The savanna-forest boundary in West Africa is one of its most remarkable geographical features. The clarity with which the boundary is defined, whether seen from the air or from ground level, is, throughout most of its length, most striking. Moreover, the most casual observer will discover that many other features, particularly of settlement and of agriculture, reach the limit of their distributions at about the same location. Not the least significant feature, in sharp contrast with many other zones of vegetation change, is that the boundary is not an ecotone, or zone of gradual change in floristic composition and community structure in response to equally gradual changes in habitat character, but ‘a mosaic of communities representative of each region’. The zone of change consists of islands, salients, and enclaves of forest and savanna, sharply separated from one another. Only one community, the ‘transition woodland’, may in any sense be considered an ecotone. Even this, in some instances at least, may be a more or less transient sub-seral community, in view of the fact that forest trees within it are generally young, whereas the savanna trees are old.