These are primarily gallery forests along the main permanent or semi-permanent watercourses (respectively the Chari and Logone, and the Batha, Batha de Laïri and Bahr Erguig Rivers).
In the south these forests are found in valleys that are sometimes deep and narrow. The fairly luxuriant vegetation contains Vitex cuneata , Syzygium guineense, Anthocleista oubanguiensis, Tricalysia okelensis, Ficus capensis, Gardenia ternifolia, Nauclea latifolia, Tamarindus indica, Diospyros mespiliformis, Terminalia laxiflora, T. macroptera and Acacia caffra var. campylacantha, with Sopubia ramosa and Arbus canascens (in the Mandoul depression) in the undergrowth. Gallery forests give way to a dry forest of Anogeissus leiocarpus with Prosopis africana, Lonchocarpus laxiflora and Acacia ataxacantha, and thence to highland tree savannah.
To the north and south of N'Djamena and as far as Lake Chad the embankments of the Logone and Chari Rivers hold small, closed, fairly impenetrable stands. Many species are widespread here: Acacia sieberiana, A. seyal, A. ataxacantha, A. caffra var. campylacantha, A. senegal, A. scorpioïdes var. nilotica, Albizia chevalieri, Cassia sieberiana, Tamarindus indica, Diospyros mespiliformis, Mitragyna inermis, Kigelia africana, Khaya senegalensis, Balanites aegyptiaca, Celtis integrifolia, Stereospermum kunthianum, Anogeissus leiocarpus, Lannea humilis and Combretum glutinosum. The undergrowth contains both thorny and non-thorny shrubs: Ziziphus mauritiana, Capparis corumbosa, C. tomentosa, Ximenia americana, Piliostigma reticulata, P. refuscens, Boscia senegalensis, Cadaba farinosa, Combretum lecardii, C. aculeatum and Cissus quadrangularis.
Away from these periodically flooded terraces the vegetation soon changes; in the south to tree savannah with Anogeissus leiocarpus and Sclerocarya birrea, in the north to shrub savannah with Acacia scorpioïdes var. adstringens, A. senegal, Cadaba farinosa, etc., and to pseudo-steppe with Andropogonae.
These gallery forests occur in the south on leached tropical ferruginous soils that are sometimes very hydromorphic with a clay-sandy to sandy-clay texture. In the north they occur on hydromorphic soils formed on recent alluvium with a clay-loam to clay-silt texture. Following deforestation and intensive cultivation, the latter evolve into alkaline halomorphic soils. In these gallery areas, tree savannah consisting of Acacia seyal and A. scorpioïdes in thick stands is found on the more clayey patches of vertisols.
Bamboo and palms
The borassus palm (Borassus aethiopium) is not very common in the Chad basin and tends to occur as isolated exemplars. It disappears north of latitude 12° N but can form thick stands wherever the water table is high and the soil texture light. Exceptional stands are found in the three regions of mid-Logone, lower Logone and east of the Bahr El Ghazal depression. As the human population grows, borassus stands give way to crops or are used to build dwellings, leaving a tree savannah with Anogeissus leiocarpus and Prosopis africana.