The Holocene vegetational history of the Nigerian Sahel based on multiple pollen profiles

Four Holocene pollen diagrams are presented from interdune depressions in the Manga Grasslands (northeastern Nigeria near 13°N). These sequences are interpreted using modern pollen spectra, statistical analyses and groupings based on phytogeographical affinity (Long Distance, Sahelian, Sudanian, Guinean). The roles of climate change and anthropogenic activity in the vegetational history of the central part of Sahel are evaluated. Particular attention is given to the separation of regional (Manga Grasslands) from local (individual depressions) vegetation trends. Humid conditions in the early and mid-Holocene (from 10,000 to ca. 3300 yr B.P.) enabled the establishment of, and sustained, swamp forest vegetation in the interdune depressions. The main taxa (Alchornea, Syzygium and Uapaca) have Guinean affinities. The surrounding dunefields consisted of open savanna with Sahelian and Sudanian (e.g., Combretaceae, Detarium) elements present. Drier conditions ca. 3300 yr B.P. produced abrupt changes in pollen stratigraphy and led to the establishment of the modern vegetation of the Manga Grasslands. Although occupied since at least 3700 yr B.P., there is little evidence of human activity in the pollen diagrams. The nomadic pastoralism practiced by the human occupants of the Manga Grasslands may be palynologically undetectable. Although the Holocene vegetational history of the Manga Grasslands appears to have been primarily controlled by climate, caution should be exercised before drawing climatic inferences from these pollen sequences. For the early and mid-Holocene differences of ca. 1000 years exist in the timing of vegetation changes between the individual depressions. While water levels in the depressions are likely to be coupled to climate, the vegetation response appears to be strongly influenced by local conditions (in particular variations in the depth of depressions and so the relative height of water table). In addition, the Guinean swamp forest vegetation of the early and mid-Holocene is unlikely to be representative of vegetation trends at this latitude, but rather developed extrazonally as a result of the particular topographic/hydrological conditions prevailing in the Manga Grasslands. The problems of lags between climate and vegetation change and the presence of extrazonal vegetation, experienced in the Manga Grasslands, are likely to be common to other Sahelian pollen sites. The palynological information presently available for this zone is deemed insufficient for detailed subcontinental scale reconstructions of vegetation and climate to be attempted