Anatomy, drying behaviour and mechanical properties of lesser used wood species from Mozambique

The study provides an overview of Mozambique timber sector, descriptive and comparative wood anatomy, drying experiments and interrelationships between mechanics, density and anatomical features of lesser used species from Mozambique. Exhaustive literature review described the timber sector as dominated by few hardwood species selectively harvested from a forest containing 118 potential wood species. The survey discussed the need to revert the current situation by proposing studies on lesser used timbers to enlarge the resource base and ensure sustainable logging practices. Afterwards, based on growing stock reported in the forest inventory, three lesser used timbers namely ntholo (Pseudolachnostylis maprounaefolia Pax), metil (Sterculia appendiculata K. Schum) and muanga (Pericopsis angolensis Meeuwen) were selected and subjected to descriptive and comparative wood anatomical studies aiming to understand their structure with regard to prospective end uses. Classical methods of wood sectioning and both light microscopy (LM) and scanning electron microscopy (SEM) were used. The results showed that ntholo and muanga are characterized by diffuse porosity, indistinct growth rings, 18 vessel/mm2, extractives in heartwood vessels, ray width (1-3 cells), % fibre proportion (ntholo 57%; muanga 58%). This set of anatomical features typifies dense timbers with recognized strength and good natural durability. Metil has shown wood structure characterized by very wide vessels without extractives recorded in low density (< 5 vessels/mm2), ground tissues dominated by thin-walled axial parenchyma (50-61%) and fibre proportion (17-27%). Metil wood anatomy is typical for light timbers with poor natural durability. Nevertheless, metil timber seems easy to impregnate with wood preservatives as demonstrated by the high uptake of 463 kg/m3. In addition, the study carried out drying experiments on ntholo boards aimed to assign adequate drying schedule. The experiment was conducted in two stages, i.e. non-symmetrical drying tests (NSD) and laboratory batch kiln drying (LBK). NSD was intended to select provisional schedule and LBK to test the performance of the selected schedule based on European standards. The results from NSD assigned provisional schedule (T6-D2) corresponding to other tropical timbers with similar drying behaviour. The LBK lasted 266 h and ensured standard quality drying described by 8.9% final moisture content, a gradient of 1.2% and 1.2 mm of casehardening. Minor deformations were recorded and twist was the largest with an average of 3.4 mm. However, in general, the drying experiment can be regarded as successful since it provided background for industrial drying. Finally, the study determined ntholo timber mechanics and thereafter examined interrelationships with density and anatomical features of ntholo through correlation and regression analysis. The results show that ntholo is a very dense timber with high mechanical strength in comparison to well known timbers. Correlation analysis revealed fibre length as the only anatomical feature significantly correlated to density and all tested mechanical properties of ntholo. The number of vessels/mm2 and %vessels were not significantly correlated to any of the measured properties but appeared to be key anatomical features for predictions under regression analysis. All tested properties of ntholo sapwood seemed to be influenced mainly by ground tissue proportions, while heartwood properties were described by more leveled anatomical predictors. The regression analyses show that both ntholo sapwood and heartwood densities are poor predictors for the tested mechanical properties, although may provide rough indication of tested properties given the observed correlations. The integrated analysis of results from the timber sector review, wood anatomy, drying behaviour and mechanical properties are expected to form a reliable background for a successful utilization of the relatively lesser explored timbers from Mozambique.