Heat, hydroclimate and herbivory: A late-pleistocene record of environmental change from tropical western Africa

Abstract

Fire shapes ecological dynamics across many regions of Africa today and plays a critical role in the maintenance of grass dominated (savannah) ecosystems in regions which could climatically support forest vegetation, such as the Dahomey Gap in western Africa. However, the importance of fire relative to other important factors (such as herbivory and moisture availability) remains relatively poorly quantified. Here we present new macrocharcoal data (particles >160 μm) from Lake Bosumtwi (Ghana) spanning the last c. 50 thousand years (ka). The macrocharcoal data are interpreted to provide evidence of high biomass consuming fires within the lake catchment (c. 52 km²). The macrocharcoal data are compared with previously published evidence of regional fires (microcharcoal), vegetation (pollen), herbivory (spores of coprophilous fungi), and moisture availability (δ ¹⁵N). The macrocharcoal data suggest three phases of increased fire severity (biomass consumption) during the last c. 50 ka: (i) 50–44 ka, (ii) 37–30 ka, and (iii) 26–10 ka. Covariance between high concentrations of macrocharcoal and grass pollen during these periods suggests that grass was likely providing most of the fuel load for the fires. After c. 10 ka macrocharcoal disappears from the sediments, suggesting high severity fires disappeared from the local landscape after this time. This decline in fire follows a decrease in herbivory within the landscape and occurs against a backdrop of increasing precipitation. We suggest that this combination of factors resulted in the loss of fire and contributed to the rise of a vegetation formation around Lake Bosumtwi during the Holocene that was unlike any seen in the last c. 500 ka.