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

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 km, began to develop during the second part of the 19th century. Subsequently, a semi-deciduous forest began to develop inside this pioneer formation. As a result, this belt now comprises a mixture of oil palms and semi-deciduous trees, which corresponds to the last major expansion phase of the Forest Domain (Maley, 2001). Data show that present-day transgressive forests progressively reduce the areas of pre-existing savannas, thereby demonstrating that former savannas were more extensive. Moreover, botanists studying present-day vegetation have provided additional evidence for savannas in adjacent sectors: north and south of the Central Congo-Guinean Forest Domain, as in southern Cameroon at Akonolinga, east of Yaoundé; at Kandara, south of Bertoua; and also in the central part of the Congo RDC. In 1985, Letouzey published a vegetation map that was in part derived by superimposing satellite images obtained in the early 80s with the first aerial photographs taken in 1950. Letouzey calculated that south of the Adamaoua Plateau, the Forest Domain increased by more 1 million hectares. Furthermore, numerous savanna fragments, enclosed within the forest, existed near the junction of the Kim and Mbam rivers (5° 20' N - 11° 20' E), where a large forest expansion was also calculated by using the same method. Compared to 1950, in 1989 the forest area increase was 21%, linked to a savanna decrease of 19%, about half that existing in 1950 (Youta Happy, 1998). Forest recovery can be very fast, as observed recently in the large savanna area remaining at La Lopé, central Gabon, close to the equator. Indeed, an aerial photograph taken in 1995 showed the complete burning of a fragment of savanna encircling the small lake of Kamalété, where coring was undertaken. The fire killed numerous trees at the forest margin. In 1999 a process of forest regeneration was already underway at the margin, characterized by the rapid growth of Anthocleista schweinfurthii, a pioneer tree with larg