Holocene vegetation history of Northeast Nigeria
Sahel zone: Manga Grasslands
The pollen profiles from the Manga Grasslands show that the contrast between open grasslands on the dunes and dense gallery forests in the wetter depressions already existed in the Early Holocene. The physiognomy of the vegetation remained more or less similar in the course of the Holocene, but its floristic composition changed in concert with climatic fluctuations. During the Early Holocene, after ca. 11,500 cal BP, swamp forests with Guinean elements were dominating in the depressions, while open savannas with Sudanian and Sahelian taxa prevailed on the dunes.
The combination of elements from different phytogeogeographical regions is a special case and can be explained with the physio-geographical conditions of the region. Due to the high groundwater table, an extrazonal vegetation with Syzygium and Uapaca established at the edge of the depressions. In spite of higher rainfall, the light and permeable sandy soils on the dunes prevented the growth of denser vegetation. At the transition fom the 7th to the 6th millennium cal BP, Guinean taxa decreased in the dune depressions, indicating growing aridity. After 3500 cal BP, the aridfication trend intensified, eventually resulting in a Sahelian vegetation comparable with the modern one.
Sudanian zone: Lake Tilla (Biu-Plateau)
In the 1980s, many vegetation ecologists were convinced that most West African savannas are man-made, degraded from former dry forests through shifting cultivation, anthropogenic bush fires, grazing and fuel wood cutting. However, the palynological data from Lake Tilla show that the existence of Sudanian savannas is not due to agriculture and pastoralism. Already at the beginning of the Holocene, savannas were dominating on the Biu Plateau, indicated by high percentage values of grasses and typical savanna trees. Regular bushfires kept the vegetation open and resulted in abundant charcoal in the lake sediments. From the charcoal alone, however, it is not possible to determine if the bushfires were natural or anthropogenic events.
At the beginning of the Holocene, after 11,500 cal BP, the savannas of the Sudanian zone harboured numerous Guinean species, probably in a mosaic with woodlands. However, large dry forests or semi-evergreen forests can be excluded even for the most humid periods of the Early and Middle Holocene. After 8000 cal BP aridity increased, the savannas became more and more dominated by Sudanian taxa, and lake levels decreased. After 4200 cal BP this trend intensified. Even for the Late Holocene, human impact on the vegetation cannot be proven unambiguously. This is due to the open character of the savannas where human and climatic factors cannot be clearly separated. Furthermore, clear anthropogenic indicators among the pollen types have not yet been identified for the West African savanna zone.
Publications
Neumann, K., K. Hahn-Hadjali & U. Salzmann (2004): Die Savannen der Sudanzone in Westafrika – natürlich oder menschengemacht? In: Albert, K.-D., Löhr, D. & Neumann, K. (eds.): Mensch und Natur in Westafrika. Abschlussbuch des Sonderforschungsbereichs 268: 39-68. Weinheim (Wiley/VCH). pdf
Neumann, K. & U. Salzmann (2001): Feuerökologie der westafrikanischen Savannenlandschaft. Würde die Savanne ohne Brand existieren? - In: Busch, B., Goldammer, J.G. & Denk, A. (eds.): Feuer. Schriftenreihe Forum 10: 280 – 289. Köln (Wienand).
Salzmann, U. (2000): Are savannas degraded forests? - A Holocene pollen record from the Sudanian zone of NE-Nigeria. – Vegetation History and Archaeobotany 9, 1-15.
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.
Waller, M. & U. Salzmann (1999): Holocene vegetation changes in the Sahelian zone on NE Nigeria: The detection of anthropogenic activities. – Palaeoecology of Africa 26, 85-102.
Dissertation
Salzmann, U. (1998): Zur holozänen Vegetations- und Klimaentwicklung der westafrikanischen Savannen. Paläoökologische Untersuchungen in der Sahel- und Sudanzone NO-Nigerias. Doktorarbeit. Julius-Maximilians-Universität, Würzburg. Berichte des Sonderforschungsbereichs 268, 13, 1999.
