difficulties: they may be blown away or more easily picked up by birds, and the use of sowing machines may be impossible. For these reasons, the wings are always removed whenever they are larger than the seed.
De-winging is almost always carried out by rubbing. In the past, seeds were spread on a floor and sometimes lightly moistened by sprinkling them with water, after which, flailing by light leather flails was sufficient to remove wings. This method is no longer used, because it involves the risk of fermentation of moist geed or the development of fungi.
Hand rubbing the seed in a sack is the safest method, but it is practicable only when small amounts of seed are involved. Many types of de-winging machines have been developed and are in use, from the simplest types, which are operated by hand, to those suitable for large plants, which work semi-automatically, and give a continuous output. They are almost all rotating devices.
Rotatory de-wingers may have brushes or knobs. In the first type seed is pressed by brushes moving against the walls of a cylinder. In the second type rotating knobs or pads force the seed to pass through narrow outlets, and leave the wings. The distance between brushes or knobs can be regulated according to the size of the seed. Brushes may be of steel, nylon or tampico, which have different degrees of hardness; cylinder walls may be of corrugated wire or perforated plate, or lined with canvas or rubber.
FIGURE 14. - Rotating de-winger with rubber knobs.
A de-winging machine may be a drum of webbing revolving in a hammock of the same material. Another de-winger (Figure 14) is made with rubber pads rotating in a rubber cylinder. Figure 15 shows a modem de winger which is used in many German plants.
If the clearance between knobs or brushes and the walls is too narrow, or if the brushes are too hard, the seed may be damaged either by pressure which breaks the seed coat; by scarification; or by heating caused by friction. Great care should be taken to avoid this. The main difficulties to be overcome are: a proper regulation of the clearance; the settlement of the best revolving speed; and the attainment of a regular seed supply.
For some very sensitive seeds, cleaning before dewinging is recommended, to remove any particle of hard material which could injure the seed coats during handling.
CLEANING OPERATIONS
The different cleaning operations may be carried out separately or at the same time. Modem cleaning machines often combine different handling methods so that the cleaning process may be both effective and quick. These operations are given below.
Fanning or winnowing
A strong air current blows away fight impurities, cone particles, dust, empty seeds, while good seed and heavier particles are not removed or fall down at once. Winnowing is often carried out, especially in agriculture, by throwing seed into the air on windy days. In winnowing machines the air current may be either horizontal and blow through the seed which falls gradually, or vertical and blow across a moving screen on which seed passes.
Screening
Largely used in many separating operations: by passing through screens with meshes of different sizes, the seed is separated from impurities which are larger or smaller than itself. Screens may be of metal net or of perforated plate. Shaking screens are often used. Screens generally work in series of different mesh or hole size.
Separation by centrifugal force, or by vibration
If seed is centrifuged in an open cylinder, heavier particles go up first along the walls and escape: variation in rotation speed provides progressive separation of the particles of different weight. The same effect is obtained by vibration.
Flotation
When put into water, good seed sinks, while light impurities and empty seed float on the surface and can easily be skimmed off. This simple separation method requires a subsequent seed drying, but it is very effective with some species, such as acorns.
Combine cleaners
Cleaning methods separate good seed from undesirable impurities or empty seeds in three ways: by size (screening); by specific gravity (flotation, vibration); and by centrifugal force. These different methods are often combined in modem equipment. Screening and winnowing are almost always used, and sometimes carried out simultaneously. There are many such machines (Figure 16).
Mechanical shaking and separation by air blowing are used, a vibrator type seems to be especially effective in cleaning larch seed. For hardwoods, fanning mills have been developed. Vacuum and cyclone systems have been successfully used in cleaning seed, but up to now such special handling methods are rarely used.
All material, from the rough fruit or cone to the fully cleaned seed, should pass from one handling to the next quickly, in order to avoid any possibility of contamination or mixture with undesirable matter, and any remoistening which would necessitate further processing.
Transport should be done as far as possible by mechanical means: conveyors, transport bands and blowing pipes should be used in the different phases of handling, depending on the size and the type of material. In general, cones are transported by chain elevators or by bands; for seed, blowing by pipe is very convenient. These blowers should be self-cleaning, in order to avoid any possibility of mixtures, chiefly between seeds of the same species but from different provenances.
All handling, but especially tumbling and cleaning, produces a large amount of dust. De-dusting apparatus should be connected to all transport equipment, as well as to all handling machines. Vacuum pipes should collect all dust from where it is produced, inside the kiln, inside other machines or at their air outlet, and convey it to a cyclone and a special cell. This keeps the work room clean all the time and makes working conditions safer. Transport equipment should carry cleaned seed up to the store. Packaging into containers for storage or for shipping could also be done by automatic bagger.
Seed should be kept dry during all the different stages of handling, but particularly at the end of the process, where it is prepared for storage or shipping.
Seed handling should be carried out in a warm, well ventilated room. In effect, many cleaning machines use the heated air coming from the kilns, thus Insuring a continuous drying process. But, in any case, before storing or shipping seed, moisture should be checked and especially if the seed is to be stored in closed containers.
Testing the actual seed moisture is rather difficult. Laboratory moisture testing may give very exact results but always takes rather a long time. Furthermore, the specialized staff required is not normally available within the seed extractory. In practice, testing should quickly give the moisture content of seed at the moment of packing. For this purpose, some electronic moisture testers have been developed, based on the electric capacity or resistance; some types give the degree of moisture of seed inside the container. Infrared moisture balances are also used. All the above mentioned testers give results by direct reading but their precision is somewhat limited: further development of such apparatus is desirable.
In storage, seed should have a low moisture content (generally of 5 to 10 percent); if necessary, it should be carefully dried by submitting it to a dry draught of air. For this purpose almost all types of stationary kilns, or even a simple winnowing machine, should give immediate results. Temperature should be kept very low, however, and for some species (i.e., Abies spp.) any heating should be avoided.
(To be continued)
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The Forestry Commissioners in the United Kingdom announce that they have appointed Sir Henry Beresford-Peirse as Director. General of the Commission with effect from 1 April 1962. Sir Henry has been Deputy Director-General for the past nine years. He joined the Commission in 1929 and was Director of Forestry for Scotland from 1947 to 1953. For the last two years he has been seconded to the Forestry and Forest Products Division of FAO in Rome as Deputy Director, and his departure will be a great loss to the Organization. |
