A 37,500-year pollen record from Lake Malawcircumflex˜i and implications for the biogeography of afromontane forests.
Two 10 m cores, taken in 1986 from a trawler with a Kullenberg piston, were analysed for pollen to establish a 37 500 year pollen stratigraphy for Lake Malawi, East Africa. One core was recovered from a depth of 298 m in the Central Basin of the lake south of Mbamba Bay, while the second core was recovered from a depth of 265 m in the Southern Basin off Nkhotakota. A lowstand from 37 500 to 35 900 years BP indicated extremely dry conditions. From 35 900 to 34 000 BP montane forests were widespread indicating a cold, moist climate. Between 34 000 and 26 400 BP warm and dry conditions prevailed and forest area decreased. During the Last Glacial Maximum montane forest was widespread in the catchment, although woodlands apparently persisted at low altitudes. The expansion of the montane forest indicated that the aridity that affected equatorial Africa during this time did not affect the Lake Malawi Catchment. The cold and relatively moist conditions during the Late Pleistocene probably allowed biotic interchange between the highlands of East and West Africa via the highlands along the Zaire-Zambezi watershed and among now-isolated islands of Afromontane vegetation in the Lake Malawi catchment. The Holocene period was characterized by climate and vegetation similar to today, with indications of slightly wetter conditions between 6150 and 3000 BP and slightly drier conditions between 8000 and 6150 BP. The low percentages of montane forest pollen throughout the Holocene support the hypothesis that the montane grasslands of Malawi are not recently anthropogenic.