A focus on the last 1000 years of natural environmental changes in the tropical rainforests of West and Central Africa. Can we detect anthropogenic disturbances?

Abstract

The vast tropical rainforests of West and Central Africa were impacted by marked climatic deterioration, which mainly developed between 2500 and 2000 cal yr BP. However, the incidence of human intervention in these rainforests appears all the more speculative, although this was the period when the first stages of Bantu migration occurred towards the south. On the scale of the last millennium, which has seen increasing penetration and colonization of these Bantu peoples, one can envisage a priori a more accentuated anthropogenic pressure on the forest landscape. The combined observations of the environmental processes during the last 1000 years, as preserved in sediments from lakes, swamps, flooded forests and even oceanic environments, are presented on the basis of 24 records. The dominant number of sites in the humid forest or wooded savannas recorded only processes of relatively small size and local effects influencing vegetation cover almost simultaneously around 1000 and 500 cal yr BP; nothing is comparable with the devastating effect on a very large scale of the 2600 cal yr BP forest crisis. Conversely, other sites located in more perennial forest or savanna did not reveal any discontinuity. In all the examples of sedimentary archives considered, no out-of-phase or unusual event, possibly linked to human population and its impact, was observed. However, in more recent times, in intensive iron-ore mining/smelting areas, changes in the vegetal cover could be a legacy of charcoal production impacting forest biomass. Nevertheless, despite the clear cultural and socio-political importance of iron, its role in shaping vegetation communities in the Central African forest is generally presumed to be negligible. The oceanic causes of these climatic oscillations are more widely accepted. On a larger scale, the Medieval Warm Period and Little Ice Age are climatic anomalies visible in a number of palaeoenvironmental records worldwide and date to ∼1600–1000 cal yr BP and ∼ 700–100 cal yr BP, respectively. These events are less visible or sometimes not present in some palynological records of tropical Africa, whereas reconstructions from lacustrine records in Atlantic equatorial Africa show overall reductions in lake levels and increases in pollen belonging to light-demanding and pioneer vegetation formations in relation to these two oscillations. To reply to what was more important «the potential influence of human activities on regional climate, or the vulnerability of societies to environmental variability», it appears clearly to date, that the second paradigm is most likely. However, today, the ongoing combined efffects of climate change and human impact, including the decimation of the diversity of seed dispersers necessary to regenerate the rainforests of West and Central Africa, may have irreparable and unforeseen consequences.