31 Les/Wood, Vol. 71, No. 2, December 2022 Vol. 71, No. 2, 31-44 DOI: https://doi.org/10.26614/les-wood.2022.v71n02a02 1 Department of Forest Products Development and Utilization. Forestry Research Institute of Nigeria. P .M.B 5054, Jericho, Ibadan, Nigeria * e-mail: ojo.ar@frin.gov.ng CORRELATION BETWEEN DRY DENSITY AND SHRINKAGE IN EIGHT TROPICAL HARDWOOD SPECIES ZVEZA MED GOSTOTO IN KRČENJEM LESA OSMIH TROPSKIH VRST Adedeji Robert Ojo 1* , Samson Oluwaseun Ogutuga 1 , Lawrence Olanipekun Aguda 1 UDK 630*812:176.1 Received / Prispelo: 16.7.2022 Original scientific article / Izvirni znanstveni članek Accepted / Sprejeto: 29.9.2022 . Abstract / Izvleč ek Abstract: Eight tropical hardwood species were assessed for their density and radial, tangential and volumetric shrinkage, after which the relation between the density and different shrinkages was checked through correlation and regression. The results showed that the highest mean density was observed in Milicia excelsa with 900.63±50.13 kg/m³, followed by Afzelia africana, Nesogordonia kabingaensis and Nauclea diderichii with 831.25±41.67 kg/m³, 808.75±20.88 kg/m³ and 801.88±46.40 kg/m³, respectively. The mean density for Cassia simea was 781.88±27.71 kg/m³, Mansonia altissima 593.13±65.98 kg/m³, and Sterculia tragacantha 481.25±111.73 kg/m³, while the least density was observed in Treculia africana with 463.75±67.88 kg/m³. The highest volumetric shrinkage was observed in Nesogordonia kabingaensis with 14.71±2.28%, and the least in Cassia simea with 5.11±2.65%. It is concluded that there exists positive but weak correlation between density and the shrinkages in the eight tropical hardwood species. Keywords: wood, topical hardwoods, density, shrinkage Izvleček: V raziskavi ocenjujemo zvezo med gostoto in krčenjem za osem tropskih lesnih vrst. Raziskali smo gostoto ter radialno, tangencialno in prostorninsko krčenje osmih tropskih lesnih vrst. Rezultati so pokazali, da je imela največjo povprečno gostoto vrsta Milicia excelsa z 900,63±50,13 kg/m³, sledijo Afzelia africana, Nesogordonia kabingaensis in Nauclea diderichii z 831,25±41,67 kg/m³, 808,75±20,88 kg/m³ in 801,88±46,40 kg /m³. Povprečna gostota za vrste Cassia simea je bila 781,88±27,71 kg/m³, Mansonia altissima 593,13±65,98 kg/m³, Sterculia tragacantha 481,25±111,73 kg/m³, medtem ko je bila najmanjša gostota opažena pri vrsti Treculia africana s 463,75±67,88 kg/m³. Največje prostorninsko krčenje je bilo zaznano pri vrsti Nesogordonia kabingaensis (14,71 ± 2,28 %), najmanjše pa pri vrsti Cassia simea (5,11 ± 2,65 %). Ugotavljamo, da obstaja šibka pozitivna korelacija med gostoto in krčenjem pri osmih tropskih vrstah listavcev. Ključne besede: les, tropski listavci, gostota, krčenje 1 INTRODUCTION 1 UVOD Water is a natural constituent of a living tree, and it commonly makes up over half the total weight; that is, the weight of water in green wood is commonly equal to or greater than the weight of the dry wood substance (Haygreen & Bower, 1996). It is well known that wood is an anisotropic material which presents differential dimensional changes in the different structural directions. The magnitude of shrinkage and swelling is affected by the amount of moisture gained or lost by wood when the moisture content fluctuates between zero and the fibre saturation point (Usta & Guray, 2000). Shrinking and swelling occur as the wood changes moisture content in response to daily as well as seasonal changes in the relative humidity of the atmosphere. That is, when the air is humid wood adsorbs moisture and swells, while when the 32 Les/Wood, Vol. 71, No. 2, December 2022 Ojo, A. R., Ogutuga, S. O., & Aguda, L. O.: Zveza med gostoto in krčenjem lesa osmih tropskih vrst air is dry, wood loses moisture and shrinks. Various finishes and treatments may be used to slow this process, but, in general, they do not stop it (Eckel- man, 2012). Likewise, air and kiln drying do not pre- vent the wood from subsequently gaining or losing moisture. Thus, wood that is kiln dried to 6% mois- ture content and stored in a dry shed outdoors in a temperate climate, such as that found in Indiana, USA, will regain moisture until it eventually reaches about 12% moisture content. Under the same con- ditions in a tropical climate, the wood will come to a moisture content of about 16%. The resulting di- mensional changes in the wood are a major source of defects in furniture and other wood structures (Ojo et al., 2016). The changes in wood dimensions as a result of its shrinkage as it dries and swelling during moisture absorption are of great importance to anyone who uses wood, because wood readily takes on or gives off moisture, even from the atmosphere. When wood loses moisture below fibre saturation point (FSP), it shrinks and swells when water is absorbed. The percentage change in wood dimensions as a re- sult of moisture loss is termed shrinkage (Dinwood- ie, 1989). The observed changes in wood dimen- sions as a result of shrinkage are unequal along the three structural directions. This behaviour of wood has been documented widely by various authors (Panshin & de Zeeuw, 1980; Lausberg et al., 1995; Ogunsanwo, 2000; Ojo et al., 2016). However, Panshin and de Zeeuw (1980), not- ed that the geometric disposition of cells along the principal