Silviculture in the Congo Basin

The Congo Basin has endured centuries of timber exploitation. Early production in colonial times was centered on ebony (Diospyros spp.) and African mahogany (Khaya spp.), largely focused in West Africa. Wenge (Millettia laurentii), of the legume family, is another dark wood, also IUCN endangered, with range centered more in the Congo Basin. Coastal and riverine areas were traditionally the most heavily logged.

The most commonly logged species in the Congo Basin is okoumé (Aucoumea klaineana), of the Burseraceae family.  Aucoumea is a light demanding species, and while it regenerates well in natural treefall gaps and fallow shifting cultivation fields, it does not regenerate well in selective logging systems.  Next most common in the region is sapelli, another one of the African mahoganies (Entandrophragma cylindricum). Another species of the same genus, Entandrophragma utile, has been heavily logged and no longer ranks as a top timber species. Both of these genera are members of the mahogany family (Meliaceae), but different from the “true” mahogany of the Neotropics, Sweitenia macrohylla. African teak, or iroko (Milicia excels) is another popular timber, growing more in the savannah edge of the Congo Basin, but stocks have decreased due to heavy exploitation. Many of these species are slow growing and now listed as IUCN endangered or CITES listed for restricted trade.

Because timber markets are generally restricted to these few species, most logging in the Congo basin is selective, with 1-2 trees removed per hectare, and most logging operations operate 25-40 year rotations. However, due to the slow growth of many of the selected species harvested for timber, some researchers find these rotations unsustainable for certain species such as Entandrophragma cylindricum and ayous (Triplochiton scleroxylon). Moreover, many of the commonly logged species are light demanding and do not regenerate well in the partial shade of selective logging gaps. Entandrophragma spp. are found to regenerate in the shade but need light openings in order to grow into the canopy; seeds are also vulnerable to small mammal predation. Researchers conclude that larger canopy openings and enrichment plantings are important to sustain populations of these valuable tree. Similarly, Terminalia superba, is a valuable timber species that regenerates in gaps and abandoned agricultural land, and is characteristic of secondary rain forest. Read more on ebony and other dark African woods at wood-database.com.

Other timber species such as moabi (Baillonella toxisperma) are also used for fruits, oil, and other non-timber forest products. One study found that moabi regenerated well in small logging gaps, and also responded well to enrichment plantings. Moabi is a large seeded tree, and is dependent on animals such as elephants for seed dispersal. Because of this dispersal limitation, tree species such as moabi can be particularly vulnerable to loss of dispersal animals from hunting and forest fragmentation.

Interestingly, although much of the Congo Basin forest is incredibly diverse in tree species, some forests are mono-dominant – dominated by one species. The most widespread of these is Gilbertiodendron dewevrei, which typically comprises 80-90% of total basal area in the mono-dominant forest. Researchers note that mono-dominant forest species are usually large seeded, shade tolerant, and may be restricted to areas free of major disturbance, but are unsure on other explanations for their dominance.

Finally, an important ecological component of silviculture in the Congo Basin relates to the species diversity in logged and unlogged areas. Research finds that tree diversity is similar between the two areas, but that stem density can vary significantly. Other studies find that several large mammal species including elephants and gorillas inhabit lightly disturbed areas, but others such as chimpanzees are negatively affected by logging.


Sources:

Clark, C. J., Poulsen, J. R., Malonga, R., & Elkan Jr, P. W. (2009). Logging concessions can extend the conservation estate for Central African tropical forests. Conservation Biology23(5), 1281-1293.

Doucet, J. L., Kouadio, Y. L., Monticelli, D., & Lejeune, P. (2009). Enrichment of logging gaps with moabi ( Baillonella toxisperma Pierre) in a Central African rain forest. Forest ecology and management258(11), 2407-2415.

Ezzine de Blas, D., & Ruiz Pérez, M. (2008). Prospects for reduced impact logging in Central African logging concessions. Forest Ecology and Management, 256(7), 1509-1516.

Hall, J. S. (2008). Seed and seedling survival of African mahogany ( Entandrophragma spp.) in the Central African Republic: Implications for forest management. Forest ecology and management255(2), 292-299.

Hall, J. S., Harris, D. J., Medjibe, V., & Ashton, P. M. S. (2003). The effects of selective logging on forest structure and tree species composition in a Central African forest: implications for management of conservation areas. Forest Ecology and Management183(1), 249-264.

Hart, T. B., Hart, J. A., & Murphy, P. G. (1989). Monodominant and species-rich forests of the humid tropics: causes for their co-occurrence. American Naturalist, 613-633.

Karsenty, A., & Gourlet-Fleury, S. (2006). Assessing sustainability of logging practices in the Congo Basin’s managed forests: the issue of commercial species recovery. Ecology and Society11(1), 26.

The Forests of the Congo Basin - State of the Forest 2010, Eds: de Wasseige C., et al. (2012). Publications Office of the European Union. Luxembourg. 276 p.

Ruiz Pérez, M., Ezzine de Blas, D., Nasi, R., Sayer, J. A., Sassen, M., Angoué, C., … & Yalibanda, Y. (2005). Logging in the Congo Basin: a multi-country characterization of timber companies. Forest Ecology and Management214(1), 221-236.

White, L. J. (2001). The African rain forest. African Rainforest Ecology and Conservation, 1-29.

WWF. (n.d.). Moabi. Retrieved from http://wwf.panda.org/what_we_do/where_we_work/congo_basin_forests/the_area/wildlife/plants/moabi/

Region: 
Congo