There are many ideas regarding bush encroachment. In short, research in Namibia and southern Africa in general shows that, for Acacia mellifera (S. mellifera), establishment events (events when seeds are produced, seedlings germinate and survive) only occur after and during two to three, but probably longer, consecutive good rainfall events. Seed production requires very good rainfall to produce viable seeds. Seedlings germinate very easily (after a good rainfall event) and so the seeds never accumulate in the soil or on the surface (they are also eaten). Because of this, one year of good rainfall followed by poor rainfall will not result in establishment. The resulting germinated seedlings require good, well-spaced (not too long between successive rainfall events) to survive the first year. If seedlings survive the third and following years, many saplings can still die if the rainfall is not good. Therefore, the last significant establishment events took place in the exceptionally wet periods from around 2000 to 2013.
Because good rainfall also increases grass production, and hence good fuel loads, natural fires tend to coincide with these establishment events. These fires kill seedlings, but once saplings of around 5 to 10 years have established (the exact time depends upon the rainfall and hence growth rate) they are resistant to fire. As the saplings grow, their competition with grasses increases and eventually there is insufficient fuel for a fire (although this is not always the case). The bush thickened area cannot easily revert back to a grassy state without active intervention. These thickets can exist for centuries but as the thickets age, trees start to die back more easily from drought, frost and disease. This thicket can then open up into a grassy savanna again (through disease, frost, fire, drought and combinations of these). Naturally, farmers don’t have the time to wait for this to happen and thus feel compelled to address the problem as soon as they can to increase grass production. Figure 1 provides a simple scheme of how bush encroachment works.