Chapter 5 Extraction

Contents - Previous - Next

What it is
Guiding principles
Potential consequences of improper extraction operations
Recommended practices for extraction with ground-skidding equipment
Recommended practices for extraction with forwarders
Cable extraction systems
Recommended practices for extraction with cable systems
Aerial extraction systems
Recommended practices for helicopter extraction
Recommended practices for extraction with draught animals
Other extraction systems

What it is

Extraction is the process of moving trees or logs from the cutting site to a landing or roadside where they will be processed into logs or consolidated into larger loads for transport to the processing facility or other final destination. Several classes of extraction systems are commonly recognized, including ground-skidding systems, forwarders, cable systems, aerial systems and draught animals, among others.

In this model code of forest harvesting practice, extraction with ground-skidding equipment is discussed first since it is the most common system and also the one that tends to cause the greatest environmental problems. For the remaining systems, the recommended practices focus principally on those that differ from practices recommended for extraction with ground-skidding equipment.

Guiding principles

Regardless of the type of logging system used, extraction is a difficult, often hazardous operation that can inflict substantial damage on forest ecosystems. Considerable skill is required, on the part of both supervisors and workers, to carry out extraction operations that are efficient and safe, as well as environmentally sound.

The three most common types of damage caused to forest ecosystems during extraction are:
- soil disturbance and soil compaction, which, if excessive, may increase the potential for erosion, retard the growth of residual trees and interfere with regeneration;
- damage to streams, either caused directly, when the skidding machines cross unprotected streams or drag logs through them, or indirectly, through the introduction of sediments or spilled fuel and lubricants into the streams from skid trails and other areas of disturbed soils;
- damage to residual trees and other vegetation, which may slow forest recovery and reduce the volume and value of timber available for the subsequent harvest.

All of these problems can be reduced substantially by comprehensive harvest planning coupled with a system of operational control to ensure that the plan is followed and adapted as necessary in response to changing conditions.


A well-organized and properly supervised extraction operation should:
- optimize extraction productivity;
- ensure the safety of extraction crews and other personnel working in the vicinity of the extraction operation;
- minimize the amount of soil compaction and soil disturbance caused by the extraction operation;
- minimize damage to streams within or adjacent to the cutting unit, particularly those that flow year-round or serve as sources of local water supplies;
- minimize damage to residual trees and seedlings, especially those that are expected to make up the population of future crop trees;
- deliver to the landing or roadside all logs prepared for extraction by the cutting crew, without significant loss of volume or deterioration in quality.

Wheeled skidder gathering a load of logs for extraction in a tropical forest plantation. The integral arch will permit the front of the load to be raised off the ground during skidding.

Crawler tractor without arch preparing to skid a large log in a natural forest in the tropics. For such work, crews should be equipped with helmets and other standard safety gear.

Potential consequences of improper extraction operations

The following are some of the potential consequences of improper extraction operations:
- a poor safety record and high insurance or compensation costs;
- low production rates and high extraction costs;
- failure to deliver all logs prepared for extraction to the landing;
- loss of log volume or value as a result of the extraction process;
- excessive damage to soils and streams;
- deterioration of water quality and loss of habitat for aquatic life;
- excessive damage to residual trees and seedlings.

Recommended practices for extraction with ground-skidding equipment

Most logging worldwide is done with ground-skidding equipment. Such equipment includes crawler tractors (bulldozers), wheeled and tracked skidders and farm tractors. The following practices are recommended when such equipment is being used:

Wheeled skidder

- As a general rule, skidders are preferable to crawler tractors for log extraction. Unlike crawlers, which are general-purpose machines especially intended for use in construction and excavation, skidders are designed explicitly for log extraction. Their rubber tyres and high manouverability can reduce soil and tree damage. They are usually narrower than crawler tractors and have smaller blades so that they can pass more easily between residual trees without causing damage. The smaller blades also prevent excessive blading, which can initiate erosion.

