55 Les/Wood, Vol. 73, No. 1, June 2024 PHYSICAL AND MECHANICAL PROPERTIES OF WOOD OF PLANTATION GROWN Albizia lebbeck IN THE SAVANNAH ECOLOGICAL ZONE, GHANA FIZIKALNE IN MEHANSKE LASTNOSTI LESA VRSTE Albizia lebbeck S PLANTAŽ V GANI Enoch Gbapenuo Tampori 1* , Francis Kofi Bih ², Kwaku Antwi ², Issah Chakurah ² UDK članka: UDK 630*812:630*176.1 Albizia lebbeck Received / Prispelo: 18.1.2024 Original scientific article / Izvirni znanstveni članek Accepted / Sprejeto: 14.5.2024 . Abstract / Izvleč ek Abstract: The increasing scarcity of major commercial tropical hardwood species has necessitated the utilization of plantation grown exotic timber species as a potential means of maintaining Ghana’s foundation of timber resources. To better consider Albizia lebbeck as a substitute for wood species which are being seriously over-exploited to the point of commercial extinction, its wood properties were characterized to expatiate its utilization potentials. Three mature plantation grown Albizia lebbeck trees with diameters 45-50 cm at breast height were purposively selected and sampled at four stem height levels of tree height. The samples were sawn into the required sizes in accordance with the British standard, BS 373 (1957) for testing. The heartwood and sapwood proportions were evaluated and the samples were examined for hardness, bending strength (MOE and MOR), compression strength parallel to grain, shear strength parallel to grain, and air-dry density. All trees had a significantly higher heartwood than sapwood percentage. The air-dry density values at 12% MC were 868 kg/m³, 806 kg/m³, 695 kg/m³ and 564 kg/m³ for four sections of the stem (heights 0-25%, 26-50%, 51-75% and 76-100%). In general, the plantation grown Albizia lebbeck exhibited favourable strength values, suggesting that it is endowed with adequate properties for being an alternative species to supply the wood industry. Keywords: Albizia lebbeck, lesser-known timber species, plantation, heartwood, sapwood, wood density, modulus of rupture, modulus of elasticity Izvleček: Zaradi naraščajočega pomanjkanja lesa tradicionalnih tropskih listavcev morajo v Gani vse bolj uporabljati les eksotičnih lesnih vrst. Ta prihaja s plantaž, kar pripomore tudi k ohranjanju tradicionalnih gozdnih virov. Cilj prispevka je raziskati lastnosti lesa vrste Albizia lebbeck in odgovoriti, ali bi les lahko uporabili kot nadomestek za nekatere tradicionalne lesne vrste. V ta namen so bila izbrana tri odrasla drevesa Albizia lebbeck s premerom 45–50 cm v prsni višini, vzorce lesa pa so odvzeli na štirih nivojih debla. Vzorci so bili razžagani na zahtevane dimenzije v skladu z britanskim standardom BS 373 (1957). Ocenjen je bil delež jedrovine in beljave, nato pa je bila raziskana trdota, upogibna, tlačna in strižna trdnost ter gostota zračno suhega lesa. Pri vseh drevesih je bil delež jedrovine znatno višji od deleža beljave. Vrednosti gostote na štirih nivojih po višini debla (0–25 %, 26–50 %, 51–75 % in 76–100 % višine) pa so bile 868 kg/m³, 806 kg/m³, 695 kg/m³ in 564 kg/m³. Na splošno so bile trdnostne lastnosti lesa vrste Albizia lebbeck s plantaže ugodne, kar nakazuje, da ima les primerne lastnosti, da bi ga lahko uporabili za potrebe lesne industrije in za širšo uporabo. Ključne besede: Albizia lebbeck, manj znane lesne vrste, plantaže, jedrovina, beljava, gostota lesa, upogibna trdnost, modul elastičnosti 1 INTRODUCTION 1 UVOD Due to unsustainable forest extraction, high-value hardwoods continue to become scarc- er worldwide. The high level of global deforesta- tion has several complicated and insufficiently un- Vol. 73, No. 1, 55-68 DOI: https://doi.org/10.26614/les-wood.2024.v73n01a05 1 Tamale Technical University, Department of Wood Technology, Tamale, Ghana 2 Akenten Appiah-Menka University of Skills Training and Entrepreneurial Development, Department of Wood Science and Technology Education, Kumasi, Ghana * e-mail: enochgbapenuo@tatu.edu.gh 56 Les/Wood, Vol. 73, No. 1, June 2024 Tampori, E. G., Bih, F. K., Antwi, K., Chakurah, I.: Fizikalne in mehanske lastnosti lesa vrste Albizia lebbeck s plantaž v Gani derstood immediate consequences (Anon, 2012; Opuni-Frimpong et al., 2017; Owoyemi et al., 2017). It is known that the indiscriminate exploitation of forests, large-scale destruction of tree habitats, and adverse effects on populations are among the fac- tors which have caused a number of timber spe- cies to disappear from forests (Amoah et al., 2015). Over reliance on the traditional timber species in Ghana has also led to their extinction in forests (Amoah et al., 2012). This has therefore put pres- sure on Ghana’s forests, which are limited in extent with a total land area of about 239,000 km² and a general deforestation rate pegged at 65,000 hec- tares per annum (Husseini et al., 2020). The for- est cover of Ghana is about 0.96 million hectares (42%) of the land area, out of which open forests cover about 0.8 million hectares, while closed for- ests cover 0.16 million hectares (Ghana Statistical Service, 2021). As such, the timber species such as Khaya spp. (mahogany), Milicia excelsa (iroko, odum), Entandrophragma cylindricum (sapelli, sa- pale), Nesogordonia papaverifera (kotibe, danta), Terminalia superba (limba, afram), and Turreathus africanus (avodire) for which Ghana is known in the international markets are becoming very scarce (Ntiamoa-Baidu et al., 2001). Establishing plantations has been implement- ed to lessen the over exploitation of commercial timber species and help to restore degraded areas in Ghana (Agyeman et al., 2010). Both afforestation and plantation programmes utilizing plantation wood species have attracted a lot of attention, with