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Vegetation degradation Vegetation degradation is usually regarded as a reduction in the available biomass, and decline in the vegetative ground cover, as a result of deforestation and overgrazing. Such degradation is a major contributory factor to soil degradation particularly with regard to soil erosion and loss of soil organic matter. The term also applies in situations where the reduction is not in the quantity of biomass but in quality - for instance bush encroachment into rangelands, and the loss of palatable pasture grasses and their replacement with nonpalatable species. In such a situation the value of the land will have declined from an agricultural point of view with a decline in its livestock carrying capacity. However the degraded vegetation may still be contributing to the soil in terms of ground cover and organic matter. In practice soil degradation usually accompanies rangeland degradation but it may not be such a clearcut relationship as that associated with deforestation. A particular form of vegetation degradation affecting significant areas within some Asia Pacific countries, notably Indonesia, Papua New Guinea, the Philippines and Vietnam, is the replacement of tropical forest with Imperata cylindrica grasslands (Scherr and Yadav 1996). This typically follows forest clearing for agriculture. Poor soil management practices then lead to the onset of soil degradation with the Imperata invasion of the farmland. It is an extremely difficult weed to control by hand labour alone and has an adverse effect on crop yields often resulting in the abandonment of the field. Deforestation in small islands of Asia and the Pacific Natural ecosystems are exceptionally important for island societies, even though their total area may be minuscule. Small island forests, for example, can be absolutely critical for life support systems, especially for water supplies, subsistence wood, food and medicine, and soil stabilisation. The watersheds of small islands are far smaller than those of the Asian continent and smaller than those on the larger islands in the Asian Archipelago nations. On these smaller islands (in both Asia and the Pacific) even slight forest degradation can destroy watershed functions (Bass and Dalal-Clayton 1995). Deforestation as a result of timber extraction and the establishment of plantation crops has led to significant anthropogenic environmental transformation within the small islands of many Asian and Pacific nations. The typical process has included (Bass & Dalal-Clayton 1995): In the Pacific the process has often been exacerbated by the following: As a result, biological productivity, diversity and resilience have diminished, and much land in many islands now lies derelict (Bass and Dalal-Clayton 1995). Agrodeforestation An environmental report on the Pacific (ADB/SPREP 1992) described agrodeforestation as a major threat to sustainable development. The term deforestation is well known in the context of the destruction or removal of forests. The term agrodeforestation covers the loss of trees from within existing agricultural landscapes, an issue that has received far less attention (Clarke and Thaman 1997). Trees are an important component of many Asian and Pacific farming systems. With a combination of both cultivated and wild species there is considerable bio-diversity to be found amongst the tree species within individual traditional farming systems. The different tree species may be: In the Pacific on the larger Melanesian islands over 100 tree species are commonly integral parts of rural and urban agricultural systems, whereas atoll systems have up to 30-40 different species. Such trees serve at least 12 distinct ecological functions, have over 70 cultural and economic uses (see List 5.1.2.1), and provide up to 70% of the real income and production of rural Pacific people. Replacing the products from these trees with imported substitutes would either be impossible or too expensive. Elimination of these trees thus constitutes a major ecological, cultural and economic disaster which would undermine self-reliance and sustainability in the Pacific islands. Agrodeforestation is a problem in Asia and the Pacific as agriculture becomes more intensive and specialised. When cultivation is undertaken by hand, retaining or planting trees within fields is not a problem, but once tractors are used, then trees hinder tillage operations and are usually removed. Within a traditional mixed biodiverse farming system the number of trees per unit area may be high, whereas the numbers of an individual species may be low, given that a farm family may be able to meet its subsistence needs for a particular product from just one or two plants. As farm families become more commercially oriented there is a tendency to specialise in the production of a more limited number of cash crops (annuals and/or perennials). This results in the removal of many existing trees as they are perceived as occupying land that could be used for more "valuable" purposes. If trees are planted then they tend to be in monocultural plantations (coffee, cocoa, rubber, coconut etc) with a resulting loss in bio-diversity and the ecological benefits of the traditional mixed home garden/farm forest. Examples of agrodeforestation can be drawn from Fiji, where trees of cultural and ecological value that were traditionally almost always protected when clearing fallow land for new gardens are now disappearing from the agricultural landscape. As reported in Clarke and Thaman 1997: |