People needed to get more from the land. However, the soil is poor.But if the trees are cut down (slashed) and burned the nourishment in the trees is partly changed to ash.
For a few years plants such as maize can be grown in the soil using the ash as a fertiliser. But then the soil becomes poor again and people have to go and slash and burn another area. In this way a small number of people can change a very large area of rainforest – and when they leave it, it becomes a jungle.
