Patrice Brénac

Vegetation dynamics, palaeoenvironments and climatic changes in the forests of western Cameroon during the last 28,000 years B.P.

The lake Barombi Mbo pollen record goes back to about 28,000 yr B.P. The pollen diagram based on 82 samples is subdivided into four main pollen zones. Zone I (ca. 28,000 to 20,000 yr B.P.) is characterized by relatively high frequencies of Caesalpiniaceae and also by a montane element with Olea capensis. The climate was cool and relatively wet. Zone II (ca. 20,000 to 10,000 yr B.P.). A sharp increase in Gramineae, the main non-arboreal land pollen taxon, began around 20,000 yr B.P. and lasted until 10,000 yr B.P. During this period the forest receded, giving way to a more open vegetation, but significant patches of forest (refuges) persisted in the area. This is confirmed by isotopic analyses (δ13C of sedimentary detritic organic matter from the catchment. Until ca. 13,000 yr B.P. Olea capensis was well represented indicating a relatively cool climate. Between 13,000 and 12,000 yr B.P. a warming trend associated with a strong increase in precipitation occurred. After this an abrupt reduction in precipitation linked to an increase in seasonality, but without temperature lowering, intervened between ca. 11,500 and 10,400 yr B.P. This last phase corresponds partly to the Younger Dryas time period. Zone III (ca. 10,000 to 2800 yr B.P.). After ca. 10,400 yr B.P. the climate became very wet until ca. 3000 yr B.P. A sharp decrease in the Gramineae intervened at ca. 10,000 yr B.P.; from ca. 9500 to 3000 yr B.P. they remained very low, between 0 and 3%, and the forest trees reached their maximum extension. Most of the trees exhibited large variations with quasi-periods of around 1000 to mainly 2000 yr (ca. 2200 calendar years), which could be related to large sylvigenetic or successional cycles. In this zone the Caesalpiniaceae were relatively well represented, with a maximum extension between 4500 and 3000 yr B.P. Podocarpus, a typical tree of the montane stratiform cloud forests, exhibited very low frequencies before 10,000 yr B.P. but their relative increase during the early and middle Holocene can only be explained by its growth on distant mountains. Its maximum extension phase was roughly synchronous with that of Caesalpiniaceae. The climate was warm and wet, but cooler on the mountains. Zone IV (ca. 2800 yr B.P. to present time). Around 2800 yr B.P. a sharp increase in the Gramineae, peaking at 30 to 40% of total pollen between ca. 2500 and 2000 yr B.P., indicates a sudden phase of vegetation opening and forest retreat, accompanied by severe erosion. Alchornea, a typical pioneer taxon, increased rapidly at the same time to large frequencies because it develops abundantly in all the openings. Elaeis guineensis, originally a pioneer palm tree, follows the same pattern. The climate was warm, relatively dry, and linked to an increase of seasonality. After 2000 yr B.P. the Gramineae returned to low frequencies, around 10%, associated with a strong increase in trees, indicating that the forest expanded again but not to the same extent as in the early and middle Holocene. The climate was warm and relatively wet, rather similar to the present-day climate.

Tropical forest changes during the late quaternary in African and South American lowlands

Abstract Arboreal pollen and montane elements of Late Quaternary pollen assemblages from three lacustrine cores (West Cameroon, southeastern Amazonia and central Brazil) are correlated, by the radiocarbon chronology, with other palaeoenvironmental records in Africa and South America. We observe in both continents a well-developed dense forest at 30,000 and 9000 yr B.P. The succession of vegetation types during the Late Quaternary appeared strongly related to the regional conditions: (1) the dense forest was more or less degraded depending on the regions during the last full glacial period (20,000–15,000 yr B.P.); (2) a slow increase of tree elements is evidenced in some areas during the Late Glacial (15,000–10,000 yr B.P.), whereas short-term fluctuations occurred in central Brazil during the same time; (3) a strong regression of the forest during the middle Holocene (6000–5000 yr B.P.), in the southern tropical zone of South America, was in opposition to a full forest development in Africa. In both continents two main features characterize the tropical forest evolution: (1) Montane elements developed in the lowlands during the last glacial period and in some southern or northern regions during the early Holocene; and (2) the climate seasonality was enhanced in several regions since 8500–7500 yr B.P. For a tentative explanation, we relate the cold or cool climate, inferred by palaeoecological evidences in the glacial period and glacial-interglacial transition, to polar air-masses reaching more frequently the tropical zone. This interpretation explains the apparent contradiction between the markedly low temperature of the continental lowlands opposed: (1) at 18,000 yr B.P., to the 1–2°C lower Sea Surface Temperature of tropical oceans and (2) to the global warming during the late glacial. During the middle and Late Holocene, climate evolution was mainly influenced by the latitudinal shift of the ITCZ positions in July and January and, in South America, by short-term changes of the zonal atmospheric circulation.