The effect of radial growth rate on wood anatomical, physical, and mechanical properties of the 26-year-old sentang tree (Azadirachta excelsa (Jack) Jacobs) Planted in Jeli, Kelantan, Malaysia

It is widely accepted that in any given stand, the radial growth rate of the individual tree varies according to age, relationships with neighboring trees, and local site factors. Thus, even in the same site, each tree would exhibit different radial growth rates, which could affect the properties of the wood. In this regard, this study was conducted to investigate the properties of sentang wood and focused on examining the effect of radial growth rate on the anatomical, physical, and mechanical properties variations in the radial and longitudinal directions of the tree. Studying these properties is vital, as they are essential indicators for wood used in furniture, pulp, and wood- based composite production and are crucial determinants of suitability for structural applications. For these purposes, tree inventory was conducted on a 4.0 ha sentang site in Jeli, Kelantan, Malaysia using a 1.0 ha circular plot containing approximately 300 sentang trees, and 30 trees were randomly selected from this population. The trees were categorized into slow-, average-, and fast-growth based on their breast height diameter (DBH). The wood properties were then examined, including the fiber and vessel dimensions, moisture content (MC), density, shrinkage, and bending and compression strength along the radial and longitudinal directions of the trees. The results revealed that despite the differences in the radial growth rate, radial variation in the vessel element dimensions, air-dry density, MOR, MOE, and compression strength tend to have a typical pattern, experiencing an increase from the pith to the bark. In comparison, radial variation in the fiber dimensions, green density, and shrinkage were found to have varied patterns depending on the radial growth rate and longitudinal position. On the other hand, longitudinal variation in the fiber and vessel element dimensions tended to have varied patterns depending on the radial growth rate and radial position. Regarding the physical properties, except for the shrinkage, longitudinal variation in the MC and density was found to have a typical pattern despite the differences in the radial growth rate. The green MC experiences a decrease from the bottom to the top, while the density tends to increase toward the top of the trees. Similarly, longitudinal variation in the mechanical properties tended to have a typical pattern of an increase toward the top of the trees despite the differences in the radial growth rate. In addition, the radial growth rate seems to affect the wood properties in general. The slow-growth tree tends to have a shorter fiber length and smaller fiber diameter, reflecting a considerably higher wood density, and thus has the highest mechanical properties. In contrast, the fast-growth tree tendsto have a lower wood density and thus has the lowest mechanical properties than the other categorized trees. The anatomical properties examination results revealed that the average fiber length and diameter of the slow-, iv average- and fast-growth trees were 833 and 16.6, 964 and 21.9, and 927 and 20.7 μm, respectively. Additionally, the average vessel element length and diameter of the slow-, average- and fast-growth trees were 618 and 390, 576 and 328, and 550 and 369 μm, respectively. Regarding the physical properties, the average green MC of the slow-, average- and fast-growth trees were 42.0, 39.3, and 48.4%, respectively. The average green and air-dry wood density of the slow-, average- and fast-growth trees were 689 and 567, 699 and 587, and 679 and 550 kg/m3 , respectively. The shrinkage in the tangential direction of the slow-, average- and fast-growth trees were 3.03, 3.19, and 3.85%, respectively. The shrinkage in the radial direction of the slow-, average- and fast-growth trees were 1.83, 1.79, and 2.41%, respectively. The shrinkage in the longitudinal direction of the slow-, average- and fast-growth trees were 0.53, 0.61, and 0.82%, respectively. In the case of mechanical properties, the average MOR and MOE of the slow-, average- and fast-growth trees were 85.2 and 8,811, 84.4 and 8,043, and 79.3 and 8,314 N/mm2 , respectively. In addition, the compression strength of the slow-, average- and fast-growth trees were 36.3, 35.9, and 34.0 N/mm2 , respectively.