Crawler tractor

- Crawler tractors are often used in forestry for road construction and maintenance as well as for skidding. As a result, they have certain economic advantages compared with skidders, which have only one purpose. Because they are equipped with steel tracks and powerful bulldozer blades, crawler tractors can be used on very steep slopes. Also, operators tend to blade skid trails simply because it is easy to do so, rather than limit blading to places where it is required for safety or efficiency. The consequence is that unnecessary soil disturbance and excessive damage to residual trees and advance regeneration are all too common when crawler tractors are used for log extraction. In spite of their problems, however, it must be recognized that in many forest areas, particularly in steep terrain with large trees and high precipitation rates, crawler tractors are likely to remain the most common type of skidding machine used. To minimize problems when such machines are used, therefore, effective training of the tractor operators and their careful supervision are essential.

Farm tractor

- The use of farm tractors is a reasonable option when trees are relatively small and the terrain is not exceptionally rugged. Because these vehicles are not designed specifically for logging, safety can be a serious problem, and modifications are almost always needed to improve the durability of the machines as well as to make them safe for forest work. With such modifications farm tractors can be an excellent choice for small, private forests or in developing countries where farm machinery is often easier to obtain than specialized forestry equipment. Farm tractors are particularly well suited for work in forest plantations, especially for harvesting smaller stems, as in thinnings.

-Regardless of the type of skidding equipment used, machines with an appropriate size and power configuration for the operation should be selected. The blade width should not exceed about 3 m (2 m is usually sufficient), and the machine should have a powered winch with at least 30 m of wire rope as well as an arch or other support that will suspend the end of the load off the ground to prevent logs from digging into the soil as they are being skidded.

- Where soils are easily compacted, the use of low-ground-pressure skidders should be considered to reduce soil disturbance, rutting, and compaction.

Tracked skidder

These are tracked skidders with torsion suspension systems and a design that moves the load's centre of mass forward on to the skidder for better weight distribution over the tracks. Such skidders are more expensive than wheeled skidders but produce less ground pressure and, because of the torsion suspension system, tend to pass smoothly over such ground cover as logging debris and undergrowth, leaving it largely in place. Wheeled skidders and crawler tractors are more likely to crush ground cover into the soil.

- "High-flotation" tyres or dual tyres mounted on wheeled skidders may be used insted of tracked skidders for damp or easily compacted soils. Both solutions increase the effective width of the tyres where they are in contact with the ground, thus spreading the weight of the machine and its load over a larger surface area, reducing pressure on the soil.

- Skidding operations should be suspended altogether during exceptionally wet weather. Skidding during such periods increases erosion and other damage, reduces operating efficiency, increases accident rates and may double or triple operating costs.

- Steep slopes should be avoided in ground-skidding operations. Although modern skidding machines are often mechanically capable of operating on quite steep slopes, doing so greatly increases soil disturbance, reduces operating efficiency and leads to faster depreciation of the machine. Slope limits will vary from region to region depending on soil conditions, precipitation and other factors. Many countries restrict ground skidding to slopes of less than 30 percent (17°) except for short distances. Steeper slopes that are sustained over long distances should be harvested by cable or aerial systems that have the ability to suspend logs above the soil.

- A system of designated skid trails should be used. Environmentally, skid trails are the most troublesome aspect of ground-skidding operations. In many parts of the world, the conventional approach to log skidding involves the skidding machine wandering through the forest after felling has been completed, searching for logs to drag to the landing. When a log is encountered, the operator hooks it to the machine and then drives to the landing, sometimes along the same path, but other times in a different direction that the operator believes will lead more quickly to the landing. In tropical forests, where undergrowth is often dense, the machine operator may even keep the blade down most of the time, particularly if a crawler tractor is being used, thus creating a path of deeply disturbed soil wherever the machine travels! This can result in an astonishingly dense system of skid trails. Although skid trails commonly represent 20 to 40 percent of a harvested area, several studies have found that 60 percent or even 80 percent of the harvested area was covered in skid trails after selection harvesting operations had been completed. When a designated skid-trail system is used, skid trail locations are included in the harvest plan. They are first clearly flagged on the ground before the cutting operation begins, using the topographic planning map, a compass and a device to measure slope angles. With the skid trails thus marked, the cutting crew can reduce extraction costs by aligning felled trees along the skid trails and felling damage to future crop trees can be reduced by placing the crowns of the felled trees within or alongside the skid trails whenever possible.