the aim of balancing the current tropical timber markets and reducing the excessive utilization of the forests, (Bosu et al., 2006). The creation of planta- tion forests by planting exotic timber species would have numerous advantages, such as substituting natural forests for timber supply, restoring land- scapes that have been damaged by deforestation, and offering ecological benefits, like sequestering carbon dioxide to lessen global warming (Onilude et al., 2020). As a result, it is acknowledged that for- est plantations are important for their conservation value, for restoring areas that have been damaged, and for easing the burden of extraction on current- ly existing forests. As a result of all this, plantation forests are now widely regarded as a component of the conservation triangle, which also includes high-yielding plantations, regulated wild forests, and ecological reserves. In tropical Africa, exotic timber species which are not native in Ghana are mostly cultivated as part of agroforestry initiatives aimed at minimizing soil erosion, managing runoff to prevent desertification, and restoring severe- ly damaged areas (Ogunwusi, 2002). Additionally, they aid in the manufacture of sawn timber, fire- wood, and, occasionally, pulp and paper (Hooper et al., 2005). Since lumber is one of Ghana’s most easily ac- cessible resources, forest plantations have been of interest since the 1920s (Foli et al., 2009; Odoom, 2002). At the time, planting mostly native species in the high forest zone (HFZ) was the typical practice. The few exotic species that were grown in the HFZ were primarily brought in for mining purposes, as well as to provide fuelwood for boilers used to gen- erate electricity and near densely populated areas (Nichols et al., 2006). Starting from 1951, a lot of exotic species were planted in the Savannah Zone (SZ) and Dry Semi-Deciduous Forest Zone (DFSZ) in order to offer fuelwood, poles, and lumber alterna- tives (Amoah et al., 2012; FAO, 2002). Consequent- ly, plantations of hardwoods in the SZ’s northern region are projected to span 2.553 hectares and are mainly used for the generation of fuelwood and the preservation of the environment (Odoom, 2002). The nation’s natural forest is one of its many abundant resources (Opuni-Frimpong et al., 2004; Nichols et al., 2006), and it provided the nation’s distinct climate and environmental conditions up until their recent overexploitation. The increasing market demand for tropical hardwoods both locally and internationally is over- whelming. As such, the amount of wood that may be supplied for the lumber industry without endan- gering the forests exceeds the maximum capacity of Ghanaian sawmills (Appiah, 2003; Hooper et al., 2005). It is therefore imperative that the planting and use of plantation grown exotic timber species should be adopted on a large scale, if it is intended to replenish the depleting forest resources and as well as reduce the urgent demands on the remain- ing known species in the forest. As such, the cur- rent thinking is that industry players in the wood sector must consider the commercial value of the lesser used exotic species to minimize the high de- pendence on the commercial timber species. This is possible by obtaining the full utilization of the less- 57 Les/Wood, Vol. 73, No. 1, June 2024 Tampori, E. G., Bih, F. K., Antwi, K., Chakurah, I.: Physical and mechanical properties of wood of plantation grown Albizia lebbeck in the savannah ecological zone, Ghana er-known timber species to substitute for the lim- ited known tropical hardwoods that remain in the forest, and which are gradually becoming extinct. The main motives for encouraging forest plan- tations in the nation are to lessen the strain on the natural forests while adding to their conservation, as well as making sure the nation can satisfy the de- mand for forest products from its expanding pop- ulation (Oteng-Amoako, 2006). As such, Ghana’s forest plantation initiatives have long been biased toward the establishment of exotic monoculture plantations (Opuni-Frimpong et al., 2008). Indeed, some two decades ago Odoom (2002) asserted that the majority of plantations now in existence in tropical nations, including Ghana, were dominated by exotic species plantations. The inclination to- wards foreign timber species may stem mostly from their rapid growth and relative lack of pests and diseases, as well as better fire resistance, than the numerous native species (Bobby et al., 2012). Fur- thermore, a greater variety of options are provided by foreign species because their silviculture is typi- cally better understood than that of native species. Exotic timber species are thus the preferred plant- ing stock, owing to their rapid growth and immedi- ate economic return (Berger, 2006). It has therefore been proposed that the harvesting of abundant but lesser-known/used plantation grown exotic timber species needs to be gradually encouraged and in- creased in the country. Accordingly, plantation grown exotic species represent over 45% of the ex- isting plantations in Ghana (Nanaag, 2012). More- over, there are over 500 lesser-known species of timber size in the forests of Ghana which have nev- er been used commercially (Oteng-Amoako, 2006). However, since the properties of plantation timbers are different from those of naturally grown ones, there is a need to investigate their properties and variability (Gorišek et al., 2018), as the present lack of reliable information makes predictions about their structural application hazardous and liable to gross errors. To address this gap in knowledge, the aim of this study is to assess selected physical and mechanical