The Reforestation of Africa?

Adam G. West, Guy F. Midgley, William J. Bond
2012 South African Journal of Science  
There is a global obsession with deforestation, and not without reason, given the lessons of recent human activity in tropical systems. With this in mind, a recent paper in Nature by Higgins and Scheiter 1 poses 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
more » ... '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 either grasses or trees could dominate. These bi-stable systems are highly prone to the infamous 'tipping point', an abrupt switch to an alternate stable state from which the original ecosystem does not easily recover. C 4 grassland and savanna ecosystems spread globally only in the last 8 million years 6 -a spread that was primed by an extended period of low atmospheric CO 2 levels. Today, C 4 grassland and savanna ecosystems contain some of Africa's most iconic biodiversity and support a large fraction of Africa's human population. Together with subtropical savannas in other parts of the world, these ecosystems are vast enough to have a major impact on the earth system. However, fossil fuel emissions have now driven atmospheric CO 2 levels higher than those experienced by plants for at least the last 800 000 years, 7 and possibly several million years. If emissions continue on a 'business-as-usual' path, by mid-century, CO 2 levels will exceed those last seen more than 25 million years ago -far predating the rise of grasslands and savannas. Could these elevated CO 2 levels result in a widespread and abrupt shift from grassy to woody vegetation across Africa? Higgins and Scheiter 1 attempted to answer this question using an adaptive dynamic global vegetation model (aDGVM). 8 Mechanistic models of this kind use established plant physiological mechanisms and spatially explicit climate data to simulate potential vegetation through modelling the growth of individual plants of a variety of 'functional types' (e.g. grasses, shrubs and trees), the outcome of competitive interactions between individuals, and processes such as disturbance by fire. Ultimately, vegetation types are categorised according to the relative cover and/or biomass of plant functional types. What makes their 'adaptive' approach unique, compared with other DGVMs, is that they model the ability of plants to adapt their phenology and growth so as to change their allocation of carbon to different plant compartments such as roots, stems and leaves. This model is a key advance that represents, more credibly than earlier DGVMs, the mechanisms behind the escape from the savanna fire trap. As a consequence, by turning fire on and off in their simulations, they are able to specifically identify 'bi-stable' areas which could support either trees or grasses. Using the A1B emissions scenario from the Intergovernmental Panel on Climate Change Special Report Emissions Scenarios, Higgins and Scheiter projected large shifts in 'savanna-complex' vegetation types from 1850 to 2100. Key projections were:
doi:10.4102/sajs.v108i11/12.1448 fatcat:i3bruxijjjax3de7man3sgm2ru