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The report presents findings of a case study on forest harvesting in natural forest of the Congo. It is part of a series of case studies published by FAO in the field of forest harvesting. This study is the first one carried out in collaboration with a private industrial enterprise.

The overall objective of the study is to contribute to the development of sustainable forest management in the tropics through the establishment of credible data on forest harvesting practices and harvesting impacts in tropical high forests.

The study aims at the establishment of reliable data on a ground harvesting system in the tropics, using power saws, crawler tractors and wheeled skidders. The study has been carried out in co-operation with a large private contractor operating a concession in the Republic of Congo. The concession consists of approximately 150,000 hectare of closed-canopy, broad-leaved forest located in the Chaillu Massif in southern Congo, at the border to Gabon. The annually harvested area is approximately 15,000 hectare. Annual rainfalls in the region average approximately 1,800 mm.

The case study consists of a study inventory, a harvesting performance study and a harvesting impact assessment. The size of the study area is 150 ha, subdivided into three harvesting compartments of 50 ha each. The terrain is slightly mountainous, with water courses and seasonal swampy areas between elevations. The average harvesting intensity is only 5-6 m3 net log volume or approximately one tree per hectare. The main commercial species is Okoumé (Aucoumea klaineana), which is a medium density timber and is used for the production of peeled veneer, mainly for plywood. The low concentration of harvestable trees played an important role in harvesting efficiency and site impact.

The study inventory of all trees revealed an average density of 455 trees per hectare greater than 10 cm dbh, of which 3.3% (15 trees) is Okoumé. The proportion of Okoumé is significantly increasing in higher diameter classes. During harvesting operations, all Okoumé trees above 80 cm dbh and showing adequate stem quality were felled and removed.

The wood volume of 96 Okoumé logs from 93 trees harvested in the study area was computed in order to establish losses during felling and bucking. The average net volume of all 96 logs is 5.8 m3. The total recovery, expressed as net log volume compared to the standing stem volume (including stump, up to the first branch of the crown) is 70%.

Out of the study area, three well defined stands of 59.5 hectare total were selected for the harvesting impact assessment. The total wood volume felled and removed from this area is 345.6 m3 net log volume, the average skidding distance is 403 meters. Crawler tractors were used for short hauls and for concentrating logs while wheeled skidders were used for the long haul to the landing.

Damages to the residual stand occur during felling, skidtrail construction and log skidding, and were classified for study as: crown damages, bark damages and uprooted or broken trees. Felling damages were recorded on 30 felling sites. The average damage frequency is 17.7 damaged trees per felled tree. Expressed in terms of damages per recovered log volume, this is approximately 3 damaged trees per cubicmetre removed. Damage to residual Okoumé trees is 3.3% with the majority of damaged stems in the higher dbh classes. The total number of felling damages of all trees is 17.3 per hectare.

Skidding damages occur with an average frequency of 11.5 trees per hectare. The proportion of Okoumé damaged by skidding is 2.8%. On average 212 trees are damaged per kilometre skidtrail, of which 5.9 are Okoumé, again concentrated in the higher dbh classes. 46% of all skidding damaged trees were fully or partly uprooted. The average number of skidding damages per felled tree is 11.8. This equals an average of two damaged trees per cubicmetre removed net log volume.

In total, the number of felling and skidding damages per hectare is 29, the number of damages per felled tree is 30 and the number of damages per cubicmetre net log volume removed is 5. The overall damage frequency for Okoumé in all diameter classes is 7.2%, and of those trees 40-80 cm dbh (immediate future crop trees) is 9%. The damage frequency for all species and size classes is 6.3%.

The soil disturbance survey revealed a total disturbed area of 8.4% of the annual coupe area. Felling sites account for 3.8%, skidtrails for 2.7%, secondary roads for 1.0%, primary roads for 0.7% and landings for estimated 0.2%. Other facilities such as worker's camps, workshops and the private airport are not regarded since they are used as the infrastructure of several annual coupes. A more detailed soil survey on felling sites, skidtrails and landings, using two disturbance classes (depending whether mineral soil is slightly or fully exposed), shows that 0.9% of the annual coupe is seriously disturbed by skidtrails and landings and other 5.8% is slightly disturbed by felling sites and skidtrails.

Under the prevailing conditions, the observed forest operation could be called "low impact" by definition due to the low removal rate of approximately one tree per hectare. However, as with any harvest operation gains are possible toward the objective of sustainable forest practice by placing high priority on harvest planning and assessment.

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