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The purpose of this case study, which is the first one of the kind in the Congo, is to document first hand information on time and work studies as well as information provided by technical and management staff of the enterprise SOCOBOIS. The technical aspects of harvesting and their related impacts were the main consideration. These included felling, skidding, transport, wood recovery rates, and damage associated with felling and skidding. The case study was carried out in the southern forests of the Congo. In this region, harvesting concentrates almost exclusively on Okoumé (Aucoumea klaineana), an important species for veneer production for the plywood industry. The average harvesting intensity in the study area is approx. 6 m3 net log volume per hectare.

The discussion of results follows the structure of the recently published FAO Model Code of Forest Harvesting Practice.2 This approach was chosen because the model code provides a comprehensive guideline and logical structure through all aspects of forest harvesting. Although it is intended to serve as a general model for the preparation of national or regional codes of practice, it raises many important issues that are also discussed at the forest management and forest industry levels. For this discussion the model code thus serves as a checklist and the basis for proposals to reduce harvesting impacts.

Two assumptions underlie the FAO Model Code of Forest Harvesting Practice. The first is that (for forest utilisation), "it is first necessary to establish which practices are technically and economically feasible." Once this is known, the legal and political feasibility must be verified. The second basic assumption is, "that it is possible to conduct forest harvesting operations in ways that are consistent with the needs of sustainability. A necessary condition for the sustainable management of forests is that utilisation, and the activities associated with it, must not compromise the potential of forests to regenerate properly and to provide products and services that are essential for the well-being of both current and future generations. ... If it were easy, the majority of forest harvesting crews around the world would already be doing it, and they are not."3

Concerning harvesting, the model code identifies four ingredients as essential if forests are to be managed on a sustainable basis:

It should be borne in mind that, because of time constraints during the field work for this study, only some of the technical aspects of forest harvesting could be studied and analysed. Social and economic aspects of the harvesting activities were not observed in detail. Furthermore, the assessment of technical data is not easy, since standards for "good forest harvesting practices" have not yet been completely developed for most forest types and in particular for the forests under discussion. The variability of forest types and harvesting systems makes any comparison with results from other studies difficult. In the following chapter the results of the study will be discussed and important elements and facts observed during field work will be highlighted. The investigations related to the environmental impact of harvesting operations have been limited to impacts caused by felling and skidding on the residual forest.

Harvest Planning

Harvesting plans are a key element of well-managed forests. The forest harvesting plan should answer the questions: what has to be done, why should it be done, where, when, by whom, and how. Harvest plans are of two types: strategic (long-term) and tactical (short-term). Both, the strategic plan and the tactical plan should consist of a written plan and of maps of different levels of detail.

The strategic harvest plan should show or describe the following features: forest cover types, important topographic features, infrastructure, protection areas, harvesting areas, problem areas, areas of non-forest land use, communities, the planned silvicultural treatment, the types of harvesting equipment to be used, an estimate of the timber volume to be removed from each coupe, a time schedule, and descriptions of any potential problems. Detailed information concerning the forest transportation system (including road design parameters for different conditions, locations, and specifications for major stream crossings, etc.) should be part of the written strategic plan.

The short-term tactical planning normally provides details of the operations that are to be carried out in the annual coupe. Its elements should include a topographic survey, a map accurately showing the boundaries of the harvest area, the location of water courses, swamps or other areas of wet soils, gullies, rock outcrops, protected areas, and other important features. On the topographic map, special management areas in which cutting is either to be prohibited or restricted should be delineated. A detailed transportation and extraction system for the operation should be laid out. The topographical map should have the trees to be harvested marked on it. Such a plan will include the haul roads, the landings, and the skidtrails. Special attention, because of environmental problems, should be paid to streams and stream crossings. If specific harvesting equipment is to be used, this should be determined.

