Harvesting is carried out by chainsaw felling in combination with a ground skidding system and temporary landings. In this chapter, harvesting operations such as planning, road construction, felling, extraction, landing operations, and transportation are introduced.
Regulations and limitations
Legal guidelines for forest harvesting in the Congo are set in the `Code Forestier' (Law No. 004/74 of 04.01.1974) The code, which deals with administrative, technical, and legal instructions, provides the legal framework for all forest operations and is currently under review. Harvesting in the concession of SOCOBOIS is limited by minimum diameter requirements. The currently applied minimum diameter for Okoumé is 80 cm. After this first harvesting, the concession is to be closed down for a number of years in order to regenerate. Presently, an enterprise does not automatically have the opportunity to re-harvest a concession.
Harvesting is further limited by weather conditions. Although harvesting operations are allowed during all months of the year, production usually declines during the rainy season (October-November and March-April) due to difficult terrain conditions and inaccessibility of parts of the forests. Road construction is normally interrupted during the rainy season.
An economically limiting factor for harvesting is the considerable transport distance of approximately 500 km from the forests to the mill site. The harvest manager has to ensure that only roundwood of suitable quality for the production process is transported to the mill.
The concession (150,000 ha) is divided into "annual coupes". An annual coupe is the area foreseen for harvesting in a certain year. A pre-harvest survey, called a "prospection" is usually carried out 2 years before harvesting commences. Only harvestable trees (above 80 cm dbh) are counted and the number of harvestable trees per compartment is noted. It is a requirement of the concession license that the results of the prospection must be submitted to the forest authorities, before obtaining a harvest permit.
The case study was carried out in the 1995 coupe "VMA95" (VMA = Volume Maximum Annuel) which consists of 15,550 ha of primary closed forests. The topography of the area is characterised by hills with maximum slopes of 40-50% on short sections. The hills are separated by small creeks and temporary swampy areas (Figure 5-2).
Coupe VMA95 (15,550 ha) is subdivided into 311 harvesting compartments of 50 ha each. The orientation of the grid is south-north and compartments are identified by rows (A-V) and columns (1-117). The boundaries of these compartments are established during the pre-harvest survey by the prospection team. Each compartment is approximately 500 m x 1,000 m. The division into compartments is carried out by a group of 7-8 men with machetes who clear all boundaries of shrubs at the rate of about three km per day.
The most important species being harvested in the concession is Okoumé. Due to their limited occurrence, all other commercial species have relatively little importance. The number of harvestable Okoumé trees in each of the 311 harvesting compartments ranges from less than 10 to more than 100 trees or, from 0.2 to 2.0 harvestable trees per hectare.
According to the pre-harvest survey (prospection), the distribution of harvestable trees in the 1995 coupe is as follows:
Table 4-1: Harvestable trees in annual coupe VMA95 (15,550 ha)
Total Number of Trees
Trees per ha
Trees per compartment (50 ha)
According to this survey, the average density of harvestable trees in coupe VMA95 is less than one tree per hectare.
The following maps were available for harvest planning and operations:
· the Congolese national geographic map (1:200000);
· the VMA map (1:20000), prepared by the enterprise at the annual coupe level. It shows compartment boundaries, the number of harvestable trees by species and water courses;
· the road planning map (1:50000), prepared by the enterprise at the annual coupe level. It shows the existing and the planned primary and secondary roads, the compartment boundaries, the location of landings, and main water courses.
A major challenge for the enterprise is presented by the lack of topographic maps. Maps at a scale between 1:10000 and 1:50000 showing forest cover, important topographic features, contour lines, stream locations, and protected areas would be of great assistance.
The objectives of a proper road layout are convenient access to the forest, minimisation of soil erosion, and minimisation of the area required for roads and landings. The basis for the transportation and extraction layout is the VMA map containing the number of harvestable trees in each compartment. An effort to reduce haul distances is made through a road layout oriented toward the compartments with the highest timber density. Since topographic information from public maps is either not available or unreliable, an extensive reconnaissance of the terrain by an experienced forest engineer and an assistant is required. The engineer develops the road layout directly in the forest. The design follows the contour of the land as much as possible and attempts to avoid streams wherever possible. The resulting quality of the road layout is quite good.
Second pre-harvest survey and harvesting plan
Shortly before harvesting commences, a second survey of harvestable trees is done by a second crew consisting of 5-6 men. From this survey the operation plan for the felling crews is developed. The number and location of harvestable trees is reported to the felling operation manager, who decides the deployment of the felling crews.
Road planning and construction are done entirely by the enterprise. The machinery available for road construction consists of crawler tractors (Caterpillar D6 and D7), graders (120G/120B), loaders (966C), and trucks (Benz 1113). Road construction is normally accomplished one year in advance of harvesting in order to allow the roads to dry. The following types of facilities exist within the concession area:
· access roads (long term, more than 10 years)
· primary haul roads (long term, during the concession)
· secondary haul roads (short term, up to three years)
· skidding trails (short term, few days up to several months)
· landings (short term, up to several months)
The trees on the road construction site are marked, felled, and deposited alongside the road. Commercial trees are crosscut and integrated into the production process. The bulldozer then starts clearing the site and preparing the roadbed. Simple wooden culverts are constructed for small seasonal water flows crossing the road. They consist of three logs, of which two are the basic layer with the third log on top. Since gravel is not available in the vicinity of the concession, the road base consists of uncompacted natural soil only.
