Part II
The ecology of savanna vegetation in Africa is governed largely by the ecology of grasses. Savanna grasses with the C4 photosynthesis pathway are superbly adapted to cope with erratic rainfall. They can grow rapidly during times when soil moisture is adequately available, exploiting pulses of nutrient release while overcoming the limitation on growth by low atmospheric concentrations of carbon dioxide. Hence they can out-compete tree seedlings in extracting soil water from the topsoil layer where most nutrients occur. For juvenile trees to establish amid the dense mat of grass roots, they must extend their roots to greater depths in the soil and expand their growth to times earlier and later in the wet season than grasses. Tree saplings are repeatedly set back by having the accumulated carbon burnt back. To escape the fire trap, tree saplings must build up sufficient resources below ground to enable them to elevate their foliage above the flame zone. Grasses lose only dead top-hamper to fires and suffer from its accumulation in the absence of fires or grazing. Grasses grow both beneath and between tree canopies. Dense evergreen foliage would be required to shade out grasses, but such trees are restricted to locations that seldom burn.
Not all woody plants have deep roots and grasses vary in how tall they grow and hence how much fuel they generate to support hot fires. Too little research has been done on individual species ecologies, especially in their root deployment below-ground. Soil fertility along with rainfall underlie a functional subdivision between dry/eutrophic savanna typified by thorny acacias with finely subdivided leaves and broad-leaved savanna woodlands prevalent where soils are sandy and rather infertile. This savanna subdivision is not replicated outside of Africa.
Within African savannas the tree cover can vary locally from open grasslands to embedded patches of forest and thicket. This is governed largely by the redistribution of rainwater within local landscapes and its effects on plant growth and fire incidence. The annual production of grass biomass above ground is almost linearly dependent on the annual rainfall total. If rainfall is halved during droughts, there is an equivalent reduction in the amount of food produced to support grass-dependent herbivores. Deciduous trees produce their new leaf crop each year ahead of the rains and thereby remain less influenced by the current season’s precipitation. Tree populations turn over slowly, in response to long-term climatic trends, although shrubs are shorter-lived. The grass cover responds more dynamically to between-year changes in rainfall. Little attention has been paid to the growth patterns of the plants labelled ‘forbs’, which encompass a range in growth forms from annual flowers to underground trees and much in between.
Trees get excluded from upland regions on ancient land surfaces where weathering has produced hardpan layers and climatic conditions are cooler, although not too dry to support woody plants. Elsewhere, water logging prevents trees from growing. The relative roles of temperature, precipitation, soil structure and fire patterns in generating extreme grassy states still need to be disentangled. Pleistocene oscillations in tree pollen indicate that soils do not form a rigid restriction. Variable levels of atmospheric carbon dioxide potentially make an additional contribution.
Our perception of Africa’s savanna vegetation is biased by the fact that we live within an interglacial interlude, a brief respite from the cooler and drier conditions that prevailed through most of the Pleistocene. Not only that, but industrialisation and fossil fuel burning have pushed atmospheric carbon dioxide to levels last reached during the Pliocene 5 million years ago. Our human ancestors evolved under conditions that were somewhat different from those we modern humans have experienced during the Holocene.
These are the take-home messages from the chapters forming this section of the book:
1.The prevalence of savanna vegetation formations is climatically controlled by seasonal restrictions in rainfall.
2.Spatial heterogeneity in the woody vegetation cover is an inherent functional feature.
3.The dry/eutrophic savanna subdivision governed by bedrock geology is exclusive to Africa.
4.The expansion of C4 grasses was primarily an adaptive response to intermittent soil moisture, generating pulsed releases of soil nutrients.
5.The C4 pathway enables savanna grasses to dominate tree seedlings in competition for soil moisture.
6.Recurrent fires contribute additionally to suppressing tree growth towards maturity.
7.Grassland–forest–heathland mosaics were more widespread during glacial advances than during the current interglacial interlude.
8.African savannas are fundamentally disequilibrium ecosystems that persist in shifting patch mosaics at various scales.
Figure II.4
Luxuriant grassland of red grass, central Serengeti NP, Tanzania, pending grazing by large herbivores.