Wood structure and properties vary between and within species . The adaptive mechanical performances of wood structure among different species in relation to tree phylogeny and other functional traits have rarely been discussed . Among the huge diversity of tropical species, wood density (of dried biomass) has often been used as a measure of maximal growth rate and of relative shade tolerance. Fast-growing, shade-intolerant species have lower wood densities [27,28]. Within a species, faster growth is usually associated with lower density, especially in softwoods, although many exceptions can be found, e.g., in oak, faster growth is associated with higher density 
Another complicating factor when considering wood structure is that wood is not homogeneous within the radial cross section . Variability due to the presence of several different types of wood can be observed. These different types of wood include: reaction wood (see below), early and late wood (specializing, respectively, in transport and support), juvenile wood (the wood formed from a juvenile cambium ), and heartwood (the central wood that does not conduct sap and is impregnated with chemicals as a result of secondary metabolism occurring in the sapwood) . Although such variability within the cross section is very common, the specific geometrical pattern of these different types of wood depends on species and genetic backgrounds, as well as environmental conditions and the stage of ontogeny. For example, juvenile wood is often less dense and stiff than normal wood , but the contrary can also be found . The adaptive interest for tree mechanical safety of such radial variations in wood density has been discussed by Schniewind , Wiemann and Williamson , and Woodcock and Shier .
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