properties of a plantation grown Albizia lebbeck (L.) Benth, from the Savannah ecological zone from the Tamale fuel wood plantation reserve which lies between latitudes 9° 16 and 9° 34 north and longitudes 0° 36 and 0° 57 west in Ghana. Albizia lebbeck, belongs to the family Fabace- ae. It is a fast growing medium-sized, multipurpose deciduous tree species with a spreading umbrel- la-shaped crown of thin foliage and finely fissured, greyish-brown bark. On good sites, it can attain an average maximum height of about 18 to 30 m, and 50 to 80 cm diameter at breast height. The tree grows best on moist, and well-drained soils (Tiwari et al., 2020). The fragrant, cream-colored flowers develop on lateral stalks in rounded clusters about 5 to 7.5 cm across the many threadlike, spreading, whitish-to-yellow stamens tipped with light green, borne at the ends of lateral stalks 4 to 10 cm long. The fruits, flattened pods 10 to 20 cm long and 2.5 to 3.8 cm broad, are produced in large num- bers and each contains several seeds. Immature pods are green, turning straw-colored on maturi- ty, usually 6 to 8 months after flowering. Seeds are small, oblong, approximately 9 by 7 mm long and broad, compressed, and light brown in color. The leaves, seeds, bark and roots are all used for tra- ditional medicine (Balkrishna et al., 2022). It was introduced as an ornamental and plantation tree throughout the tropics and northern subtropics (Inside Wood, 2024). 2 MATERIALS AND METHODS 2 MATERIALI IN METODE 2.1 SAMPLE PREPARATION 2.1 PRIPRAVA VZORCEV Three (3) mature plantation grown Albizia leb- beck trees were purposively sampled from the Sa- vannah ecological zone from the Tamale plantation reserve in Ghana. The age of the plantation Albizia lebbeck trees was about 45 years. Trees with similar diameters (45-50 cm) at breast height (1.3 m from the ground level) were felled and the merchantable length of the clear bole of each tree was measured and divided into four parts of the same size (0-25%, 26-50%, 51-75% and 76-100%). A stem sectional disc approximately 7.5 cm in thickness was cut at each end of the divided sections for the determina- tion of the heartwood and sapwood proportions, and air-dry density at 12% moisture content. An experimental study in the laboratory was employed to test the material properties. The wood specimens were prepared at the Tamale Technical University Wood Technology Workshop and every 58 Les/Wood, Vol. 73, No. 1, June 2024 Tampori, E. G., Bih, F. K., Antwi, K., Chakurah, I.: Fizikalne in mehanske lastnosti lesa vrste Albizia lebbeck s plantaž v Gani test was conducted at the Council for Scientif- ic and Industrial Research’s (CSIR) of the Forestry Research Institute of Ghana (FORIG) at the Timber Mechanics and Engineering Laboratory. The speci- mens have been produced following British Stand- ard BS 373, which specifies the use of an Instron machine to test small clear specimens of timber for mechanical properties (MOE, MOR, compres- sion parallel to grain, shear parallel to grain, and hardness) and density at 12% MC. To ensure that the heartwood and sapwood portions of each bil- let were extracted, great care was taken in marking and sawing (Figure 1). The sawn planks were care- fully chosen from visually verified defect-free areas to prepare the specimens. Following sawing, each wood sample was further ripped into the appropri- ate sizes for testing, and the boards underwent ad- ditional planning. 2.2 HEARTWOOD AND SAPWOOD PROPORTIONS 2.2 RAZMERJE JEDROVINE IN BELJAVE To determine the heartwood and sapwood proportions, lines were drawn on the surface across the disc (South, South-East, East, North-East, North, North-West, West and South-West) as a reference point for the measurements to follow. A pencil dot was then marked directly on the line of each heart- wood and sapwood borderline as shown in Figure 2. With the aid of a microscope, the borderline was determined on the basis of the natural color differ- ence between the heartwood and sapwood. Each disc’s diameters (heartwood and sapwood) were carefully identified and linear measurements were taken to the nearest ± 0.001 mm using a digital ca- liper, steel ruler and a tape measure in order to de- termine the heartwood and sapwood proportions as follows: TotalD isc Surface Area R   2 (1) Where   3 142 . R RRRRRRRR   1 2345678 8 Heartwood Surface Area r   2 (2) Figure 1. Albizia lebbeck trees used for the study from Savannah ecological zone of Ghana. Slika 1. Drevesa vrste Albizia lebbeck, uporabljena za študijo, z ekološkega območja Savannah v Gani. 59 Les/Wood, Vol. 73, No. 1, June 2024 Tampori, E. G., Bih, F. K., Antwi, K., Chakurah, I.: Physical and mechanical properties of wood of plantation grown Albizia lebbeck in the savannah ecological zone, Ghana Where r   rrrrrrrr 1 2345678 8 Therefore Sapwood Area Rr ;    22 (3) 2.3 PHYSICAL PROPERTIES OF Albizia lebbeck WOOD 2.3 FIZIKALNE LASTNOSTI LESA VRSTE Albizia lebbeck 2.3.1 Moisture Content (MC) 2.3.1 Vlažnost lesa (MC) The green specimens that were obtained from all merchantable parts of the stems (0-25%, 26- 50%, 51-75%, and 76-100%) were sawn into blocks 20 mm × 20 mm × 20 mm to determine the mois- ture content (MC) using the oven-dry method. The test specimens were instantly weighed using an electronic balance once they were prepared. Af- ter being oven-dried at 103 ± 2 °C, the specimens were cooled in desiccators and weighed again us- ing an electronic scale. Until the weight remained constant, the process was repeated. The propor- tion of the wood’s oven-dry weight was used for determining the moisture content (Panshin & de Zeeuw, 1980). To determine the moisture content, the original weights (W 1 ) and the oven-dry weight (W 2 ) were used to calculate the MC: MC WW W x %   12 2 100 (4) Where MC = moisture content, W 1 = initial weight of sample (g), W 2 = oven-dry weight of sam- ples (g). 