Extent of paleofires and past human settlements in the current rainforest patchwork of the Northern Republic of Congo

Objectives The origins of the patchwork of lowland semi-deciduous forests in the Northern Republic of Congo were apprehended. The aim of this study was to show that dense forests suffered fewer disturbances than the open canopy forest types. Old disturbances associated with fires and human settlements seem to have had and still have today a substantial impact on the physiognomy and the composition of the forest types. Understanding past forest dynamics is a major component to predict the effects of both present climate change and human activities. Method Fifteen locations were studied along a 400-km-long SW-NE gradient in the NW of the Congo Basin through the ERA-net BiodivERsA CoForChange project. An anthraco-archaeological study was carried out in association with floristic inventories of three strata. Two main geology substrates were considered: the Mesozoic sandstones and Quaternary alluvial deposits. The abundance of charcoal fragments and human artifacts were evaluated by a network of 1-m-deep boring augers (n =208) and a 1.5-m-deep reference soil pit (n =15) in each site. A rating system was used to quantify the abundance of charcoal, charred Elaeis guineensis seeds and other artifacts (ceramic and metallurgic slag). Estimations were based on 20-cm-depth intervals (augers) or on pedological layers (pits). Twelve radiocarbon dating were performed in the major disturbed layers of each vegetation type studied. Results Two main groups of vegetation were highlighted according to the relative openness of the woody stand, the importance of the woody regeneration, and the development of the herbaceous cover. The two dense forests sampled were preferentially found in the northern part: the dense forest with Manilkara mabokeensis and Haumania dankelmaniana, and the Gilbertiodendron dewevrei forest on dry land. Most parts of the understorey exhibited the woody regeneration. The southern part presented more openness including the largest areas of open canopy vegetation types. Three forest types were identified: the Macaranga barteri pioneer forest, the open canopy vegetation type with Aframomum and Marantaceae, and the sparse forest with Megaphrinium macrostchyum and/or Haumania liebrestisiana. The understorey was a dense thicket of giant herbs belonging to the families Marantaceae and Zingiberaceae, causing a very scarce woody regeneration. Regardless the auger depth, charcoals were more profuse in the soils of the southern open canopy vegetation types (2-Way ANOVA, F=5.46, p=0.02). As in pit layers, charred oil palm nuts were more plentiful in the soils of these vegetation types (Mann-Whitney test, p