We aimed at analysing some functional trade-off or associations between wood traits among the diversity of tree species in a tropical rain-forest and how they are related to the ecological functioning of species, along a gradient from pioneer to shade tolerant species. 22 species have been studied at the early sapling stage in the natural site of Paracou in French Guiana . Wood basic density, hydraulic traits (specific conductivity and vulnerability to embolism) and mechanical traits (modulus of elasticity and maximal strain at fracture limit) have been measured. Species sampling cover a wide range of mechanical and hydraulic performances. Wood basic density varies from 0.4 to 1. There is a narrow, and not strongly associated to the species ecology, relation between the modulus of elasticity and the wood density. As known by wood technologists, such a relation is strongly determined by physical causes, as wood density mainly represent the material porosity. Mechanical safety measured by the maximal strain at fracture limit, which is quite independent to the modulus of elasticity, is also positively correlated with wood density. As expected since specific conductivity is linked to the fourth power of lumen diameter, specific conductivity is negatively but less strongly correlated to basic density. All the species are hydraulically vulnerable that is not surprising in a wet environment as the tropical rainforest, but the pioneer species are less vulnerable. Thus, the less hydraulically vulnerable woods are also the more conducting and the less mechanically stiff and deformable. At this level of sampling (the range of tree species that coexist in a wet tropical rainforest), there is no absolute physical necessity of trade-off between hydraulic safety and performance, neither between hydraulic performance and mechanical stiffness. Therefore, results obtained at wider scale, i.e. by comparing species on a wide range of environmental conditions from wet to dry forests, cannot be extrapolated : some trade-off or associations depend strongly on the sampled environment variations, while others as association between density and Young's modulus are more stable . According to their needs in their environment, pioneer species in the Paracou site combine a low wood density, a high hydraulic performance and a lower hydraulic vulnerability of wood. The relations are mainly influenced by some extreme behaviour and actually, traits varies quite independently with a great variability. Such results concerning wood are discussed in relation with the stem performance that integrate wood and geometrical or size effects.