The aim of this chapter is to summarize experience in the collection of dry-zone acacia seeds relevant to the FAO/IBPGR Forestry Project. It is based primarily on Australian data, but information from other countries has been used when pertinent. Emphasis is placed on seed collections for research, but with some changes the techniques described are applicable to bulk seed collection for afforestation programmes.
The chapter has been arranged as a series of sequential steps that should be followed, or at least considered, when collection is undertaken.
‘Provenance, with reference to clonal or seed material, is the geographical area and environment in which parent trees grew and within which their genetic constitution has been developed through artificial and/or natural selection. Although this purely forestry term has no place in the formal taxonomic hierachy, it may, for natural, indigenous forest, and depending on the nature of the populations under consideration and the size and distinctness of their geographical area or ecology, be equivalent to an ecotype, a geographical race, a variety or perhaps more frequently to part of a cline’ (Burley and Wood 1976).
In forestry, the practical and economic importance of matching species and provenance with site and end use has been convincingly demonstrated through a number of internationally co-ordinated species and provenance trials (Palmberg 1980, 1981). The longest-running trials testing a range of Eucalyptus camaldulensis provenances on a wide range of sites showed that the gains in productivity which can be achieved simply by selecting the best-adapted provenances for prevailing environmental conditions may amount to several hundred percent.
While there have been few systematic studies of geographic variation in acacia species, the available evidence (2.21) suggests that substantial intra-specific variation exists. Plans for afforestation with acacias, therefore, should be preceded by taxonomic and genecological exploration followed by systematic sampling throughout the area of natural distribution and evaluation in field trials established over a range of potential planting sites. These trials will indicate the existence and nature of interactions between species, provenance and site and should lead to the efficient utilization of available genetic resources. The choice of provenance may make the difference between a successful plantation and total failure.
The ease with which the boundaries of provenances of an acacia species may be defined depends on the distribution pattern of the species in question. If a species is restricted to small discrete sites, the term ‘provenance’ may be synonymous with ‘site’ and can be reliably applied. The problem of delineating provenances is much more difficult when species occur over extensive areas having fairly uniform ecological conditions. A. aneura, for example, grows widely over the inland plains of Australia. In such a case any provenance boundaries are quite arbitrary.
Special problems associated with the collection of seed to represent provenances of acacias include hybridization and uncertainties in identifying closely related species as discussed in 3.14. Hybrids may be hard to detect in the field and it is therefore essential for later verification that seed of individual trees should be kept separate and that herbarium voucher specimens should be taken from each tree.
It is essential to record the place of origin of all material (seeds, herbarium voucher specimens, photographs and other data) as this forms the basis of provenance research and future action.
Once the broad objectives of the collection programme have been defined, it is essential that ample time be allowed to plan an efficient and practical collection strategy.
Comprehensive collections for research will almost certainly require more detailed planning than routine bulk collections and may require a lead time of from one to several years depending on circumstances. An example of the inputs and time required to plan a major collection can be seen in the FAO/IBPGR Forestry Project (FAO 1980). After three years, the way is now almost clear for the majority of first-stage collections to proceed.
For scientific collections the extent of funding for a particular programme and the availability of experienced personnel will be primary considerations and will dictate what can be achieved in the time allowed. The following steps are usual during forward planning.
A literature search may provide information on the natural distribution of a species, its habit, ecology, genetic variability and on fruiting and flowering times in different parts of its range. A visit to Regional Herbaria to examine relevant herbarium specimens may add significantly to the published information on species distributions, variability and reproductive phenology. Personal contact with botanists, field foresters and others who are actively studying or utilizing a species can be rewarding in building up an overall picture of the species of interest.
If the collections are wide-ranging and cross political boundaries, official contacts with the forest services in the states or countries concerned may be essential at an early stage to develop a good working relationship of mutual benefit to all parties. Such action will usually be welcomed by the local forest service if it is assured of free access to information gathered at this and later stages of exploration and seed collection (Burley and Wood 1976). Concerning the FAO/IBPGR Forestry Project specificially, official contacts have already been made in the countries collaborating in the project (FAO 1980) and pertinent information is being provided through a series of handbooks of which this is one.
