BÉREAU, MOÏSE1, ÉLIANE LOUISANNA1, TÉTÊ BARIGAH & JEAN GARBAYE2.
1INRA, Station de Recherches Forestières, BP 709, 97387 Kourou, French Guiana. 2INRA, Centre de Recherches forestières de Nancy, 54280 Champenoux, FRANCE.
Dicorynia guianensis and Eperua falcata are two major timber Caesalpiniaceae of the tropical rain forest in French Guiana which differ markedly in their regeneration patterns: the former exhibits an aggregative habit with seedlings surviving and growing better under the parent trees, while the latter is dispersed with seedlings surviving better at a distance from mature trees of the same species. Several hypotheses have already been put forward to explain such a difference, as allelopathy or light microclimate, but no convincing results have been obtained yet. Here we explore a third hypothesis which concerns mycorrhizas.
Both species are strictly endomycorrhizal, with the typical hyphal coils, few arbuscules endomycorrhizal type observed in most trees of the Guiana forest; however, they have very different rooting habits, root morphologies and cortex structures, suggesting that they might elect different fungal symbionts of various effectiveness. The hypothesis that the availability of adequate fungal propagules in the soil determines the seedling fitness is presently addressed with a series of pot experiments.
The first results show that D. guianensis seedlings are more growth-responsive to endomycorrhizas than those of E. falcata ; they are particularly mycorrhiza-dependant under the illumination conditions which prevail in the forest. The roots of the corresponding mature trees are an effective endomycorrhizal inoculum for seedlings of D. guianensis but not for those of E. falcata when compared with soil inoculum. This tends to support he hypothesis and new experiments are planned, including placing germinating seeds in well-defined niches in the forest floor and monitoring the kinetics of their mycorrhizal colonization.