2.3.2 Wood Density 2.3.2 Gostota lesa The air-dry density for each sample (20 mm × 20 mm × 20 mm) was determined for parallel stem sections. The wood samples were air-dried in or- der to reach an appreciable percentage of mois- ture content before they were oven-dried at 103 ± 2 °C with intermittent weighing until a constant oven-dry mass (W 0 ) was obtained. After that, the specimens were conditioned for 120 days at 20 °C and 65% relative humidity to achieve a moisture content of roughly 12%. The specimens’ masses and dimensions at 12% MC were utilized to deter- mine the air-dry density:   � m V (5) Where ρ = air-dry density (kg/m³), m = air-dry mass (kg), V = volume of wood sample at 12% MC (m³). 2.4 MECHANICAL PROPERTIES 2.4 MEHANSKE LASTNOSTI For the mechanical properties testing, the test specimens were prepared to the sizes and orienta- tions required by the British Standard, BS 373. Sam- ples were obtained from the tree sections (for each of the three trees). Having air-dried the specimens to an appreciable amount of moisture content they were conditioned to achieve the 12% moisture content and were stored for testing in a controlled chamber. Figure 2. Lines drawn across disc surface for the measurement of the heartwood and sapwood por- tions. Slika 2. Črte, narisane na prečno površino diska za merjenje deležev jedrovine in beljave. 60 Les/Wood, Vol. 73, No. 1, June 2024 Tampori, E. G., Bih, F. K., Antwi, K., Chakurah, I.: Fizikalne in mehanske lastnosti lesa vrste Albizia lebbeck s plantaž v Gani 2.4.1 Modulus of Elasticity (MOE) and Modulus of Rupture (MOR) 2.4.1 Modul elastičnosti (MOE) in upogibna trdnost (MOR) The three-point bending (central loading) sys- tem on an Instron Universal Testing Machine was utilized to perform the static bending tests (mod- ulus of elasticity and modulus of rupture). The ma- chine applied the loading automatically at a rate of 6.5 mm/min. The machine recorded the applied load and related deflection every 0.1 N intervals. The test piece was loaded at this speed until failure occurred. The maximum load at failure as well as the maximum load at the limit of proportionality was recorded by the computer component of the Instron Universal Testing Machine with reference to the outer points of loading. The duration of the test was 90 ± 30 seconds. The modulus of elasticity, E, is computed as: E PL A  1 1 1 2 4 (6) Where E = Young’s modulus, i.e. modulus of elasticity (N/mm²), P 1 = load applied at the limit of proportionality (N), A = area of cross-section of beam normal to direction of load (mm²), ∆ 1 = de- flection at mid-length at limit of proportionality (mm), L = distance between supports (mm). The highest load a wood sample can withstand before breaking is known as the modulus of rup- ture. A test approach identical to that described for MOE was employed to determine the MOR. The modulus of rupture R, is calculated as: R PL bd = 3 2 2 (7) Where R = modulus of rupture (N/mm²), P = maximum load applied at the midpoint of the sam- ple (N), L = distance between supports (mm), b = breadth of test piece (mm), d = depth of the test sample (mm). 2.4.2 Compression Parallel to Grain 2.4.2 Tlačna trdnost vzporedno z vlakni The parallel to longitudinal grain method (BS 373, 1957) was used to determine the resistance to compression. The sample sizes were divided into 2 cm × 2 cm x 6 cm sections using the 2 cm stand- ard. Every specimen was examined to confirm that the testing apparatus was built of the appropriate materials and that the rectangular test piece was parallel, smooth, and normal to the axis. Through- out the whole test duration, the plates that held the test component were parallel to one another. To ensure that correct findings were produced, sev- eral checks were carried out. In compliance with BS 373, a total of 240 samples were tested using the Instron Universal Testing Machine. The duration of the test was 90 ± 30 seconds. This formula is used to calculate the compressive stress at maximum load: C P A = (8) Where C = compressive stress at maximum load (N/mm²), P = maximum load (N), A = cross sec- tional area of sample (mm²). 2.4.3 Shear Parallel to Grain 2.4.3 Strig vzporedno z vlakni BS 373 (1957) was followed in the conducting of the test, with the standard specifying that the sample sizes were 5 cm x 5 cm x 5 cm. Two hundred and forty (240) samples of each billet’s sapwood and heartwood were examined using the 100 kN load cell capacity of the Instron Universal Testing Machine. The crosshead moved at a steady pace of 0.635 mm/min as the load was applied. The grain’s longitudinal direction and the shearing direction were parallel. The item was subjected to the load until it broke. The Instron Universal Testing Ma- chine automatically documented the load at which failure occurred. The duration of the test was 90 ± 30 seconds. Shear parallel to the grain (V) is com- puted as follows: S = P A (9) Where S = shear (N/mm²), P = maximum load (N), A = area in shear (mm²). 