Towards an understanding of West African montane forest response to climate change
Résumé
Aim Our aims were to provide a detailed chronology of past vegetation changes in the tropical mountains of west-central Africa since the Last Glacial Maximum and to discuss montane forest responses to climate change in terms of distribution and biodiversity. Location Lake Bambili, western Cameroon (05°56′ N, 10°14′ E, 2273 m a.s.l.). Methods Fossil pollen was extracted from a long, continuous and well-datedsedimentary sequence extending back 20,000 years. Pollen diversity and evenness estimates were based on rarefaction analysis and on Simpson's (1/D) index. Results Forest was nearly absent from high elevations during the last glacial period. It expanded gradually from 18.4 cal. kyr bp, interrupted only by reversals coincident with the Heinrich 1 (H1) and Younger Dryas (YD) climatic events, and then suddenly collapsed at 3.3 cal. kyr bp. This switch from forest to grassland communities occurred within a time span of three centuries. After 3.3 cal. kyr bp only short and taxon-poor forest phases are recorded, specifically between 2.7 and 1 cal. kyr bp and in recent centuries. Main conclusions Our data indicate that the mountain forests of Cameroon are recent, resulting from plant migrations from various sources from 18.4 cal. kyr bp onwards. Their progressive development closely followed intensification of the Atlantic monsoon, leading to the formation of a dense forest between 11.7 and 3.3 cal. kyr bp. The composition of this forest included the co-occurrence of species whose ranges do not overlap today, and indicates the potential for the development of unique iterations of climax forests through time. These forests appear to be very sensitive to Northern Hemisphere climate change, with phases of expansion/contraction coeval with the boreal deglacial signal, including widely recognized events such as H1, the YD and the 8.2 ka event. From 8.2 cal. kyr bp onwards, the progressive opening of the canopy is likely to reflect increased seasonality in rainfall related to more frequent El Niño/Southern Oscillation (ENSO) events. The final step of this history was the collapse of the montane forest at 3.3 cal. kyr bp. During the last millennium, very severe disruptions have meant that only the most rapidly dispersing species could spread, contributing to a drastic loss of forest biodiversity that has been accentuated by human impact in the last few centuries.