As Rennie (1971) indicates, conceptually, the aim of mechanized site preparation is very simple, ¡.e., to facilitate the best possible regeneration or growth. For a particular forest manager concerned with a particular forest situation, the feasible mechanized techniques may be very restricted. The local conditions will be instrumental in balancing out the social benefits against the economic benefits. The selection of the right type of machines and the most appropriate use of the plantation also play an important role in the achievement of objectives. Field operations, should be organized to maximise the efficient use of the selected machines. The road and ride pattern of the mechanical equipment should match the spacing in the plantation.
The availability of competent personnel to operate the machinery and organize and carry out maintenance and repair tasks is essential for a successful operation. When planning, the following factors should be taken into consideration: the availability of labour and equipment; cost efficiency; scale of operation; quality of work and timeliness; and environmental consequences.
In watershed rehabilitation projects, mechanized site preparation can be divided into three phases:
- Removal or clearing the existing herbaceous and woody vegetation (grass, shrub, bush or trees). This can be either done with or without root extraction;
- Disposal of vegetative debris by windrowing, chopping and mixing with soil or burning;
- Soil cultivation either on strips or entire areas in conjunction, where necessary, with soil and water conservation techniques.
This operation uses crawler bulldozers equipped with front-end sharp angle or straight blades to cut and fell single trees at or near ground level.
- To eliminate or minimize the competition for nutrients, moisture and light between the existing woody vegetation and the new plantation.
- To quickly achieve medium-scale land clearing.
- For medium-scale clearing operations in upland wood or brush country with sparse
standing trees. The bulldozer blade can be
used for various purposes, and can be immediately turned to a new task after felling the
trees.
- Where manual felling is expensive and the necessary machinery is available.
- Where the topography and soil conditions are suitable for a mechanized operation.
Although the angled and the straight blades are considered suitable for small and medium trees respectively, trees with broad roots are best felled by using the end bit of the angle blade (Komatsu Ltd.). A sharpened and angled KG blade can be effectively used for shearing woodland trees up to 30 cm diameter or larger (Chapman and Allan, 1978). The disadvantage of this technique when used for felling large trees is the need to excavate and cut off the roots which slows down the operation.
On level sites, trees should be felled in the direction the trunk is leaning, or in the direction in which most of the branches are pointing. On slopes, trees should be felled along the contour line or uphill. In the case of large trees with a broad root system, the roots on the side of the felling direction should be cut first. The roots are then cut on the opposite side from which the tree will fall, and the hole is refilled. The tractor should approach the tree slowly with the blade fully raised until contact is made with the trunk and then the tree should be pushed with the engine set at maximum speed and full power. Reference can be made to specific manuals in this field which contain tables detailing the time needed to fell trees according to their diameter.
This operation usually requires some sort of excavation for exposing and cutting the broad root systems of large trees which results in extensive soil disturbance. Caution should be exercised when using this method on vulnerable sites such as sloping lands with erodible soils. The heavy machinery and blades required for this operation usually cause soil compaction and should be avoided on shallow and stony soils.
This operation entails the extensive felling of brush or thicket growth using heavy rolling choppers which consist of a large drum with cutting blades towed by a tractor (Chapman and Allan, 1978).
- To improve the nutrient balance and physico-chemical properties of the soil by chopping the woody vegetation into small pieces and mixing the debris into the soil.
- To destroy the competing vegetation and facilitate root penetration.
- To increase the organic content and infiltration capacity of top soil, and facilitate the penetration of rain-water into the deep rooting zone by subsoiling (Donmez, 1984). In many cases, the chopping operation leaves a mulch of chopped vegetation on the ground surface which protects the soil from the beating action of rain drops and splash erosion.
- On sites where a dense growth of shrubs is the dominant competing vegetation and needs to be cleared.
- On dry and poor sites with shallow sandy soils where a bushy, vegetation dominates.
- Where bushy vegetation, such as maquis in the Mediterranean region, or chaparral in north America, covers large areas.
- Where manual labour is expensive.