A farm tractor being used to skid logs. Note the protection for the driver and the device for raising the front end of the logs off the ground.

Wheeled skidder equipped with high-flotation tyres (1000-mm wide) to reduce tyre pressure on wet soils.

Excessive skid-trail density in a natural forest in the tropics. Such practices can lead to heavy soil erosion and sedimentation of streams. These problems can be reduced by using a system of designated skid trails and requiring skidders to remain on the designated skid trails at all times.

- Skidding machines should remain on the designated skid trails at all times. Studies in many countries, including several in the tropics, have demonstrated conclusively that it is both unnecessary and uneconomical to drive the skidder to every log; by staying on the skid trail and pulling the winch line out to the log, significant reductions in soil disturbance, soil compaction and damage to residual trees and advance regeneration can be achieved.

- Choker-setters should be trained to preset chokers, locate the best approach to the log and pull the winch line out to the log while the skidder remains on the skid trail in order to facilitate the system of designated skid trails. In conventional operations, the winch line is often wrapped directly around the log to be skidded, rather than using a separate choker cable or chain. This causes a delay during which the skidder sits idle. Pre-setting of chokers not only eliminates this delay, but it also permits easier collection of a number of logs that can be skidded simultaneously.

- Skid-trail construction with the skidder or crawler blade should be minimized. In some countries, including several in the tropics, blading of skid trails has been prohibited altogether except when necessary to ensure safety. Guidelines developed by CIRAD-Forêt and tested in West Africa, South America and Southeast Asia stipulate that skid-trail construction should be limited to hand-cutting of brush and trimming of stumps to ground level. The brush cuttings can then be placed in the skid trail to provide a protective mat over which the skidders will travel. In addition to protecting soils, such procedures can also reduce costs by eliminating the time bulldozers spend unnecessarily blading skid trails.

- Skid trails should generally be as straight as possible, curving where necessary to reach timber marked for cutting or to avoid wet or unstable soils and steep hillsides. On slopes steeper than about 30 percent (17°), skid trails should be angled across the slope rather than running straight up and down it. The skid-trail width should be the minimum feasible; 4.5 m is often recommended as an appropriate maximum width when large, heavy machines are being used. Tight corners should be avoided in order to protect trees and other vegetation standing near the skid trail

- Where feasible, uphill skidding may be preferable to downhill skidding since it tends to disperse runoff water into the surrounding vegetation, whereas downhill skidding tends to concentrate it at the landing. The logs being skidded are easier to control when the skidding direction is uphill, and there is also less of a tendency for operators to attempt slopes that are too steep for safe skidding. On the other hand, pulling uphill requires more power and the vehicles may churn the soil. Operators may also tend to use the blade more frequently to remove topsoil and improve traction. Whether to specify uphill or downhill skidding thus requires an evaluation of tradeoffs, and the decision will usually represent a compromise.

- Skid trails, as well as skidding machines, must be kept out of streamside buffers entirely. Where harvesting of trees from within buffer strips is permitted, the trees should be felled away from the stream and extracted by winching.

- Skid trails should never cross streams or gullies, unless it is absolutely unavoidable. Where this must be done, the crossing should be made at a site where there is a rock base if possible. The streambed should be protected with logs or a temporarily installed culvert.

- Operators should be encouraged to use lubricating oils that are environmentally benign, such as vegetable oil, when feasible.

- Adequate drainage in the form of drainage ditches and cross-drains should be provided for skid trails and other areas where soils have been disturbed after the harvesting operation has been completed. These areas should be left in a state that will facilitate their rapid revegetaion. If necessary, they should be ripped up and sown with grass or other fast-growing plants.

Recommended practices for extraction with forwarders

Forwarders are extraction machines that carry a load of logs completely off the ground, either within their own frame or on a trailer; therefore, they typically cause less soil damage than ground-skidding equipment. Usually they include a mechanism, such as a hydraulic or mechanical crane, for self-loading and unloading of logs. Most forwarders have been designed explicitly to utilize the latest information on ergonomics and to promote safety. They are frequently used in combination with mechanized felling and processing equipment, although this is not essential. Recommended practices for extraction with forwarders include the following:


- Forwarders work best when logs are approximately uniform in size.