When all available information is to hand, it should be collated and summarised. Data on natural occurrence is best plotted on maps detailed enough to show the main communications systems (e.g. roads, rivers, railways and airstrips), topography and other data that may assist in the selection of collection sites.
If the species is little known, or known to present problems to the collector, a field reconnaissance of species variability, natural distribution and flowering and seeding habits may be a necessary preliminary to planning the ultimate seed collection programme.
Within its own country, the local forest service may be in the best position to undertake this phase, reporting to the collector information on the likely size and time of maturation of the seed crop and advising on an itinerary and travel arrangements best suited to the fruiting times and available access.
In the interests of time and economy the biosystematic exploration of the species has frequently had to be combined with the collection of seeds for provenance trials. A single combined exploration and seed collection expedition cannot be expected to furnish all the answers on variation.
Depending on the degree of variability, a number of preparatory reconnaissance missions and repeated seed collection expeditions over several years may be necessary. This is the strategy planned for the FAO/IBPGR Forestry Project, where provision is made for follow-up collections should new information demonstrate gaps in the original coverage (Palmberg 1981).
For most of the acacias the time interval between flowering and seed maturation is long enough to give the collector, having checked the flower crop, time to prepare for seed collection. For the acacias in the FAO/IBPGR Forestry Project the time interval is usually 6–12 months depending on species (see Table 1), but is as short as 3 months for A. albida. Significant inter-and intra-provenance variability in the length of the ripening period has been noted in some species (e.g. A. aneura), and in such cases regular monitoring of the progress of maturation may be desirable.
Maturity of seeds of dehiscent acacias is usually indicated by the cracking of the dark-coloured pods containing seeds that are hard-coated and dark in colour. With indehiscent species the darkening of the pod is also the main indicator of seed maturity. In some instances seeds are viable before they have fully ‘matured’ i.e. when the seedcoat is still relatively soft and pale in colour, and the swollen pods are only just beginning to turn brown. Collection of ‘green’ acacia seed has been advocated by several workers as a means of arresting severe bruchid attack and avoiding the need for presowing treatment. This technique has been found to work well with A. mearnsii (Isikawa 1960), A. senegal (Kaul and Manohar 1966), A. tortilis subsp. raddiana (Karshon 1975), and several Australian species (such as A. salicina W. McReaddie pers. comm.).
Two simple field tests used in combination can be employed to gauge the degree of maturity of ‘green’ crops:
take a sample of pods and leave them to dry in the sun for a day or so. If the seed remains swollen in the pod and is turning dark in colour, the seed is usually sufficiently mature to collect. If, on the other hand, the seed loses moisture and contracts it would be unwise to collect it.
make transverse cuts through several seeds. The embyro should be firm and swollen whilst the seed coat should not collapse when cut.
The collection of immature seed may not be applicable to every species and it would be prudent to run a series of viability tests to prove the technique before applying it to unfamiliar species. There have been no systematic studies to determine if an after-ripening period exists in acacias (Cavanagh 1980a).
The time between fruit maturation and seed shed varies considerably from species to species. As a general rule it is better to collect early (i.e. as soon as seed maturation allows) rather than late and risk loss of the crop through dehiscence, insect attack or other agencies. Seed shed is certainly hastened by hot, dry weather, and the collector must remain vigilant if he is to collect the crop at the right time.
There are many advantages in collecting seeds in years of good flowering and seed set. In good years the viability of the seeds collected is likely to be high, there are few restrictions on the selection of trees, and if well-separated trees are chosen, there is less chance of inbreeding. The time required to gather a specified amount of seed is shorter and hence the operation is more efficient. Even in good seed years a species may not produce good crops throughout its entire range and to obtain a complete representation, seed may have to be accumulated for two or more years.
Some generally accepted sampling methods for species introduction and first-stage provenance trials are given below:-
Species introduction trials are designed to rapidly screen a large number of species of possible value but about which comparatively little is known. The objective is to eliminate species which show little promise in order that the resources, which are usually limited, may be concentrated on the critical testing of a reduced number of the more promising species.
