Plantations of exotic species are a major source of wood for timber and paper industries in Africa; indigenous species (hardwoods) are used mostly for fuelwood, for charcoal production, and in traditional construction. Some indigenous species have very specialized uses, including making furniture, musical instruments, railway sleepers, and carvings. In such cases, natural stands are harvested by removing large and viable trees, with residual stands left to recover. It is likely that natural stands would take 60 or more years before they produce trees of harvestable size. Exotic species, on the other hand, can take less than 10 years to about 30 years to produce poles and sawtimber-quality trees. The most common introduced species are pines (mostly Pinus patula), eucalypts, and cypress (Cupressus spp.).
In the 1980s, fast-growing exotic plantations were regarded as the solution to Africa's dwindling fuelwood resources. Many projects performed poorly, however, because of poor species choice, lack of species trials, limited site characterization, and unforeseen pests (such as goats and termites). Conversion of extensive areas of woodland, shrub, or savannas to forest plantations has had ecological consequences that have not been fully assessed for Africa. Elsewhere, extensive plantations lead to increased transpiration and reduced runoff, so catchment yields can be reduced. Although there are signs of this process in many parts of Africa, no data have been collected. Eucalypts, in particular, have been suspected of drying landscapes and lowering water tables (FAO, 1985).
In many countries in Africa, forest plantations are established in marginal areas (to avoid competing with agriculture), and often in complex terrain (mountainsides and plateaus). Soil erosion after timber harvesting leads to siltation problems in rivers and reservoirs and can cause serious problems in hydroelectric generation. Plantations traditionally have been managed to maximize financial returns. Erosion problems associated with clear-cutting and log extraction will need to be addressed in future silvicultural planning, especially where logging is being carried out in catchment areas that are important for drinking water or hydroelectric generation.
The widespread use of exotic conifer species has increased the incidence of exotic forest pests, such as aphid pests (Katerere, 1983). These include the Eurasian pine woolly aphid, Pineus pini (Macquart); the Holarctic pine needle aphid, Eulachnus rileyi (Williams); and the European cypress aphid, Cinara cupressi (Buckton). The cypress aphid was first reported in Africa in Malawi in 1986 (Chilima, 1991); by 1990, it had spread to several countries, including Kenya (Owuar, 1991). In Africa, the aphid attacks the exotic plantations of Cupressus lusitanica, as well as indigenous species such as Juniperus procera and Widdringtonia nodiflora (Mulanje cedar)-Malawi's national tree. In Kenya, about half of the approximately 150,000 ha of plantation forests is Cupressus lusitanica. The aphid thus poses a considerable threat to the timber industry (Orondo and Day, 1994). Data are being collected to assess the extent of damage and any correlations with climatic factors in eastern and southern Africa (Chilima, pers. comm.).
Plantations may be most vulnerable to climate change through increased stress resulting from drought, which makes conditions ideal for new or old pests and diseases. In a matter of a few years, an important species can be wiped out-before control measures are developed or new species found to replace it. No data exist on how plantation species in Africa might respond to increases in CO2 concentrations. It would seem likely, though, that improved WUE associated with a CO2 fertilization effect would boost productivity.
Other reports in this collection