Inter- / inside-tree variations of reference wood characteristics of high value-added Central African timber species

The Central African wood sector is characterised by a steady but sometime short-lived commercialisation of new timber species on the local market as well as for regional or international trading. Wood manufacturers must identify the most suited species according to their own processing constraints in relationship with the specific wood properties of the selected species. According to these operators, wood variations quite only correspond to differences between species. However, important variations of physical and mechanical properties are observed on sawn timber, leading to bitter technological problems during processing. Some reference characteristics currently used to qualify technological behaviour of tropical species have been determined according to international standards or specific CIRAD's standard sampling procedure The analysis of the variations of some of these properties (shrinkages, fibre saturation point, hardness, Young's modulus) brought out the respective effect of several factors: basically, type of botanical species, but also differences between trees (linked to any diameter and age effect, even if very often, trees age is not known), and variations inside trees. Inter-species variations are frequently very high, but for some Central African timbers (i.e. Cylicodiscus gabunensis / OKAN, Pterocarpus soyauxii / PADOUK, Staudtia stipitata / NIOVE), intra-species variations can be higher. In particular, it is usually considered that intra-trees variations have not to be taken into account for timber from natural forest, and only concern plantations species. However, high variations inside trees have been observed for some species explaining some technological problems occurring during processing. These results shown that the study procedures to qualify tropical woods from plantations in one hand, and tropical woods from natural forests in the other hand, can be quite similar in some cases. Intra-species wood variations of timber from natural forest must be taken into account to improve appropriateness between their quality and their potential utilisation. (Résumé d'auteur)