The concept of species introduction trials as single plots of one seed source of each species, using such seed as happens to be available, has largely been superceded through the recognition of the importance of intraspecific variation and hence seed origin. When a species has a wide natural distribution, a collection from a single locality may provide a very poor indication of the potential of the species and it is now generally recognized that in introduction trials several provenances may be required. Although it may seem desirable to make a range-wide provenance collection representing perhaps 100 localities, this is not practicable at the stage of species trials. Edwards (1963) suggested that for an unknown species with a wide distribution several provenances should be tried, including at least one from (a) where it reaches its finest development, (b) part of the range which closely matches the new conditions, and (c) part of the range which extends the range already sampled in an important direction or forms an acceptable alternative to (b). These correspond to the ‘optimum’, ‘matching’ and ‘marginal’ sample sites as discussed by Turnbull (1975). The inclusion of provenances from marginal or isolated sites is very important in indicating the extent of genetic diversity in a species and may have considerable practical significance because of their adaption to limiting climatic and edaphic factors (e.g. drought resistance or tolerance to alkaline or saline soils).
The sampling of ‘optimum sites’, ‘matching sites’ and ‘marginal sites’ is generally carried out in an attempt to save resources such as time, labour, and land which are required for comprehensive provenance trials. It is appropriate if there are a large number of possible species for a site and when a preliminary indication of the potential of a number of species is required. The choice of seed sources for initial screening trials of species requires careful study of the natural distribution of the species and the environmental factors operating within that range (Burley and Wood 1976).
Once there is some indication that a species may be of value on a particular site, but the extent of the variation in it is largely unknown, it is usual to conduct comprehensive provenance trials using seed from many sources widely scattered throughout the natural range. The number of seed sources sampled will depend on the extent of the natural distribution, the genetic diversity of the species, and the resources available to sample and carry out the research.
Efficient sampling schemes can be devised only if there has been a preliminary study of variation patterns within the species in its natural habitat. Biosystematic studies ideally should precede the collection of seeds for provenance research, but frequently sampling schemes have to be devised without the benefit of prior knowledge of the variation pattern. In these circumstances more samples are necessary to ensure adequate representation of the variation in the provenance trial.
When a species is being sampled for the first time and little or nothing is known of the variation pattern, sampling frequency should not be high. Burley and Wood (1976) suggested that at least five or six collection sites are needed to represent both the limits and centre of the geographical range.
The advantages and limitations of coarse grid sampling i.e. the super-imposition of a square grid on a map of the known range of the species to identify regularly-spaced sampling sites is discussed by Turnbull (1975) and Burley and Wood (1976). For species with wide and continuous distributions, such as many of the dry-zone acacias (see Fig. 5), some form of flexible grid sampling is probably the best. The sampling pattern should aim to capture the range of variation in a species as it is known, the range of environments in which it occurs (latitude, altitude, rainfall etc.) and must consider the practical limitations imposed by local conditions and available resources.
Having decided the approximate location of the collection site, a stand of trees must be chosen from which the individual trees will be selected. Good, recent aerial photographs are of the greatest value in this task and can save days of ground reconnaissance (Burley and Wood 1976).
The chosen stands should be representative of that part of the species' range and as far as practicable be isolated from related species, so that there is a minimal risk of hybridization. There should be sufficient trees in the stand to ensure ‘adequate’ cross pollination and so that the chance of the selection of individuals closely related genetically (e.g. half-sibs) is low. The application of such criteria to some acacia species which are found as isolated trees in deserts may be difficult. In such instances individual tree lots may be kept separate or combined with others covering a considerable area to represent a ‘stand’.
The main aim is to capture as much as possible of the potentially valuable genetic variation within a population (Turnbull 1975). For the FAO/IBPGR Forestry Project the following standard has been adopted: sampling within each population will be done at random and seed will, whenever possible, be collected from a minimum of 25 trees spaced no less than 100 m apart to minimise relatedness of the mother-trees. Special caution is required for within-population sampling of species which are able to reproduce by root suckers, such as A. albida (Palmberg 1981).
