Báo cáo nghiên cứu nông nghiệp " Seed collection procedures for Acacia Seed Orchards and Seed Production Areas "

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The collection of seed from planted trees in a Seed Orchard (SO) Seed Production Area (SPA) generally requires access to seed-bearing branches at heights. High lift machinery or climbing methods can be used to obtain required working heights. High lift machinery is the more efficient option, reducing the risks associated with climbing and chainsaw operation. In cases where high lift machinery is not available or too expensive, climbing is a viable option. Working at heights involves considerable risks, but with proper equipment, training and management practices those risks can be minimised....

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Seed collection procedures for Acacia Seed Orchards and Seed Production Areas John Larmour ATSC CSIRO FFP 1. Introduction The collection of seed from planted trees in a Seed Orchard (SO) Seed Production Area (SPA) generally requires access to seed-bearing branches at heights. High lift machinery or climbing methods can be used to obtain required working heights. High lift machinery is the more efficient option, reducing the risks associated with climbing and chainsaw operation. In cases where high lift machinery is not available or too expensive, climbing is a viable option. Working at heights involves considerable risks, but with proper equipment, training and management practices those risks can be minimised. Climber with safety harness and rope straps around Acacia mangium tree bole. Ladder is secured to bole. 1
2. Management of Acacia seed orchards Management of Acacia seed orchards varies depending on the species, site and requirements of future seed collections. Management strategies should take into consideration the health of the trees, practical and efficient methods of removal of large quantities of seed-bearing material and the long-term impact of repeated seed collections. The size and shape of the trees on which seed collections are undertaken varies depending on species, age and previous management strategies. Stocking rates, determined by initial planting density and subsequent cycles of selective thinning, influence tree form. Trees grown at wide spacing (fewer than 200 trees per hectare) develop wider crowns and heavier, larger lateral branches. Tropical acacia species can be managed by pollarding, removing the tops to promote lateral seed-bearing branches at a lower height (6-10 metres) than if the tree was left with a dominant leader (20+ metres on a good site). This height reduction makes a big contribution to improving safety by reducing the dangers of falling. Management strategies for acacia seed production areas have to consider the relative short-lived nature of acacia trees compared to other forestry tree species. Mature trees can be highly susceptible to borers and heart rot and can start to decline in health after 10-15 years, especially after repeat removal of seed bearing branches. The future seed bearing capability of a tree is influenced by the quantity of branches and leaves left after harvesting. Pollarding at lower heights has advantages in reducing the working height of future collections but increases decreases the size of seed crops that will develop in the first year or two after pollarding. Trees in the orchard should be pollarded at the same height to avoid uneven competition from neighbouring trees. Depending on the size of the stand and demand for seed, harvesting can be scheduled on different sections of the orchard to achieve a sustainable harvesting regime. For example, a 3-hectare seed orchard could be divided into three 1-hectare blocks which are heavily pollarded once every three years, in sequence. The initial height of trees when seed collections are first undertaken impacts on the seed collecting and management strategies. In the development of seed orchards to produce improved seed, the trees must first be grown to a reasonably large size so that inferior trees can be identified and felled to improve the genetic quality of the stand. This is typically done in two or three stages, with the first thinning at about 2 years after planting when the trees are 6-10 m tall and the second thinning at age 3-4 years when the trees are 10-15 m tall. Only after these selective thinnings is the orchard or seed production area suitable for collection of improved seed. The first commercial seed collection is made from the superior trees retained in the orchard after two selective thinnings. This collection is carried out when the retained trees that are bearing a heavy seed crop resulting from flowering that has occurred after 2
the second selective thinning. This collection will involve pollarding trees that are 15-20 m high, to a chosen height of say 6, 8 or 10 m. Seed pods are then collected from the felled tops by workers on the ground. After pollarding, the pollarded trees then re-grow lateral branches. After a year or so, these branches flower and set seed, and subsequent harvests of seed can be obtained by felling seed-bearing braches. These later harvests are easier as the re-grown branches are smaller and lower then the original tops. 3. Safe tree climbing and felling for seed harvest from pollarded tops With safe climbing practices and good techniques it is practical to cut the tree at say 10 m above ground, pollarding the stem just below the first seed crop. Subsequent collections can reduced the trees height, pollarding the tree 2-3m lower than the first cut. Rot can commonly occur in the cut bole and work its way down the bole. By removing the infected area each time seed collections are undertaken it reduces the risk of the rot causing structural weakness in the tree. Alternatively if the trees are to be pollarded at lower heights of 5-6 m , subsequent collections can be undertaken at heights that allow for easier seed harvesting. Acacia mangium seed orchard pollarded at 6m. One tree pollarded at 12m at left rear showing retained branches and leaves. Climbing and seed harvesting; Tree climbing and working at heights require specialised equipment and training to minimise risks, which are substantial. Equipment is available that can allow climbers safe access to most tree canopies, with free movement of hands to undertake work whilst safely secured to the tree. The equipment and techniques required to ascend and work safely in trees vary depending on the situation. Safety equipment is required to secure the 3