directions is mainly responsible for this observation. The moisture content is the water contained in wood and it is a natural constituent of all parts of a living tree and makes up about half of the total weight (Hossain et al., 1991). Desch and Dinwood- ie (1983), reported that timber of living trees and freshly felled logs contains a large amount of water which has a profound influence on the properties of wood, such as weight and strength. Wood is also liable to attack by some insects and fungi when the moisture content is high. Density is the amount of wood substance per unit volume. Panshin and de Zeeuw (1980), Din- woodie (1981), and Desch (1988), explained that the density of wood is a function of the cell wall thickness and also depends on the level of cell wall development. In research activities, densities are frequently reported on an oven-dry weight and volume ba- sis. At any other condition the moisture content has a marked effect on density (Kellog, 1981). As the moisture content increases from the oven-dry condition up to fibre saturation point, the weight increases and as a result of swelling so does the volume (Kellog, 1981). Meanwhile, high density is associated with thick fibre walls and a higher pro- portion of fibres. These are the qualities which contribute to water absorption and resultant di- mensional changes (Shrivastava, 1997). It is there- fore necessary to ascertain that in the absence of any other data about the dimensional stability of a particular species, wood density is used as a guide to its utilization by establishing the relationship between the density and dimensional stability of some tropical hard wood species so as to serve as guide for their general technical application. The eight tropical timbers assessed in this study are as follows: Nauclea diderrichii (African peach; opepe) is an evergreen tree that reaches a height of about 30-40 m and a diameter of 0.9-1.5 m; bole cylindrical, slender, straight and branchless, rising to 20-30 m and a broad spherical crown with thick foliage (Orwa, 2009). It is a commercial timber of West Africa. The wood is yellow and darkens slightly when exposed to light. It is semi-heavy and of medium hardness. Because of its good mechanical properties and natural durability, which can be enhanced by pre- servative treatment, it is sought after as a timber for outdoor uses (harbour works, railway sleepers), buildings (carpentry, floors, facings, indoor and outdoor woodwork) and for cabinet making. The wood is also suitable for fence posts and bridges as it is moderately termite-resistant and resistant to fungi and marine borers (Orwa, 2009). The wood is strong and moderately hard to hard. At 12% moisture content the modulus of rupture is 85–166 N/mm², modulus of elasticity 10,490–14,660 N/mm², compression parallel to grain 52–78 N/mm², shear 8.5–17 N/mm², cleavage (7–)12–24 N/mm, Janka side hardness 5790–7260 N, Janka end hardness 7140–9160 N and Cha- lais-Meudon side hardness 3.0–8.6. (Protabase, 2010). 33 Les/Wood, Vol. 71, No. 2, December 2022 Ojo, A. R., Ogutuga, S. O., & Aguda, L. O.: Correlation between dry density and shrinkage in eight tropical hardwood species Milicia excelsa (African teak; iroko) is large de- ciduous tree, growing up to 50 m high and 350 cm in trunk diameter. The trunk is often buttressed and can be branchless for up to 20 m (Barwick, 2004). The heartwood is pale yellow to yellow, darken- ing on exposure to yellowish or greenish brown or sometimes to chocolate brown. The wood is some- what greasy and is odourless (Protabase, 2022). The wood is of medium weight, moderately hard, of good durability, being resistant to fungi, dry wood borers and termites. The wood is a highly valued commercial timber in Africa, for which de- mand is also high. It is used for construction work, shipbuilding and marine carpentry, sleepers, sluice gates, framework, trucks, draining boards, outdoor and indoor joinery, stairs, doors, frames, garden furniture, cabinet work, panelling, flooring and profile boards for decorative and structural uses. It is also used for carving, domestic utensils, musi- cal instruments and toys. As it is resistant to acids and bases, it is used for tanks and barrels for food and chemical products and for laboratory benches. It is used as sliced veneer but only rarely as rotary veneer. The wood is also used as firewood and for making charcoal (Protabase, 2022). At 12% mois- ture content, the modulus of rupture is 75–156 N/ mm², modulus of elasticity 8300–13,300 N/mm², compression parallel to grain 42–65 N/mm², shear 5.4–14.1 N/mm², cleavage 10.3–20.9 N/mm, Janka side hardness 4400–5610 N, and Janka end hard- ness 5360–6640 N (Protabase, 2010). Afzelia africana (afzelia) is an evergreen, small to fairly large tree up to 40 m tall; bole branchless for up to 20 m, usually straight and cylindrical, up to 150(–200) cm in diameter. Like other Afzelia spp., the wood of Afzelia africana is characterized by excellent stability with little susceptibility to varia- tions in humidity and a good natural durability. The wood is durable and treatment with preservatives is unnecessary, even for usage in permanently hu- mid conditions or in localities where wood-attack- ing insects are abundant. This makes it an excellent wood for use