Les palmiers des forêts tropicales humides du Bas Tocantins (Amazonie brésilienne)

Vingt et une espèces de palmiers réparties en 9 genres ont été recensées sur 10,56 ha dans les forêts de l'est amazonien, vallée du Tocantins, Etat du Parà. La densité moyenne de palmiers (ou d'axes pour les espèces multicaules) de hauteur égale ou supérieure à 1 m est de 602 par hectare. Une comparaison avec deux autres inventaires situés dans le centre et l'ouest de l'Amazonie révèle la pauvreté en espèces et genres de palmiers et leur faible densité dans ces forêts de la région orientale. (Résumé d'auteur

Life forms of Amazonian palms in relation to forest structure and dynamics

Two Amazonian terra firme forests present contrasting structures and palm compositions : one is characterized by tall and broadtrunked trees up to 50 m in height and 2.5 m in DBH, and by tall arborescent palms ; the other by more modest-sized trees up to 40 m in height and 1.3 m in DBH, and by abundant acaulescent and smaller arborescent palms. This comparison strongly suggests that forest structure and dynamics influence life forms, palms size, and species diversity. Gap size, which mainly depends on the size of upper-canopy trees, determines the intensity of light reaching the understory and thus influences the development of arborescent palms. In this analysis, acaulescent forms are regarded as an adaptation to forests with small trees and resultant small gaps. (Résumé d'auteur

L'organisation comparée de deux forêts d'Amazonie brésilienne

A comparison of two "terra firme" amazonian forests shows different patterns of spatial organization. In one case multistorey canopy appears to be correlated with a coarse grained mosaic of clearings and with an important lateral heterogeneity in the forest. On the other hand, a fine grained mosaic of clearings and relative homogeneity of the forest seems associated to a closely-packed uniform canopy. These different patterns of spatial organization affect settling and growth conditions of the different species according to their specific light demands. This leads to the question whether the structural organization of the forest does or does not play a role in the distribution of the species as suggested by the case of acaulescent palm-trees in Central Amazonian forests. (Résumé d'auteur