Regional deforestation and stream biogeochemistry in the southwestern Brazilian Amazon basin

Rapid and large-scale deforestation in the humid tropics has led to increasing concern about its impacts on stream ecosystems and biogeochemistry. This dissertation examines the spatial and temporal patterns of the land uses associated with deforestation in a region of the southwestern Brazilian Amazon basin, and quantifies the impacts of those transformations on stream nutrient concentrations. The dissertation's central argument is that deforestation for regional economic development is fundamentally different from deforestation in small experimental watersheds, and that this difference requires theoretical development to account for the spatial and temporal scaling behaviors of human activities in a basin context.; A multi-scale, probabilistic analysis of land uses demonstrates that deforestation in small watersheds is primarily a process of vegetation conversion and cattle establishment, and that this process occurs relatively gradually over the span of more than 10 years. In watersheds larger than approximately 1000 km2, deforestation also includes urban areas that have potentially significant effects on stream biogeochemistry. Contrary to expectations based on experiments in small watersheds, stream nutrient concentrations do not increase and gradually decline following clearing, but rather increase gradually but monotonically with time. Comparison of Amazonian streams with the more industrialized south of Brazil suggests that stream nutrient impacts will likely continue to increase as the Amazon becomes more urbanized.; Investigations at the field scale demonstrate that runoff processes and nutrient transport mechanisms differ from forested conditions, primarily due to trampling by cattle. Infiltration-excess overland flow is shown to be an important process delivering nutrients to streams.; The dissertation demonstrates that vegetation conversion itself has relatively modest impacts on stream nutrients compared with other processes such as cattle establishment and urbanization that occur during regional economic development. The human impact on stream biogeochemistry is shown to be highly scale-dependent, suggesting that policies designed to prevent human impacts should differ according to the size of the targeted watershed.