Key messageInterlocked grain and basic density increase from pith to bark in Bagassa guianensis and greatly improve trunk torsional stiffness and wood tenacity in the radial plane.AbstractTrees modulate their building material, wood, throughout their lifetime to meet changing mechanical needs. Basic density, a widely studied wood property, has been proved to be negatively correlated to growth rate and is then considered to reflect the diversity of species growth strategies. An alternative way for trees to modulate growth strategy at constant construction cost is changing the organisation of their fibre network. Interlocked grain, the result of a periodic change in the orientation of the fibres in the tangential plane, is found in numerous tropical tree species. In this study, we first describe the variations in basic density and interlocked grain occurring during ontogeny of Bagassa guianensis, a fast-growing Amazonian species, and analyse their influence on the local mechanical properties of wood at the tissue level. The observed radial patterns and properties are then incorporated in a finite element model to investigate their effect on mechanical properties of the trunk. We report extreme and highly reproducible concomitant radial variations in basic density and interlocked grain in all the sampled trees, with grain angle variations ranging from -31 degrees to 23 degrees. Such changes in wood during ontogeny allows trees to tailor their growth rate while greatly improving resistance to torsion and reducing the risk of splitting.