In africa, tropical wet climates transition into seasonally wet/dry tropical woodlands, followed by another transition into:

Into tropical savannas or steppes, in Africa, tropical wet climates transition into seasonally wet or dry tropical woodlands, followed by another transition.
The tropical savannas are identified by tall grasses and occasional trees. It can result from either climate, soil conditions, animal behavior, or agricultural practices, which limit the occurence of trees.

The answer is the magnetosphere.