In his phytogeographic mapping of Cameroon, Letouzey (1985) distinguished six areas: congolese, congo-guinean, soudanian, sahelian (from South to North), as well as the atlantic coastal zone, and the mountain domain.
The Yaoundé-Sangmélima-Abong Mbang triangle belongs to the domain of the semi-deciduous congo-guinean forest, characterized by Stertuliaceae and Ulmaceae; this area is surrounded by the grass and shrub savannah in the North, the atlantic permanent forest in the West and South, and the Congolese forest in the East. In fact, the forest observed in the upper Nyong basin appears rather discontinuous, with islands surrounded by zones with degraded vegetation. The most characteristic species of the congo-guinean forest are Lophira alata and Pycnantus angolensis, or Gibertiodendron dewervei and Bailonella toxisperma near the Dja reserve, whereas saxicol groups are described in the forest shooting zones (after clearing) ; the swampy depressions are often colonized by Raphia cf montbuttorum. The marshy forest with Stertulia subviolacea present in the upper 80 km of the Nyong course progressively changes towards downstream into aquatic grasslands with Echinochloa pyramidalis.
In the Nsimi catchment, hills and sidehills are covered by secondary forest; the virgin forest is no more found in the region actually, except in rare scraps; a lot of areas have been deforested for growing traditional food crops like manioc, macabo, banana, peanut, corn… These plots, where no fertilizer is used, get into fallow after some years, whereas new deforested plots are put into cultivation. The swampy depression is mainly occupied by raphia trees.