While an ideal standard or objective must be stated it would be rare, given the variability of nature, that it can be adhered to at every collection site. A random sampling procedure, for example, may be difficult to employ in many areas because of over-riding constraints such as the available collection methods, variable seed crops, irregular stocking and difficult terrain. The collector, while allowed some degree of judgement in selecting seed trees, must be on-guard not to bias the sample in any one direction (e.g. towards trees with seed near the ground).
The intensity of sampling is where the collector may need most flexibility. While 25 widely spaced trees is a desirable minimum sample to represent heterogenous populations, scant crops may dictate a much smaller sample. An arbitrary number of say 5 seed trees may be set as the absolute minimum sample. This problem will face the seed collector when collecting acacia species that occur as isolated trees widely separated from one another in desert areas (e.g. A. tortilis). In this case the procedure followed is likely to be determined largely by the practical limitations of accessibility, time, funds and staff.
The collector will require some knowledge about maximum and minimum seed requirements per provenance if he is to allocate his time to most effect. In areas with prolific crops he may quickly collect the maximum requirement and move on so that he can spend longer obtaining the minimum requirement where crops are small.
Ideally each tree should provide equal quantities of viable seed to give a bulk collection representing the provenance; but as individuals vary in seed and fruit size, and the seed in viability, the best that can be done is usually to collect equal quantities of the fruit (Lines 1967). If the seeds from each parent are kept separate until after extraction and germination testing it may be possible to manipulate the quantity of seed from each tree in the bulked mix to give equal representation of viable seeds.
The preservation of the identity of individual trees through the collection and extraction phases often requires considerably more effort than bulking the collections. If the effort is made, then certain advantages accrue: biosystematic study of genetic variation both within and between populations is feasible; it is possible to manipulate the bulking process to equalise the amount of viable seed from each tree if the seed must be bulked to form a provenance mix prior to sowing: hybrids can be eliminated. It is not always possible to detect trees bearing hybrid seeds in the field; if seedlots are kept separate then, following the germination of small samples of seed from each tree, any seedlots showing evidence of hybridization can be eliminated before the main trial is established.
At this stage in the procedure all available information will have been collected on which to decide how and when the collection should be organised and the resources of staff, equipment and funds that will be needed.
To make best use of usually limited resources, the seed collector needs a clear statement of objectives. Species and provenance priorities, the standard of documentation required, the number of individual tree samples necessary to represent a provenance, and the amount of seed required per provenance are points that need to be decided at this stage. Wherever possible flexibility should be worked into the objectives to allow the collector to use his judgement in the face of unforeseen circumstances. For example, it would be highly wasteful for collectors to return empty handed from remote areas because of a too narrow definition of aims and procedure when alternative and potentially valuable seed harvests were available for collection.
Early selection of the collection team is recommended so there is ample time for those involved to familiarise themselves with the project and the species and, if necessary, undertake training in the methods they will be expected to use in the field. When expeditions are undertaken in foreign countries, language training and familiarization with local habits and customs may be essential to the success of the collections. Long delays in the issue of essential documents, such as entry visas, work permits, and permits for importation of items of collection equipment (e.g. rifle and ammunition etc.) are not uncommon.
A major collection programme may require the services of a project co-ordinator, preferably a specialist on the species concerned, supported by several team leaders. The team leaders should have expertise and experience in seed collection methods and may be provided by the country participating in the project. The necessary labour force should be recruited in the area of operation since they may also serve as guides and interpreters.
However the project is structured, it is of vital importance that the team be enthusiastic, fully briefed on primary objectives and any alternatives and have a good working knowledge of the species. The often difficult nature of the job requires these individuals to be physically fit, adaptable, and able to make sound on-the-spot decisions in the face of unforeseen problems.
The field data have two main functions; to provide the information necessary for the relocation of the site for further work, and to provide the background essential for the interpretation of the results of the experiments.