The only public map currently available for strategic planning in the study area is the National Geographic Map at a scale of 1:200000. This does not provide sufficient details for harvest planning. For example, the location and size of many important topographic features is inadequate. Streams, swampy areas, protected areas, and locations of communities are inadequately mapped. Accurate forest cover maps and forest inventory data are not available. Two years before harvesting starts a commercial survey (prospection) is carried out by the enterprise. Knowledge of the species composition of the forests, of their regeneration potential, and of growth dynamics of the harvested forests is limited.

For tactical planning the company prepared sketch maps for each harvest unit, consisting of important terrain features, locations of harvestable trees, and the road layout. The purpose of these maps is to reduce unnecessary movements and time loss. These maps reduce the number of surveys, increase their efficiency, and improve the information flow among survey, felling, and skidding crews. Since the sketch maps have to be done entirely by the survey crew, their accuracy is limited. Any improvement or the preparation of printed maps would be time consuming and requires experience, surveying skills, and a thorough understanding of the requirements of proper harvest planning.

Harvest planning in the study area seems to be relatively well developed in relation to the tools that are available to the enterprise and the operating crews. Roads, for instance, are located at ridge-tops, following the harvesting units with the highest timber density and avoiding streams wherever possible.

The successful intro-duction of refined har-vest-ing practices requires a broad accep-tance of methods and good motivation of the workforce. While moti-vation may be achieved through financial incen-tives, the acceptance of new methods can only be achieved through a thorough understanding of what has to be done and why it is needed.

Other undertakings where harvest planning would certainly improve current practices are: organisation of operations, reduction of harvesting damage to future tree crops, road engineering in connection with improved drainage systems, harvesting assessment (success management), and follow-up actions.

Road Engineering

According to the FAO Model Code, the objective of forest road engineering is the provision of convenient, low cost access to the forest and simultaneously minimise soil erosion, stream sedimentation, and area consumption. Protected areas of any kind are to be avoided.

Improved road engineering practices can be realised through the employment of experienced forest engineers in road surveying, construction, and supervision. The results of improved practices include minimising the total length of roads, reduction of the total area of disturbance associated with roads, avoidance of areas of wet soils and high erosion risk, minimising excavation and embankment, adequate compaction and stability of the road base, construction of adequate road drainage, revegetation of roadside slopes, avoidance of water and buffer strips, and road gradients kept near optimal. Reduced impact roading includes the utilisation of stable ridge-top locations for roads, the proper maintenance of all construction elements, and the closing of secondary roads. Ripping the surface of closed roads may be required for better revegetation.

In general, forest concessionaires in the Congo have to establish all infrastructure needed for the wood harvest of the forests, including roads, bridges, workshops, and workers' camps. Where required for long-distance transport, the enterprise must also maintain national roads. Without these additional efforts, long-distance transport would become very unreliable and slow.

Road density and area consumption are parameters typically used for the evaluation of the forest road network. For both, characteristic relationships were established (see Table 6-13). The average density of primary roads is 1.8 m/ha and for secondary roads it is 3.9 m/ha, thus 5.7 m/ha in total. The total surface consumption due to harvesting amounts to 8.4 % of the total annual coupe (see Table 6-18). The largest share of the area consumption is for felling (45%), which has relatively low impact on the soil. The next highest portion is for skid trails (32%), which can have a significant negative impact on soils and forest regeneration due to the associated soil compaction. Primary and secondary roads account for 20% of the area consumption, and landings for approximately 3% (estimated).

Some reduction in the area used for roads could be realised with a further optimisation of the road and skid trail layout (see Extraction). However, due to the transport requirements of a ground skidding system the total length of roads required for operations in a particular forest area is typically much higher than that required for other extraction techniques such as cable or airborne systems. Some significant reduction in the area used for roads might be possible through a reduced clearing width for primary roads. Currently the average clearing width is 40 m. The arguments that this clearing width is required to allow roads to dry out and to avoid road blockage by falling trees should be examined. Good tropical forest management practices suggest that harvesting operations should preferably be carried out in the dry season in order to avoid unnecessary erosion, water pollution, and damage to roads. Doing this would permit a much narrower clearing width in connection with forest roads than is currently applied.

