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.