Plant migration and plant communities at the time of the “green Sahara”

Around 8500 cal years BP, at the time of the maximum of the African Humid Period, lakes and wetlands expanded in the present- day Sahara while large paleodrainages were formed or re-actived, in response to an orbitally-induced increase in monsoon rainfall. It has been suggested that the direct consequence of this increase in rainfall was the northward displacement of the Sahara/Sahel boundary, thought to have reached 238N in central and eastern Africa. Here, we show a more complex situation characterized by an increase in biodiversity as the desert accommodated more humid-adapted species from tropical forests and wooded grasslands: tropical plant species now found some 400 to 500 km to the south probably entered the desert as gallery-forest formations along rivers and lakes where they benefited from permanent fresh water. At the same time, Saharan trees and shrubs persisted, giving rise to a vegetation that has no analogue today. In this article, we present distribution maps of selected plant species to show both the amplitude of the vegetation change compared to the present and the composition of the past plant communities. We also estimate the migration rate of tropical plant taxa to their northernmost position in the Sahara. This study is based on the use of several data sets: a data set of the modern plant distribution in northern Africa and a data set of modern and fossil pollen sites (from the African Pollen Database, http://fpd.mediasfrance.org/ and http://medias.obs-mip.fr/apd/).