The reconstruction of African tropical grassland history during the late Holocene can be carried out using phytolith analysis. Fossil phytolith assemblages from Lake Guiers, in the Sahelian region of Senegal, and from Lake Sinnda, in the Guineo-Congolian region of Congo were investigated. The results are interpreted on the basis of modern phytolith assemblages from the same regions and compared to pollen data previously obtained. Tall or short grass associations are discriminated by their phytolith index lph(%)=saddle/(cross + dumbel + saddle), while the density of shrubs and trees is indicated by relative proportions of the dicotyledon phytoliths. The phytolith data emphasize that, in the Guineo-Congolian region around Lake Sinnda, the driest phase of the late Holocene occurred between 4000 and 1200 yr B.P., commencing with the opening of the dense forest and its replacement by a short grass savanna. From ca. 1000 yr B.P., wetter climatic conditions developed, as represented by the setting up of a tall grass savanna woodland. The modern shrub and tall grass savanna was developed ca. 700 yr B.P. In the Sahelian region around Lake Guiers, the driest phase occurred after about 2000 yr B.P. and has not been followed by moister conditions. A tall grass savanna woodland was gradually replaced by a shrub and short grass savanna which still occurs. A short period of development of swampy vegetation, which can be related to a lake level change, interrupted the semi-arid adaptation of the vegetation, between about 2000 yr B.P. and the present.

This review paper synthesizes the recent published palaeoecological results obtained in Atlantic Equatorial Africa (ECOFIT program) on the history of forest ecosystems and inferred climate changes during the past 4000 years. Evidence are mainly provided by pollen analysis carried out at nine sites from Congo, Cameroon and Ghana, locally supported by macroflora remains, phytoliths, diatoms, 13 C and mineralogical data. At all the sites, except Lake Bosumtwi (Ghana), following a large expansion of rain and mesophilous forests until 3000 years BP, a major change is registered, affecting floristic composition, structure and geographical distribution. According to the hydrological sensitivity of the different sites, local openings of the forests with development of heliophilous formations and/or isolated enclosed savannas are observed at the most humid sites; complete disappearance of forested formations at the driest. The agreement between pollen records, hydrological and hydrobiological data definitely demonstrates that an arid event has been the primary driving factor of this change and is responsable for the main features of the modern landscapes in Atlantic Equatorial Africa. Moreover, the most recent palaeoecological data obtained in Congo (Lake Sinnda), indicate that this Late Holocene increasing aridity was of longer duration, from 4000 to 1300 years BP, and more progressive than previously inferred. A new expansion of forests is locally detected c. 900-600 BP despite increased human impact.

Pollen analysis of two cores from the Lake Sinnda, located in one of the driest areas of the southern Congo, reveals a history of vegetation and climate in this region during the past 5000 yr. A major change centered around 3000–2500 yr B.P. is indicated by an abrupt decrease in forest pollen and by a corresponding increase in grassland pollen. Concurrent drying up of the lake shows that climate, in particular aridity, was the major cause of this change. This paleoclimatic reconstruction conforms with evidence for drier conditions in other parts of western equatorial Africa, such as the development of isolated enclosed savannas and of heliophilous forested formations. The aridity is recorded more fully at Lake Sinnda than at the previous studied ones. It probably lasted longer, from 4200 to 1300 yr B.P., and was more progressive than previously inferred. The aridity predates agriculture marked by pollen of the oil palm at Lake Sinnda.

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Isolated savannas enclosed by forest are especially abundant in the eastern part of the Congolese Mayombe. They are about 3000 years old, and were more extensive some centuries ago. The boundary between forest and savanna is very abrupt, as a consequence of the numerous savanna fires lit by hunters. Floristic composition and vegetation structure data, organic carbon ratios, ΔC and δC measurements presented here show that forest is spreading over savanna at the present time and suggest that the rate of forest encroachment is is currently between 14 and 75 m per century, and more probably about 20-50 m per century. As most savannas are less than 1 km across, such rates mean, assuming there are no changes in environmental conditions, that enclosed savannas could completely disappear in the Mayombe in about 1000-2000 years.

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In an area of savanna-forest (S-F) mosaic of Cameroon, at Kandara near Bertoua, an enclosed savanna bordered by young semi-deciduous forests was selected for detailed studies of vegetation and soil carbon isotope compositions with a view to estimating the rate of forest advance into savannas. Forest floristic composition and structure were analysed in small plots along two S-F transects and within two large stands. Tree species counts and basal area (BA) measurements gave convergent results defining (1) an edge forest with low BA values that forms an irregular strip parallel to the S-F border, (2) a large colonization zone zone composed of pioneer species (Albizia species, with individuals of very large diameter) and (3) a mature forest composed of abundant Rinorea individuals and large individuals of Triplochiton scleroxylon and Piptadeniastrum africanum. Carbon stable isotopes were deter mined from organic matter of soil profiles sampled at various depths in savanna, colonization-zone and mature forest. In the deep soil horizons (40-50 cm) of the colonization-zone profiles, δ13C values similar to those of the present savanna reveal the past existence of a large-tree savanna. In subsurface horizons (15-20 cm), δ13C values intermediate between those of savanna and mature forest prove the encroachment of the forest ecosystem on savanna. Using 14C measurements, the mean residence time (MRT) of soil organic matter of these last horizons was determined with precision owing to the atmospheric 14C pulse from nuclear bomb tests prior to 1964. By assuming an exponential age distribution of organic compounds and by taking account of MRT and remaining carbon from the savanna, the coloniza tion zone was found to be 60-80 y old. The age of the colonization zone being the same near the present savanna and near the mature forest, it seems that the forest advance was probably not a linear process but would result from the coalescence of Albizia thickets born in savanna.