Croissance des plantes en sous-bois de forêt dense humide (Makokou, Gabon)

Abstract

The growth of tree saplings, juvenile lianas, and understorey shrubs was studied in a rain forest of north-east Gabon from 1980 to 1985. Five hundred individual plants belonging to more than one hundred species were marked along two transects, one located inside the undisturbed forest, and the other in a recent tree gap. The mortality of these plants during the 64 months’ time interval, the growth in height, the percentage occurence of traumas, and the average leaf lifespan, were measured. The mortality of shrubs was lower (25 % in the understorey, and 51 % in the tree gap), than that of tree saplings (38 % and 50 % respectively), and of juvenile lianas (57 % and 72 % respectively). The overall mortality was higher in the tree gap (66 %) than in the undisturbed forest (49 %). The tallest individuals suffered a lower mortality than the smallest ones. The overall growth increment of young plants in the tree gap was twice that of the plants in the undisturbed forest. The lianas displayed the best growth increment, followed by tree saplings. At the community level, however, the handicap of a slower growth rate for the trees is offset by the higher mortality rate of lianas. The traumas observed in surviving individuals were, in 1985, slightly more frequent on the tree saplings than on shrubs, one replacement shoot was formed on the injured plants of the understorey, while extra new shoots were more frequent on plants of the tree gap. The average lifespan of leaves was 4.4 years in undisturbed forest, and 3.7 years in the tree gap ; it is slightly longer in shrubs than in trees. The maximum leaf lifespan estimated was 10 years. It can therefore be concluded that growth rates vary a great deal from species to species in the rain forest studied. These variations are, however, limited in the understorey, owing to the long «resting periods» of many species. In a tree gap, many shrub species can die out, but the trees benefit from better growth conditions.