Common causes of disturbance leading to degraded primary and secondary forest are excessive wood exploitation; overharvesting of non-wood forest products; overgrazing; and destructive natural disturbances such as forest fires, storms, and hurricanes (ITTO 2002). As a result of such disturbances, forest structure is not significantly damaged, but there may be poor unders-tory development and absence of young age classes of canopy species (especially of those that had been extracted from the forest). In some cases, there may be invasion by light-demanding species, such as grasses and bamboos. The regrowing forest may differ in species composition and in physiognomy from the original primary forest.
Degraded primary and secondary forests today represent a major source of forest products in several countries (see Chap. 5). For example, in Costa Rica, the area covered by these types of forest is estimated to be more than 600,000 ha. In the Philippines almost all the dipterocarp forests are now highly degraded. Similar situations are found in Sri Lanka, Nepal, the Brazilian Amazon, and West Africa. The World Bank has estimated that about 300 million people depend on degraded or secondary forests for their livelihoods (ITTO 2002).
Possible choices for the restoration or rehabilitation of degraded and secondary forest may include:
• Leave to regenerate (for example, as part of a conservation strategy).
• Manage for wood production or multiple use, with the same silvicultural techniques as for natural forest management (see Chap. 5).
• Manage as part of an agroforestry system.
• Enrich by planting with species of economic and/or ecological value.
We have already discussed the first three options in this and the preceding chapter. Enrichment planting is discussed in the next section.
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