Analysis of the climatic gradient of continentality and identification of plant communities in central African rain forests using the mega-transect method

The main objectives of this work are to: create a phytogeographical synthesis of data gathered over 10 years in ECOFAC Project (Conservation et Utilisation Rationnelle des Ecosystemes Forestiers d'Afrique Centrale), test the relevance of data collected along transects for a phytosociological study, identify the main forest types and the environmental variables responsible for their differentiation, and compare results for two forest strata (diversity, plant communities and physiognomy). Five sites have been chosen at increasing distances from the ocean: Ndote (6 km), Monte Alen (58 km) and Nsork (160 km), all three in Equatorial Guinea, the Dja (340 km, in Cameroon) and Ngotto (840 km, in Central African Republic). The different phytogeographical entities of the lower-guinean sub-center of endemism are thus represented in this scheme. The phytosociological samples used are 320 meter long homogeneous sections of transects. For each section, two arborescent strata are analysed on surfaces which are adapted to their density: trees with DBH (Diameter at Breast Height) greater than or equal to 10 cm, over 0.16 ha, and DBH greater than or equal to 70 cm, over 1.6 ha. The environmental variables considered are altitude, distance from the ocean and distance from the equator. Their influence is studied by using a canonical correspondence analysis. Eight plant communities have been described and classified in a hierarchical three-level system. Through constant similar sampling, it is observed that the lower stratum has more species than the upper stratum. The minimum area is not reached at the end of a releve, especially within the upper stratum, because of the homogeneity constraints and the living space of these big trees. This stratum does not allow as clear a classification as the lower stratum, although we find characteristic species. Moreover these character species can hardly be identified because of the presence of species transgressing from other types of vegetation. As for the physiognomy, the decrease of the leaf dimension is more significant for the vertical gradient, from the lower to the upper stratum, than for the climatic gradient of continentality. The fact of taking into account several strata strongly improves the quality of the results, thanks to the complementarity of floristic composition, the information on the dynamics of populations, and especially thanks to the complementarity of the responses to micro- and macro-climatic variables.