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The transgressive behaviour of the African rain forests during the two last millenia

Maley J., Doumenge C.. 2012. In : Fondation Simone et Cino d. Colloque de l'Académie des Sciences, Impact d'une crise environnementale majeure sur les espèces, les populations et les communautés : la fragmentation de la forêt africaine à la fin de l'Holocène, Paris, France, 1-2 mars 2012. s.l. : s.n., p. 44-45. Colloque de l'Académie des Sciences/Impact d¿une crise environnementale majeure sur les espèces, les populations et les communautés : la fragmentation de la forêt africaine à la fin de l'Holocène, 2012-03-01/2012-03-02, Paris (France).

The forest fragmentation period which occurred during the second part of the third millennium BP (Maley, 2002, 2012), triggered a significant expansion of pioneer forest formations, as apparent at Barombi Mbo and other sites. Several data show the more seasonal character of the climate during this perturbated phase, as in south Cameroon, with the cultivation of Pennisetum, a Sahelian cereal, by Early Iron Age people (Neumann, 2012). These pioneer forest formations favoured the forest recovery which began in early second millennium BP. Pollen data from a core collected in northern Congo revealed a vegetation history similar to that outlined previously from other sites, and in particular a brief savanna extension episode dated ca. 2500 yr cal. BP. These data reinforce the possibility that a corridor of mainly savanna could have spread briefly in the Sangha River Interval across the Central Forest Domain (Maley, 2010; Doumenge, 2012). This pollen record, which continues until the present time, shows first a rapid and significant expansion of pioneer taxa. Subsequently, from the beginning of the last millennium BP, the progressive development of numerous, more mature trees occurred, which belong to the present-day forest habitat. These data confirm the beginning of forest recovery in the early second millennium BP, a transgressive phenomenon which continues up to the present-day. This transgressive process probably stopped during the Little Ice Age, between ca. 15 to 18th centuries, as shown by pollen data from Gabon (Ngomanda, 2007). However, expansion re-started markedly near the end of the 18th c., a process that was also observed in West Africa. For example, different historical sources in the southern Ivory Coast indicate that from the end of the 18th century until the end of the 20th c. the forest expanded northwards by 50 to 80 km (Fairhead, 1998). In West Cameroon, near the savanna border, a natural oil palm belt of 10 to 20 km width and extending over ca. 150 k

Mots-clés : forêt tropicale humide; histoire naturelle; écologie forestière; dynamique des populations; afrique

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