Some erosion observed during the field work is due to inadequate road drainage. On steep road sections, cross-drains and roadside ditches would drain water quickly from the road structure and thus increase the durability of the road surface. Any investment in a well-designed road has to be seen in relation to the utilisation period of the road and its annual maintenance costs. A relatively short utilisation period, followed by a shut-down of the road, might justify relatively low road standards. But this mainly economic evaluation must also consider environmental aspects to avoid erosion and sedimentation damage.


All cutting should be done so as to ensure the safety of workers, minimise the damage to the residual stand, soils, and streams. The merchantable wood volume and the value of logs should be maximised.

In the case study three aspects of felling were examined: damages to the residual stand, total wood recovery, and felling time.

Felling canopy gaps account for 45% of the total surface area disturbance of the entire forest harvesting phase; this equals 3.8% of the annual coupe. Harvestable trees usually belong to the dominant tree layer and therefore have considerable height and crown size. Felled trees often create relatively large canopy gaps and substantial breakage of other trees. The area considered disturbed includes both the felled and broken trees. The occurrence of climbers, which tend to bridge between tree crowns, results in the breakage of neighbouring trees during cutting operations. This climber induced damage also increases the total affected area.

Felling damages were recorded on 30 felling sites. Damages associated with felling can be crown damages, bark damages, and uprooted or broken trees. On average, 17.7 other trees were damaged while felling one harvestable tree. More significant is the felling damage frequency in terms of the wood volume extracted. The number of felling damages per 1000 m3 extracted net volume is approximately 3000. Since very few climbers originate on the harvested tree, there seems to be very little opportunity for improvements through cutting climbers in advance.

Through the application of directional felling techniques, damage to future crop trees might be avoidable in a number of instances. However, only limited positive effects on the overall damage statistics can be expected from this. Certainly, directional felling would greatly increase the safety of the felling crews. Future crop trees would be better protected by marking them permanently, for instance with flagging or paint.

A significant improvement of the current practices with regard to erosion and water pollution could be achieved by avoiding felling across water courses or swampy zones. This might be difficult on steep slopes, but in most cases trees could be felled laterally or along the contour line. This would not only protect water, but also minimise timber breakage that sometimes occurs due to difficult crosscutting in situations on irregular terrain.

Wood recovery during felling was 86%. In other words, 86% of the standing stem volume was skidded to the landing site. The losses of 14% are in stump wood, which remains standing in the forest, and in stem wood that does not meet quality requirements. The possibilities for increasing felling recovery are very limited, since the stump wood is currently not considered useable. The entire stem up to the first branch could be considered for use since the curved parts of a log can often be used in the peeling process. Some unnecessary timber breakage could be reduced by the introduction of improved crosscutting techniques. Further recovery improvements might be possible through better co-ordination and feedback between mill requirements and the quality and dimensions of harvested logs. The usability of the logs in a tree should always be determined before felling starts. Through a better selection of harvestable trees, volume losses could be reduced and the economics of the transport operation would improve. Environmental impacts and harvesting economics certainly improve when trees that are largely unusable because of decay or quality are left standing in the forest. This would require a detailed assessment of the forest-mill supply chain, clear quality standards, and a permanent feedback and communication link between the mill and the forest.

The felling analysis shows the average distribution of time required for each work element (Table 6-9). It is obvious that a relatively high proportion of time (24%) is required for reconnaissance and an equally high proportion is required for topping. It can be expected that by using a tree location map the time required for reconnaissance would decline considerably. The reconnaissance time recorded in the study is the walking and searching time for the chainsaw operator only; the time that the guide needs for locating a harvestable tree is much higher. Time is lost because of misunderstandings between the survey crew and the felling crew. Improved crosscutting techniques would certainly reduce the time required for the topping operation along with reduced timber breakage. All efficiency improvements will result in higher productivity only if adequate motivation is offered and continuous supervision is secured. Without incentives and supervision the success of training efforts or organisational improvements is questionable.


