The Dahomey Gap: an abrupt climatically induced rain forest fragmentation in West Africa during the late Holocene

Abstract

The Dahomey Gap, a savanna corridor interrupting the zonal West African rain forest, did not exist during the mid-Holocene. The pollen diagram from Lac Sélé (7°9'N, 2°26'E) indicates that in southern Benin a semi-evergreen rainforest prevailed between c. 8400 and 4500 cal. yr BP. The mid-Holocene marine transgression caused a spread of mangrove forest along the inland lagoons. Pollen analysis and geochemistry indicate that the Dahomey Gap became established at the onset of the late Holocene due to an abrupt climatic change between c. 4500 and 3400 cal. yr BP. Drier climatic conditions led to a rapid deterioration of the rain forest and subsequent spread of Sudano-Guinean savannas. A return to wetter climatic conditions between c. 3300 and 1100 cal. yr BP resulted in a rise in the lake level and a renewed spread of forests into the savanna. During this time the Dahomey Gap consisted of a forest-savanna mosaic with a high number of pioneer tree taxa including the oil palm Elaeis guineensis After c. 1100 cal. yr BP the lake level dropped again and the Lac Sélé profile indicates drier environmental conditions resulting in the establishment of an open savanna which persists until present. The palaeorecord from Lac Sélé suggests that the role of humans in shaping the West African savannas has been overestimated. The opening of the Dahomey Gap and spread of the oil palm E. guineensis can now be confidentially attributed to climatic change and was not initiated by humans.

Five thousand years of tropical lake sediment DNA records from Benin

“…BP, evident in a 287 more or less constant fine sedimentation due to this lower rainfall. The pollen 288 assemblages presented in Salzmann and Hoelzmann (2005) also confirm a high lake 289 level between 3,000 years cal. BP and 1,000 years cal.…”

“…The pollen counts used for comparison with sedDNA were previously published by 234 Salzmann and Hoelzmann (2005). Our sediment core was collected from the same part 235 of the lake.…”

Pastoralism and Socio-technological Transformations in Northern Benin

“…Benin Republic is part of a fragmentation within the Guinea rain forest block, known as "Dahomey gap". This is a forest relic causing climate anomaly with a decline in annual rainfall, a reduction of the sea level and air surface temperatures (Booth, 1958;Salzmann and Hoelzmann, 2005). During the period 1971-2000, the number of rainy days averaged 140 in the south and 80 in the extreme north of the country (Boko et al 2012: 3).…”

Tree species composition and stand structure of woody savanna in Dahomey Gap

“…Annual precipitation of 1000-1200 mm in the Dahomey Gap is much smaller than surrounding rainforests with more than 2000 mm (in Nigeria and Cote dʼIvoire). The cause of this phenomenon remains under debate in palaeoclimatology and palaeogeography (Dupont andWeinelt 1996, Salzmann andHoelzmann 2005).…”