The data to be gathered at each collection site must be specified and systems must be developed to make its reliable collection as easy as possible for the man in the field. While the opportunity should be taken to gather as much potentially-valuable research material as possible (e.g. wood specimens and gum samples may be desirable with some acacias), these additional operations take time, often require special equipment and could over-load the party at the expense of the main priority, seed collection from a large number of trees.
The use of specially prepared field data sheets is recommended to ensure that systematic records are obtained from all sites. The standardised ‘Seed Collection Report’ sheet adopted for the FAO/IBPGR Forestry Project is given in Appendix 3. Significant features of the ‘Seed Collection Report’ appropriate to all such records of provenance collections are -
Locality, latitude, longitude and altitude
These facts are essential to define the provenance area. They should be stated accurately and concisely so that future collectors can return with certainty to the same collection site. Distances from the nearest town, village or geographical feature (river or mountain), the forest district or any other fact that will assist future field parties should be recorded. Maps (hand drawn if others are not available) and aerial photographs showing the stands and the position of seed trees are useful and should be filed with the data sheets for ease of reference.
Aspect, slope, climate, soils and associated species
These site features help build up a picture of the environment and ecology in which the trees grow, and may assist in the interpretation of experimental results. The collector should always be on the look out for relevant comment from the locals on the past history of the area and its climate. The recording of tolerance to such features as alkalinity or salinity in the soil, seasonal inundation etc. may be of later importance.
Individual tree descriptions
The main characteristics of each tree (e.g. height, diameter, stem form, branching habit) and the number of trees in a provenance are of obvious importance in any future tree breeding work. Photographs are a very useful adjunct to the written descriptions.
Of particular importance when dealing with little known, variable species, is the collection of a herbarium voucher specimen to allow later scrutiny by botanical experts. Care needs to be taken in handling the specimens and it is best, after drying, to despatch them to base at the earliest opportunity as they are susceptible to damage.
Other data that may be of great value to future collectors are crop and seed details, collection methods etc.
Security and labelling
It is paramount in provenance work that foolproof systems be adopted to maintain the identity and purity of each collection. Systems must be adopted to eliminate the possibility of any seedlot being contaminated with seed of another, through all steps from collection to registration in the seed store.
Careful labelling through each process is, of course, essential. A well-proven method in use by CSIRO is based on the party leader allotting numbers to his collections in numerical sequence and prefixed by his initials (e.g. in Appendix 3, SM218 is the two hundred and eighteenth seed or botanical collection undertaken by S. Midgley). The number that is allotted to a particular tree appears on and in the collection sacks, seed bags, on the herbarium voucher specimen, against the photograph and identifies any other sample or recording of that tree. The provenance number is not allotted until the seed is ready for storage (see Appendix 5).
The safe keeping of the ‘Seed Collection Report Data’ sheet is essential in this as in other systems. As an added insurance against loss of the sheet, the collector keeps a summary of the days work in a field diary which is kept separate from the data sheet file.
In developing an itinerary for the collection team, ensure that the team, along with its equipment, reaches the collection region well in advance of the proposed date for the commencement of collections. Two to four weeks may be required to organise the sequence of operations in a particular region. Extra permits may have to be obtained, labour recruited and trained, and reliable transport arranged.
While an itinerary is important as a guide it must, of course, be flexible to account for the unexpected problems that invariably arise in the field. Over-intensive schedules may lead to loss of accuracy and attention to detail, and a lowering of morale.
Decisions on equipment to be used must be taken at an early stage if long delays in purchase or delivery are anticipated. A check list of equipment in use by the Seed Centre, CSIRO Division of Forest Research, Canberra is given in Appendix 4, and a selection of this equipment illustrated in Fig. 8.
Essential documents, such as entry visas, work permits, and permits for importation of items of collection equipment (e.g. rifle and ammunition) must be arranged well in advance as long delays in their issue are not uncommon. A study must be made of the regulations governing collection, export, introduction and movement of seed within and between countries. Official permits may be required for any of these procedures. Airlines may require special documents before they will accept seed for transport, and it is important to comply with customs and quarantine regulations at the point of entry to avoid delays or, at worst, the destruction of seed by officials.
