Ecosystem effects of tropical deforestation across soil types

Few studies have explicitly examined the effects of tropical deforestation beyond the first cycle of disturbance, yet most tropical landscapes do not simply revert to a mature forest state after use. Understanding the consequences of repeated disturbance for carbon and nutrient cycling is essential to predict feedbacks between tropical forests and the global climate system. Response to disturbance is likely to vary with both soil type and climatic regime. Using four case studies, I examined changes in biomass accumulation rate and soil nutrient dynamics over two to ten cycles of shifting cultivation, the cause of half the deforestation world wide. Soils included oxisols, ultisols and mollisols in Mexico, Bolivia, Brazil and Indonesia. Annual rainfall varied from 900 to 4000 mm per year. In the latter three countries, biomass accumulation rate did not decline over two to four cycles of biomass burning, cultivation and fallow (ca. 30 to ca. 80 years). In Indonesia, biomass accumulation rate declined by 11% after six to ten cycles. In Mexico, the driest site, it declined by 38% from the first to the third cycle (ca. 35 years). Soil nutrient dynamics may explain this difference. Non-occluded phosphorus fractions doubled over the first four cycles in Indonesia but declined by 27% after three cycles in Mexico. The soils and climate differ, suggesting that drier forests are more sensitive to repeated disturbance. Alternatively, as suggested by Shang and Tiessen (1997) carbonate-rich mollisols may impede organic matter turnover, limiting phosphorus mineralization. A final possibility is that the human component of this coupled human-natural system is fundamentally different in the two realms: recent colonists in Mexico versus long-settled indigenous farmers in Indonesia.