- In comparison with ground skidders, the productivity of forwarders is less sensitive to extraction distance. Typically, therefore, the economic extraction distance is two to four times that of ground-skidding equipment. The density of haul roads required in the forest can thus be reduced when forwarders are used instead of skidders.

-Forwarders require a somewhat better track than skidders because the load must be kept relatively level in order to avoid tipping. In hilly terrain, the forwarder tracks should go directly up and down the slope. Stumps and undergrowth in the track should be cut down to ground level, and the cut brush can be thrown in the track to help protect the soil. A modest amount of blading is necessary where forwarders are intended to travel along hillsides. It is generally unsafe to use them on tracks with grades that exceed about 30 to 40 percent (17° to 22°).

- High-flotation tyres should be used to reduce soil compaction in areas of wet or easily compacted soils.

- One justification for the use of forwarders is that the construction of landings is often unnecessary. The forwarder can unload its load of logs directly on to a truck or it can deck the logs alongside the haul road for later pick-up by the trucker.

- As with skid trails, forwarding tracks must be closed and protected from soil erosion when the operation has been completed. Drainage ditches and cross-drains should be constructed to divert water from the tracks, and exposed soil should be revegetated.

Cable extraction systems

Cable extraction systems are fundamentally different from other extraction systems. In cable systems, one or more suspended cables are used to convey logs from the felling site to the landing. The cables are operated by a winching machine (also called a yarder or hauler), which may be installed either at the landing or at the opposite end of the cableway, often on a ridgetop. In partial-cutting operations, a narrow corridor is cleared as a pathway for the operating cables, which are then strung along the corridor and suspended above the ground with a "spar tree" located near the winching machine. The spar tree can be either an actual tree or a steel tower. To obtain additional clearance above the ground, the cables may also be suspended in a "tail tree" located at the opposite end of the cableway. If necessary, intermediate supports for the cables can be provided by additional trees located at intervals along the cableway.

A great many different types of cable extraction systems have been developed in the century or so since the first systems were introduced. Differences among these systems include variations in the configuration of the cables as well as in the methods for conveying logs to the landing and for returning empty chokers to the felling site.

From an environmental point of view, cable systems that are considered to be low-impact systems generally fall into the category of "skyline" systems, also called cable-crane systems. These systems utilize a skyline cable, which serves as a support to elevate the load of logs above the ground. The skyline cable is mounted on the spar tree and runs to an anchor fixed at the opposite end of the cableway. A wheeled carrier mechanism, called a carriage, rides along the skyline cable to carry the load to the landing and to transport empty chokers back to the felling site. The load is attached to the carriage by another cable called the skidding line, and some mechanism is usually provided for increasing the length of the skidding line so that it can be pulled laterally out from the cableway to reach the logs that are to be yarded. The load is then skidded laterally to the cableway by reducing the length of the skidding line (usually by spooling it on to a drum). After the load reaches the corridor, the carriage is conveyed to the landing. If the landing is uphill from the felling site, this is done by pulling the carriage uphill with a "mainline" cable powered by the winching machine. If the landing is downhill from the felling site, the load is moved to the landing by unspooling the main-line cable so that the load will travel to the landing under the force of gravity.

Although significant variations on this theme exist, in general, cable systems that can be classified as skyline or cable-crane systems have the following characteristics:

- an elevated skyline cable that is used to support the load of logs;

- a carriage riding along the skyline cable that conveys the load of logs to the landing and returns the empty chokers to the felling site;

- some mechanism for pulling line laterally from the cableway to reach the logs to be yarded, and for laterally yarding these logs to the skyline corridor;

- during "main yarding" along the skyline corridor, the load of logs remains either fully or partially suspended in the air, thus reducing or eliminating soil disturbance within the cableway;

- the power source does not travel along the ground as in the case of ground-skidding systems. An important advantage of cable systems in general, this permits cable systems to be used under more extreme conditions, such as steep terrain or in areas with permanently wet soils, where ground-skidding systems would be neither economically feasible nor environmentally acceptable.

One type of "skyline" cable system commonly used for extracting logs in steep terrain. This system has been set up to lower the logs to a landing near the "tail tree" at the bottom of the figure.

A skyline carriage with a fully suspended load of logs.