in pleasure-crafts, especially for keels, stems and panels, for bridges, as well as interior fit- tings. The wood is also valued for joinery and pan- elling, both interior and exterior, parquet floors, doors, frames, stairs, furniture and sporting goods. The wood is also used as firewood and for charcoal production (Gerard & Louppe, 2011). The heartwood is orange-brown to golden brown, becoming red-brown upon prolonged expo- sure, sometimes with darker streaks. It is distinct- ly demarcated from the whitish to pale yellow, up to 8 cm wide sapwood. At 12% moisture content, the modulus of rupture is 105–145(–200) N/mm², modulus of elasticity (9100–)14,000–17,000 N/ mm², compression parallel to grain 57–85 N/mm² (Gerard & Louppe, 2011). Cassia siamea (Senna siamea; yellow casia; kassod tree) is large-sized tree species, with a height up to 20-25 m, diameter 50-60 cm. Its stem is cylindrical, twisted. The bark is grey, with regular small and narrow cracks, sometimes segments are formed due to the stem twisting at some points. (SSR-VINA, 2022). Sapwood and heartwood have different colours; sapwood is pale yellow, heart- wood got yellowish-brown to blackish brown. Sen- na siamea wood can be used for structures requir- ing high durability in construction, and transport (SSR-VINA, 2022). The pressure strength along the grain is 615kg/ cm², the static bending strength is 1520kg/cm², splitting strength is 20kg/cm, collision bending is 0.64, Janka hardness 1,490 lbf (6,640 N), modulus of rupture 12,440 lbf/in2 (85.8 MPa), elastic modu- lus 1,581,000 lbf/in2 (10.90 GPa), crushing strength 10,150 lbf/in2 (70.0 MPa) (Wood database, 2018). Mansonia altissima (African black walnut; Afri- can walnut) is an evergreen medium-sized to fairly large tree up to 45 m tall; bole branchless for up to 30 m, up to 100(–150) cm in diameter, generally straight, cylindrical. The wood is used for general and high-quality joinery, cabinet work, furniture, turnery, decorative veneer and handicrafts. It is also used in construction for doors and windows, in railway coaches and shop fittings, and for boxes and crates (Ohene-Coffie, 2008). The heartwood is yellowish brown to dark grey-brown or even dark brown, often with purple, reddish or greyish green streaks, often in alternat- ing light and dark bands. It fades on exposure to a somewhat dull brown. It is distinctly demarcated from the 2–4(–6) cm wide, white to pinkish sap- wood. The grain is usually straight and the texture fine. The wood is moderately lustrous (Ohene-Cof- fie, 2008). At 12% moisture content the modulus of rupture is (61–)114–177(–183) N/mm², modu- lus of elasticity 9320–12,800 N/mm², compression 34 Les/Wood, Vol. 71, No. 2, December 2022 Ojo, A. R., Ogutuga, S. O., & Aguda, L. O.: Zveza med gostoto in krčenjem lesa osmih tropskih vrst parallel to grain 43–68(–96) N/mm², shear 6–15 N/ mm², cleavage 9–23 N/mm, Janka side hardness 5690–7470 N and Janka end hardness 5740–7470 N (Protabase, 2010). Treculia africana (African breadfruit) is a large, slow-growing, evergreen tree with a dense, spread- ing crown; usually growing 15–30 m tall but with some specimens up to 50 m. The bole is fluted. It is a very valuable food crop in Africa, providing a nutritious protein and oil rich food. It is thus of- ten grown in and around African villages where it is commonly harvested for its edible seeds, which are sold in local markets. Trees are often protect- ed when land is cleared for agriculture (Ken Fern, 2022). The heartwood is yellow with very narrow pale sapwood that is very dense, fairly elastic and flexible, rather heavy, with a fine, even structure. It is suitable for furniture, carving, turnery and in- lay wood, as well as for pulp and papermaking. The wood is used for fuel and making charcoal (Ken Fern, 2022). The wood density of the species is 674 kg/m³ (Keay, 1989). The wood is fairly elastic and flexible (Torelli & Čufar, 1995) and has an even structured fine grain, and saws and planes very eas- ily. Sterculia tragacantha (African tragacanth) is a tree to about 28 m high, the bole sometimes with buttresses, girth up to 1.5 m by 10–15 m long, bear- ing a crown of pseudo-whorled branches. It used for building materials (made from the bark), fibre (wood), pulp and paper (wood), carpentry and related applications (gum, wood), farming, forest- ry, hunting and fishing equipment (gum) (Burkill, 1985). Nesogordonia kabingaensis (danta) is a medi- um-sized to large tree up to 45(–50) m tall, mostly evergreen but sometimes briefly deciduous. The bole is usually straight and cylindrical, branchless for up to 25 m and 80(–120) cm in diameter, with narrow buttresses up to 3 m high (Oyen, 2005). The heartwood is pale brown to purplish brown with a tendency to become lighter on ex- posure to light, distinctly demarcated from the pale brown to pink sapwood, which is 2–5(–10) cm thick. The grain is straight or interlocked, the tex- ture fine. At 12% moisture content the modulus of rupture is 108–183(–231) N/mm², modulus of elas- ticity (7800–)10,900–16,200 N/mm², compression parallel to grain 45–75 N/mm², shear 8–16 N/mm², and cleavage 13–31 N/mm (Oyen, 2005). The species Nauclea diderrichii, Mansonia al- tissima , Nesogordonia kabingaensis, Cassia sim- ea, Afzelia africana and Milicia excelsa are all well known, while Treculia africana and Sterculia tra- gantha are less familiar. They grow in the natural forests in Nigeria, and due to the afforestation and reforestation efforts of the government and private individuals a number of plantations of the species are being established. The trees are very important in Nigeria and elsewhere because the woods are hardwood and naturally durable, and thus suitable for various construction purposes. The general objective of this study is to es- tablish the relationship between the density and shrinkage/swelling of the wood of the selected spe- cies, and thus its specific objectives are as follows: determination of density, evaluation of the radial, tangential and volumetric shrinkage of the wood and their correlations with the density. 