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Two sites situated in the Sahelian and Sudanian zones of NE-Nigeria were chosen for pollen analysis. A sediment core from an interdunal depression in the eastern Manga Grassland provides information on the Holocene vegetation history of the Sahel between c. 9600-3400 B.P. The 3 m pollen record indicates an open savanna during the mid-Holocene. The presence of Sudano-Guinean taxa, which were mainly restricted to the interdunal depressions, points to more humid conditions. Already before c. 4000 B.P., a slow change towards drier conditions and the establishment of the modern Sahelian vegetation is visible in the diagram. This development was accompanied by high fire frequencies. A 16 m core from a crater lake (Lake Tilla) in the Sudanian zone of NE-Nigeria provides a pollen record which can be dated back to approximately 11-12000 B.P. Preliminary pollen spectra show a relatively constant pattern with a dominance of grass pollen even during the middle Holocene.
La coupure que l'on constate dans les forêts denses africaines au niveau du Togo et du Bénin soulève de nombreuses questions sur les rapports forêt/savane au cours des derniers millénaires. Le projet Dahomey Gap vise à y répondre, par une approche pluridisciplinaire intégrant botanique et histoire de la végétation dans le Sud-Bénin et le Sud-Ouest du Nigéria. A partir de travaux déjà avancés sur la végétation du Bénin et des premières prospections palynologiques, quelques aspects du projet sont exposés ici: tels la caractérisation des différentes formations forestières du Bénin, les premiers résultats sur l'histoire holocène de la végétation du Sud-Bénin (mangroves, forêts, savanes) enregistrée depuis près de 7000 ans dans les lagunes.
Numerous ecologists postulate that West African savannas are mostly the result of degradation of formerly closed forests. This hypothesis can only be tested by palaeoecological investigations. The palynological results summarised in this paper document the history of the Sudanian and Sahelian savanna of NE-Nigeria during the last 11.500 years (uncal. BP). Both sites investigated provide evidence for the persistence of savanna throughout the entire Holocene. Patches of closed dry forest may have occurred, but never completely displaced the savanna vegetation. Humid conditions during the early and mid Holocene (from 10.000 BP onwards) caused a rapid spread of Guinean and Sudanian taxa into the northern vegetation zones. A slow return to drier climatic conditions between ca. 6800 BP and ca. 5500 BP can be recorded at both sites. Finally, between 3800 BP and 3300 BP a strong aridification resulted in the establishment of the modern vegetation zones. In both the Sahelian and Sudanian zone the vegetational changes appear to have been primarily controlled by climatic changes, whereas the effects of human activities remain palynologically silent even for the late Holocene.