The primary causes of bush encroachment on savannahs include a reduction in the frequency of fires and overgrazing of livestock. When the grass layer on savannahs loses its competitive advantage and its ability to utilise nutrients and water efficiently, higher infiltration of water and nutrients into the sub-soil results; a situation that benefits bush and tree species, allowing them to predominate (de Klerk 2004). Bush encroachment is also accompanied by a change in the dominant grasses: perennial grasses are often lost, being replaced by annual species often of inferior quality for livestock (Scholes 1997, Rothauge 2007). Annual grasses are generally less productive than perennial grasses. Thus, animal production on an annual grass sward is very precarious and less sustainable.
Another important theory is the state-and-transition model, which says that savanna ecosystems are event-driven, where rainfall and its variability play a more important role in vegetation growth (and composition) than the intensity of grazing. This model implies that bush encroachment is not a permanent phenomenon, and that a savanna can be changed to its grass-dominated state by favourable management or environmental conditions (Doughill et al. 1999). Woody plants establish themselves after dry periods followed by a few wet years, and then maintain themselves by utilising most of the water. Rather than a gradual annual increase in numbers, the general rule is that woody plants establish in large numbers during certain years and at varying intervals (Donaldson 1969).
Thus bush encroachment can occur rapidly and may be triggered by management practices and natural events, or a combination of these factors. >>> Read More