Archaeobotany and vegetation history in SW Cameroon
Introduction
The objective of our archaeobotanical research – in close cooperation with the archaeological working group of Prof. Manfred Eggert, University of Tübingen - was to better understand the emergence of agriculture and its relationship with environmental change in the Central African rainforest during the critical period of the first millennium BC. Already in the early 1990s it was hypothesised that the expansion of Bantu speakers into the Central African rainforest had been connected to a major desiccation period iduring this period. However, there was no sound archaeological evidence and almost nothing was known about the subsistence of the first ceramic producing groups in the rainforest. Based on linguistic data it was assumed that they cultivated tuber plants and plantain. 
In cooperation with the Frankfurt University geographers (Prof. Jürgen Runge, project ReSaKo) we also investigated sediment cores from the alluvial plains of the rivers Ntem, Sanaga und Nyong. Especially significant were the pollen cores of Nyabessan, covering the first millennium BC, and several cores with sediments dated to the Last Glacial Maximum (LGM).
First millennium BC: Bantu expansion and climate crisis
Based on numerous fruits, seeds, charcoal and phytoliths from archaeological pit sites, a new unexpected scenario emerged. Charred remains of pearl millet (Pennisetum glaucum), a typical crop from the dry savannas, were found in two sites dated between 400 and 200 BC. Bambara groundnut (Vigna subterranea), another savanna crop, was also present. These plants came from more northern savanna regions, and linguistic evidence makes it highly probable that pearl millet was brought into the rainforest by Bantu speaking groups.
But how was it possible to cultivate these crops? Under modern climatic conditions with almost continuous rainfall pearl millet could not thrive in the rainforest. But in the second half of the first millennium BC, distinct climate change enabled the cultivation of savanna crops. After 500 BC, the dry season became longer and more severe, although the total annual rainfall remained high. Increasing seasonality is visible in the pollen diagram of Nyabessan: pioneer forests with Trema orientalis. Trema orientalis is a pioneer species able to withstand longer desiccation periods. However, the rainforest was not replaced by savannas: Neither in the pollen diagram nor in the charcoal samples savanna indicators could be found. Probably a mosaic of mature and pioneer forest was prevailing, with open grass vegetation only in settlement areas. substituted to a large extent the mature rainforest.
Changing climate and vegetation offered favourable conditions for the settlers. Under a longer dry season, pearl millet could be successfully cultivated, and clearing of pioneer forest is much easier than cutting the large hardwood trees of the mature rain forest. Furthermore, in the pioneer forest light demanding useful trees were thriving, such as oil palm (Elaeis guineensis) and Canarium schweinfurthii. Fruit remains of these oil producing species are abundant in the archaeological sites.
The first millennium AD
The agricultural system with pearl millet was only an episode of 200 to 400 years in the Central African settlement history. Towards the end of the first millennium BC, annual precipitation became more regular and the rain forest regenerated. Shorter dry seasons were unfavourable for the cultivation of pearl millet which was probably substituted by plantains and tuber plants. However, hard archaebotanical evidence for these crops has not yet been found in the sediments of the excavated pits
Late Pleistocene fluvial and vegetation history
The alluvia of the Ntem, Nyong and Sanaga fluvial systems in southern Cameroonian record repeated fluvial activity fluctuations during the Late Pleistocene. We applied a multi-proxy approach on alluvial stratigraphies dated between 22.4 and 13.0 ka, including remote sensing, sedimentological and morphogenetic methods, phytoliths, sponge spicules, 14C and δ13C data. A distinct NE-SW gradient of landscape and fluvial dynamics around the LGM can be drawn, with evidence for the persistence of extended fluvial rain forest refuges only in the Ntem area. The Sanaga and Nyong catchment areas were characterized by unstable vegetation subject to fire, including grasslands, woodlands, and gallery forests with bamboo thickets. In spite of increasing rainfall after 16.4 ka, persisting landscape instability played the major role for fluvial system dynamics and vegetation development until 13.0 ka, before rain forest expansion set in at the beginning of the Holocene.
Publications
Höhn, A., S. Kahlheber, K. Neumann & A. Schweizer (2007): Settling the rain forest – the environment of farming communities in Southern Cameroon during the first millennium BC. - In: Runge, J. (ed.): Palaeoecology of Africa 28. London (Taylor & Francis), 29-41. pdf
Kahlheber, S., Höhn, A. & K. Neumann (in press): Plant use in southern Cameroon between 400 BC and 400 AD. - In: Fuller, D. & M.A. Murray (eds.): Flora, Past Cultures and Archaeobotany in Africa. Walnut Creek (Left Coast Press).
Kahlheber, S., Bostoen, K. & K. Neumann (2009): Early plant cultivation in the Central African rain forest: first millennium BC pearl millet from South Cameroon. - Journal of African Archaeology 7(2): 253-272.
Neumann, K. & E. Hildebrand (2009): Early bananas in Africa: The state of the art. - Ethnobotany Research and Applications 7: 353-362. pdf
Neumann, K., K. Bostoen, A. Höhn, S. Kahlheber, A. Ngomanda & B. Tchiengué (2011, in press): First farmers in the Central African rainforest: A view from southern Cameroon. Quaternary International, doi:10.1016/j.quaint.2011.03.024
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.
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.