2 MATERIALS AND METHODS 2 MATERIALI IN METODE 2.1 DETERMINATION OF WOOD DENSITY 2.1 DOLOČANJE GOSTOTE LESA Two discs, 20 cm long were cut from each of the species at the base and top. Selection of rep- resentative samples for test was carried out from the central planks obtained from each of the discs to give 16 planks from where test samples for all the experiments were obtained. Ten samples were collected for each of the species from the bark to the pith resulting in eighty (80) test samples. For the determination of density, test samples of di- mensions 20 x 20 x 60 mm were produced from the central planks. The test samples were oven-dried to a constant weight, and the density was thus de- termined as given below in accordance with the American Standard for Testing Materials (ASTM D) 2395 (1983): D M V kg m =       3 (1) 35 Les/Wood, Vol. 71, No. 2, December 2022 Ojo, A. R., Ogutuga, S. O., & Aguda, L. O.: Correlation between dry density and shrinkage in eight tropical hardwood species Where: D = density M = weight of the wood V = volume of wood 2.2 DETERMINATION OF PERCENTAGE SHRINK- AGE 2.2 DOLOČANJE KRČENJA Test specimens of 20 x 20 x 60 mm were pro- duced, which were then properly aligned and de- noted ‘T’ and ‘R’ for tangential and radial planes, respectively. They were soaked in water for 48 hrs in order to get them conditioned to moisture above the fibre saturation point (FSP). Specimens were re- moved one after the other, and their dimensions in wet condition were taken to the nearest millimetre with the aid of a vernier calliper. The percentage shrinkages along the two planes were measured af- ter specimens had been oven-dried as S DD D x SO S = − 100 (2) Where: S = shrinkage % D S = dimensions at saturated condition D O = dimensions at oven dry condition Volumetric shrinkage is approximately equal to the sum of radial and tangential shrinkage, as given below: SSS vR T =+ (3) Where: S V = volumetric shrinkage S R = radial shrinkage S T = tangential shrinkage This is in accordance with the approximations done by Dinwoodie (1989). Anisotropy S S T R = (4) 2.3 EXPERIMENTAL DESIGN AND DATA ANALYSIS 2.3 EKSPERIMENTALNA ZASNOVA IN ANALIZA PODATKOV The design adopted for the experiment was a completely randomized design with ten replica- tions each and the data obtained were subjected to analysis of variance (ANOVA), correlation, linear and non-linear regression. 3 RESULTS AND DISCUSSION 3 REZULTATI IN RAZPRAVA 3.1 DENSITY 3.1 GOSTOTA The highest mean density was observed in Milicia excelsa with 900.63±50.13 kg/m³, followed by Afzelia africana, Nesogordonia kabingaensis and Nauclea diderichii with 831.25±41.67 kg/m³, 808.75±20.88 kg/m³ and 801.88±46.40 kg/m³, respectively. The mean density for Cassia sim- ea was 781.88±27.71 kg/m³, Mansonia altissima 593.13±65.98 kg/m³, and Sterculia tragacantha 481.25±111.73 kg/m³, while the least density was ob- served in T reculia africana with 463.75±67.88 kg/m³. According to Brandon (2005), who classified wood species based on their density (the density of seasoned timber is usually measured for clas- sification purposes at 12% air-dry MC) as follows: exceptionally light – under 300 kg/m³, light – 300 to 450 kg/m³, medium – 450 to 650 kg/m³, heavy – 650 to 800 kg/m³, and very heavy – 800 to 1000+ kg/m³. Based on this classification, Milicia excelsa, Afzelia africana, Nesogordonia kabingaensis and Nauclea diderichii are termed very heavy density wood while Cassia simea to be heavy and Manso- nia altissima , Sterculia tragacantha and Treculia af- ricana have medium density wood (Table 1). According to Jane (1970), wood from different parts of a tree is noted to show differences in den- sity, and according to Panshin and de Zeeuw (1980) this variation exists horizontally from the pith to the periphery, and vertically from the base to the crown of the tree. According to the United nations Food and Agriculture Organization (1985), timber should be graded hard, intermediate or soft, cor- responding to high, medium and low densities. The technical limits between the grades are: high densi- ty above 500 kg/m³, medium density between 500 36 Les/Wood, Vol. 71, No. 2, December 2022 Ojo, A. R., Ogutuga, S. O., & Aguda, L. O.: Zveza med gostoto in krčenjem lesa osmih tropskih vrst and 350 kg/m³, low density less than 350 kg/m³, and only high density timber is acceptable for structural purposes. Chaffe (1991) reported that high cellulose con- tent in wood is a good indication of high density and low lignin content. Density varies greatly de- pending on the anatomical structure of the wood. However, the results of ANOVA show that there is a significant difference among the wood species (Table 2) regarding density. The results of corre- lation analysis indicated that there was a positive correlation between density and tangential shrink- age (0.236) at a 0.01 level of probability, and a pos- itive but not significant correlation among density, radial shrinkage and volumetric shrinkage (Table 3). 