The road carriageway is shaped by graders. The only available surfacing material is laterite. With few exceptions, the maximum road gradient in the concession does not exceed 15%. Although roadside ditches are used on slopes, no cross-drains are applied to the surface to channel water away from the road structure. Surface erosion is a common problem during the rainy season, which results in considerable maintenance effort throughout the year. It is worth mentioning that surface erosion is not limited to steep sections. It also occurs on flat terrain and seems to be related to the absence of a sealed surface rather than to the road gradient. The enterprise also maintains the national road, which is used for long-distance log transport.
The average cleared width for main haul roads in the forest is 40 m, while the carriageway does not exceed 7-10 m. Secondary roads (feeder roads) are cleared to a width of 25 m and the carriageway is not more than 4-7 m. The cleared width for skidtrails usually does not exceed 4 m. This width reflects the minimum space required for skidder and tractor operations.
Felling and topping
Felling is carried out by a felling crew consisting of three persons: chainsaw operator, helper, and guide. The chainsaw operator wears personal safety equipment (safety boots and helmet) to protect himself against falling branches and other hazards. The guide identifies harvestable trees and determines the next felling site. He also provides information on the location and number of felled trees to the skidding crew. Before felling begins, the helper cleans the bark along the sawkerf to remove sand and stony material.
The chainsaw (Stihl 070) is usually equipped with a 75 or 90 cm blade. Felling does not utilise special techniques such as sidenotches, undercuts, or auxiliary equipment such as wedges to determine felling direction. The operator starts in the middle of the stem cutting the entire cross section. He continues with the buttresses, starting with the one in the assumed felling direction. However, due to difficult felling conditions the resulting, actual direction of fall is highly variable. The falling tree, climbers, and re-springing neighbouring trees pulled down by climbers, make felling dangerous and stressful for the crew.
After felling, topping of the crown is done by the helper. Some timber breakage occurs due to inappropriate cutting technique, especially if difficult "hangers" must be cut. For legal reasons and work control, the logs and the stump are numbered and recorded with species and the number of the harvesting compartment.
The average performance of a felling crew is 20 trees per day; the range is from 13 to 30 trees. Normally the enterprise operates three felling crews simultaneously, which amounts to an average of 60 felled trees per day. Under the assumption of an average volume per tree of 6 m3, this is a total of 360 m3 prepared per day (net log volume).
Skidding is carried out by two Caterpillar D6H tractors and two Caterpillar 528 wheel skidders that are used in various combinations. Usually a crawler tractor works together with a wheel skidder. In this combination, the extraction of the logs is carried out in two phases. The crawler tractor first pulls the logs directly, or by winch, from the felling site to the skidtrail. This produces a concentration of logs at the skidtrail. During the second phase, the wheel skidder is used for long distance skidding; pulling the logs to the landing. All tractors and skidders are equipped with front blades and single cable drums. Only wheel skidders have rear fenders. Cables are used only for short inhaul distances, up to 15 m.
Skidtrail planning is done by a guide during operations. The guide locates felled trees and marks a way for the skidder. The most important criterion for the layout of skidtrails is avoiding water and swampy areas. Wet soil bearing capacity is not sufficient for heavy equipment and negative environmental impacts may occur.
Skidtrails can be established by both the D6H and the 528. The skidders avoid trees above 30-40 cm dbh during skidtrail construction. The skidder usually drives within 5-20 m of the felled tree and pulls the log with the winch. The maximum skidding load is usually two logs (gross volume 12-15 m3. Logs are skidded either to a road or to a temporary landing.
Landing operations consist of crosscutting, sorting, measuring, and loading logs. Since harvesting in the concession is concentrated mainly on a single species and one product (Okoumé logs in veneer quality), landing operations are rather simple and standardised.
Depending on the distance between the harvesting site and the next road, landings are established either directly alongside the road or at temporary sites in the forest. A secondary road must then connect the temporary landing site with the main road.
The logs are crosscut upon arrival at the landing. Defects at the log-ends that occur due to felling and skidding and possible decay are removed. Then all logs are measured, re-numbered and marked with paint for later recognition. Measurement is carried out by one person using a meter stick and volume tables. Volume calculation is based on the mean log diameter. Spraying of fungicides and insecticides is also done to protect against damage.
Logs are crosscut to a maximum possible transport length of 15 m. Usually the log lengths range from 8 to 15 m. Logs are loaded onto trucks by a wheeled front-end loader. Further log length optimisation takes place after transportation to the log yard of the mill.
Transportation of logs to the mill in Dolisie is by heavy duty trucks (Benz 1928/2628, payload up to 60 metric tons). A truck-trailer backpack system is used. This allows the truck to carry the trailer while travelling unloaded. The transport distance is approximately 500 km. Approximately 100 km of this distance is tarmac surface and the rest either bare soil or a layer of laterite. The national road used for the long distance transport crosses the Chaillu mountains; gradients of 12-15 % occur on short sections. Due to the relatively long transport distance, difficult road conditions, and the high risk of accidents, the enterprise puts emphasis on proper road maintenance throughout the year. This reduces repair cost for trucks and increases reliability of operations.