Two skyline corridors on a steep hillside (left). This is a natural forest in the tropics where selection harvesting has been completed and the logs have been extracted by skyline under full suspension. Skid trails in another part of the forest from which a similar volume per hectare has been removed but where extraction was done with crawler tractors (right). Note the much higher levels of soil disturbance and canopy opening.

Recommended practices for extraction with cable systems

- Cable extraction requires skilled personnel who are qualified to plan and lay out cable systems, as well as those who are capable of supervising and carrying out the actual extraction work. Much of the detailed planning can be done during the tactical harvest planning stage, as long as accurate, large-scale topographic maps are available. Whenever cable systems are to be used, it is essential that sufficient time be allotted to permit the necessary advance planning so that the operation can meet its environmental objectives at a reasonable cost. Managers should also recognize that, if planning is done properly, some areas of the forest will probably not be harvested even though they have been classified as production forest. Where the timber is less dense, the species less valuable or the topography too difficult, the most sensible choice may be to leave the forest un-harvested.

- Only skyline systems are recommended for most situations. Other cable systems, such as the "high-lead" system, which is commonly used in some parts of the world, do not adequately suspend logs above the soil, and because of this they cannot normally reach dogs that are more than a few hundred metres from the landing. Skyline systems, by comparison, can access timber at distances of up to 1 000 m or occasionally even more from the landing. This allows a considerable reduction in the density of haul roads required to serve a specific area of forest.

- Skyline systems, including both single-span and multispan configurations, are the only types of cable systems that are generally recommended for use in selection harvesting. In the past, high-lead systems were used extensively in Southeast Asia for selection harvesting of tropical forests on steep terrain. Occasionally, when the crew was highly skilled and closely supervised, these operations were quite satisfactory. For the most part, however, the damage to the residual stand was so extensive that high-lead logging was judged to be totally incompatible with selection harvesting in tropical forests. Some countries went so far as to ban the use of cable extraction systems altogether. In retrospect, this was a mistake; the damage caused by ground-skidding equipment operating in steep terrain is usually far greater than that caused even by high-lead systems. And when skyline systems are used with proper planning, training and operational control, impacts on soils and on the residual stand can be as low as or lower than those of any extraction system except perhaps when helicopters are used.

- Winching machines with power capabilities that are appropriate for the terrain and the size of logs being extracted should be selected.

- Cable systems are not only used over steep terrain but also in areas such as swamp forests where ground-skidding equipment would cause environmental damage. In such situations, the yarder is typically mounted on a floating barge. Extreme care must be taken when harvesting operations are carried out in such areas because of the particularly fragile nature of their ecosystems.

- After harvesting has been completed, cross-drains should be constructed at intervals along any cableways or lateral yarding tracks where the soil has become exposed.

Aerial extraction systems

Balloon extraction systems
Helicopter extraction systems

Aerial extraction systems are those that fully suspend logs in the air throughout the entire extraction process. Skyline cable systems do not meet this criterion in general, because they do not normally suspend logs during the lateral phase of yarding, and the logs may also occasionally drag along the ground at intervals during main yarding, even when full suspension is intended, depending upon the length of the logs and the amount of clearance between the ground and the skyline cable.

Two types of aerial log-extraction systems are currently in use: balloon systems and helicopter systems. Several others have been proposed, but thus far they have not got past the experimental stage.

Balloon extraction systems

These are cable systems that use balloons filled with a lighter-than-air gas (usually helium) to provide the lifting force that fully suspends the load of logs during extraction. Cables spooled on to drums that are mounted on a winching machine are used to pull the balloon and its load to the landing and to return the balloon, with the empty chokers, to the felling site. Experience with balloon logging suggests that it is only feasible for clear-felling operations, although new developments currently being tested may change this in the future. Balloon logging also requires cutting units that are large enough to cover the high fixed costs associated with installing the balloon system at a landing. Where these conditions can be met, balloon systems cause very little soil disturbance and can reduce road construction substantially since they are capable of reaching very long distances from the landing. Because of the high initial investment and the specialized equipment and crew skills required, however, very few balloon extraction systems are currently in use.

