The Reforestation of Africa?

Abstract

1poses a challenging question for African ecologists and environmentalists: do we, in the subcontinent, face not a contraction, but a vast and inevitable expansion of subtropical tree cover, driven by levels of CO 2 that have not been seen in the past several million years? Higgins and Scheiter’s paper warrants our attention because it projects, for the first time, the continent-wide implications of the decade-old hypothesis, originally formulated by South African ecologists, of CO 2 -driven woody expansion in fire-prone savannas. 2 The paper supports concerns that the expansion of woodlands and forest may be an imminent threat to ecosystem structure, function and biodiversity across extensive landscapes in the sub-continent. 3 If its projections are correct, then we stand on the brink of massive ecosystem change in the ‘savanna–complex’ vegetation (i.e. tropical grasslands, savanna and forests) of Africa. But how credible are these projections? Unlike more intensively researched temperate ecosystems, the vegetation structure and land cover of huge tracts of sub-Saharan Africa may be highly sensitive to increasing levels of atmospheric CO 2 . Vast areas of the subcontinent are currently dominated by C 4 grasses – a photosynthetic mode that owes much of its competitive advantage to the low CO 2 levels of pre-industrial and, even more so, glacial times. 4 Grasses do not require the large amounts of carbon that woody plants do to support their photosynthetic tissue. This low carbon demand for growth allows grasses to outcompete woody plants under low CO 2 conditions by building up a flammable layer of grass fuel – the savanna fire trap – that immolates slower growing woody plants and maintains the system in its grassy state. Under high levels of CO 2 , trees are thought to regain the advantage, escaping the fire trap and converting the system into forest. 2 This mutable balance of trees versus grasses, mediated by atmospheric CO 2 levels and fire, results in ‘bi-stable’ systems 5 in which