The objectives of well-organised and properly supervised extraction are: optimisation of extraction productivity, safety, minimisation of soil compaction and disturbance, minimisation of water pollution, minimisation of damage to residual trees (especially to future crop trees) and the delivery of all logs prepared by the felling crew.

To achieve these objectives the FAO Model Code provides some technical recommendations on the selection of skidders and crawler tractors. Although for log extraction the use of wheel skidders is preferred compared to crawler tractors, the FAO Model Code states that: "In spite of their problems, however, it must be recognised 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."

An estimated 2.5% of all trees in the study area were damaged during skidding operations. This equals 11.5 trees per hectare or 212 trees per kilometre of skid trail. The average proportion of Okoumé damaged was 2.8%. In terms of wood volume extracted, this is approximately 2000 damaged trees per 1000 m3 extracted. Of all damaged trees, 46% were fully or partly uprooted. The total skidding damage frequency of 2.5% of all trees is directly related to the proportion of the total disturbed area attributed to skid trails, 2.7% (Table 6-10).

The number of skidding damages is strongly related to the area required for skid trails. The total number and the proportion of skidding damages could therefore be significantly reduced by a reduction of the skid trail area. The skid trail layout (Figure 6-6) shows an almost optimised connection of harvestable trees. Slight reductions of skid trail area seem to be possible through increasing winching distances and through intensified planning. This would replace the current zigzag layout by a trail system with a planned main skid trail. However, the potential for a significant reduction of the total area used for skid trails seems to be very limited (see harvest planning). The skid trail layout used by the crews in the study area results from crew experience in harvesting this type of forest, the low harvesting intensity, and the relatively easy terrain conditions.

A further possibility for reducing the number of damages associated with extraction is a critical evaluation of harvesting conditions with regard to extraction efficiency. In some cases it might be better to omit individual commercial trees that require long skidding distances or that are difficult to harvest because of steep terrain or difficult soil conditions. In this way careful harvest planning could improve the damage statistics as well as the economics of harvesting. The average skidding distance is shown for each zone of the study area in Table 6-12. The highest average skidding distance occurred in Zone C. In terms of metres per tree the skidding distance was twice as high as in the other compartments and the relative damage frequency increased accordingly. Harvest planning should determine whether harvesting isolated trees is economically justified when all technical and environmental aspects are considered.

Many damages to residual future crop trees are due to excessive blading in the skid trails. The largest share of blading damage is attributed to crawler tractors, but wheel skidders can also cause considerable damage if operator awareness or motivation is low and the trees to be protected are not clearly marked.

In some forests, the disturbance associated with forest harvesting is considered a beneficial contribution to accelerated and improved regeneration. Preliminary observations during field work did not confirm this hypothesis, at least with regard to the regeneration of Okoumé. Very few Okoumé seedlings and saplings could be found along skid trails and on felling sites in an area harvested three years ago. In the closed canopy, the density of Okoumé seedlings and saplings seems to decrease with increasing distance from the Okoumé seed tree. It is recommended that the current harvesting practices with regard to silvicultural requirements of Okoumé and its regeneration be evaluated. The long tradition of harvesting in the Congo would certainly facilitate this work, since it gives the opportunity to conduct research in areas harvested by the same enterprise up to 30 years ago.

Landing Operations

The objectives of a well designed, properly constructed, and efficiently operated landing are: safety, cost minimisation, landing size minimisation, water protection, and proper transfer of logs to the transport system.

The design and location of the landings should be established during harvest planning, preferably in connection with road planning. Temporary roadside storage is recommended where feasible. Landings should be as small as possible, taking into account the need to unhook logs from extraction equipment, sort logs, store them temporarily, and provide for the loading of trucks.

