Unlike temperate forests, where forest fire is essential to continued forest health and even species reproduction, fire is not a natural part of a rain forest cycle. Nearly every rain forest fire which has been observed from satellites is adjacent to human habitation, and the remainder are likely adjacent to illegal logging roads.
How soil depletion occurs in rain forests
Tropical rain forests have very poor topsoil, because most nutrients are stored directly in the trees and other plants instead. The conventional slash-and-burn techniques commonly used along the edges of the Amazonian rain forest often remove these trees first, which removes valuable biomass from the burn.
As well, the removal of the tree roots opens up this area of soil to erosion. Most nutrients which make it into the soil from the ash are likely to be washed away by the torrential tropical rain, which also increases the acidity of the soil.
As a result, slash-and-burn in rain forests usually leaves behind only enough nutrients for one to two crop seasons. After that, the soil is so depleted that nothing can grow in it.
Even in temperate forests, ongoing slash-and-burn agriculture can result in large areas of land which have been permanently deforested, as is the case in most of Southern Europe. Where the forested region is semi-arid, removing the trees will also alter the weather, which in turn will make the region even more arid. This practice can result in desertification.
Preventing soil depletion through slash-and-char techniques
In contrast to slash-and-burn, using slash-and-char techniques can make rain forest soil unusually rich. This is especially true when practiced over many years.
The difference is that instead of removing biomass from the soil, slash-and-char converts biomass into biochar, a highly stable form of organic carbon which can then be mixed with the soil. Where this practice occurs in conjunction with an active and ongoing settlement, bone and manure are also added to the soil over time.
Over many years, this combination will result in terra preta, which is among the richest soils on Earth. Terra preta is extremely stable, to the point of being the only known topsoil capable of renewing itself.
However, terra preta takes many centuries to become established. Every known patch of terra preta dates back at least 500 years. Some may be up to 2,500 years old. These patches of terra preta are the result of centuries of indigenous soil management. They cannot be quickly duplicated on deforested soil.
Restoring soil in rain forests
A short-term method of restoring soil in rain forests is to fertilize heavily. However, this must be repeated multiple times for every crop season in order to produce anything more than a subsistence crop, because the fertilizer will constantly wash away in the tropical rain. This is not economically feasible in most deforested rain forest areas.
Rain forest can be restored on degraded soil by replanting the deforested land. This is not as simple as planting a monoculture of seedlings. A dynamic, resilient forest ecosystem must be reintroduced to the land by gradually introducing new species, usually starting with shrubs to anchor the soil in place so that other species can become established. In many parts of the world, the area undergoing reforestation must also be protected from illegal charcoal harvesting. In this way, a sustainable forested area can be established in roughly 40 years.