61 Les/Wood, Vol. 73, No. 1, June 2024 Tampori, E. G., Bih, F. K., Antwi, K., Chakurah, I.: Physical and mechanical properties of wood of plantation grown Albizia lebbeck in the savannah ecological zone, Ghana 2.4.4 Janka Hardness 2.4.4 Trdota po Janki The BS 373 (1957) was followed while conduct- ing the test. The specimen was chopped tangen- tially and radially to measure 5 cm x 5 cm x 15 cm for the hardness test. A total of 240 sapwood and heartwood samples were subjected to a hardness test fixture on the Instron Universal Testing Ma- chine. The fixture consists of an 11.3 ± 2.5 mm di- ameter steel ball at one end of a steel bar. The hem- ispherical end of the steel bar (steel ball) enters the test piece when a load is applied. The Instron ma- chine automatically records the failure load as the amount of force required to drive the steel ball’s hemispherical end into the test piece to a depth of 5.6 mm, i.e. equal to the steel ball’s radius. The determined maximum force required to indent the steel ball halfway into the wood was used to de- termine the Janka hardness. Merely the tangential and radial surfaces were determined. The radial and tangential surfaces that most closely resem- bled the actual radial and tangential directions of the grain were selected for testing. 2.4.5 Data Analysis 2.4.5 Analiza podatkov Analysis of Variance (ANOVA) was used to compare the results from the radial portions and to determine whether the differences found between the sapwood and heartwood were significant. The statistical tool used for the analyses was SigmaPlot version 14.0. Descriptive statistics consisting of means with standard deviations were presented for each tree section used for the study. 3 RESULTS AND DISCUSSION 3 REZULTATI IN RAZPRAVA 3.1 HEARTWOOD AND SAPWOOD PROPORTIONS 3.1 DELEŽ JEDROVINE IN BELJAVE The mean percentages of heartwood and sap- wood proportions were 74% (26%), 75% (25%) and 74% (26%) for trees 1, 2 and 3, respectively (Fig- ure 3). The bottom portion of each tree record- ed a greater percentage of heartwood and the heartwood portion of the stem decreased from the bottom to the top of the stem whereas the sapwood portion of the stem increased from the bottom to the top, which is consistent with Qadri and Mahmood (2005). Heartwood is more durable than sapwood and less subjected to attack by in- sects, stain and mould producing fungi (Elzaki et al., 2012). The heartwood is also coloured and there- fore considered more decorative than the light coloured sapwood (Hassan et al., 2007; Taylor et al., 2002; Zia-Ul-Haq et al., 2013). 3.2 PHYSICAL PROPERTIES (DENSITY) OF Albizia lebbeck WOOD 3.2 FIZIKALNE LASTNOSTI LESA (GOSTOTA) VRSTE Albizia lebbeck Air dry density at 12% moisture content (MC) of three plantation grown Albizia lebbeck trees, showed the following results at different levels in Figure 3. Average percentage (%) of heartwood and sapwood in the test trees. Slika 3. Povprečni delež (%) jedro- vine in beljave v testnih drevesih. 62 Les/Wood, Vol. 73, No. 1, June 2024 Tampori, E. G., Bih, F. K., Antwi, K., Chakurah, I.: Fizikalne in mehanske lastnosti lesa vrste Albizia lebbeck s plantaž v Gani the stems: tree 1 recorded an average density of 842, 790, 659 and 573 kg/m³, tree 2 recorded 871, 788, 699 and 562, while tree 3 recorded an aver- age density of 892, 841, 726 and 561 for sections 0-25%, 26-50%, 51-75% and 76-100% respectively (Table 1).The mean difference between two close sections, for example 26-50% compared with 51- 75%, was statistically insignificant. However, the mean difference for sections which are far apart, for example 0-25% and 76-100%, was statistically significant (Table 2 and 3). This trend was also re- ported by Moya and Ledezma (2003), as support- ed by the findings of Forest Products Laboratory (2010). For radial variation, the heartwood displayed higher air-dry density than its corresponding sap- wood portion. There was a significant difference in density between the heartwood and sapwood in all the trees studied. According to Onilude et al. (2020), wood generally increases in density during the transformation from sapwood to heartwood. The change in density is ascribed to deposition of extractives such as phenols and quinines, which en- hances the durability of wood. Factors that might have had ab influence on the density values include the growth rate, plantation site, climate and geo- graphical location (Sasmal et al., 2013; Wanneng et al., 2014), and as we showed the position in the Table 1. Axial variation of air-dry density at 12% MC (kg/m³) of plantation grown Albizia lebbeck. Preglednica 1. Aksialna variacija gostote zračno suhega lesa plantažno gojene vrste Albizia lebbeck. Tree Section Density at 12% MC (kg/m³) Tree 1 Tree 2 Tree 3 0–25% 842.11 (±81.26) 871.41 (±77.66) 892.42 (±100.63) 26–50% 790.02 (±67.36) 788.08 (±64.00) 841.00 (±77.33) 51–75% 659.59 (±69.28) 699.85 (±63.57) 726.34 (±73.71) 76–100% 573.59 (±67.58) 562.40 (±54.64) 561.11 (±51.46) Average value and standard deviation in parentheses. Table 2. ANOVA for comparison of density between heartwood and sapwood portions. Number of repli- cates (N), standard deviation (Std. Dev), standard error of the mean (SEM), degrees of freedom (df), sum of squares (SS), mean square (MS), F-statistic (F), P-value (P). Preglednica 2. Analiza variance (ANOVA) za primerjavo gostote med jedrovino in beljavo. Legenda: število ponovitev (N), standardni odklon (Std. Dev), standardna napaka povprečja (SEM), stopnje prostosti (df), vsota kvadratov (SS), srednji kvadrat (MS), F-statistika (F), P-vrednost (P). Descriptive statistics One Way RM ANOVA Tree portion N Missing Mean Std. Dev SEM Source of variance df SS MS F P T1 Heart 10 0 847.53 33.36 10.55 Between Subjects 9 14006.95 1556.33 T2 Heart 10 0 845.95 31.91 10.09 Between portions 5 1922348.81 384469.76 834.71 <0.001 T3 Heart 10 0 840.21 34.90 11.04 Residual 45 20727.14 460.60 T1 Sap 10 0 486.74 12.71 4.02 Total 59 1957082.89 T2 Sap 10 0 482.68 14.07 4.45 T3 Sap 10 0 490.48 12.29 3.89 63 Les/Wood, Vol. 73, No. 1, June 2024 Tampori, E. G., Bih, F. K., Antwi, K., Chakurah, I.: Physical and mechanical properties of wood of plantation grown Albizia lebbeck in the savannah ecological zone, Ghana tree, too. These qualities can be compared to those of the most well-known timber species, including Milicia excelsa (odom), Khaya spp. (mahogany), Cylicodiscus gabunensis (denya), Piptadeniastrum africanum (dahoma), and Aningeria spp. (asan- fena) that are in short supply in the timber markets. 