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

Abstract

Around 8500 cal years BP, at the time of the maximum of the African Humid Period, lakes and wetlands expanded in the presentday 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,

Humans as Agents in the Termination of the African Humid Period

“…Climate models consistently predict that changes in orbital precession weakened inland monsoon flow over the northern Sahara (>20 • N) by 8200 years BP, and this effect spread southward (to 12 • N) over the next 3500 years (Claussen et al, 1999(Claussen et al, , 2013de Noblet-Ducoure et al, 2000;Foley et al, 2003;Renssen et al, 2006Renssen et al, , 2012Liu et al, 2007;Hély et al, 2009;Watrin et al, 2009;Lézine et al, 2011b;Harrison et al, 2015). Ecological boundary conditions in regions affected by the termination of the HCO have been attributed to their position relative to the Intertropical Convergence Zone (ITCZ)-once the rain belt migrated southwards, their predominant source of moisture was substantially reduced (Liu et al, 2007;Castañeda et al, 2016).…”

Hybrid insolation forcing of Pliocene monsoon dynamics in West Africa

“…Nevertheless, δD wax studies from offshore of NW Africa covering the African Humid Period (∼ 15-5 ka) yield a robust humid signal among different records (Niedermeyer et al, 2010;Collins et al, 2013;Kuechler et al, 2013;Tierney et al, 2017), although the vegetation of this African Humid Period had no modern analogue (Watrin et al, 2009). Watrin et al emphasize that instead of a homogenous latitudinal shift of vegetation zones as a whole, individual plant species likely have an advantage over others.…”

Holocene changes in African vegetation: tradeoff between climate and water availability

“…Climate simulations from general circulation models have highlighted the role of land surface conditions including vegetation cover and open water surfaces, and their feedbacks (Claussen and Gayler, 1997;Hoelzmann et al, 1998;Krinner et al, 2012), in amplifying the influence of orbital forcing on precipitation changes and the establishment of a so-called "Green Sahara". Using probability density functions (pdf s) performed on dated paleohydrological records, Lézine et al (2011a) have shown that paleolakes related to increased monsoon rainfall during the Holocene extended up to 28 • N, while the maximum expansion of lacustrine conditions occurred at 8.5 cal ka BP (between 12 and 5 cal ka BP) and reached roughly 25 • N. Subsequently, with the dessication of this system from 7.5 cal ka BP onward, shallow water bodies and swamps became more prevalent between 16 and 23 • N. Using a similar statistical approach, Watrin et al (2009) have shown that tropical plant taxa may have migrated north by 5 to 7 • latitude compared to their modern distribution in response to increased monsoon rainfall. Instead of having moved as communities in response to climate change (Hoelzmann et al, 1998), they appear to have behaved independently, each migrating at its own speed.…”

“…Unlike Watrin et al (2009), who focused their study on a few selected taxa, we use here the complete set of pollen data from the African Pollen Database, as well as recently published pollen records (Lézine et al, 2011b). Pollen taxa have been grouped into four main phytogeographical groups (GuineoCongolian, Sudanian, Sahelian, and Saharan, respectively) in order to show the broad-scale changes in the vegetation distribution through time.…”