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

C Hély,A.-M Lézine,Apd Contributors

doi:10.5194/cp-10-681-2014

Abstract

Abstract. Although past climate change is well documented in West Africa through instrumental records, modeling activities, and paleo-data, little is known about regional-scale ecosystem vulnerability and long-term impacts of climate on plant distribution and biodiversity. Here we use paleohydrological and paleobotanical data to discuss the relation between available surface water, monsoon rainfall and vegetation distribution in West Africa during the Holocene. The individual patterns of plant migration or community shifts in latitude are explained by differences among tolerance limits of species to rainfall amount and seasonality. Using the probability density function methodology, we show here that the widespread development of lakes, wetlands and rivers at the time of the "Green Sahara" played an additional role in forming a network of topographically defined water availability, allowing for tropical plants to migrate north from 15 to 24° N (reached ca. 9 cal ka BP). The analysis of the spatio–temporal changes in biodiversity, through both pollen occurrence and richness, shows that the core of the tropical rainbelt associated with the Intertropical Convergence Zone was centered at 15–20° N during the early Holocene wet period, with comparatively drier/more seasonal climate conditions south of 15° N.

Highlights

During the early to middle Holocene, the so-called “African Humid Period” (de Menocal et al, 2000), parts of the hyper-arid Sahara were vegetated (e.g., Hoelzmann et al., 1998) and inhabited by humans (Kuper and Kröpelin, 2006)
Using probability density functions 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 and reached roughly 25◦ N
The maximum extent of this group clearly follows the maximum extent of lacustrine area during 6000 years, with a rapid northern expansion until 9 cal ka BP followed by a stable period of 1500 years, a southern retreat starting from 7.5 cal ka BP onward

Summary

Introduction

During the early to middle Holocene, the so-called “African Humid Period” (de Menocal et al, 2000), parts of the hyper-arid Sahara were vegetated (e.g., Hoelzmann et al., 1998) and inhabited by humans (Kuper and Kröpelin, 2006). 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”. 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. The consequence of this was the relatively diverse vegetation assemblages characterized by the co-occurrence of species whose ranges do not overlap today

Methods

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Conclusion