The equipment consists of a chopping machine with cutting blades or hammers. Towed by a crawler or 4 x 4 rubber-tyred tractor the machine should cut the bushy vegetation into small pieces and mix the debris into the soil. The makes and types of choppers vary from small to very large drum types and include multible drums pulled in tandem. Cutting and crushing efficiency can be increased by filling the drum with water to increase the weight. For maximum chopping efficiency, a constant speed of at least 8 km per hour should be maintained by using a direct-drive power unit (Chapman and Allan, 1978).
Although the size of chopper and the drawbar horsepower needed for a given operation is determined by the density and size of the vegetation, the following general rule of thumb (Chapman and Allan, 1978) can be used:
|
Chopper filled with water |
Diameter of woody stems |
DBHP* |
|
4.5 ton |
5 cm | 35 - 60 |
|
8.0 ton |
8 cm | 50 - 75 |
|
11.0 ton |
10 cm | 70 - 125 |
|
16.0 ton |
larger than 10 cm |
up to 250 |
The topography and slope gradient are the determining factors in designing the working system. Details of the procedure are summarized as follows: (E. Donmez, 1984) "With gradients of 0-30 percent, a chopping machine can be used in circular passes with a 90-110 hp, 4 x 4 rubbertyred tractor. In this system, the chopping operation must start from the middle point of the area to be cleared and continue in circular passes. The machine must begin working from the highest part of the area so that the machine can take advantage of the slope. The chopping machine is driven by the power-take-off (P.T.0) shaft of the tractor. Therefore, tractors with this type of machine should never work uphill. The chopping machines can work in runs lying parallel to the contour lines of the new clearing point. With gradients of 31-60 percent, the chopping machine must be used with a crawler tractor having a P.T.O. shaft. The vehicle should start from the top and chop downhill, so that the empty runs go uphill." (Photo l).
The selection of the machinery and equipment should be made on the basis of the vegetation, soil and terrain. On steep slopes with shallow soils, ploughing should be done along the contour lines when mixing chopped residue into the soil. Sometimes the clearing and soil preparation can be combined into a single operation. Studies have shown that single chopping is of little value, but double chopping with a well-timed interval is quite effective. The incorporation of debris improves the height and diameter growth, whereas treatments which entailed the removal of this material from the site resulted in less growth (Bernier and Winget, 1975). Compared to other site preparation techniques, chopping causes a significant reduction in sediment production and has less effect on water quality.
This operation combines the clearing of brush cover and soil preparation for afforestation, by disking, disc-ploughing and strip ploughing.
- To obtain benefits which can arise from cut brush vegetation and other debris being mixed into the soil.
- To improve the nutrient content of poor soils by increasing their organic matter content.
- To improve physical and hydrological soil properties which in turn result in decreased surface run-off, erosion and sediment delivery.
- To improve the height and diameter growth of seedlings and trees to be planted.
- To make the site preparation cost-effective by combining the vegetation clearing and soil preparation into a single operation.
On sites where brush vegetation measuring 1 to 3 cm diameter at ground level is growing on poor soil.
Light vegetative cover such as brush, can be efficiently broken up and incorporated into top soil by disc-doughing the entire area. In various regions of Turkey, for example heavy-duty semi-trailed disc ploughs are used for ploughing the brush over the entire area at gradients of 0 to 30 percent, by passes parallel to the contour lines. The disc-ploughs are attached to 4 x 4 rubber-tyred tractors with 90-120 hp engines. Discploughing provides deeper soil preparation than disking alone. (Donmez, 1985). The discs are built in such a way that, in stony soils, one disc can rise over an obstacle while the rest of the discs go on working, thus easing the strain upon the machine (Goor and Barney, 1972).
Another method used for combined land preparation operations is disking the brush vegetation over the entire area. Heavy duty trailed disc harrows attached to a 4 x 4 rubber-tyred tractor are effective when used on gradients of 0 to 30 percent. It is recommended that the first run be downhill and the second run be on the contour lines.
Cultivation or ploughing over the entire area is an expensive operation and should be practiced on level sites where there is no likelyhood of erosion and sediment delivery. On sites where the topography is steep, the land is cultivated only on strips along the contour lines by building trenches or, more commonly, gradoni, at gradients up to 40 percent. Strip-ploughing is commonly carried out using double-furrow heavy-duty tine ploughs attached to a 4 x 4 tractor.
