Vegetation and Change: Introduction and Western Africa

Abstract

About 50 million years B.P. in Subsaharan Africa a large turnover of bovids occurred which Vrba (1985) attributes to the spread of open grasslands, but historically recent changes in Africa’s vegetation are primarily related to forests. Being generally deep-rooted, forest trees obtain their moisture from lower levels than other vegetation and transpire enormous quantities of moisture into the air. If this transpiration exceeds the normal amount of evaporation on open ground the tree growth will lower the water table, although the moisture of the surface soil in a forest is greater than that of open ground. Highly fissured rocks, such as some limestone formations, are capable of acting as important reservoirs and if penetrated by tree roots will lose far more water than if covered by shallow vegetation, but root penetration into some types of rock improves water penetration. Whether forests affect the water supply of rivers and streams depends upon the character of the soil, subsoil, and underlying rock, nature of the rainfall, slope of the land, and composition of the forest. Humus formed in forests tends to increase the moisture-absorbing capacity of the soil and although on level ground this will not affect runoff appreciably, on hill slopes it is of great importance as it allows more rain to percolate into the soil, reducing the extent of floods and causing runoff to be more regular. On dry gravelly or sandy sites forests improve the water supply, on wet sites they reduce it, while on intermediate sites there is little appreciable effect although certain water-demanding exotic species will reduce the water supply there (Nicholson 1929a).