3.2 RADIAL SHRINKAGE 3.2 RADIALNO KRČENJE The highest mean radial shrinkage was ob- served in Nesogordonia kabingaensis with 6.98±1.63%, followed by Nauclea diderichii, Milicia excelsa and Mansonia altissima with 4.36±1.64%, 3.98±1.02% and 3.96±1.34%, respectively. The mean RS for Cassia simea was 3.65±1.56%, Afzelia africana 3.12±2.24%, Sterculia tragacantha 3.71±1.06% and for Treculia africana 3.30±1.23% (Table 1). Ogunsanwo (2000), in his work on Triplochi- ton scleroxylon, and Choong et al. (1989) as well as Poku et al. (2001), all reported significant differ- Table 1. Mean values of the parameters from the eight hardwood species; density = oven dry density, ST, SR, SV – tangential, radial and volumetric shrinkage. Preglednica 1. Povprečne vrednosti parametrov lastnosti lesa za osem tropskih listavcev; density – gostota lesa v absolutno suhem stanju, ST, SR, SV – tangencialni, radialni in prostorninski skrček, anisotropy – anizotropija krčenja. Species Density (kg/m³) S T (%) S R (%) S V (%) Anisotropy Nauclea diderrichii 801.88 a ±46.40 4.44 a ±1.42 4.36 a ±1.64 8.79 a ±1.67 1.32 a ±1.00 Mansonia altissima 593.13 c ±65.98 3.96 a ±1.34 5.20 ab ±1.74 9.16 a ±1.55 0.88 a ±0.49 Treculia africana 463.75 b ±67.88 3.30 a ±1.23 3.91 a ±1.49 7.21 ab ±1.42 1.08 a ±0.76 Sterculia tragacantha 481.25 b ±111.73 3.92 a ±1.59 3.71 a ±1.06 7.64 ab ±0.96 1.24 a ±0.71 Nesogordonia kabingaensis 808.75 a ±20.88 7.74 b ±1.40 6.98 b ±1.63 14.71 c ±2.28 1.15 a ±0.20 Cassia siamea 781.88 a ±27.71 3.77 a ±3.21 3.65 a ±1.56 5.11 b ±2.65 2.09 a ±0.10 Afzelia africana 831.25 a ±41.67 4.30 a ±2.01 3.12 a ±2.24 7.42 ab ±3.02 2.01 a ±1.57 Milicia excelsa 900.63 d ±50.13 4.34 a ±0.90 3.98 a ±1.02 8.32 a ±1.34 1.17 a ±0.45 Values with the same letters are not significantly different from one another. Vrednosti z isto črko niso značilno različne. Table 2. Analysis of variance of all parameters. Preglednica 2. Analiza variance vseh parametrov. F-values SV df density SR ST SV Anisotropy Species 7 79.07* 5.8112* 5.9619* 19.756* 0.76ns Error 72 Total 79 * = significant at 0.05 level of probability ns = not significant at 0.05 level of probability * = značilno pri stopnji verjetnosti 0,05 ns = ni značilno pri stopnji verjetnosti 0,05 Table 3. Pearson correlation matrix for the tested parameters. Preglednica 3. Matrika s Pearsonovimi korelacijski- mi koeficienti za testirane parametre. D S R S T S V 1.00 0.107 0.236* 0.191 1.00 0.232* 0.671** 1.00 0.644** 1.00 D S R S T S V ** Correlation is significant at the 0.01 level * Correlation is significant at the 0.05 level ** Korelacija je značilna na ravni 0,01 * Korelacija je značilna na ravni 0,05 37 Les/Wood, Vol. 71, No. 2, December 2022 Ojo, A. R., Ogutuga, S. O., & Aguda, L. O.: Correlation between dry density and shrinkage in eight tropical hardwood species Table 4. Linear and non-linear models of the relationship between oven dry density and shrinkage (S T , S R , S V – tangential, radial and volumetric shrinkage) for each of the species. Preglednica 4. Linearni in nelinearni modeli za proučevanje zveze med gostoto lesa in krčenjem (S T , S R , S V – tangencialni, radialni in prostorninski skrček) vsako vrsto. Species Models ST SR SV Nauclea diderrichii Simple Linear y = -7.967x + 837.2 R² = 0.059 y = 17.77x + 724.4 R² = 0.396 y = 11.42x + 701.3 R² = 0.169 Exponential y = 837.9e-0.01x R² = 0.063 y =726.9e0.022x R² = 0.397 y =707.6e0.014x R² = 0.164 Logarithmic y = -11.4ln(x) + 817.6 R² = 0.025 y = 58.76ln(x) + 720.1 R² = 0.341 y = 88.21ln(x) + 611.5 R² = 0.142 Polynomial y = -10.58x2 + 55.96x + 781.2 R² = 0.234 y = -0.392x2 + 21.43x + 716.9 R² = 0.397 y = 3.413x2–48.16x + 952.8 R² = 0.232 Power y = 817.3x-0.01 R² = 0.028 y = 722.9x0.073 R² = 0.343 y = 634.1x0.108 R² = 0.137 Mansonia altissima Simple Linear y = 11.73x + 546.6 (R² = 0.056) y = 12.85x + 530.1 R² = 0.053 y = 22.32x + 388.6 R² = 0.275 Exponential y = 537.4e0.023x R² = 0.057 y = 520.8e0.025x R² = 0.053 y = 398.2e0.042x R² = 0.265 Logarithmic y = 47.69ln(x) + 529.6 R² = 0.052 y = 12.85x + 530.1 R² = 0.053 y = 226.1ln(x) + 95.09 R² = 0.318 Polynomial y = -1.357x2 + 24.15x + 520.9 R² = 0.057 y = 19.12x2–176.5x + 974.9 R² = 0.165 y = -16.05x2 + 330.1x–1049. R² = 0.534 Power y = 519.3x0.094 R² = 0.054 y = 505.8x0.097 R² = 0.035 y = 226.3x0.434 R² = 0.309 Treculia africana Simple Linear y = 32.89x + 355.1 R² = 0.377 y = -7.649x + 493.6 R² = 0.028 y = 17.56x + 337.1 R² = 0.136 Exponential y = 364.4e0.07x R² = 0.366 y = 494.0e-0.01x R² = 0.036 y = 357.4e0.034x R² = 0.114 Logarithmic y = 55.04ln(x) + 403.3 R² = 0.178 y = -6.03ln(x) + 471.4 R² = 0.001 y = 113.5ln(x) + 241.6 R² = 0.128 Polynomial y = 33.18x2–161.3x + 586.5 R² = 0.882 y = -9.652x2 + 69.81x + 357.6 R² = 0.219 y = 3.794x2–34.50x + 508.3 R² = 0.144 Power y = 404.4x0.115 R² = 0.168 y = 471.4x-0.02 