The combination of vegetation clearing and cultivation or soil preparation methods is cost-effective if used under the proper conditions. Disking brush vegetation over the entire area should only be practiced on flat terrain with sandy, sandy loam or loamy sand soils in order to minimize the erosion hazard on such soils. Disc-ploughing as compared to disking only, is a deeper cultivation technique and is recommended for sites with deep and medium textured soils. It should be avoided on steep terrain with erodible soils near lakes, reservoirs, main stream channels or waterways in a watershed. This method of ploughing over an entire area, however, has an adverse effect upon the water quality. Strip-ploughing in conjunction with any type of terracing technique can be effectively used on sloping land. It provides better growth by conserving moisture and controlling erosion .
Vegetation clearing and removal of roots by single tractor operation
This is a mechanical stumping and felling operation which entails pushing or pulling over standing trees and at the same time extracting the tree roots from the soil. The operation is carried out by a crawler tractor.
- To remove existing trees on the planting site without using digging and scraping techniques, thereby minimizing soil disturbance.
- To improve soil conditions and stimulate the growth of planted trees by minimizing the competition for nutrients, water and light.
- On sites where there is degraded coppice, maquis or similar small diameter vegetation with large root systems. It is commonly practiced in Turkey and other Mediterranean countries where the maquis and/or degraded coppice are the dominant types of vegetation.
- May also be adapted for rain forests or heavy vegetation by using two tractors operating individually and following each other in consecutive passes.
The felling operation is carried out using a crawler tractor equipped with a front-mounted rake and a top pusher bar or frame designed for the site conditions in question. (Photo 2). In degraded forest or coppice, the tractor should place the pusher bar or frame as high up the bole of the tree as it can reach and push the tree over.
Next, the roots and main laterals should be ripped out of the ground by lowering and applying the rake. In this way, the tractors may push the trees along to a pre-determined piling line or elsewhere for windrowing. The operation should be repeated by reversing the tractor before approaching the next tree. The type of vegetation and root system should be taken into consideration when choosing the size and power of the tractor.
A medium or large size crawler tractor with a front-mounted rake may be suitable for the stumping of large trees growing on clay soils. In such cases, the rear-mounted rippers should be lowered into the soil and the tractor should circle the tree to cut lateral roots. This facilitates the subsequent pushing operation. The design of the rake, its tooth length and the spacing between the teeth are all important factors for a successful land clearing operation (Photo 3).
A particular adaptation of this system for rain forest regions is practiced using a tractor with a KG angled blade without the knife edge and hydraulic tilt rams. The operation can be accomplished in two consecutive stages by two individually operated tractors. The first tractor with the blade and hydraulic tilt ram should advance through the forest pushing over all the undergrowth and small trees while the second tractor with a stringer should follow the same path pushing over the remaining large trees. The two tractors should work around the forest in circular paths with about 100 m distance between them (Chapman and Allan, 1978).
From the standpoint of soil disturbance and the removal of nutrient-rich top soil, vegetation clearing by the front-mounted rake poses less problems than the single tractor operation equipped with a dozer blade.
Type of vegetation and soil characteristics such as depth, texture, stoniness and terrain are the determining factors for selection of the type of operation and the particular design of the rake.
The length of the rake and the spacing between its teeth should be taken into careful consideration. If the teeth are spaced too close together the rake may scrape the top soil and cause more disturbance and harm than necessary. A rake with 68 cm long teeth on deep, medium and fairly heavy-textured soils which were spaced 30-33 cm apart proved to be efficient in Turkey when used for coppice clearing. However, the dimensions of the rake can vary according to the type of the vegetation to be cleared. This sort of mechanized operation for land clearing is not recommended on steep slopes or sites with shallow and stony soils.
In rain forests, a Caterpiller D 5 fitted with a blade cannot knock down or pull big trees. It, therefore, becomes necessary to extract the roots by use of the blade and then cut them off with the strippers. (Dibbits, 1974). Although the method is cheaper than manual stumping its advantages are outweighed by excessive soil disturbance.