3.3 MECHANICAL PROPERTIES 3.3 MEHANSKE LASTNOSTI The average MOE of the wood of all the trees under study was 14356 N/mm², 14071 N/ mm², 13322 N/mm², and 12367 N/mm² for all the tree sections 0-25%, 26-50%, 51-75% and 76- 100%, respectively. Also, the MOR of all the trees had an average of 129 N/mm², 115 N/mm², 112 N/ mm² and 101 N/mm² for the sections, respective- ly. Moreover, the average strength in compression parallel to grain of all the trees recorded 59 N/mm², 50 N/mm², 49 N/mm² and 46 N/mm² for the sec- tions. Furthermore, shear parallel to grain for the trees under study recorded an average of 21 N/ mm², 20 N/mm², 20 N/mm² and 17 N/mm² for the sections, respectively. Finally, the average hardness property obtained for all the trees recorded 11 kN, 11 kN, 9 kN and 7 kN for sections 0-25%, 26-50%, 51-75% and 76-100%, respectively (Table 4). Axial variation in the parameters strongly suggests that all the mechanical properties of all three trees de- creased from the bottom portion of the trees to the top portion. 3.3.1 Modulus of Elasticity 3.3.1 Modul elastičnosti Axial variation of all the plantation grown Albi- zia lebbeck strongly suggests that the MOE values reduce from the bottom portion of the tree to the top portion, with an average strength of 14356 N/ mm², 14071 N/mm², 13322 N/mm², and 12367 N/ mm², respectively, as shown in Table 3. However, the difference between the means was insignif- icant. This result confirms the assertion that the axial variation of some timber species decreases significantly along the bole height from the bottom portion to the top (Chulet et al., 2010). The analysis Table 3. Pairwise Multiple Comparison of density between heartwood sapwood. Degrees of freedom (df), t- value (t), P- value (P). Preglednica 3. Večkratna primerjava parov povprečij gostote med jedrovino in beljavo. Legenda: stopnje prostosti (df), t-vrednost (t), P-vrednost (P). Comparison df t P P<0.050 T1 HEARTWOOD vs. T2 SAPWOOD 364.852 38.014 <0.001 Yes T2 HEARTWOOD vs. T2 SAPWOOD 363.275 37.849 <0.001 Yes T1 HEARTWOOD vs. T1 SAPWOOD 360.793 37.591 <0.001 Yes T2 HEARTWOOD vs. T1 SAPWOOD 359.216 37.426 <0.001 Yes T3 HEARTWOOD vs. T2 SAPWOOD 357.529 37.251 <0.001 Yes T1 HEARTWOOD vs. T3 SAPWOOD 357.056 37.201 <0.001 Yes T2 HEARTWOOD vs. T3 SAPWOOD 355.479 37.037 <0.001 Yes T3 HEARTWOOD vs. T1 SAPWOOD 353.470 36.828 <0.001 Yes T3 HEARTWOOD vs. T3 SAPWOOD 349.733 36.438 <0.001 Yes T3 SAPWOOD vs. T2 SAPWOOD 7.796 0.812 0.962 No T1 HEARTWOOD vs. T3 HEARTWOOD 7.323 0.763 0.949 No T2 HEARTWOOD vs. T3 HEARTWOOD 5.746 0.599 0.960 No T1 SAPWOOD vs. T2 SAPWOOD 4.059 0.423 0.965 No T3 SAPWOOD vs. T1 SAPWOOD 3.737 0.389 0.909 No T1 HEARTWOOD vs. T2 HEARTWOOD 1.577 0.164 0.870 No significance level = 0.05 stopnja značilnosti = 0,05 64 Les/Wood, Vol. 73, No. 1, June 2024 Tampori, E. G., Bih, F. K., Antwi, K., Chakurah, I.: Fizikalne in mehanske lastnosti lesa vrste Albizia lebbeck s plantaž v Gani of variance (ANOVA) shows that there is an insig- nificant difference with regard to the modulus of elasticity between the tree sections within each in- dividual tree of species. 3.3.2 Modulus of Rupture 3.3.2 Upogibna trdnost In terms of axial variation, the species’ bending strength, i.e. modulus of rupture, values typically decrease from the base to the apex of the tree. There was a noticeable marginal drop in MOR from the base to the top of the tree. This supports ear- lier claims (Ayarkwa et al., 2000; Uma et al., 2009) that strength qualities increase with decreasing moisture content. When comparing the heart- wood’s modulus of rupture sectionally, the mean difference amongst the trees was negligible. Most heavy construction species, including essa (104 MPa) and dahoma (109 MPa), compare favorably to the average mean values of MOR reported for Albizia lebbeck (129 N/mm², 115 N/mm², 112 N/ Table 4. Means of mechanical properties of plantation grown Albizia lebbeck. Preglednica 4. Mehanske lastnosti lesa vrste Albizia lebbeck, gojene v nasadih. Mechanical Properties Tree sections 0-25% 26-50% 51-75% 76-100% MOE (N/mm²) Tree 1 14688 (±1950.12) 14289 (±1885.69) 12212 (±1257.56) 11932 (±1465.17) Tree 2 15087 (±1230.09) 14746 (±1321.94) 14667 (±1285.54) 14022 (±1644.44) Tree 3 13292 (±2160.50) 13178 (±2529.27) 13087 (±1939.03) 11147 (±4024.22) MOR (N/mm²) Tree 1 128.65 (±12.62) 108.90 (±27.23) 108.43 (±16.68) 98.35 (±18.27) Tree 2 135.77 (±15.54) 121.06 (±14.38) 118.87 (±14.00) 115.28 (±18.87) Tree 3 122.70 (±24.44) 114.41 (±23.71) 108.46 (±21.04) 89.46 (±34.97) Compression Parallel to grain (N/mm²) Tree 1 66.63 (±9.66) 49.33 (±4.60) 47.60 (±5.55) 46.09 (±3.95) Tree 2 54.70 (±6.43) 52.69 (±5.66) 51.59 (±4.48) 45.94 (±4.65) Tree 3 54.23 (±5.43) 49.28 (±3.95) 47.65 (±2.25) 46.92 (±6.53) Shear parallel to grain (N/mm²) Tree 1 21.45 (±3.64) 20.02 (±7.24) 18.78 (±5.26) 15.18 (±3.91) Tree 2 21.45 (±2.59) 20.52 (±3.98) 20.49 (±2.76) 19.94 (±4.77) Tree 3 21.33 (±3.40) 20.80 (±2.42) 19.88 (±2.90) 16.88 (±3.88) Janka Hardness (kN) Tree 1 12.53 (±4.00) 12.31 (±8.30) 9.57 (±3.23) 7.07 (±2.01) Tree 2 8.70 (±2.15) 8.68 (±4.47) 6.12 (±1.96) 5.08 (±3.05) Tree 3 11.15 (±2.40) 10.83 (±2.42) 10.43 (±2.27) 8.18 (±1.00) Average value and standard deviation in parentheses. Povprečna vrednost in standardni odklon v oklepajih. 