Abstract
Site web mis à jour le 8/12/03
The study of the distribution of Orchidaceae in Central Africa is surveyed along a transect extending over 800 km from São Tomé in the Gulf of Guinea to the Dja Reserve in South-East Cameroon.  Inventories have been established on different scales in five zones along this transect. Cultivation in these areas of 6168 Orchidaceae in 9 shadehouses has yielded a harvest of fertile samples at flowering time. We have thus identified 3947 vouchers at the level of species. These samples represent 361 taxons, divided in 55 genus, of which 77% are epiphytes and 23% terrestrial. Among these taxons, 35 are new for science. The environments richest in Orchidaceae are mature rainforests (187 taxons), secondary plant formations (169 taxons) and inselbergs (149 taxons).
Taxonomical, ecological and phytogeographical study of the Orchidaceae in Atlantic Central Africa
Key words: São Tomé, Principe, Cameroon, Dja Reserve, Equatorial Guinea, Gabon, Cristal Mountains, Oyem, taxonomy, phytogeography, chorology, forest refuge, foehn effect, epiphyte, phorophyte, inselberg.
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The distribution of epiphyte plants in São Tomé and Principe is significantly correlated with that of terrestrial plants. This correlation varies depending on the sampling strategy (grids, localities, altitude). On the archipelago the distribution patterns of epiphytes and terrestrials are similar and can be explained by identical environmental parameters. If we consider the variations of the floristic composition at different altitudes, we observe no discontinuity between the vegetation of medium altitude and that of submontane forest.  This suggests a continuous transition between these two types of vegetation.

The interpretation of the distribution of Orchidaceae in the Lower Guinea subcentre of endemism shows similarities between their distribution areas and White’s vegetation map (1983). We propose here to consider the three oceanic islands of the Gulf of Guinea (São Tomé, Principe and Annobon) as a distinct phytogeographical area and to connect them with the Guineo-Congolian Region. The flora of the Orchidaceae on São Tomé originates from Principe and to a lesser extent from Rio Muni (Equatorial Guinea). This conflicts with the concept of an independent origin of the flora on the islands of the Gulf of Guinea and with a stepping stone migration along the volcanic line of Cameroon. The submontane flora of São Tomé derived from lower altitude rather than from that of the afromontane flora. This is made evident by the lack of discontinuity between these types of vegetation and by their large affinity.

Along our transect, the altitude, the continentality and the chorological vegetation type constituted significant parameters which explain the distribution of the Orchidaceae. One of the patterns observed with these plants matches the location of a forest refuge located by several authors in the North of Gabon and Rio Muni. We can explain this similarity by a foehn effect due to the presence of a mountain chain along the ocean. This phenomenon seems to explain not only the distribution of epiphytes in Central Atlantic Africa and in São Tomé, but also the presence of forest refuges in these places.
In the Dja Reserve, the parameter which best explains the variability of floristic composition of the epiphyte groups is the phorophyte species. It represents almost 50% of the variability explained by the parameters that we considered, i.e. the type of vegetation, the vertical distribution according to Johansson’s scheme (1974) and the type of phorophyte. This result suggests a strong correlation between the epiphytes and the phorophyte in areas with low precipitations.
As suggested by Reitsma’s scheme (1992), inselbergs contain a large number of facultative epiphytes. On the other hand, we do not find any similarity between the epiphyte groups from the canopy and those from the inselbergs.
Dinklageella scandens P.J. Cribb & Stévart, a new species from Sao Tomé (Photo: J. Lejoly)