In the annual coupe, two types of landings were found: roadside landings where a relatively small log volume was skidded directly to the main road and forest landings where larger log volumes had to be collected, sorted, and loaded. In the study area only roadside landings were used. The overall area consumption for landings is relatively low; thus the potential for area reduction is limited. The total area for landings is estimated at 0.2% of annual coupe VMA95. With regard to the requirements of environmentally sound forest harvesting, the size and organisation of landings should be evaluated since landings are potential sources of water pollutants like fuel, oil, and soil sediments.

The volume recovery of the truck-ready log in relation to the standing tree volume was computed. The average log recovery was 70% of total stem, 14% stem wood loss occurred during felling operations and 16% lost occurred during crosscutting at the landing site (Table 6-8). It should be noted that the full volume (100%) used in this study is the stem volume, including the stump, up to the first branch of the crown (see Chapter 4.3).

Other studies use different assumptions for the 100% volume. A recently published ITTO study on forest recovery in four tropical countries calculates the recovery based on all parts of the tree above the ground including crown wood to a top diameter of 20 cm. In the ITTO study, the average log recovery is 53.5%. Stem cut-offs, buttresses, and the stump account for 20.2% and crown wood for 26.3% of the losses in the ITTO study. If crown wood is extracted from the calculation, the proportion extracted as logs amounts to 72.3% of the stem volume, which is comparable to the 70% total recovery obtained in this case study. If a similar proportion of crown wood is assumed for the Okoumé trees in the Congo, the recovery result would be estimated at 50% based on the total tree volume to a 20 cm top diameter.

If an increased use of stem cut-offs and stump wood is not feasible for processing, the utilisation of residues for purposes other than industrial roundwood should be considered in order to improve the overall utilisation rate of the felled trees. The problems with the utilisation of residues in this particular concession are: the remote location, the size of the residues, the associated transport problem, and the work required to break down the residues to a usable size (for fuelwood, etc.).

Harvesting Assessment

According to the FAO Model Code a harvesting assessment is a systematic check undertaken to determine the degree to which a harvesting operation has followed the harvesting plan and met its stated objectives while complying with established standards of practice.

Critical items that should be examined during the harvesting assessment are: directional felling and crosscutting techniques; compliance of harvesting operations with harvesting plan; timber volume losses; location and condition of roads, landings, and skid trails; soil disturbance; and effects of the operation on the residual forest.

Currently a systematic harvesting assessment is not part of harvesting operations in the concession area. However, together with a harvest plan, the harvest assessment is an essential requirement and pre-requisite of sustainable forest management. Based on a written plan, harvesting operations should be effectively controlled if timber production, work quality, and environmental impacts are of concern.

The development and adjustment of current practices to meet the challenges of sustainable forest management include several steps. The adjustment process could start with a thorough review of the current organisation of harvesting, including important factors that have a significant impact on operation efficiency and work quality. These factors include work planning and organisation, internal communication, incentives, motivation, and training of staff.

A second step is the identification of potential areas for the improvement of harvesting costs. Strategies for improvement have to be developed and discussed with regard to their priority, technical feasibility, and economic efficiency. The establishment of more thorough harvesting plans, in combination with a permanent harvesting assessment, should be considered.

Harvest planning would benefit from more detailed large-scale maps, which include contour lines, water courses, swamps, etc. If the company is to prepare these maps, the additional costs accruing from the planning and surveying effort have to be compared with the possible economic and environmental benefits of an improved op-eration. The combination of low harvesting intensity with a low value species certainly complicates the introduction of improved planning tools. Not only harvesting tech-niques, but silviculture, taxes, fees, working conditions, and wages are of concern. Quali-fied government authorities should be involved in the discus-sion at each stage.

2 Dykstra, Dennis P. and Heinrich, Rudolf. 1996. FAO Model Code of Forest Harvesting Practice. Rome, FAO. 85 pp.

3 Ibid. p. 5-6.

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