All the vegetation of Río Muni is classified as "low- and medium-altitude tropical rainforest" on the 1959 AETFAT UNESCO map, and as a "mosaic of wetter and drier types of lowland rainforest" on the updated UNESCO map (White, 1980). These mixed tropical forests are like those found in the first and second forest zones of Gabon and a basic feature is the presence of okoumé (Aucoumea klaineana), which is found everywhere except in some north-western and north-eastern areas. Although some exemplars of the species have been reported in Cameroon, on the border with Río Muni, it is essentially native to the Republic of the Congo, Gabon and Río Muni. A commonly accepted explanation for its presence is that it is typical of forests that have regenerated after the primary forest was cut down for agricultural purposes by the much larger population of previous centuries (Capdevielle, 1969). Indeed, regenerated okoumé is often widespread on land that has reforested naturally after cultivation has been abandoned.
Other species typical of these forests are calabo (Pycnanthus angolensis or trade ilomba); Myristicaceae such as ekum (Coelocaryon klainei), Scyphocephalium ochocoa and bokapi (Staudtia stipitata or trade niove); Burseraceae such as asia (Dacryodes buettneri or trade ozigo), Dacryodes spp., abe (Canarium schweinfurthii or trade aielé), and Santiria trimera; numerous Caesalpiniaceae, sometimes found in pure or nearly pure stands, such as andjung (Monopetalanthus spp.), Didelotia brevipaniculata, Gilbertiodendron spp., palisander or rosewood (Guibourtia ehie), oveng (Guibourtia tessmanii or trade bubinga), ekop-ekaba (Tetraberlinia spp.), elondo (Erythrophleum spp.) and Dialium spp. and Irvingiaceae such as eves (Klainedoxa spp.), Desbordesia spp. and andok (Irvingia spp.). Other typical or endemic species are akoga (Lophira alata or trade azobe), Coula edulis (in primary forests), Sacoglottis gabonensis and Scytopetalum klaineanum. Meliaceae such as embero (Lovoa trichiliodes or trade dibetou), samanguila (Khaya ivorensis or coast mahogany), abebay (Entandrophragma utile or trade sipo) etom (E. cylindricum or trade sapele mahogany) and ndongomanguila (E. angolense or trade tiama) are found in smaller numbers. Akom (Terminalia superba or trade limba) and ayus (Triplochiton scleroxylon or trade obeche or samba), two softwood species important in the international market, are found in secondary forests; however, they are not as common here as in other African countries and are therefore not as economically important.
Elobon (Mitragyna ciliata or trade abura or bahia) is common on marshy ground along rivers and is exploited. There are mangroves, including Rhizophora racemosa, but their extent is not known.
According to the second version of the UNESCO Vegetation Map of Africa, the island of Bioko is covered with a "mosaic of lowland rainforest and secondary grassland" and with "undifferentiated montane forest" on the highest slopes of the volcanoes. The island's relief makes extraction difficult from most of the forests. However, before 1968 abeng or morera (Chlorophora excelsa or trade iroko) was being extracted for local use, as well as softwood species such as calabo (ilomba), ekum and nsu (Daniellia spp.) for veneering.