Helicopter extraction systems

Conceptually, these are quite similar to ground-skidding systems, except that, because the transport machines are helicopters, logs are flown from the felling site to the landing rather than being skidded along the surface of the ground. Helicopters used for logging can be grouped into three classes: those with rated lifting capacities for external loads of 2.7 tonnes or less are considered "light-lift" helicopters; those that can lift 2.7 to 5.4 tonnes are "medium-lift" helicopters; and those with greater lifting capacities (currently up to 11.3 tonnes for commercially available, nonmilitary aircraft) are "heavy-lift" machines.

Helicopter logging is accomplished by suspending below the aircraft a "tether line" (also called a tag line) of wire rope to which chokers can be attached. In some operations a grapple hook is used rather than chokers. The tether line is typically between 30 and 100 m in length, depending upon topography and the height of any trees above which the helicopter must hover, among other factors. Long chokers are used, and these are pre-set. The ends are then brought together to make up loads that are estimated as being slightly less than the helicopter's lifting capacity.

Typical configuration of a balloon extraction system.

When a load has been prepared, the helicopter flies to the hooking point, lowers the tether line to the ground and hovers while the load is attached to the tether line. Upon receipt of a signal from the loadmaster, the helicopter lifts the load above any residual trees and then flies to the landing. The logs are placed on the landing and the chokers are released by remote control from the aircraft. The helicopter then returns to the felling site for the next load of logs. A round trip between the landing and the felling site typically takes from two to five minutes.

It is widely recognized that direct operating costs for helicopter logging are many times higher than those for other types of extraction systems. Fuel costs alone are enormous - the largest heavy-lift helicopters consume nearly 2000 litres of high-octane fuel per hour! Added to this, the cost of the aircraft itself, replacement parts, maintenance, insurance and crew salaries can make hourly expenditures for a single heavy-lift helicopter higher than those for a medium-sized sawmill. On the other hand, the great range of the helicopter, which can serve an area several kilometres in radius from the landing, makes it possible to eliminate a great deal of road construction and thereby reduce not only costs but also a major source of environmental damage.

In addition, helicopter extraction causes virtually no soil disturbance and no dam age to residual trees other than that which occurs during the cutting operation or that is associated with the landings and transport operations. For some situations, therefore, particularly where high-value timber is to be extracted, the considerable cost of helicopter logging can be justified by the combination of savings from reduced road construction and the ecological value of very low environmental impacts.

Certainly, helicopter logging is practical only in special circumstances, but where those circumstances apply it can be an attractive alternative to other extraction systems.

Recommended practices for helicopter extraction

- Helicopters, especially heavy-lift machines, are capable of production rates that far exceed those associated with most other extraction systems. An aircraft that delivers 5 m³ of logs to the landing every four minutes will produce 400 m³ in eight hours, even allowing 20 minutes per hour for maintenance and refuelling. In good weather and with optimal flying and working conditions, helicopter operations have been reported that have produced as much as 2 000 m³ in a day. Such high production rates put a great deal of stress on haul roads, work crews, transport vehicles and support facilities. Operations involving helicopter extraction must therefore be planned thoroughly from the very beginning, and plans must be flexible enough to adjust to rapidly changing conditions.

Typical configuration of a helicopter extraction system

- Because of the high production rates, the number of workers employed on helicopter operations is necessarily much larger than for most other logging systems. This means, for instance, that a large number of workers may be involved in cutting operations that are being carried out simultaneously over a wide area. Coordination and constant supervision are therefore especially important in order to reduce hazards while at the same time maintaining productivity at the rate needed to avoid costly delays.

- Operations must be planned so that helicopters carrying suspended loads never fly directly above workers on the ground and never follow a route that would put workers in danger from dropped or aborted loads. Helicopter crews must be made aware of the fact that workers using motorized equipment such as chainsaws may not be able to hear the helicopter approaching.

- Crews working on the ground cutting timber and pre-setting chokers are not always visible from the air, especially in partial-cutting operations. Where there is a possibility that such crews may be working near the helicopter's flight path, provision should be made for the crews to report their location to the aircrew at regular intervals or whenever necessary. Usually this is done by providing all crews with two-way radios and teaching them the protocol required for radio communication. This also has the advantage of providing a means of rapid communication in the event of an emergency. Wearing brightly coloured clothing can also help make the field workers visible to the aircrew.

