Ecotopes des arbres et gradients écologiques verticaux en forêt Guyanaise

The ecotope has been defined by Whittaker et al. (1973) as the « full range of adaptations to external factors of both niche and habitat » . This term is the only one among those used by animal ecologists which can be applied to plants without extensive redefinition. An example is then given, based on the analysis of a plot of an evergreen rain-f orest in French Guiana. The study of the growth patterns of trees (architectural models), together with that of the forest structure, during successive developmental and successional stages, makes possible a better understanding of the dynamics of tropical sylvigenesis. In an evergreen rain-forest environment, the climate itself does not exert a limiting influence upon forest vegetation. On the contrary, the growth of individual trees, as well as forest succession, are strongly influenced by the micro-climatic feedback effects of developing vegetation. The stratification of a mature rain-forest is both the cause and the result of this biological regulation, which acts through the modulation of the vertical gradients of light and humidity and manifests itself via « inversion levels » . From the beginning of the regeneration cycle (a tree-fall for instance) to its end (a stratified forest with a small net production), numerous transient ecotopes can be distinguished. The plant types proposed at the Kandy Symposium (biological nomads, scar species, sedentary species) appear to play the same role in this biologically determined microclimate as Raunkiaer’s biological types in temperate macroclimates : they represent different ways of reacting to the environment by different strategies of energy and nutrient transfer. The importance of a fundamentally dynamic concept of the equatorial rain-forest is finally stressed, the architecture of every living tree being at every moment the phenotypic expression of a compromise between the inborn growth-pattern of the species and the constraints of the local environment