R² = 0.004 y = 295.9x0.224 R² = 0.107 Sterculia tragacantha Simple Linear y = -22.10x + 568.1 R² = 0.099 y = 60.91x + 255.1 R² = 0.332 y = -2.491x + 534.2 R² = 0.013 Exponential y = 595.0e-0.06x R² = 0.084 y = 232.0e0.185x R² = 0.303 y = 534.4e-0.00x R² = 0.013 Logarithmic y = -34.2ln(x) + 522.4 R² = 0.054 y = 244.2ln(x) + 170.3 R² = 0.420 y = -18.9ln(x) + 553.5 R² = 0.015 Polynomial y = -9.615x2 + 37.15x + 505.6 R² = 0.138 y = -74.71x2 + 621.7x–721.9 R² = 0.672 y = 2.589x2–40.01x + 667.6 R² = 0.032 Power y = 521.5x-0.10 R² = 0.047 y = 178.4x0.745 R² = 0.387 y = 555.3x-0.03 R² = 0.015 38 Les/Wood, Vol. 71, No. 2, December 2022 Ojo, A. R., Ogutuga, S. O., & Aguda, L. O.: Zveza med gostoto in krčenjem lesa osmih tropskih vrst Species Models ST SR SV Nesogordonia kabingaensis Simple Linear y = -7.817x + 869.2 R² = 0.273 y = -5.706x + 848.5 R² = 0.200 y = -5.892x + 895.4 R² = 0.413 Exponential y = 871.2e-0.01x R² = 0.273 y = 848.7e-0.00x R² = 0.196 y = 899.3e-0.00x R² = 0.408 Logarithmic y = -57.0ln(x) + 924.6 R² = 0.276 y = -41.9ln(x) + 889.2 R² = 0.206 y = -83.0ln(x) + 1031. R² = 0.431 Polynomial y = 0.335x2–12.78x + 886.9 R² = 0.273 y = 0.259x2–9.625x + 862.6 R² = 0.202 y = 0.580x2–22.30x + 1008. R² = 0.440 Power y = 932.8x-0.07 R² = 0.276 y = 891.8x-0.05 R² = 0.201 y = 1062.x-0.10 R² = 0.426 Cassia siamea Simple Linear y = -3.671x + 795.7 R² = 0.181 y = -6.415x + 807.8 R² = 0.063 y = -5.760x + 815.4 R² = 0.064 Exponential y = 795.1e-0.00x R² = 0.180 y = 807.4e-0.00x R² = 0.063 y = 814.5e-0.00x R² = 0.062 Logarithmic y = -18.0ln(x) + 800.0 R² = 0.329 y = -22.6ln(x) + 812.9 R² = 0.041 y = -43.0ln(x) + 856.9 R² = 0.099 Polynomial y = 0.745x2–12.59x + 811.8 R² = 0.280 y = -6.455x2 + 52.86x + 680.5 R² = 0.148 y = 10.06x2–131.3x + 1192. R² = 0.379 Power y = 799.4x-0.02 R² = 0.324 y = 812.4x-0.02 R² = 0.041 y = 857.6x-0.05 R² = 0.095 Afzelia africana Simple Linear y = 1.969x + 822.7 R² = 0.009 y = 4.403x + 817.5 R² = 0.056 y = 3.310x + 806.6 R² = 0.057 Exponential y = 821.4e0.002x R² = 0.010 y = 4.403x + 817.5 R² = 0.056 y = 805.3e0.004x R² = 0.064 Logarithmic y = 18.81ln(x) + 805.6 R² = 0.047 y = 24.28ln(x) + 808.6 R² = 0.176 y = 31.82ln(x) + 770 R² = 0.111 Polynomial y = -4.708x2 + 49.52x + 722.5 R² = 0.222 y = -3.664x2 + 41.90x + 752.7 R² = 0.331 y = -1.561x2 + 28.64x + 717.5 R² = 0.209 Power y = 804.7x0.023 R² = 0.050 y = 807.8x0.029 R² = 0.187 y = 770.2x0.039 R² = 0.120 Milicia excelsa Simple Linear y = 21.91x + 805.4 R² = 0.157 y = 8.825x + 865.4 R² = 0.032 y = 15.17x + 774.3 R² = 0.165 Exponential y = 809.2e0.024x R² = 0.163 y = 865.7e0.009x R² = 0.032 y = 782.6e0.016x R² = 0.169 Logarithmic y = 93.44ln(x) + 765.4 R² = 0.166 y = 40.23ln(x) + 846.3 R² = 0.047 y = 115.7ln(x) + 657.0 R² = 0.169 Polynomial y = -9.791x2 + 105.7x + 633.4 R² = 0.177 y = -19.18x2 + 160.7x + 582.8 R² = 0.165 y = -2.082x2 + 47.29x + 654.6 R² = 0.170 Power y = 774.2x0.103 R² = 0.172 y = 848.0x0.043 R² = 0.047 y = 687.5x0.127 R² = 0.173 39 Les/Wood, Vol. 71, No. 2, December 2022 Ojo, A. R., Ogutuga, S. O., & Aguda, L. O.: Correlation between dry density and shrinkage in eight tropical hardwood species ences between radial and tangential shrinkage on lesser-used hardwood species from Ghana. Laus- berget et al. (1985), reported that this could have been caused by the presence of ray cells on the radial plane, with their horizontally aligned cells producing a restraining effect on radial shrinkage. However, Panshin and de Zeeuw (1980) noted that it is related to the rapid reduction of the microfibril angle in the cell wall. The results of the analysis of variance of radial shrinkage show that there were significant differ- ences among the wood species at a 0.05 level of probability (Table 2). However, the results of cor- relation analysis indicate that there was strong and positive significant correlation between radial and tangential shrinkage (0.644**) at a 0.05 level of probability (Table 3), but of all the models devel- oped none of them has a good fit because of the low R 2 for all the species (Fig. 1 and Table 4) The observed changes in wood dimensions as a result of shrinkage are unequal along the three structural directions. This behaviour of wood has been documented widely by various authors (Pan- shin and de Zeeuw 1980; Dinwoodie, 1981; Laus- berg et al., 1995; Ogunsanwo, 2000). However, Pan- shin and de Zeeuw (1980) noted that the geometric disposition of cells along the principal directions is mainly responsible for this observation. Osadare (2001) observed that the noticeable variations in wood properties are influenced prin- cipally by (i) the changes in activities of cambium as it grows older, (ii) genetic constitutions which govern the form and growth of the tree, and (iii) environmental influences. However, the interac- tion of these factors made it difficult to attribute the observed variations in wood properties to only a single factor. The variability of wood character- istics within individual trees is basically related to changes resulting from ageing of the cambium and modifications imposed on the cambial activity by the environmental conditions, genetic and silvicul- tural effects, as noted by Evans (1991). 