In slash-and-burn agriculture, the felled forest land is used to grow subsistence crops such as cassava, beans, and fruits. When these farm plots are abandoned and if they are not disturbed, they eventually return to forest, but the process takes a long time. The frequent weeding typical of slash-and-burn agriculture means that succession begins and is curtailed many times before sites are finally abandoned. These repeated weedings cause striking shifts in the composition of the regrowth. Most notably, the number of woody pioneer species declines after each weeding, whereas the density of forbs and grasses increases. Forbs and grasses are able to germinate, flower, and set seed in the interval between weedings and therefore can build up high plant densities and large seed banks. In contrast, the woody pioneer species that are established from seeds surviving the burn are weeded from the site before they have had time to produce more seeds. Because the agricultural practices of cutting, burning, and weeding largely eliminate mechanisms of on-site regeneration, the only way for forest species to establish on farms is through seed dispersal. This dependence on seed dispersal slows succession, because many of the animal species that routinely disperse seeds of forest species do not frequent large forest openings. Approximately 200 years are required for abandoned farm plots to reach mature forest proportions (Saldarriaga, 1985). That forest regeneration proceeds at all on abandoned farm sites is the result of two important factors:
Slash-and-burn clearings are relatively small (usually =1 hectare). Hence, seed dispersal distances are short.
The period of farm use is relatively short (±3 years). Hence, safe germination sites, e.g., slash piles (the shaded, moist zones along the sides of decaying logs), and soil nutrients are still available to ensure the establishment of some forest trees.
Forest regeneration on intensively used cattle pasture in Amazonia is more problematic, because the disturbances there can have the following characteristics:
They are frequently prolonged and can therefore result in a highly compacted soil and allow time for the decomposition of slash, which normally provides important microhabitats for the establishment of seedlings.
They can involve repeated burning and weeding, which eventually destroy all means of on-site woody regeneration and further homogenize the site, eliminating establishment microhabitats.
They can be very large and, hence, make seed dispersal from distant forests extremely unlikely.
Regeneration to forest occurs naturally in Amazonia following natural disturbances and small-scale human-induced disturbances, but where disturbances are severe, human intervention may be necessary to ensure reforestation. Restoration efforts are most needed on abandoned mine sites and highly degraded pastures.
The most extensive work on abandoned mines is being done by Oliver Henry Knowles, a resident ecologist for the Vale do Rio Dulce Bauxite Mining Company