The term "primary forest" is used to designate a forest that has fully recovered formany past disturbance, and the structure and function resemble a forest that has never been cut. The term "secondary forest" is used for those forests that have recently regenerated following a natural (hurricanes, landslides) or human-induced (logging, clear-cutting, and replacement by agriculture or other land use) disturbance. Secondary forests in the tropics occupy larger and larger areas as the area of primary forest decreases and agricultur al areas are abandoned due to unsustainable practices. For example, in Central America, secondary forests are rapidly growing on abandoned pasture lands (Kaimowitz 1996).
The area of secondary forests has been estimated to increase at a rate of about 1 million ha/year (Achard et al. 2002). The type and rate of formation of secondary forests varies from region to region. There are two broad categories of secondary forest. One is residual forest that has been cut over once or more in the past 60-80 years. Because they have never been completely felled, they retain some of their former characteristics. The other type, called fallow or "volunteer", is forest that has regrown after being clear-cut for agriculture, pasture, timber extraction, or some other use. Because volunteer forests are composed largely of pioneer species, they lack both the structure and the composition of a mature forest. About 55% of secondary forests in the tropics as a whole are cutover forests and 45% are fallow (volunteer) (Wadsworth 1997).
Most secondary forests that developed after selective timber extraction are located in tropical Asia (47%), followed by tropical America (32%) and tropical Africa (21%) (Brown and Lugo 1990). The structure and composition of these secondary forests vary according to forest age, site fertility, previous land use, and distance from seed sources. The economic potential of each secondary forest for management for timber has to be determined with inventories that take into consideration the marketability of possible products. In general, secondary forests that originated from abandoned agricultural lands are closer to centers of human population, and therefore access and markets are not a problem. Most of the species growing as volunteers are heliophilous (light-demanding) and therefore likely to respond positively to sil-vicultural treatments such as thinning and liberation. A potential disadvantage of secondary forests as timber sources is the relatively lower market value of the timber species present, especially in comparison with primary forests. However, the markets change as the supply of more valuable timbers diminishes. Many secondary forests are good sources of fuelwood, non-timber forest products, pulp, and timber for local use.
In addition, tropical farmers have long depended on secondary forest fallows to restore productivity to land worn out by cultivation. When a secondary forest replaces a crop or pasture, the production of biomass by the vegetation and the cooler soil temperatures under the forest canopy contribute to the addition of organic matter to the soil. Typically, in most tropical humid areas, fallow periods of 5-15 years are required for soils to recover organic matter so that soils can be farmed again (Van Wambecke 1992). The type of secondary vegetation and the species composition influence the rate of recovery of soil fertility and the specific nutrient inputs to the soil. Farmers often enrich the fallow with fruit trees or other useful species, thus making use of the fallow and sometimes even accelerating the recovery of soil fertility.
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