Forests and woodlands are remarkable ecosystems. They have high productivity rates-over 800 gC/m2/yr in moist tropical forests (WCMC 2000)-and support rich and diverse animal and plant communities that, together, provide resources and opportunities sustaining livelihoods and commercial operations.
Forests and woodlands provide resources and environmental services at global, regional and local levels. At the global level, evapotranspiration and cloud cover over tropical rain forests play a role in maintaining a thermal balance in the earth's atmosphere. Forests also filter out pollution and are a sink for atmospheric carbon dioxide, thus helping to mitigate global climate change. Loss of forests and woodlands can contribute to local and perhaps regional climate variability (BSP 1992, Laurance 1998, IPCC 1998) because, when forest is cleared, both the albedo (proportion of solar radiation which is reflected by the earth's surface) and local temperatures rise.
It is estimated that about 50 per cent of the precipitation in the Amazon basin originates from evapotranspiration; the proportion of rainfall recycled in Central Africa may be as high as 75 to 95 per cent. Clearing of tropical forests is implicated in reductions in local rainfall in many areas, including Côte d'Ivoire and the Gambia
Clearing of forests can affect evapotranspiration and hydrological cycles because trees (and especially trees in tropical rainforests) recycle much of the rainwater that falls over an area. It is estimated that about 50 per cent of the precipitation in the Amazon basin originates from evapotranspiration; the proportion of rainfall recycled in Central Africa may be as high as 75 to 95 per cent (BSP 1992, Laurance 1998). Clearing of tropical forests is implicated in reductions in local rainfall in many areas, including Côte d'Ivoire and the Gambia (Park 1992, WCMC 1992). Deforestation can also set off a chain of events that can result in intensified droughts affecting areas in other regions or sub-regions that might be more vulnerable to an increase in dry spells. For example, it has been argued that the prolonged droughts affecting the Sahelian areas of Northern Africa are caused, in part, by the destruction of forests in Western Africa (Park 1992).
|Box 2d.1 The value of forests in Madagascar|
|Source: Kremen Niles,Dalton, Daily, Ehrlich, Fay, Grewal|
Forests and woodlands also regulate soil and water quality, protecting the soil from erosion and contributing to its fertility, intercepting rainfall and channelling run-off, and maintaining the balance of elements and nutrients in the air, soil, water, and organisms. They prevent silting of water downstream, and control the drought-flood cycles in rivers. Major hydroelectric schemes can suffer if these phenomena are disrupted, resulting in a lower capacity for power generation that can affect industries and their ability to provide employment.
The destruction of forests upstream of mangrove forests can harm the mangroves by causing increased sediment loads in rivers, and by contributing to global warming (Wass 1995). In coastal areas, mangrove forests protect coastlines and riverbanks by stabilizing sediments and controlling erosion. They also absorb the impact of waves and of storm floods, regulate salt intrusion inland and trap sand preventing its moving onto the land behind beaches. In addition, mangrove forests protect coral reefs and beaches, absorb pollution from the ocean and provide a habitat for many species of commercial fish.
The value of all of these 'ecosystem functions' is not easy to estimate in monetary terms. Furthermore, the benefits accrue to the global community and not only to local communities or users of forest resources. There is now increasing recognition of their importance and of these functions and of the need to harvest forest products on a sustainable basis in order to maintain them (FAO 1999). A recent study of forests in Madagascar has highlighted both the short-term and long-term values of different uses of forests, as detailed in Box 2d.1.
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