climber to the tree bole at all times. It is critical that training in the correct operation and techniques of the equipment utilized is provided. Incorrect use or maintenance of the equipment can lead to serious accidents. Options for climbing to the required working height are: ladder climbing spikes, or single rope techniques. Climbing spikes are not generally recommended in an acacia seed orchards as repeat climbing with spikes can damage tree boles. However the use of spikes can allow for initial access up a tree bole to clear branching to allow for the use of ladders for repeat collection. Single rope technique involves fixing a climbing rope over a secure point in the tree crown and ascending the rope. This is generally not suitable for acacias as the quality of the high point cannot be determined from the ground, which could result in falls. There are various types of safety harness and associated straps available. The harness and straps recommended for seed harvesting are tree surgery harnesses, with static rope straps. These are more adaptable than safety harnesses which are designed for utility workers such as linesmen working on power lines etc. The tree surgery harness and ropes are better adapted at ascending and securing the climber onto a tree, which has branches and other obstructions. The straps are lengths of 11mm static rope, which secure to the harness via aluminum karabiners and are adjusted by mini ascender systems. The mini ascenders allow for the rope to be easily adjusted around the tree bole and over branches. A good quality saw, designed for green wood, carried in a sheathed pouch attached to the harness is required for clearing unwanted small branches as the climber ascends, and for felling of tops or branches once working height is reached. The use of chainsaws when climbing is not generally recommended due to the restricted nature of the working environment. In some situations, such as working in large, mature trees where climbers can have some freedom of movement, chainsaws allow for much greater efficiency in removing branches. Additional training and equipment is required for the safe operation of chainsaws at height. To gain access to the required height or to where the climber can use tree branches to assist with the climbing, a ladder can be used. The tree bole must be free of branches to allow for the ladder to be placed against the tree bole. Either a extendable ladder (~10m reach) or Wi-Be type sectional ladders which are specifically designed for tree climbing are recommended. The Wi Be come in sections of around 4.5m in length that can be strapped securely to the trees and the lengths are fitted together as the climber ascends the tree. Ladders can also be purposely constructed, bamboo being the preferred material. The ladder is placed up as close to the tree bole as possible. The climber places the rope strap that is attached to the harness around the ladder and tree bole and adjusts to a holding position. The rope holds both the climber and ladder in position. The climber 4
ascends to about 3-4 m height then tightly secures the ladder to the tree bole. If the ladder is extended above 5-6m a second secure point is made from ladder to tree bole. If branches impede the ladder these can be removed by various methods- long handled pruning saw, climbing with spikes or removing branches above ladder before extending ladder section. The climber must always have a rope strap secured around the tree bole, with the climber’s weight on the strap to prevent falls. The climber must be aware of the strength of the tree bole and not use un-sound, weak or small tree boles to hold their weight or rely on branches to support weight. Two rope straps are used to get over tree branches when ascending/descending. When a branch/s is encounter the climber throws the second rope around the tree bole above the obstacle and tightens the rope until the climber’s weight is taken off the first rope and transferred to the second rope. As the climber ascends the rope is adjusted to support the weight. The climber can use tree branches to assist with climbing to required heights. Branches can easily break and the rope strap is required around the tree bole for support and safety. With most of the weight supported by the tree bole the climber can safely step onto small branches to climb the tree. If the removal of the seed bearing material is to be undertaken by pollarding the tree, extreme care needs to be taken when felling the tree bole as there will be a significant weight involved. The climbers must be proficient in tree felling before attempting to pollard trees at heights. Techniques should first be mastered by tree felling at ground level. The direction of fall and falling without the bark and sapwood ripping is critical to the safety of the climber. The climber needs to be able to judge the weight distribution of the tree and preferred direction of felling and fell the tree without hitting other trees. Pollarding is undertaken whilst the climber is attached to the tree in a stable, well- balanced position that does not require support from hands or arms. The climber’s weight should be taken up by the rope strap, supported by the tree bole of sufficient size and structural integrity to hold the weight. The pole strap should be adjusted so that the climber has the ability to move quickly around the tree bole to avoid falling material if necessary. 5