The method selected for seed collection will depend on many factors but, particularly in the case of research collections, the need for high standards must always be kept in mind. Excessive economy in the seed sampling and collection is, therefore, unacceptable.
The collection of acacia seed in the arid zone can be conveniently discussed under three headings: seed collection from natural seed fall, standing tree, and felled trees.
|Fig. 8||A selection of collecting equipment: From top left: Plant press with specimen and tags, small bag containing clean seed, large collection bag, 2 × 2 m collection sheet, leather gloves, bow saw, secateurs, flexible saw, fine sieve with bottom pan, large sieve, throwing rope with weight.|
The seeds of dehiscent and the pods of indehiscent acacia species can be collected from the ground after shedding. This practice is cheap, does not require skilled labour and ensures that only mature seeds are collected. A rake and sieve (optional) and collection container are the only items of equipment required. This technique is in regular use for the collection of the pods of the indehiscent African species such as A. albida, A. nilotica and A. senegal and for the collection of the relatively large seed of A. caven in South America.
While this method may prove useful in some situations (Poggenpoel 1978), the technique has several disadvantages of particular relevance to the arid zone where acacia seeds are often an important source of food.
The high level of predation of the seed and pods of acacias on the ground is a major disadvantage (see 3.4), and uncertainty over the identity of the parent tree is a consideration that may totally exclude its use to provide seed for research except in situations where trees are isolated from their neighbours.
Some refinements of the basic technique have proved useful in the collection of seed of species that mature and shed their seed over a short period. These can range from the spreading large sheets of plastic, canvas, hessian or like material around the base of the tree or on racks above the ground to minimise predation, to elaborate structures like the metal funnel designed for A. aneura collection in Queensland (Fig. 9).
|Fig. 9||Funnel for trapping A. aneura seed near Charleville in Queensland, Australia.|
The collection of seed from standing trees is the normal method employed for provenance research. One of the main advantages over felling (method 4.53) is that the trees are not immediately lost as a future seed source, and, when warranted, the seed trees can be marked for future reference. It is usually fairly easy to collect from standing dry-zone trees as they are of open form and relatively small.
A widely used method of collection is simply to climb into the crown of the tree and with the use of a saw or similar implement remove seed-bearing limbs. Unaided climbing is not feasible where tree stems are thorny or weak and there is always an element of danger. The use of safety belts is highly recommended.
Items of equipment that may facilitate access to the crown of small trees include -
(a) vehicles with strong roof carriers often provide a quick and easy means of gaining access to the crowns and fruits of small trees (Fig. 10)
|Fig. 10||Collecting acacia seed from the roof carrier of the collection vehicle.|
(b) portable ladders. Well-designed ladders provide a quick and safe means of reaching the live crowns of trees up to about 15 m tall. There are many designs and the type selected should be compatible with tree habit and local conditions of topography and access. For small trees, a light wooden or aluminium single section or free standing ladder, 6–8 m long, which will fit easily on a standard vehicle roof carrier is usually appropropriate.
The use of these and more sophisticated methods for gaining access to tree crowns is discussed by Turnbull (1975).
(a) Manual collection
This may prove very effective where trees are small and their branches low enough for collectors to reach them easily from the ground.
When dealing with closed pods the branches may be bent over collection sheets (say 4 × 3 m tarpaulins) spread on the ground and the pods stripped by hand (Fig. 11). Thick leather gloves may be needed when handling thorny species (Fig. 12). Where the crop is fully mature and near dehiscence, sticks are often used for beating the branches to shake down seeds or dislodge the fruit. The use of pruning shears or secateurs for cutting off thin and sometimes acutely ascending and out of reach branches can be effective in gaining additional seed (Fig. 13).
(b) Pole implements
Where the crop is out of reach for hand picking, the use of various pole implements may be appropriate. Poles with shears, saws or hooks of various designs attached to one end are commonly employed for detaching the fruit or cutting seed-bearing limbs. Light and rigid bamboo, aluminium or plastic poles, 4–6 m in length, are common. (Figs. 14 and 15). In order to reach beyond the 6–8 m range of single poles, multi-stage telescopic poles with a shear on the end have been developed. These methods require careful location of the ground sheets so that pods and seeds are not lost on impact.
