The most significant reasons for deforestation

The most significant reasons for deforestation are the spread of shifting cultivation, increasing plantation agriculture and large-scale ranching, logging, mineral extraction and road and dam construction.
Only the impact of agricultural practices will be discussed here.
According to Arnold (1987) and shifting cultivation is the main cause of deforestation, causing 70, 50 and 35 per cent of the deforestation in Africa, Asia and tropical America respectively.
Data reported by Malingreau and Tucker (1988), however and suggest that these estimates are incorrect and that conversion to pasture is at least as significant as shifting cultivation.
Salati and Vose (1983), for example, believe that of the 8 x 10 6 km 2 of tropical forests that have been converted to agriculture and some 3 × 10 6 km 2 are now under shifting cultivation while 3.5 × 10 6 k 2 have been converted to pasture.
Nevertheless and the data reflect the magnitude of the impact of agriculture on tropical forest areas.
Sometimes   the movement of farmers into these regions is actively encouraged by government policies; elsewhere it is an indirect result of policies that have forced peasants off their lands in order to develop cash cropping and ranching, a process that began with colonial settlement (section 4.4).
In South America, for example, landownership especially in the developed coastal zones is concentrated in a small proportion of the population who are engaged in plantation agriculture or ranching.
As a consequence and there is a growing population of landless farmers whose only alternative to migrating to the ghettos of the major cities is to invade the forests.
Shifting agriculture is not in itself environmentally harmful, as is evidenced by the long and generally successful history that this type of agriculture has enjoyed in tropical regions.
The clearance of small areas of forest, followed by burning to release the nutrients stored in the biomass, ensures a sufficiently fertile soil to provide an adequate crop for 2–3 years.
Problems arise, however, when it becomes impossible to allow a long enough fallow period between cropping for the forest to redevelop and thus restore the nutrient content of the ecosystem.
This occurs because most tropical rainforest biogeochemical cycles involve a tight intra-system relationship between the living biomass and the litter which contains the major reservoirs of nutrients; this is in contrast to the biogeochemical cycles of temperate and boreal forests wherein the soil plays a much more significant role in nutrient recycling (see Mannion 1986e for a more complete discussion of these relationships).
More intensive use of tropical forests is occurring where pressure on land is so acute that fallow periods of insufficient duration are creating environmental degradation.
Scott (1987), for example, has examined the impact of more frequent clearance that is now being practised by the Campa Indians in the Gran Pajonal area of central Peru, where primary and secondary forest is interspersed with savanna and grassland.
The Campa Indians have been forced into the more remote regions of Gran Pajonal due to the migration of Peruvian settlers from the coastal and Andean cities.
Traditional Campa agriculture is based on garden or chacra plots which are used mainly for the cultivation of manioc, although plantains, bananas and sweet potatoes and beans are also grown on a smaller scale.
Initially, garden plots of c. 1 ha are created by felling (slashing) and the felled trees are burnt after a period of drying.
While any one group may cultivate two or three different gardens of varying ages at any one time and the nutrient stocks are rapidly depleted and within 3 years the plot is abandoned and secondary forest begins to reestablish.
Scott (1987), on the basis of biomass and nutrient data, believes that a 15-year fallow period is adequate to allow the replenishment of nutrients stocks to support a 2-3-year cycle of cultivation.
However, new colonists in Gran Pajonal are employing a fallow period of only 7 years, which is likely to cause long-term nutrient depletion so that maintenance of crop productivity will only be possible with the addition of artificial fertilisers.
Similar problems are occurring in Rondonia in the southwest of Brazil, where the Polonoroesti road-building project is attracting some 70 000–80 000 settlers per year as the forest is opened up.
The area has already lost 30 per cent of its forest cover (Earth Report 1988).
Many of the new colonists are, as Fearnside (1985) has commented, using a fallow period that is much too short for sustained agriculture.
In many cases the annual crops planted in the first few years are rapidly being replaced by pasture which itself is frequently proving to be uneconomic and environmentally detrimental.
Further large-scale deforestation is occurring in response to government-sanctioned  development projects, many of which involve cash cropping and ranching.
In Indonesia, for example and there is a government policy to encourage the migration of people from the densely populated islands of Java, Madura and Bali to the more sparsely populated outer islands of the archipelago.
This programme and titled Repelita, has been discussed by Ross (1986) who indicates that it operates via a series of 5-year plans.