65 Les/Wood, Vol. 73, No. 1, June 2024 Tampori, E. G., Bih, F. K., Antwi, K., Chakurah, I.: Physical and mechanical properties of wood of plantation grown Albizia lebbeck in the savannah ecological zone, Ghana mm² and 101 N/mm²) for all three trees (Ayarkwa et al., 2000). Albizia lebbeck belongs to class S3, ac- cording to Bolza and Keating’s (1972) classification. 3.3.3 Compression Parallel to Grain 3.3.3 Vzdolžna tlačna trdnost The average compressive strength results ob- tained for Albizia lebbeck were 59 N/mm², 50 N/ mm², 49 N/mm² and 46 N/mm² (Table 3, Table 4 and Table 5) for trees 1, 2 and 3, respectively, com- pares favourably with the values of most timber species used for heavy construction. Some of these include odum (Milicia exelsa) with a compressive strength of 52 MPa, ofram (Terminalia superba) with 33.80 MPa, iroko (Chlorophora spp) with 32.62 MPa, emeri (Terminalia ivorensis) with 35.00 MPa and dahoma (Piptadeniastrum africanum) with 23.00 MPa (Bosu et al., 2006; Forest Products Lab- oratory, 2010; Appiah-Kubi et al., 2012). Farmer (1972) classed the compressive strength parallel to the grain as follows: very low, low, medium, high, and very high when the strength values are under 20 MPa, 20–35 MPa, 35–55 MPa, 55–85 MPa, and over 85 MPa, in that order. As a result, the com- pressive strength of Albizia lebbeck is rated as me- dium in this classification. 3.3.4 Shear Strength Parallel to Grain 3.3.4 Vzdolžna strižna trdnost The shear strength comparison indicated a sig- nificant mean difference between the heartwood and sapwood. This suggests that the radial varia- tion strength of heartwood is greater with regard to resisting failure than that of the sapwood. Though the axial shear strength properties marginally de- crease from the bottom portion to the top portion of the tree (21 N/mm², 20 N/mm², 20 N/mm² and 17 N/mm²), as depicted in Table 3, for all the trees. This trend confirms an earlier assertion by Hassan et al. (2007) and Sasmal et al. (2013). The strength values obtained for plantation grown Albizia leb- beck trees (19.72 N/mm²) compare favourably to those of most heavy construction species, including denya (Cyclidiscus gabunensis) at 11.10 MPa, daho- ma (Piptadeniastrum africanum) at 9.60 MPa, and asanfena (Aningeria altissima) at 9.50 MPa, (Antwi et al., 2014). 3.3.5 Janka Hardness (Radial and Tangential) 3.3.5 Trdota po Janki (radialna in tangencialna) There was no significant variation among the axial positions along the bole (0-25%, 26-50%, 51- 75% and 76-100%) for hardness (Table 3, Table 4 and Table 5) for trees 1, 2 and 3, respectively. Ev- idently, these results demonstrate the ability of the various parts of all the trees of the plantation grown Albizia lebbeck to resist indentation. The re- sistance of Albizia lebbeck to indentation. i.e. Janka hardness, was relatively high (9.22 kN) for all the trees and it can be classified as a class IV hardwood, and thus used for high-class furniture production. 4 CONCLUSIONS 4 SKLEPI The examination of the plantation grown tim- ber species Albizia lebbeck, exotic in Ghana, has provided useful information in terms of the full utilization potential of this species as a good possi- ble substitute for the limited known tropical hard- woods with similar properties, which are now fac- ing extinction. Based on the findings of this study, the following conclusions were made. The heart- wood and sapwood proportions (74%, 26%) of the plantation grown Albizia lebbeck varied considera- bly and statistically significantly in all the trees stud- ied. The studied trees with a diameter of 40-50 cm generally had a greater percentage of heartwood than sapwood, suggesting that enough heartwood can be obtained from the stems for furniture and structural utilization. Heartwood is desired for fur- niture and other engineering purposes in the trop- ical zones where biodegradation by organisms is common. However, the sapwood of some species of timber could perform equally well when prop- erly treated. It was also observed that the plantation grown Albizia lebbeck species exhibited a mean density of 734 kg/m³ for all the trees under study. This reveals that Albizia lebbeck has the potential required by the furniture and construction industries to serve as a substitute for the rarer timber species. The mean density found in the present study is com- parable to that reported for other timbers, such as Celtis mildbraedii, Celtis zenkeri, Petersia africana (essia), and Nesogordonia papaverifera (danta), which have densities of 781 kg/m³, 743 kg/m³, 66 Les/Wood, Vol. 73, No. 1, June 2024 Tampori, E. G., Bih, F. K., Antwi, K., Chakurah, I.: Fizikalne in mehanske lastnosti lesa vrste Albizia lebbeck s plantaž v Gani 738 kg/m³ and 712 kg/m³, respectively, and which are mostly used in the Ghanaian furniture and con- struction industries (Ofori et al., 2009). In terms of the mechanical properties, the mean values for the modulus of elasticity (13528 N/ mm²), modulus of rupture (114 N/mm²), maximum compressive strength parallel to grain (51 N/mm²), shear parallel to grain (19 N/mm²), and hardness (9 kN) varied significantly along the sampling height. Generally, all the mean values decreased along the tree height from the bottom to the top portion. The results showed that the MOE and MOR values were high and compared favorably with those of other commercial tree species, which confirmed the suit- ability of the plantation grown Albizia lebbeck spe- cies for various furniture and construction work. Moreover, the compressive strength, resist- ance to shear and hardness values of Albizia leb- beck mean this wood has relatively high resistance to deformation and indentation, and there is al- ways a strong correlation between wood hardness and strength. It can therefore be concluded that the plantation grown Albizia lebbeck species could be used for medium to heavy timber structures. The values recorded for the mechanical properties of the wood species have validated its suitability for use in high-quality furniture products and other in- terior applications. 