Are modern savannas degraded forests ? - A Holocene pollen record from theSudanian vegetation zone of NE Nigeria

A pollen record from a crater lake (Lake Tilla, 10 degrees 23'N, 12 degrees 08'E, c. 700 m asl) in the Sudanian zone of northeast Nigeria provides evi dence for the persistence of woodland savanna throughout the Holocene. Wett er conditions from c. 10,000 B.P. to c. 6800 B.P. enabled the establishment of a dense Guinean savanna, though the occurrence and rapid spread of the montane element Olea hochstelleri indicates cool climatic conditions prior to c. 8800 B.P. Patches of closed dry forest may have existed, but never co mpletely displaced the savanna vegetation. Grass fires were frequent throug hout the Holocene and were probably important in promoting the open charact er of the vegetation. From c. 6800 B.P. onwards a gradual floristic change from a Guinean to a Sudano-Guinean savanna and a lowering of lake levels po int to drier environmental conditions, which intensified around 3700 B.P. H uman impact might have caused increasing sedimentation rates from c. 2500 B .P. onwards. The pollen diagram of Lake Tilla reflects a history of the sav anna which appears to have been primarily controlled by climatic changes. T he lack of unambiguous pollen indicators might be the reason why human acti vities remain palynologically hidden even for the late Holocene.