3.3 TANGENTIAL SHRINKAGE 3.3 TANGENCIALNO KRČENJE The highest tangential shrinkage was observed in Nesogordonia kabingaensis with 7.74±1.40%, followed by Mansonia altissima (5.20±1.74%), Nauclea diderichii (4.44±1.42%) and Milicia excel- sa (4.34±0.90%). The mean tangential shrinkages for Sterculia tragacantha, Afzelia africana, Tre- culiaafricana were 3.92±1.59%, 4.30±2.01% and 3.91±1.49%, respectively, while the least tangen- tial shrinkage was observed in Cassia simea with 3.77±3.21% (Table 1). According to the classifica- tion of Bolza and Keating (1972), TEDB (1994) and Upton and Attah (2003), the tangential shrinkage values are classified as small (3.5-5.0%), medium (5.1-6.0%), large (6.1-8.0%) and very large (above 8.0%) The results of the analysis of variance of tan- gential shrinkage show that there were significant differences among the wood species at a 0.05 level of probability (Table 2), while the follow-up analy- Figure 1. Wood den- sity and radial shrink- age of the eight tropi- cal wood species. Slika 1. Gostota lesa in radialno krčenje osmih tropskih lesnih vrst. 40 Les/Wood, Vol. 71, No. 2, December 2022 Ojo, A. R., Ogutuga, S. O., & Aguda, L. O.: Zveza med gostoto in krčenjem lesa osmih tropskih vrst sis shows that there was no significant difference among the species except for Nesogordonia ka- bingaensis (Table 1). However, the results of the correlation analysis indicated that there was a strong and positive significant correlation between tangential shrinkage and radial shrinkage (0.232*) at the 0.01 level of probability, and a positive but not significant correlation between tangential and volumetric shrinkage (Table 3). Likewise, of all the models developed none of them had a good fit with the data because of the low R 2 for all the spe- cies (Fig. 2 and Table 4). The greatest dimensional shrinkage occurs along the tangential plane, followed by shrinkage along the radial plane while longitudinal shrink- age has been widely reported to be the smallest, ranging from 0.1 to 0.3% (Desch, 1988; Dinwoodie, 1989). The suitability of wood for various end uses has been linked with the tangential/radial shrink- age ratio (ST/SR), also known as anisotropy. Pan- shin and de Zeeuw (1980) noted that a low value of T/R is synonymous with high suitability of wood for end uses. In this study, it is observed that the anisotropy found for the eight species is low, as can be seen in Table 1. 3.4 VOLUMETRIC SHRINKAGE 3.4 PROSTORNINSKO KRČENJE The highest volumetric shrinkage was observed in Nesogordonia kabingaensis, at 14.71±2.28%, followed by Mansonia altissima (9.16±1.55%) Nauclea diderichii (8.79±1.67%) and Milicia excel- sa (8.32±1.34%). The mean volumetric shrinkag- es for Sterculia tragacantha, Afzelia africana, and Treculia Africana were 7.64±0.96%, 7.42±3.02% and 7.21±1.42%, respectively. The least volumet- ric shrinkage was observed in Cassiasimea, at 5.11±2.65% (Table 1). Poku et al. (2001), recorded volumetric shrinkages of 7.51%, 11.51% and 6.21% for Alstonia boonei. Pterocarpus macrocarpus and Ricinodendron hendelotti, respectively, while Kiaei and Samariha (2011) obtained 12.39% for Ulmus glabra grown in Iran. The wide disparity among the eight species could be attributed to their densities and probably the presence of more biomass in latewood cells, as noted by Chudnoff (1976). However, many other factors, such as spiral grain and latewood propor- tion, also affect the variation in the shrinkage of wood (Pilura et al, 2005; Walker, 2006) The results of the analysis of variance of tan- gential shrinkage show that there were significant differences among the wood species at a 0.05 lev- el of probability (Table 2). Moreover, none of the models developed in this work had a good fit be- cause of the low R 2 for all the species (Fig. 3 and Table 4). 4 CONCLUSIONS 4 SKLEPI The use of the eight tropical hardwood spe- cies for this study has provided useful information Figure 2. Wood den- sity and tangential shrinkage of the eight tropical wood spe- cies. Slika 2. Gostota lesa in tangencialno kr- čenje osmih tropskih lesnih vrst. 41 Les/Wood, Vol. 71, No. 2, December 2022 Ojo, A. R., Ogutuga, S. O., & Aguda, L. O.: Correlation between dry density and shrinkage in eight tropical hardwood species regarding the density, shrinkage and anisotropy of the wood and the relationships among them. Based on the findings of this study, the following conclusions were made. Nesogordonia kabingaen- sis shrinks more than the other species studied. Along the radial and tangential plane, there was a positive correlation between wood density, radial, tangential, and volumetric shrinkage but a weak co- efficient of fitness R 2 for the linear and non-linear models for all the species. Statistically significant differences among the eight species could not be established for all the studied parameters. 