5 SUMMARY 5 POVZETEK V nasadih na ekološkem območju Savannah v Gani gojijo več eksotičnih lesnih vrst, za katere imamo malo informacij o lastnostih njihovega lesa. Znano je, da je trenutno že veliko dreves na plan- tažah zrelih za posek, a jih še vedno ne sekajo za komercialne namene. Medtem beležijo v Gani vse večje pomanjkanje lesa glavnih znanih komercial- nih tropskih listavcev, zlasti tistih z gostejšim lesom. To spodbuja potrebo po uporabi lesa eksotičnih li- stavcev s plantaž, kar bi lahko delno zadostilo po- trebam države po lesu. Med potencialno zanimivi- mi drevesnimi vrstami je Albizia lebbeck, ki bi lahko nadomestila nekatere bolj znane komercialne lesne vrste. Zato moramo bolje poznati lastnosti lesa te vrste in možnosti njegove uporabe. Namen te štu- dije je bil določiti nekatere fizikalno-mehanske last- nosti lesa vrste Albizia lebbeck s plantaže na ekolo- škem območju Savannah, Tamale, v Gani. Specifični cilji raziskave so bili ovrednotiti deleže jedrovine in beljave in določiti nekatere fizikalne lastnosti (go- stoto zračno suhega lesa) ter mehanske lastnosti (vzdolžna tlačna trdnost, vzdolžna strižna trdnost, trdota po Janki, upogibna trdnost in modul elastič- nosti) in njihovo variabilnost vzdolž debla. Izbrana so bila tri zrela drevesa vrste Albizia lebbeck s premeri od 45 do 50 cm v prsni višini (1,3 m). Drevesa so bila posekana in izmerjene so bile tehnične dolžine hlodov od baze drevesa do nivoja krošnje. Iz posameznih debel so bili izžagani hlodiči za raziskave na štirih nivojih glede na višino v drevesu (0–25 %, 26–50 %, 51–75 % in 76–100 % višine). Vzorci za določanje fizikalnih in mehanskih lastnosti so bili razžagani na zahtevane velikosti v skladu z britanskim standardom BS 373 (1957) za testiranje majhnih čistih vzorcev lesa. Označevanje in žaganje je bilo opravljeno tako, da so pridobi- li vzorce beljave in jedrovine iz vsakega segmenta raziskanih dreves. Za pripravo vzorcev so bile izbra- ne deske brez napak kot so grče, odklon vlaken in poškodbe, ki jih povzročajo biološki škodljivci ter abiotski dejavniki. Po žaganju smo deske še dodat- no skobljali in skrbno pregledali z vidika morebitnih napak. Po sušenju do zračne suhosti so bili vzorci lesa izdelani v delavnici za lesno tehnologijo Teh- niške univerze Tamale (Tamale Technical Universi- ty Wood Technology Workshop). Vlažnost lesa pri zračni suhosti in v absolutno suhem stanju ter go- stoto lesa smo določili v Laboratoriju za gradbeni- štvo in les (Construction and Wood Laboratory of the Akenten Appiah-Menka University of Skills Tra- ining and Entrepreneurial Development, Kumasi, Ghana). Vsi ostali testi so bili opravljeni v Laborato- riju za lesno mehaniko in inženiring Inštituta za goz- darske raziskave Gane (FORIG), Sveta za znanstvene in industrijske raziskave (CSIR) (Timber Mechanics and Engineering Laboratory, of the Forestry Resear- ch Institute of Ghana (FORIG) of Council for Scienti- fic and Industrial Research (CSIR)). Približno 7,5 cm debeli koluti s štirih nivojev v deblu posameznega drevesa so bili uporabljeni za oceno deleža jedrovine in beljave. Izmerjeni so bili premeri kolutov in premeri delov z jedrovino vzdolž več radijev za izračun deležev jedrovine in belja- ve. Upogibna trdnost (MOR) in modul elastičnosti (MOE), vzdolžna tlačna in strižna trdnost ter trdota 67 Les/Wood, Vol. 73, No. 1, June 2024 Tampori, E. G., Bih, F. K., Antwi, K., Chakurah, I.: Physical and mechanical properties of wood of plantation grown Albizia lebbeck in the savannah ecological zone, Ghana so bili določeni za zračno suh les, pri pribl. 12 % le- sni vlažnosti. Rezultati so pokazali, da ima Albizia lebbeck danih premerov višji delež jedrovine (74,04 %) kot beljave (25,96 %). Les je dokaj gost in ima gostoto zračno suhega lesa 869 kg/m³, 806 kg/m³, 695 kg/ m³ in 565 kg/m³ na nivojih debla 0–25 %, 26–50 %, 51–75 % oziroma 76–100 % aksialne višine (pregle- dnica 1). Na splošno je Albizia lebbeck imela dobre trdnostne lastnosti (Preglednica 4), kar kaže na to, da bi jo lahko uporabili kot alternativno vrsto za os- krbo lesne industrije. Očitno lahko plantaže dreves vrste Albizia lebbeck izkoriščamo za pridobivanje kvalitetnih žagarskih sortimentov. Sicer je les jedro- vine temno rjav, prepreden s temnimi in belimi od- tenki, to daje jedrovini dober videz in dekorativno teksturo, kar je pomembno za rabo v notranjih pro- storih in za izdelavo pohištva. Čeprav gostota lesa variira v aksialni smeri dreves, in od spodnjega do zgornjega dela drevesa upada, razlika povprečij go- stote med testiranimi četrtinami delov debel ni bila značilna (t-test; p > 0,05). Srednje vrednosti me- hanskih lastnosti so bile primerljive z vrednostmi lesnih vrst, kot so denia (Cyclidiscus gabunensis), dahoma (Piptadeniastrum africanum), asanfena (Aningeria altissima) in druge, ki jih uporabljajo v Gani za gradbene namene. Na splošno je bila trdo- ta lesa vrste Albizia lebbeck razmeroma visoka, kar nakazuje, da je les uporaben za pohištvo in druge notranje konstrukcije. ACKNOWLEDGEMENT ZAHVALA The authors are grateful to the management and staff, especially Seidu Haruna of the Forestry Research Institute of Ghana – Fumesua, Kumasi for making their furniture test laboratory available for the study. REFERENCES VIRI Amoah, M., Appiah-Yeboah, J., & Okai, R. (2012). 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