Part II - The Savanna Garden: Grassy Vegetation and Plant Dynamics

Summary

The tropical savanna biome, defined most simply by the coexistence of trees and grasses, covers nearly half of Africa’s surface south of the Sahara (Figure II.1). Much attention has been given to explaining why the tree cover doesn’t close up. But while the shade cast by tree canopies can suppress grasses, if sufficiently dense, sunlight is not the main limitation where savanna vegetation formations prevail. It is water in the soil, supplied seasonally and somewhat erratically within seasons by rainfall, and redistributed spatially. Competition among trees and grasses operates primarily underground in the rooting zone and thus out of sight. It takes place amid the mat of grass roots and the roots of woody plant seedlings probing for soil moisture and the mineral nutrient resources that this water conveys. The competitive interaction enters a second stage once established tree saplings emerge from the grass layer, only to be burnt back periodically by the recurrent fires sustained by the grasses. While awaiting a sufficient interval between fires to raise their foliage above the flame zone, juvenile trees are exposed to further tissue losses and damage from browsing herbivores. Grasses are superbly adapted to accommodate variable rainfall, withstand fires and tolerate herbivory, as the chapters forming this section will reveal. The feature defining savanna formations is specifically the presence of a grass layer sufficiently well-developed to support fires. Hence grasslands lacking trees are functionally allied in a broadened category of tropical grassy biomes.¹ What needs explanation is where and how woody plants manage somehow to establish and persist in regions where grasses dominate.