The information used for the documentation is bound to the following subjects: Palynology, Paleobotany, deep-sea-cores, and isotope analysis. The locations of the used data giving direct and indirect information about the vegetation cover during the LGM are shown in the data-base page.
Because of the rare data concerning the LGM the amount of precipitation with its annual distribution had to be calculated, derived from SST reconstructions following dated foraminifers and radiolarians (PFLAUMANN & SARNTHEIN personal communication, Schneider 1991). During winter (NH) the SSTs reached 24-26°C and were therefore about 2°C lower than today. But in summer (NH) the temperatures were approximately 5-6°C lower than today with an average of 17-18°C. Particularly coastal areas between 5°S and 5°N revealed a precise reduction of athmospheric water vapour content during summer. The areas between 5° and 10°S are characterized by a smaller reduction of water vapor as the higher SSTs of SH-summer coincide with its rainy season. Therefore the precipitation losses must have been approximately 20-30% lower (Lautenschlager 1991).
On the basis of the reconstructed climatic water-budget and in relation to the present climate - vegetation condition a map showing the distribution of vegetation for the LGM is derived (ANHUF 2000).
The transition from open dry forests, today found in the central and southern Sahelian areas, to open tree savannahs was shifted to 13°N at the western coast, almost to 10°N in the northeastern edge of Ivory Coast and to 12°N in the region of Kaduna (Nigeria). This means that it lies further south than today (3-4°). The transition from grass savannahs to the diffuse vegetation of the southern Saharan border lies at 15°N in Senegal and at nearly 14°N in the central part of todays Sudanian Zone (north of Kano/Nigeria). This northern frontier of the grass savannah in West Africa is comparable to the southern border of moving sand dunes during the Pleistocene (Talbot 1984).
The 1,800 mm precipitation limit of the LGM most probably coincides with the present locations of the 2,600 mm isohyetal line. Thus there was only a very narrow strip of land east of Douala (Cameroun) reaching southward until Libreville, in which ombrophile and hyper-ombrophile evergreen rainforests could survive. The borderline of the semi-deciduous rainforests may have extended to a line approximately half way between Douala and Yaounde at about 4°N. Southwards these forests were able to expand until Port Gentil (Gabon) along the coast whereas towards the interior of the country they reached as far as Brazzaville (Congo). Also in the eastern central area and the southeastern edge of the Congo-Basin the conditions must have been similar with enough humidity for the existence of semi-deciduous rainforests.
The major part of the Congo-Basin and an approximately 400-600 km wide strip along the coast of Angola as well as southeastern Zaire - in accordance with the reconstruction of the precipitation - was dominated by open and partly by more dense dry forests. Open grass expands into the area of river systems that might be interpreted as flooded tributaries of the Congo and the Ubangi.
Open dry forests dominated the entire East African Highlands. Merely along the edge of the Western Rift and northeast of Lake Victoria remains of semi-deciduous rainforests could be found during the LGM. The tree- and grass savannahs of the generally dry highlands were only interrupted by islands of dry montane forests. The aridity increased towards the east. The Danakil-Plain, the Ogaden, large parts of Somalia as well as the northeastern part of Kenya were dominated by a desert-like climate. The sub-alpine and alpine zone of the mountain region was clearly shifted down up to 1,000 m below their contemporary position covering considerably larger areas than today.
Between 20,000 and 18,000 yr BP the vegetation in southeast Tunisia was obviously drier than today. An open steppe vegetation dominated by Artemisia spp. covered the entire area between the Dorsale and the Mediterranean coast. In the North, the steppes reached as far as to the Mediterranean sclerophyllous forests (Brun 1979, 1985), today receiving annual precipitation amounts of 400 to 500 mm. The climatic conditions during the LGM caused a considerable precipitation decrease in central and southern Tunisia. The present-day 400 mm isohyete more or less equates the 200 mm isohyete of that period. At the west side of the Middle Atlas annual precipitation did not exceed 400 to 500 mm (today 930 mm), but the coastal plains north of the Middle and High Atlas Mountains were considerably more humid. The southernward shift of mid-latitude wind-belts caused a luff-position of the north-west African coastal areas therefore receiving sufficient rainfall even under generally cooler climatic conditions. Pollen of evergreen oaks, in Morocco even deciduous oaks, cypresses, and pines appear in the profiles. The altitudinal vegetation belts of the Atlas orogen are composed of very humid, humid and semi-dry formations along the northern exposition and generally drier formations along the leeward positions.
Go to data-base for 18,000 yr BP
© 2000 Department of Physical Geography, University of Mannheim
Editor: Birgit Schröder