Mid-Holocene vegetation dynamics with an early expansion of Mauritia flexuosa palm trees inferred from the Serra do Tepequ,m in the savannas of Roraima State in Amazonia, northwestern Brazil

摘要

Transition zones between forest and savanna in northern South America are important areas for improving our understanding of ecosystem dynamics and climate change. The uniquely available mid-Holocene sediment deposits from the Serra do Tepequem plateau in Roraima State, northwestern Brazil, were used to analyze past forest-savanna dynamics through pollen, spores, microcharcoal and loss on ignition (LOI). In this newly studied landscape, two distinct periods of vegetation, fire and climate dynamics have been recorded. The first phase from ca. 7,570 to 6,190 cal BP, with the dominance of savanna vegetation in particular with Poaceae and Cyperaceae and some small forest patches with Moraceae/Urticaceae, Alchornea and Schefflera, indicates a relatively dry period. Based on the microcharcoal concentration and influx data, frequent regional fires occurred at that time. The second phase from ca. 6,190 to 4,900 cal BP shows a change in the vegetation composition with an increase of Ilex, Schefflera and Fabaceae. In this period forest expanded, while savanna became reduced, reflecting an increase of wetter conditions. The fire frequency was markedly lower. The first occurrence of Mauritia flexuosa palm was at ca. 7,300 cal BP and an early expansion occurred at around 6,600 cal BP. This early expansion of M. flexuosa showed a development that was in opposition to the increase of fire and savanna expansion found in other regions in northern South America. The increase of wetter conditions in Serra do Tepequem in the mid-Holocene confirms other results found in savannas of Colombia and Venezuela between 7,000 and 6,600 cal BP.