Abstract
Faidherbia albida and Balanites aegyptiaca are two characteristic and emblematic species of the semi-arid zone of Chad. They are of great socio-economic importance in all their natural areas of distribution due to their multiple uses. The present exploratory study consisted in understanding the determinism of variations in the wood characteristics of the two species in relation to their variations in growth conditions. The aim is to gain a better understanding of the heterogeneity of the technological behaviour of the wood of these two species in order to make better use of them in reforestation processes and to broaden their field of local use. The study was carried out using two approaches. The first consisted in determining, using conventional and standardised methods, a selection of physical, mechanical and chemical characteristics of wood taken from two Chadian climatic zones (Sahelian zone and Sudanian zone) and one Senegalese climatic zone (Sahelian zone), and in studying their variations according to growing conditions linked essentially to climatic differences. The second consisted of studying the radial variations in physical and mechanical characteristics and in the isotopic composition of carbon 13 (δ13C), as a function of tree growth, by means of measurements taken from micro-samples in the tree rings.The results of the first part of the study showed variable trends for both species for all the characteristics studied.For Faidherbia albida, infradensity, density, drying shrinkage and the three mechanical characteristics studied are significantly higher in the Senegalese Sahelian zone than in the two Chadian provenances. The same trend is observed for extractives and lignin content, but cellulose and hemicellulose content are significantly higher in the Chadian Sahelian zone.For Balanites aegyptiaca, the trends are variable across properties, with an inverse effect of provenances for tangential and radial shrinkage, and no marked trend for volumetric shrinkage. The fibre saturation point is low for all three provenances (below 25%). In the Chadian Sudanian zone, the density is significantly higher than in the other two zones, while the compression strength and the longitudinal modulus of elasticity are lower. Lignin and hemicellulose levels are higher in the Chadian Sahelian zone.GC-MS analysis of extractives showed marked differences between the two species on the one hand and the three provenances on the other. These differences could be related to those observed between provenances and species for certain physical and mechanical properties of wood such as drying shrinkage, PSF, density and modulus of elasticity. However, the effect of each extractable on these properties remains to be established, taking into account the probable combined impacts of these different compounds.The second part of the study showed that the average increment was slightly higher in the Sudanian zone than in the Sahelian zone, which reflects the differences in climatic conditions that affect tree growth.For Faidherbia albida, the lack of significant difference in δ13C between the two provenances can be explained by the fact that this species is a phreatophyte. For Balanites aegyptiaca, the average δ13C in the Sahelian zone is significantly lower than in the Sudanian zone, a trend that is the opposite of what would be expected. This result tends to show that this species has a good efficiency and opportunistic water use strategy.The study of the relationships between the variables taken in pairs gave contrasting results.