Past tree cover of the Congo Basin recovered by phytoliths and δ13C along soil profiles

Abstract

Abstract The tropical moist forests (TMF) of the Congo Basin are extremely diverse in terms of structure and functional diversity. Previous paleoecological work suggests that these forests have experienced dramatic changes over the last millennia, related to climate or humans. These disturbances still influence today's repartition of forests and savannas as well as species distributions. The objective of this study is to explore the sensitivity of phytolith assemblages, compared to the δ 13 C of soil organic carbon (SOC), to reconstruct past tree cover of the present TMF. Large transects across different geological substrata and forest communities were explored. In total, 18 soil profiles were investigated for pedogenic features, and 53 radiocarbon dates from SOC and charcoals were obtained. Phytolith extractions from modern soils and along four soil profiles were performed. The δ 13 C of SOC and phytolith assemblages were interpreted in terms of tree cover changes. One of the most interesting results highlighted by this study was the strong match between phytolith tree cover and the δ 13 C signal; as it not only aids the interpretation of δ 13 C soil signals, but also puts into question the transport behavior of phytoliths in soil by processes that are still poorly understood. The δ 13 C SOC method has been successfully used to study major vegetation changes in Africa, but has never been previously constrained with another proxy of tree cover. This study confirms the value of a multi-proxy approach to investigate past vegetation changes in African tropical moist forests, and shows that SOC δ 13 C and phytolith signatures in soil profiles display the same dynamics and are complementary. They suggest that the majority of the present forested sites never experienced a true savanna phase.