(c) Throwing rope with weighted end
A 5 mm diameter sash cord or nylon rope 25 m in length with a 400 gram weighted end (e.g. a small bag of sand) can be thrown over small branches, which are then broken off by holding the two ends and pulling. Branches up to 12 m from the ground can be reached using this method. Skill is required to throw the rope over the selected branch and in the correct position for ease of breakage.
|Fig. 11||Stripping of A. aneura pods by hand.|
|Fig. 12||Hand stripping A. victoriae pods using a heavy leather glove.|
|Fig. 13||Pruning shears in use for removing seed-bearing branches.|
|Fig. 14||A pole pruner in use on A. aneura seed collection in Australia.|
|Fig. 15||A long-handled pruning shear for collecting A. peuce seed in Australia.|
(d) Flexible saw
A flexible saw has recently been developed called a ‘High Limb Chain Saw’. Details of this implement are available from Green Mountain Products Inc., Mullar Park, Norwalk, CT 06852, U.S.A.
The flexible saw, which is 1.2 m long and has a metal ring at each end for securing ropes, is similar in appearance and principle to the chain of a chain saw. A throwing rope is placed over the required branch and the saw with heavier rope tied at each end is then drawn up. Two operators working in unison then pull on the saw downwards and outward. Branches up to 20 cm diameter can be brought down quickly and easily. The method is not applicable to trees with acutely angled branches.
The advantages of using a rifle to shoot down seed-bearing limbs from tall eucalypts is discussed by Boland et al. (1980). This technique has proved useful for the collection of acacia pods from large specimens in moist forests (e.g. A. mangium) but is usually unnecessary in dry-zones because of the small stature of most trees.
This is a cheap and easy method of collecting large quantities of seed but to fell trees just for seed collection, particularly in dry-zones where trees are already scarce, requires exceptional justification.
The mixture of seeds, pods and branches collected from each tree may need some further breaking down on the collection sheet to reduce bulk before bagging and temporary storage. The method used will depend on species and crop maturity. Hand stripping, beating with sticks or mallets, trampling underfoot, and the use of secateurs to separate the pods from the branches may be used singly or in combination. The bulk of large collections can sometimes be reduced by packing the dry pods into sacks and running the wheels of a vehicle over them several times.
Branches, twigs and other impurities should be removed as far as possible before the fruit is bagged. This is usually a manual operation (Fig. 16).
The need for careful labelling at all stages of this operation cannot be overstressed. The identity of each individual tree sample must be maintained from the sheet to the bag. Each bag should be clearly labelled both inside and out.
|Fig. 16||An A. aneura collection being prepared for bagging.|
(Photo: J. O'Connell, University of Utah, USA)
The transport of mature acacia pods presents few problems. Dry or almost-dry pods can be loosely packed in fairly closely woven hessian sacks, well-tied and arranged in the vehicle to allow adequate air circulation between them. In dry weather, the regular rotation of sacks on the roof carrier of the collection vehicle is often ideal and allows the completion of the drying process. Care must be taken that the sacks do not develop holes that allow seed to spill out. This is most likely in lighter cotton or linen sacks and when a significant amount of sharp twig material remains attached to the pods.
Long transport periods for ‘green’ pods should be avoided, as the high moisture content encourages the development of micro-organisms, fermentation and overheating. When the pods have to be carried long distances extra care must be given to ensure free movement of air around the sacks and the pods should be emptied and spread-out in the sun at every available opportunity. The use of insecticidal and fungicidal dusts may arrest deterioration, the former being particularly important where bruchids are a problem.
Extracted seed should be placed in clean cotton bags, the identity marked directly on the bag, and a label placed inside. Several small bags of seed may be placed in strong outer bags of canvas or similar material for protection and ease of handling. If it is necessary to despatch the valuable research collections of seed from the field to base, it may be advisable to split each seedlot into at least two parts to travel separately. This will reduce the chances of losing an entire collection through accidents en route. When seedlots are split each bag must be labelled to indicate the number involved (e.g. 1 of 4).