Since 1989, researchers of the Goethe University Frankfurt have studied the vegetation history of West Africa, exploiting different archives and using several complementary methods. From archaeological sites in the Sahel of Burkina Faso, Nigeria und SW Cameroon and from the sediments of Ounjougou (Mali) charcoal, fruits, seeds and phytoliths have been used for vegetation reconstructions. The investigated pollen profiles come from Oursi (Burkina Faso), the Manga Grasslands and Lake Tilla/Biu-Plateau (Nigeria), Lac Selé (Benin), and Nyabessan (SW Cameroon).
The number of sites containing sediments of the cold and arid Late Pleistocene is very limited. This much is known: Between 23,000 and 11,500 cal BP the Sahara/Sahel boundary was lying 300 km further south, and rainforest only survived in a few small refugia, e.g. in SW Cameroon. After 15,000 cal BP precipitation increased, but climatic conditions remained unstable, and a distinct change of the vegetation took place only at the beginning of the Holocene, after 11,500 cal BP.
In the Early Holocene, woody plants from the South spread slowly along rivers and lakes, while the Sahelian savannas of Ounjougou and in the Manga Grasslands were characterized by annual grasses and a sparse tree cover. During the Early and Middle Holocene climatic optimum, the Biu Plateau, Ounjougou, and the Chaîne de Gobnangou (SE Burkina Faso) were covered by floristically diverse Sudano-Guinean savannas. Sudanian elements were also thriving in the Sahelian grasslands further north. Frequent bush fires kept the savannas open and prevented the large-scale development of dry forests. Southern Benin, where the Dahomey Gap separates the West African and the Central African rainforest areas today, was covered by semi-evergreen forest.
Already from 8000 cal BP, Guinean taxa disappeared from the area around Lake Tilla: a first indicator for decreasing precipitation. A distinct dry spell is visible in the pollen diagram of Lake Sélé in S Benin where savannas substituted the semi-evergreen forest after 4500 cal BP, thus creating the Dahomey Gap. Further to the North, the shift towards drier vegetation types comparable with the modern ones is dated somewhat later: Lake Tilla 4200 cal BP, Ounjougou 3800 cal BP, Manga Grasslands 3500 cal BP, Oursi 3200 cal BP, and Chaîne de Gobnangou 2800 cal BP.
The special development in SW Cameroon is illustrated by the pollen profile of Nyabessan. Between 2500 and 2200 cal BP the rainforest witnessed a serious crisis and was partly substituted by fast growing pioneer formations. Most probably precipitation was not lower during this period, but its seasonal distribution had changed, with a distinct dry season unfavourable for the growth of evergreen rainforest trees.
As a consequence of increasing Late Holocene aridity, pastoralists emigrated from the desiccating Sahara into the Sahel after 4500 cal BP. Agriculture and pastoralism led to a profound transformation of the landscape, but this is hard to detect in pollen diagrams. The development of a cultural landscape is clearly visible in charcoal spectra from archaeological sites in the Sahel of Burkina Faso, where indicators for agroforestry appear from the early first millennium AD. In the 20th century, over-exploitation and increasing aridity resulted in a distinct degradation of the vegetation, especially in the West African Sahel; numerous plant species present in archaeological samples dated before 1500 AD have disappeared from the modern Sahel.
Ballouche, A. & K. Neumann (1995): A new contribution to the Holocene vegetation history of the West African Sahel: pollen from Oursi, Burkina Faso and charcoal from three sites in northeast Nigeria. - Vegetation History and Archaeobotany 4: 31-39.
Eichhorn, B. & K. Neumann (im Druck): Holocene vegetation change and land use at Ounjougou, Mali. - In: Fuller, D. & M.A. Murray (eds.): Flora, Past Cultures and Archaeobotany in Africa. Walnut Creek (Left Coast Press).
Frank, Th., P. Breunig, P. Müller-Haude, W. Van Neer, K. Neumann, R. Vogelsang & H.-P. Wotzka (2001): The Chaîne de Gobnangou in SE Burkina Faso: archaeological, archaeobotanical, archaeozoological and geomorphological studies. Berichte zur Allgemeinen und Vergleichenden Archäologie 21, 127-190. pdf
Höhn, A. (2007): Where did all the trees go? Changes of the woody vegetation in the Sahel of Burkina Faso during the last 2000 years. – In: Cappers, R. (eds.): Fields of Change. Progress in African Archaeobotany. Groningen (Barkhuys), 35-41.
Höhn, A. & K. Neumann (2011, im Druck): Shifting cultivation and the development of a cultural landscape during the Iron Age (0-1500 AD) in the Northern Sahel of Burkina Faso, West Africa. Insights from the archaeological charcoal. Quaternary International.
Neumann, K., A. Fahmy, L. Lespez, A. Ballouche & E. Huysecom (2009): The Early Holocene palaeoenvironment of Ounjougou (Mali): Phytoliths in a multiproxy context. -Palaeogeography, Palaeoclimatology, Palaeoecology 276: 87-106.
Ngomanda, A., K. Neumann, A. Schweizer & J. Maley (2009): Seasonality change and the third millennium BP rainforest crisis in Central Africa: a high resolution pollen profile from Nyabessan, southern Cameroon. - Quaternary Research 71, 307-318.
Salzmann, U. & M. Waller (1998): The Holocene vegetational history of the Nigerian Sahel based on multiple pollen profiles. – Review of Palaeobotany and Palynology 100(1-2), 39-72.
Salzmann, U., P. Hoelzmann & I. Morczinek (2002): Later Quaternary climate and vegetation of the Sudanian zone of NE-Nigeria deduced from pollen, diatoms and sedimentary geochemistry. – Quaternary Research 58, 73-83.
Salzmann, U. & P. Hoelzmann (2005): The Dahomey Gap: an abrupt climatically induced rain forest fragmentation in West Africa during the late Holocene. - The Holocene 15(2), 190-199.
Sangen, M., K. Neumann & J. Eisenberg (under review): Climate induced fluvial dynamics in tropical Africa around the LGM? Alluvial sediments, phytoliths and sponge spicules from southern Cameroon. - Quaternary Research.