Climber in position to fell tree. Back cut made, final cut underway. The first cut (back cut) is made facing the direction which the tree is to be felled and ~0.5m above the rope strap. The cut should be made just over one third through the diameter of the tree. If the diameter of the bole is large and direction of tree fall is not certain a scarf can be made by making an angled cut on top of the back cut that removes a wedge of wood. The next cuts are to bring the back cut around the sides of the bole, cutting through the bark and sap wood to a depth of 1-2 cm. The final cut is made coming through the tree slightly above the back cut. The tree should be held by ‘holding wood” in the center of the bole. Holding wood is what is left supporting the tree and if felling procedures are correct the tree will slowly start to fall in the desired direction until the holding wood snaps cleanly off. The climber has to be aware of any shifting of weight in the tree. If the tree is going to fall away from the climber in the desired direction the gap of the cut should increase. If the weight shifts back on to the saw, the tree is falling towards the climber and defensive measured are needed. A rope can be secured 2-3 m above the cut prior to felling to allow ground support to assist by pulling the top in the direction of felling. 6
Climber pollarding tree at 6 m height with the assistance of a rope pulling the top in the desired direction. The most hazardous problems that can be encountered are: Ripping of bark If the back cut is not correct, either too shallow or angled wrongly, the bark and sapwood can rip away from the tree as the tree falls. The bark can catch on the climber’s supporting rope strap and can pull the climber out of the tree. The tree-top above the cut 7
can also remain supported by the outside section of the tree in the direction of the fall, causing the tree to slide back into the climber. Solution- Correct back cut. Incorrect back cut causing bark ripping. Hang up in other trees If the felled tree hits and hangs up in another tree crown the bole can slide back into the climber. Solution- correct judgment of falling direction and assistance of rope and ground crew to guide direction of tree fall. Misjudgment of preferred direction of falling If the tree is falling back in the direction of the climber the restrictive environment of the climber secured to the tree can prevent an escape route to avoid falling tree. Solution- climber to be aware of the shifting weight of the tree whilst cutting. If the tree’s weigh is shifting the climber can change position and modify cuts to prevent bark ripping. The first back cut should always be less than half way through bole diameter so if weight shifts holding wood is still present in the center of the bole. Rope and ground crew can also assist in shifting weight distribution of the tree and guide direction of tree fall. 8
Felled tree showing broken holding wood. The back cut is the cut on top of the log near hat. Once the tree has been pollarded, future seed collections should mainly involve the removal of seed bearing branches of relatively small diameters. The climber can reach a secure high point and prune branches with either a hand saw or long handled pruning saw. 9
Acacia mangium tree one year after pollarding at 9 m above ground 4. Seed collection After pollarding, seed bearing pods are picked from the felled section of the tree and collected into bags. There were varying stages of pod maturity present. Green pods that are swollen and fully formed, containing green seed of full size will produce viable seeds (seeds that will germinate) if after-ripening conditions are correct. Green pods that were undersized and flat can produce no viable seed. It is important that the seed collectors can recognize the different stages and understand what can produce viable seed, as there were large quantities of green crop along with brown and open pods. Leaving behind green crops that will yield viable seed will reduce quantity of seed harvested from each tree. On the other hand, picking unviable pods will impact on productivity, not just in the picking but subsequent drying, extraction and cleaning processes. 5. Drying Pods are spread out in a shallow layer (no more than 5 cm depth) on plastic sheeting for 2 days in a well-ventilated, dry and covered location, protected from sun and rain. A shed with a dry floor is ideal. This allows for green pods to after-ripen. Pods are then placed in the sun to fully dry. Sun-dry until pods are fully open and all seed coats are hard. 10
Pods drying in the sun 6. Seed Extraction When pods are fully dry, after around four days of drying in the sun, seeds can be removed from the pods. A cement mixer if available can be used to break seeds away from pods. Loads of pods are placed in the mixer along with several large (>1 kg) blocks of heavy wood, and the mixer operated for several minutes to break up the pods. Material is then separated using varying sized wire mesh. Sieving tables are constructed using the mesh, to separate unopened pods and debris from the seed in various stages using sequentially smaller mesh sizes. Sieves with more precise openings matching required sizes can be constructed cheaply out of plastic containers and drilled using various sized drill bits. Sieving can be used to remove most of the smaller and larger fractions from the seed. Dust-masks are needed as dust released during this process can cause respiratory problems. Cement mixture used to break seeds free from dried pods 11
Removing coarse debris from seed using table sieves. 7. Seed cleaning Traditional winnowing techniques and placing seed in water and floating off lighter material removes additional material. It is important that the viable seeds have hard seed coats before submerging in water and that the seeds are quickly dried afterwards. Physically rubbing well-dried seed across small fraction wire mesh removes most of the arils. The seed still retained an unacceptable amount of contamination for commercial sale and further work is required. Final cleaning can be done by hand if low-cost labour is available (remaining sticks and pod fragmentsare individually picked out from a pile of seed). A good worker can clean up to x kg of seeds per day. 12
Fine sieving seed Winnowing seed Seed after seed extraction and final cleaning 13