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Significance

The spread of root diseases in native rainforests is usually restrained by differences in local conditions, such as soil type, plant density and diversity, and microorganisms. Clearing forests upsets this restraint. When E. J. Corner described Fomes noxius (now Phellinus noxius) as a new species in 1932, he said it was usually found in cleared or disturbed areas; recent surveys tend to support this. Since the beginning of the 20th century, many plantations of rubber, tea, cocoa, coffee, oil palm, and mahogany established on cleared forests sites have been damaged or destroyed by P. noxius.

As growing populations in tropical countries convert agricultural land to other uses, remaining native forests are cleared for plantations, large-scale farms, and subsistence agriculture (Figure 19). Tropical soils are quickly depleted of nutrients, however, and may be lost to erosion during heavy monsoon rains. Further, the presence of fungal pathogens such as P. noxius can destroy even the most carefully managed venture. Some scientists are also concerned with loss of genetic diversity, both by the open killing of endangered species and by destruction of their habitats. An estimated 70% of the world's species live in tropical forests and these forests are being cleared at the rate of 0.5 to 2% per year (Figure 20).


Figure 19		Figure 20