The African rain forest vegetation and palaeoenvironments during late quaternary

Abstract

This review paper presents first the main pollen results on the vegetation history of the rain forest during the late Quaternary.-The Lake Bosumtwi record (Ghana) shows the disappearance of rain forest from the base of the core (ca. 28 000 yr BP) to ca. 9000 yr BP. During this time interval the vegetation was of montane type with sparse clumps of trees. There is synchronism between montane vegetation disappearance and rain forest reappearance. This phenomenon occurred abruptly around 9000 yr BP.-The Lake Barombi Mbo record (West Cameroon) shows clearly that from ca. 24 000 yr BP until the present time, rain forest persisted with limited variations, and thus, this area represents a refuge area. From these data and other, one concludes that Afromontane vegetation extended to lowland during cool and humid phases.Other palaeoenvironmental data were obtained by diverse geological analyses of the lacustrine sediments. For Bosumtwi, the relatively precise reconstruction of lake-level fluctuations permitted several palaeoclimatic interpretations for the main Holocene phases.For Barombi Mbo, the evolution of total organic carbon (TOC) and total nitrogen (TON) seems to be related mainly to temperature evolution. By comparison with present-day mountain environments, TOC and TON increase in cool environments, but decrease when warmth and humidity increase, as during Holocene time, because the recycling processes speed up in the topsoil. For the same period the alteration of the soils in the catchment produced a strong increase of kaolinite. All these change intervened ca. 9500 yr BP, which is a key date in tropical Africa.In conclusion, climatic correlations between equatorial and dry north tropical Africa illustrate how changes in the forest block must have important effects on adjacent climatic zones.

An abrupt change in the African monsoon at the end of the Younger Dryas

“…New field data supported by numerous AMS age determinations confirm that the change coincided with a major lacustrine transgression following a lowstand contemporaneous with the YD [Shanahan et al, 2006]. High lake levels after the YD [Talbot and Delibrias, 1980;Shanahan et al, 2006], and the development of a diverse forest vegetation [Maley, 1991] at the time of sapropel accumulation, indicate that the climate was humid, but the lack of well-developed lamination suggests little seasonality in the runoff that supplied clastic sediment to the lake. Furthermore, the apparent absence of significant vertical mixing indicates that surface winds were too slack and seasonal temperature contrasts too small to cause significant overturn of the water column at that time [Talbot et al, 1984;Talbot and Johannessen, 1992].…”

Palaeo-precipitation is a major determinant of palm species richness patterns across Madagascar: a tropical biodiversity hotspot

“…It is well known that the dry and cold conditions during the Pleistocene glacial periods, including the LGM have caused moderate to drastic changes in tropical rainforest distributions [13,14]; notably, rainforests in Africa retracted to fairly small regions where water availability remained sufficient, i.e. rainforest refugia [14,15,20]. Pollenbased palaeoecological reconstructions and biome modelling indicate LGM persistence of rainforest in the northeast of Madagascar [16,58].…”

“…Both cooling and precipitation changes (often drying) and low CO 2 levels associated with glacial periods have been important, causing contractions of wet lowland forest [16], the extent of which remain the subject of ongoing debate [17,18]. Remnant rainforest areas have been hypothesized as glacial refugia for many organisms, for example, in Africa [19][20][21] and the New World [22]. Empirical studies have provided indirect or less commonly direct evidence that species distributions and endemism patterns can indeed be associated with Quaternary-scale habitat stability in tropical areas [7,23,24].…”

Genetic differentiation among natural populations of the Nile tilapia Oreochromis niloticus (Teleostei, Cichlidae)

“…These climatic changes (Maley, 1991) could explain the genetic structure of some fish populations in West Africa (Agnese, 1989;Adépo-Gourène et aL, 1997). In East Africa, many tectonic movements and much volcanic activity have had a major role in the formation of the different basins and then in the distribution of fish species (Fryer & lIes, 1972;Trewavas, 1983).…”