Farmers, smelters and caravans: two thousand years of land use and soil erosion in North Pare, NE Tanzania.

摘要

Slope deposits in North Pare provide evidence of two millennia of anthropogenically driven land clearance, soil erosion and land degradation. Drawing on deposit stratigraphy, soil magnetic parameters, stable carbon isotope composition and radiocarbon dating, three phases of soil erosion are distinguished characterized by distinct surface processes and increasing levels of agricultural land use. Onset of slope deposit formation in Pare since about 300 BC documents soil erosion as an immediate consequence of new land use practices associated with the spread of agriculture and iron working across northern Tanzania. By AD 500, slope deposits extended into valley bottoms and to middle slopes suggesting catchment-wide land clearance and soil erosion. In the 15th century AD, progressive anthropogenic soil erosion had exhausted the topsoil resource and material changes of the slope deposits reflect widespread subsoil erosion. The exposure of subsoils represents an ecological tipping point and triggered the transition to a new morphodynamic framework dominated by runoff-based erosion processes that are recorded as sand lenses and sand layers. The most recent deposits show ongoing accelerated erosion and severe land degradation whilst cessation of sand lens preservation indicates pre-colonial intensification of agricultural land use. Land use changes and socioeconomic transitions associated with the establishment of the Ugweno chiefdom and the 19th-century caravan trade are discussed as possible responses to imperceptible long-term land degradation in Pare. The study demonstrates that anthropogenic soil erosion and not external climatic drivers shaped landscape development in Pare and shows that the identification of environmental thresholds is essential for the assessment of resilience in human-dominated ecosystems.