Les plantes à tubercules de la forêt dense d’Afrique Centrale

Abstract

The different species of rain-forest plants developing starchy tubers were studied around Makokou (N.E. Gabon) and in the Lobaye River district (S.W. Central African Republic). A detailed description of the tuberous parts is given, particularly in wild yams (Dioscorea spp.) to help to elucidate the taxonomic status of a number of species. The unusual morphology of yams belonging to the Enanthiophyllum section required the use of three new descriptive terms borrowed from the Aka pygmies, namely «Mbolo» for a lignified plateau, «Mosway» for the fingerlike expansions, and «Yoko» for the spherical terminal edible parts. The different underground storage structures are considered as adaptive to the rain-forest environment. During the long period that most plants spend in the shade of the undergrowth (as the «set of the future», Oldeman, 1974), the energy stored in tubers is more likely to escape insect and/or vertebrate predation than the aerial parts of other plant species. Accordingly, many tuberous plant taxa, both dicots and monocots, might well have originated in rain-forest areas rather than in more seasonal woodlands or savannas. Counts were made along very narrow transects to estimate the number per hectare of stems visible above ground level (Tables III to V). The number of stems averaged 97/ha in the closed forest of the Lobaye, but can reach locally higher values (up to 24,000/ha) in open areas. In the latter places the distribution of tuberous plants was clumped, and therefore more likely to be successfully exploited by animal consumers ; most of their tubers, however, were poisonous. By contrast, the tuberous plants found in the closed forest did not contain highly toxic alkaloids and were rich in starch (up to 80 %, dry weight) and protein (up to 10 %) (Tables VII to IX). Stem densities were lower in Gabon than in the Central African Republic. An estimate of the underground tuber biomass has been made at Makokou (Table VI). The tuber biomass was even higher in the Lobaye District ; a standing crop of more than 5000 kg of wild yams can remain available throughout the year on the home range of an Aka pygmy group. The benefits of energy storage in tubers and the reproductive strategies of the various plant species concerned are discussed in relation to the selective pressures exerted by animal consumers, not forgetting human traditional hunter-gatherers.