Selection of the correct equipment, power source and machines is important for the success of the operation. well-trained technical personnel to carry out the operation are essential because even a minor mis-use of the machine and equipment may cause irreversible harm. Maintenance and efficient repair services are similarly important.
This operation is carried out by two crawler tractors with front-mounted blades or rakes pulling a heavy anchor chain. The operation may require additional lead bulldozers or follow-up tractors equipped with a tree stringer in order to push over big trees with large root systems.
- To clear the existing vegetation on an afforestation site so as to eliminate its competing effects on planted seedlings.
- To fell trees and woody vegetation and extract their roots in preparation for windrowing or piling, and soil cultivation.
In areas where a large-scale land clearing operation is necessary for a woodland or savanna-type vegetative cover.
In broad and flat areas where wood is not needed and/or on sites where a large area of trees needs to be felled in a short time with a consequent reduction in the uprooting costs per unit area.
In rain forests where it is the most cost~efficient land clearing method. However, it requires good preparation and organization and it should be noted that windrowing after chaining is more difficult than after complete hand clearing and the single tractor technique (Dibbits, 1974).
In areas where the average tree diameter is less than 40 cm. The operating capacity, including the efficiency of the back~up bulldozer is illustrated as follows (Komatsu Ltd., 1980):
| Tree diameter | Operating capacity |
| Under 20 cm. | 5 ha/hr |
| Under 40 cm. | 2.5 ha/hr |
| Under 80 cm. | 1.0 ha/hr |
Situations where use of chain techniques is not recommended
In some types of thicket because root extraction becomes difficult where the young trees tend to bend under the weight of the chain.
In rain forests or other density forested areas where the visibility is sufficiently poor to prevent the necessary coordination between the two tractors (Chapman and Allen 1978).
In areas where the trees and other vegetation are densely packed making it difficult to pile up all the felled trees.
On rugged terrain and/or on uneven ground surfaces.
The chaining technique in land clearing should be carried out with two crawler tractors with front-mounted blades or rakes, and a rear-hitched heavy anchor chain. In areas with large trees, additional lead bulldozers or follow-up tractors equipped with a tinger may also be used. In woodland or degraded forest, the two tractors should move forward at the same steady speed, dragging the chain in a U-shape on the ground felling the trees inside the chain. The width of the operation area and the distance between the two parallel tractors should vary from 15 m to 25 m depending upon the height of the trees. In this way, the trees enclosed inside the chain start falling from the back towards the centre of the area (Fig. l).
The two tractors should maintain the same speed in order to prevent the chain from hitting more than two trees at once which may exert too much strain on the tractors. In this way, the tree felling and extraction of the entire root system are carried out in one operation. The roots of big trees extending in the direction of the fall, however, may not be entirely extracted. In this case, the remaining roots should be ripped out during a subsequent windrowing operation. Alternatively, back chaining may complete the job.
The total length of the pulling chain should be at least three times the height of the tallest trees. Shackles and swivel joints must always be used to join the lengths of chain (Komatsu, 1980). Chaining is an efficient and economical method for large-scale clearing of degraded forests or savanna-type vegetation.
A study conducted in tropical rain forest in Nigeria indicated that chaining in both directions with two tractors is more cost effecient than the single tractor technique and causes less soil disturbance (Dibbits, 1974). The same study showed that windrowing is more difficult after chaining than complete hand clearing and the single tractor technique. The distance between the linear heaps must not exceed 40 metres.
The main environmental damage arising from this land clearing method is sediment. The amount of sediment produced depends upon the type of site preparation, run-off characteristics, the recovery period and the treatment area in a watershed. In the Southeast U.S.A., a study indicated that erosion as a result of site preparation ranges from 0 to 4 percent of forest land. In addition, 30 to 80 percent of the total amount of sediment from forest land was attributable to land clearing methods. (Balmer, et al, 1976).
The operation of the heavy equipment used in chaining - 3 to 4 tractors and heavy chain - can result in serious compaction when carried out during wet weather on heavy clay soils. Such areas experience difficulty in recovering their original natural structure, are slow to revegetate and suffer a reduction in site quality. The operation should, therefore, only be carried out during the dry season.