-Although the safety records of helicopter logging operators are better on average than those of ground-skidding operators, serious accidents have occurred in helicopter logging, sometimes even involving multiple fatalities. Also, because of the system's high production rates, pressure to work too swiftly may increase the frequency of accidents among the cutting crews and on the landing. This tendency must be countered by a continual emphasis on safety and good working methods.

- The safety of bystanders and visitors to the logging site must be considered with helicopter logging operations since these operations inevitably attract the attention of the public. A useful precaution is to construct a viewing area some distance from the landing, where visitors can get a good overview of the operation without being endangered.

-The helicopter must never fly over areas of habitation while carrying a load of logs or some other suspended load.

-When the helicopter's flight path is likely to carry it over or near a roadway, flag bearers must be stationed along the road in positions that will permit them to prevent entry into the area of danger until after the helicopter has passed. The operation should be organized in such a way as to minimize delays and inconvenience to the public that may result from such stoppages.

- Landings where logs are to be delivered by helicopters must be of sufficient size to accommodate a large volume of logs and also to permit trucks, log loaders and workers on the landing to stay well away from the drop zone when the helicopter is delivering its load.

- Dust can be a major problem on helicopter landings during dry periods, and may interfere with the safe operation of the helicopter. A water truck may be needed to wet down the landing periodically, especially in the log-drop zone and nearby areas.

- In addition to the landing, one or more graded areas must also be prepared as landing pads for periodic refuelling and maintenance of the helicopter. Vegetation must be cleared away from these areas to permit adequate clearance by the helicopter's rotor blades during landing and take-off. Dust on the pads must be controlled, and the pads must be kept clear of objects that could be knocked loose by the down draft and fly up into the rotor blades. The refuelling pad in particular must be located sufficiently far away from the landing that it will not constitute a hazard to personnel on the landing. Fuel storage must be provided in a way that will minimize the danger of explosion and fire, and dykes must be built around fuel containers to prevent spilled fuel from contaminating streams or groundwater supplies. Where possible, the maintenance pad should be lower in elevation than the landing and, with respect to prevailing winds, its location should facilitate an emergency landing of the helicopter if required. The flight path into and out of the refuelling and maintenance pads should not pass over roadways or other public areas nor over dwellings.

- Because of the large number of logging trucks, fuel trucks and other vehicles that will enter and depart from helicopter logging areas, road signs must be placed near junctions where vehicles will enter public roads to warn the public that heavy truck traffic may be encountered.

Recommended practices for extraction with draught animals

Log extraction with draught animals continues to be an economically attractive choice in many areas, sometimes even in industrialized countries. As compared with ground-skidding equipment, the use of draught animals, such as elephants, water buffaloes, oxen, horses and mules, has been shown to reduce soil disturbance, soil compaction and damage to residual trees significantly. Direct impacts are generally limited to the extremely narrow skidding paths used by the animals. Animal skidding is particularly advantageous for thinnings or for pulpwood harvesting, where relatively small logs are to be extracted or where products from pit sawing or other onsite processing are to be transported.

- Planning for animal skidding must allow for short extraction distances (typically 200 m or less) and relatively gentle slopes. Depending on the type of animal used, a maximum slope of about 20 to 30 percent (14° to 17°) is often suggested when skidding downhill and 10 to 15 percent (6° to 9°) when skidding uphill.

Head yoke for oxen logging

Elephant logging: hooking chokers to a load of logs.

Elephant logging: pulling a load to the landing.

- Proper harnesses are essential in order to prevent injury to the animals and to avoid cumulative discomfort over long working periods. Head yokes are recommended for ox teams as they avoid chafing of the skin and allow the animals' full pulling power to be employed.

- Devices such as skidding pans, sledges and sulkies can greatly improve productivity in animal skidding because they reduce skidding resistance and thus allow larger loads to be pulled.

- For operations in natural forest it is usually necessary to clear skidding paths for the animals. Undergrowth should be cut close to the ground by hand and thrown clear of the path. Obstacles that could pose a danger to the animals should be removed, and any stumps in the path should be trimmed to ground level.

