Secondary forests are forests regenerating largely through natural processes after significant removal or disturbance of the original forest vegetation by human or natural causes at a single point in time or over an extended period, and displaying a major difference in forest structure and/or canopy species composition with respect to pristine primary forests. Secondary vegetation is generally unstable, and represents successional stages. If undisturbed by recurrent disturbances such as grazing, tree felling, and frequent fires, secondary vegetation may slowly be invaded by primary forest trees and can eventually revert to the original type. But the speed of change depends on the frequency and intensity of disturbance, and the availability of seed parents.
On the other hand a repeated and extended disturbance may deflect the succession. For example, if a secondary forest community is subjected to fire or grazing, or if a second period of cultivation intervenes following an initial disturbance, the development may culminate in replacement of the forest by an open grassland, with or without scattered trees and bushes.
Before large areas of primary forests were destroyed by man (a very recent event), small patches in the pristine forest (mostly openings and bare spots on riverbanks) were the only habitats where secondary species could establish. When a primary forest is first logged it normally contains a high standing volume of timber, a variable proportion of which is marketable, depending upon species composition and market demand. This standing volume has accumulated over a long period. The quality and volume of commercial timber of the first cut will probably not be matched in future cuts. A return to a pristine forest is unlikely, unless the logged forest is closed to further exploitation for a century or more.
In practice the disturbance of pristine forest vegetation has been gradual. Frequencies and intensities of disturbance have built up incrementally with rapid population growth, changes in forest ownership and land use practices, and forest management systems, in response to changing market and policy forces. By the time of forest reservation, many forests had been farmed, and most forests had permanent villages or hunting camps, and in some areas a significant covering of (swidden) farms studded the forests even in the 17th century (Hawthorne and Abu-Juam, 1995). Areas close to forts and coastal trading centers in the West, Central, eastern and southern Africa had been exploited for timber and other utility products over many centuries. In some cases logs were taken from in-land areas by floatation down rivers.
Different silvicultural and management practices, including uniform, shelterwood, selection systems and salvage logging, have been tried in the region (Parren, 1991; Logie and Dyson, 1962). From the 19th century to the Second World War, only occasional large trees (1 to 1.5 m diameter) of relatively few commercial species were logged. Heavy felling occurred during and in the post-war period with expanding demand for timber in Europe and an expansion of timber industry at home. Salvage felling and creaming allowed unlimited felling of tree species in demand. In some cases this was combined with poisoning and removal of non-commercial species. Such actions posed the biggest threat to the management of secondary forests in the region.
In practice the following categories of secondary forests can be identified (Terms of Reference for this Workshop):