The high tree species diversity in tropical forests is difficult to take into account in models. The usual solution consists of defining groups of species and then adjusting a set of parameters for each group. In this study, we address this issue by allowing a species to move from one species group to another, depending on the biological process that is concerned. We developed this approach with a matrix model of forest dynamics, for a tropical rainforest in French Guiana, at Paracou, focusing on the methodological aspects. The forest dynamics is split into three components: recruitment, growth and mortality. We then built five recruitment groups, five growth groups and five mortality groups. One species is characterized by a combination of the three groups, thus yielding in total 5 X 5 X 5 = 125 possibilities, out of which 43 are actually observed. The resulting matrix model provides a better view of the floristic composition of the forest, and does not have more parameters than it would have with five global species groups. However, its predictions are no more precise than those of the matrix model based on five global groups.