Ancient charcoal archives for understanding palaeofire regimes, sedimentary processes and vegetation changes in the rainforests of Central Africa: review and challenges

On the African continent, the relationship between palaeofire regimes and vegetation changes is quite well-documented in savannas or the transitional areas from savannas to forests where human-caused burning can regulate the rhythms of natural fires and reduce their size and frequency. In contrast, this relationship remains poorly known in the vast rainforest where a growing number of palaeostudies highlight the difficulties in associating societal responses with environmental changes. Considering the vast forest biome of Central Africa, the main challenge is distinguishing between climatic drivers, vegetation changes, and possible human impacts. The recurring weakness is the uncertainty of estimating past human population densities in Africa – a parameter relatively well-documented on other continents and often essential in understanding fire regimes. As such, fires and charcoal are often primarily used to frame the question of environmental changes caused by anthropogenic impacts in the past. However, this approach appears to ignore the available evidence that in early Holocene times, human densities in Central Africa were very low, with many records suggesting the major preponderance of climatic factors, and human action being considered as an uncertain hypothesis. The archaeological remains found represent a wide variety of activities, and indicate that perceived fuelwood quality depended on activity-specific requirements. Hence, the selection or non-selection of fuelwood is related to the abundance or scarcity of locally available plant biomass. Nevertheless, “cultural biases” – in terms of species selection – can mean that charcoal assemblages do not provide valid reconstructions of past vegetation. In addition, other issues with such archaeological deposits include the potential absence of age-depth relationships due to soil profile mixing (pedoturbation) resulting from biotic or abiotic agents, with the upper soil layer even containing a mixture of both recent and ancient charcoal. However, a few privileged sites have allowed the development of robust chronostratigraphies and palaeogeographic reconstructions based on in situ charcoal. • In Central Africa, the clear relationship between aridity and natural fire activity argues against human activity being the key control on charcoal fluxes. • Rainforest fires are considered to be mainly driven by climatic factors, and as a result humans will have to adapt to them spreading in the future. • Anthropogenic ignition is not the primary origin for charcoal recorded in Quaternary soils from Central African rainforests. • In Pleistocene and early Holocene times, human densities were very low in the African rainforests. • Frequently, charcoal in the Central African rainforest is not in situ , and therefore the transport and depositional processes (abiotic and biotic) require better understanding.