Increases in woody plant densities in savanna grazing lands worldwide have resulted in a decline in the grazing capacity of these rangelands. At present, the actual cause of the problem is unknown although a vast body of literature deals with various aspects relevant to the issue. It is generally assumed, however, that the changes in the tree : grass ratio are a response to changes in soil-water development brought about by rangeland utilization. These utilization patterns differ from those under which the savanna system evolved. While changes in physiognomy are very prominent, these are generally preceded by changes in grass species assemblage. The development of species composition has been used as an indicator of rangeland condition for many years; the increase or decrease of individual species is considered in this context. Thus far, however, the shift from perennial to annual grasses has not been evaluated for its effect on soil moisture development, and subsequent implications for woody plant establishment and growth. The review presented here consolidates the existing information in order to provide a basis for understanding the woody weed encroachment problem. Keywords: bush encroachment, savanna ecology, model, vegetation composition.