- Cutting must normally be coordinated with skidding, and both should start at the back of the cutting unit (farthest from the landing) and proceed towards the landing. This avoids the animals having to climb over logging slash left behind by the felling crew.

- Animals must be fed, watered and rested at regular intervals while working or they will refuse to continue. Some animals, such as elephants and water buffaloes, require frequent baths during hot weather in order to dissipate the body heat generated by hard physical activity.

- Depending upon climatic conditions, terrain and other factors, animals may not be able to work every day and they may require relatively short working days. Often, a 20 to 25 percent reserve of animals is recommended in order to ensure that a sufficient number is available to work on any particular day.

- Although horses work quite well for log extraction in temperate climates, they are not generally suited for work in hot, humid climates such as those common to tropical forests.

-Regular veterinary care is essential whenever draught animals are used in forest work, and feeding practices must ensure that the animals' nutritional requirements are being met fully.

- After the skidding operation has been completed, skidding paths should be examined and, where necessary, cross-drains should be constructed to divert water from the paths into the surrounding vegetation.

Other extraction systems

Manual extraction
Pit sawing
Extraction by chute
Extraction by winch truck
Extraction by water

A wide variety of extraction systems have been developed for timber harvesting. Many of these are quite low-impact systems, if only because they are typically used on a small scale and thus their impacts are not widespread. No attempt is made here to recommend practices for all of these systems; there are simply too many of them and, for the most part, their application is limited to specific local regions. A few of the more common ones, however, are described below.

Manual extraction

The movement of logs by human labour is increasingly rare, except on farms where it persists even in industrialized countries. In general, it is practical only where labour costs are low, extraction distances are short and the logs or other pieces of wood being extracted are light enough to be handled easily by humans. Most manual extraction is carried out in connection with fuelwood harvesting, certain types of thinnings in plantation forests, agroforestry operations and in forests where trees seldom attain large sizes, such as most mangrove forests. Efficiency and safety can be improved both by using lifting hooks, levers and other hand tools or by pulling logs with the aid of manual sulkies. In some man grove forests, wheelbarrows have been used to assist in manual extraction. These are pushed along narrow plank roads built in the extraction areas by hand.

Manual extraction of logs from a thinning operation, using a hand sulky to reduce human effort and improve productivity.

Polyethelene chute for extracting small logs from a thinning operation.

Pit sawing

Although technically it cannot be considered "extraction", pit sawing is a manual method of converting logs into sawn wood on-site. The sawn planks are then carried out manually or by animals. Environmental impacts associated with pit sawing are usually very low, although where pits are dug the soil is exposed and erosion can begin. The impact is usually very localized, however, and its effects can be minimized by locating pits well away from streams. A more significant problem with pit sawing is the difficulties of ensuring that the pit sawyers do not overutilize the resource by cutting all of the high-value trees. Because they only work on one tree at a time, usually using hand tools, pit sawyers can be difficult to find in the forest. Ensuring that they are cutting only designated crop trees requires constant surveillance. Equally important is the protection and safety of pit sawyers when preparing and sawing the logs.

Extraction by chute

In steep terrain, gravity can be useful in rolling or sliding logs downhill from the felling site to a haul road. To reduce damage to residual timber and concentrate the logs at intervals along the haul road, chutes may be used to control the path followed by the logs. Commonly these chutes are constructed from timber or made from commercially available polyethelene tubing cut into U-shaped sections and fastened together to form a long pathway. Safety is a potential problem whenever chutes are used. Signs or flag bearers must be posted near the bottom of the chute to warn passing traffic of possible danger.

Extraction by winch truck

A common extraction method in some parts of the tropics uses light trucks with powered winches to load logs for transport right at the cutting site. A low-grade hauling road is usually cleared manually so that the truck can be driven up to the tree. Logs are loaded on to the truck with the winching system, and the truck is then driven to the next cutting site. This process continues until a full load is obtained. The truck is then driven directly to the final destination or to a landing where the logs can be consolidated into larger loads for transport by heavier trucks. Under the right soil and terrain conditions, winch-truck extraction can be a relatively low-impact system. Soil disturbance and damage to residual trees are typically less than when crawler tractors are used, for example. In general, however, the system should not be used in swampy areas or on steep slopes.

Contents - Previous - Next