Summary

The most productive agricultural lands in Amazonia are located in the alluvial plain of the Amazon River and its turbid tributaries. Geomorphological and botanical studies show how the natural forest and herbaceous vegetation development, their vertical and horizontal structure, floristic composition and dynamics, depend upon the mechanisms of fluvial dynamics.

Successional development is subordinate to the ecological conditions prevailing on each and every geomorphological unit: flood duration, internal draining characteristics, distance to the channels and aquatic dispersal of the seeds.

The successional status of the species, the importance of water in dispersal, the origin of the flora, and the peculiar adaptive ability of the Moraceae, are discussed. Moraceae, Annonaceae, Leguminosae are the families most represented in the riparian forest of western Amazonia.

The specific terminology relative to the alluvial plains is reviewed in three languages (English, Spanish and French). Várzea and igapós definitions are proposed.

The sensitivity of the environment with regard to duration, frequency and periodicity of flooding allows the prediction of serious perturbations with an eventual evolution of fluvial dynamics. If human activities provoke a modification of the water regime, an aggravation of flooding will result in an invasion of the upland poor soils for agricultural purposes.

The consequences will be destructive and irreversible, the main reason being the demographic increase of the human populations. The alluvial plain is the foster land of the Amazonian peoples. The survival of the upland forest also depends upon its preservation.

Key-words : Western Amazonia, Forest Ecology, Phytogeography, Flooded ecosystems, Alluvial plains, Fluvial dynamics, Stream ecosystems, Biodiversity, Forest structure, Climax, Successions, Environment, Agricultural lands, Resource conservation