S ub -S aharan A frica : F ood P roduction at R isk
and the relative vulnerability of large and small farms underline the inadequacy of the current understanding of the impacts that climate change may have on pastoral systems. The impacts on forage yields and livestock sensitivity to high temperatures and associated diseases, however, do highlight the sector´s vulnerability to climate change.
Projected Ecosystem Changes The impacts on livestock described in the previous section are closely tied to changes in natural ecosystems, as changes in the species composition of pastures affect livestock productivity (Thornton et al. 2009; Seo and Mendelsohn 2007). Processes, such as woody plant encroachment, threaten the carrying capacity of grazing land (Ward 2005). Thus, food production may be affected by climate-driven biome shifts. This is a particular risk to aquatic systems, as will be discussed below. Africa’s tourism industry highly depends on the natural environment; it therefore is also exposed to the risks associated with climate change. It is currently growing at a rate of 5.9 percent compared to a global average of 3.3 percent (Nyong 2009). Adverse impacts on tourist attractions, such as coral reefs and other areas of natural beauty, may weaken the tourism industry in Sub-Saharan Africa. It is believed that bleaching of coral reefs in the Indian Ocean and Red Sea has already led to a loss of revenue from the tourism sector (Unmüßig and Cramer 2008). Likewise, the glacier on Mount Kilimanjaro, a major attraction in Tanzania, is rapidly disappearing (Unmüßig and Cramer 2008).
Terrestrial Ecosystems Sub-Saharan Africa encompasses a wide variety of biomes, including evergreen forests along the equator bordering on forest transitions and mosaics south and north further extending into woodlands and bushland thickets and semi-arid vegetation types. Grasslands and shrublands are commonly interspersed by patches of forest (W J Bond, Woodward, and Midgley 2005). Reviewing the literature on ecosystem and biodiversity impacts in southern Africa, Midgley and Thuiller (2010) note the high vulnerability of savanna vegetation to climate change. Changes in atmospheric CO2 concentration are expected to lead to changes in species composition in a given area (Higgins and Scheiter 2012). In fact, during the last decades, the encroachment of woody plants has already affected savannas (Buitenwerf, Bond, Stevens, and Trollope 2012; Ward 2005). The latter are often unpalatable to domestic livestock (Ward 2005). Grasslands and savannas up to 30° north and south of the equator are typically dominated by heat tolerant C4 grasses and mixed tree-C4 grass systems with varying degrees of tree or shrub
cover (Bond et al. 2005), where the absence of trees demarks grasslands in contrast to savannas. Forest trees, in turn, use the C3 pathway, which selects for low temperatures and high CO2 concentrations (Higgins and Scheiter 2012). However, William J. Bond and Parr (2010) classify as savannas those forests with a C4 grassy understory that burn frequently. At a global scale, the rainfall range for C4 grassy biomes ranges from approximately 200 mm mean annual precipitation (MAP) to 3000 mm MAP, with tree patches associated with higher precipitation (Bond and Parr 2010). According to Lehmann, Archibald, Hoffmann, and Bond (2011), however, the wettest African savanna experiences 1750 mm MAP.
The Role of Fire Fires contribute to the stability of these biomes through a positive feedback mechanism, effectively blocking the conversion of savannas to forests (Beckage, Platt, and Gross 2009). C4 grasses are heat-tolerant and shade-intolerant, such that a closed tree canopy would hinder their growth. Efficient growth of C4 plants at high growing season temperatures allows for accumulation of highly flammable material, increasing the likelihood of fire that in turn hinders the encroachment of woody plant cover. Fire-promoting ground cover is absent in the humid microclimate of closed canopy woods, further stabilizing these systems (Lehmann et al. 2011). A further factor promoting the wider spread of savannas in Africa compared to other continents is the prevalence of mega-herbivores, as browse disturbance reduces woody plant cover in arid regions (Lehmann et al. 2011). However, grazing and trampling simultaneously reduce fuel loads and promote tree growth (Wigley, Bond, and Hoffman 2010). While short-term responses of and biological activity in African biomes are typically driven by water availability and fire regimes, in the longer term African biomes appear highly sensitive to changes in atmospheric CO2 concentrations (Midgley and Thuiller 2010). Increases in CO2 concentrations are expected to favor C3 trees over C4 grasses, as at leaf-level the fertilization effect overrides the temperature effect; this shifts the competitive advantage away from heat tolerant C4 plants, resulting in a risk of abrupt vegetation shifts at the local level (Higgins and Scheiter 2012). The effect may be further enhanced by a positive feedback loop. Trees are expected to accumulate enough biomass under elevated atmospheric CO2 concentrations to recover from fires (Kgope, Bond, and Midgley 2009). This might shade out C4 grass production, contributing to lower severity of fires and further promoting tree growth. Fire exclusion experiments show that biome shifts associated with the processes above can occur on relatively short time scales. High rainfall savannas can be replaced by forests in less than 20–30 years (Bond and Parr 2010). 49