Fig. 1. Effects of repeated slash-and-burn cultivation practices on vegetation and the carbon and nutrient contents of soil and vegetation. The average effect of the repeated slash-and-burn cultivation cycle (generally 1 yr cultivation, 5 yr fallow) on carbon and nutrient stocks (nitrogen, phosphorus, potassium, calcium, and magnesium) can be measured in vegetation and soils along a degradation gradient from rain forest to shrub fallow, mixed fallow, degraded fallow, and grassland (Brand and Pfund 1998). The time taken to pass from one stage to another is approximate, based on interviews with farmers about past cultivation cycles. The first threshold is the transformation of forest into agricultural land. The organic matter in the soil increases, but the nutrients stored in the vegetation are mainly lost. The second threshold is the appearance of invasive species. The third threshold is decisive because the soils become depleted and it is impossible to cultivate rice, mainly because of changes in texture and aluminum toxicity.