5 SUMMARY 5 POVZETEK Izbrali smo les osmih tropskih vrst: Nauclea diderichii, Mansonia altissima, Treculia africana, Sterculia tragacantha, Nesogordonia kabingaensis, Cassia simea, Afzelia africana, Milicia excelsa in raziskali gostoto, radialno, tangencialno in volum- sko krčenje ter anizotropijo krčenja ter povezave med njimi. Večina vrst je uveljavljenih, samo Trecu- lia africana in Sterculia tragantha sta z vidika last- nosti lesa manj znani. Za določanje gostote lesa v absolutno suhem stanju so bili izdelani testni vzorci dimenzij 20 x 20 x 60 mm iz centralnih desk spodnjega dela debel. Testni vzorci so bili posušeni v laboratorijskem su- šilniku do konstantne mase, gostota lesa pa je bila tako določena v skladu z ameriškim standardom za testiranje materialov (ASTM D) 2395 (1983). Izdela- ni so bili orientirani vzorci velikosti 20 x 20 x 60 mm; za merjenje tangencialnih in radialnih skrčkov. Les so namočili v vodo za 48 ur, da je dosegel vlažnost nad točko nasičenja celičnih sten (FSP). Dimenzije v mokrem stanju so bile natančno določene s pomoč- jo Venierovega merilnika. Odstotek krčenja vzdolž obeh ravnin je bil izmerjen po tem, ko so vzorci v sušilniku dosegli ravnovesno, absolutno suho sta- nje. Največjo povprečno gostoto je imel les vrste Milicia excelsa z 900,63±50,13 kg/m³, sledijo Afzelia africana, Nesogordonia kabingaensis in Nauclea di- derichii z 831,25±41,67 kg/m³, 808,75±20,88 kg/m³ oziroma 801,88±46,40 kg/m³. Povprečna gostota za les vrst Cassia simea je bila 781,88±27,71 kg/m³, Mansonia altissima 593,13±65,98 kg/m³, Sterculia tragacantha 481,25±111,73 kg/m³, medtem ko je imela najmanjšo gostoto vrsta Treculia africana s 463,75±67,88 kg/m³ (preglednica1). Največje povprečno radialno krčenje je bilo pri lesu vrste Nesogordonia kabingaensis s 6,98 ± 1,63 %, sledijo Nauclea diderichii, Milicia excelsa in Mansonia altissima s 4,36 ± 1,64 %, 3,98 ± 1,02 % in 3,96 ± 1,34 %. Povprečen radialni skrček pri vrsti Cassia simea je bil 3,65 ± 1,56 %, pri vrsti Afzelia africana 3,12 ± 2,24 %, pri Sterculia tragacantha 3,71 ± 1,06 % in pri Treculia africana 3,30 ± 1,23 % (preglednica 1). Največje tangencialno krčenje je bilo zabele- ženo pri vrsti Nesogordonia kabingaensis s 7,74 ± 1,40 %, sledijo Mansonia altissima (5,20 ± 1,74 %), Nauclea diderichii (4,44 ± 1,42 %) in Milicia excelsa Figure 3. Wood den- sity and volumetric shrinkage of the eight tropical wood spe- cies. Slika 3. Gostota lesa in prostorninsko kr- čenje osmih tropskih lesnih vrst. 42 Les/Wood, Vol. 71, No. 2, December 2022 Ojo, A. R., Ogutuga, S. O., & Aguda, L. O.: Zveza med gostoto in krčenjem lesa osmih tropskih vrst (4,34 ± 0,90 %), srednje tangencialno krčenje je imel les vrst Sterculia tragacantha, Afzelia africana in Treculia africana (3,92±1,59 %, 4,30±2,01 % in 3,91±1,49 %), medtem ko je bilo najmanjše tangen- cialno krčenje opaženo pri vrsti Cassia simea s 3,77 ± 3,21 % (preglednica1). Največje prostorninsko krčenje so opazili pri vrsti Nesogordonia kabingaensis s 14,71 ± 2,28 %, sledijo Mansonia altissima (9,16 ± 1,55 %), Nauclea diderichii (8,79 ± 1,67 %) in Milicia excelsa (8,32 ± 1,34 %), povprečno prostorninsko krčenje za vrste Sterculia tragacantha, Afzelia africana, Treculia africana je bilo 7,64±0,96 %, 7,42±3,02 % oziroma 7,21±1,42 %. (preglednica 1). Za les osmih raziskanih tropskih listavcev smo ovrednotili tudi povezave med gostoto absolutno suhega lesa in krčenjem lesa, vendar nismo mogli potrditi statistično značilne povezave. Na podlagi raziskav ugotavljamo, da se les vrste Nesogordonia kabingaensis krči bolj kot les drugih vrst. Determinacijski koeficient R 2 je bil v vseh pri- merih prenizek, da bi lahko potrdili zvezo med go- stoto lesa ter radialnim, tangencialnim in prostor- ninskim krčenjem. REFERENCES VIRI American Society for Testing and Materials D 2395-83 (ASTM) (1983). Standard test methods for specific gravity of wood and wood- base materials. West Conshohocken, Pennsylvania. Reappro- ved in 2007, 9. Barwick, M. (2004). Tropical and subtropical trees–A worldwide en- cyclopaedic guide. London: Thames & Hudson. Bolza, E., & Keating, W. G. (1972). African timbers – the properties, uses and characteristics of 700 species. Melbourne: CSIRO, Di- vision of Building Research, 1-9. Brandon, J. (2005). Properties of wood. Builders guide to aircraft materials – Wood, plywood and adhesives modules. AUF Au- stralia, 9. Burkill, H.M. 1985. The useful plants of west tropical Africa, Vol 5. Sterculia tragacantha Lindl. [family STERCULIACEAE]. Herba- rium: Royal Botanic Gardens, Kew (K). URL: https://plants.jstor. org/stable/10.5555/al.ap.upwta.5_294 (30.7.2022) Chaffe, S. C. (1991). A relationship between equilibrium moisture content and specific gravity in wood. Journal of institute of wood science, 12(3), 119-122. Choong, E. T., Fogg, P . J., & Shoulders, E. (1989). Effect of cultural treatments and wood types on some physical properties of long leaf and slash pine wood. Wood and fibre science, 21(2), 193-206. Chudnoff, M. (1976). Density of tropical timbers as influence by cli- matic life zones. Commonwealth Forestry Review, 55(3), 203- 217. Desch, H. E. (1988). Timber: Its structure, properties and utilization. 6th Edition. London: Macmillian Education, 410. Desch, H. E., & Dinwoodie, J. M. (1983). Timber: Its structure, pro- perties and utilization. 6th Edition. London: Macmilliam Edu- cation, 410. Dinwoodie, J. M. (1981). Timber: Its nature and behaviour. New York: Van Nostrand Reinhold, 190. Dinwoodie, J. M. (1989). Wood: Nature’s cellular, polymeric fibre composite. London: The Institute of Metal, 138. Eckelman, C. A. (2012). The shrinking and swelling of wood and its effect on furniture. Forestry and natural resources. FRN 163. Purdue university cooperative Extension Service. Department of Forestry &Natural Resources 1159 Forestry Bldg. West La- fayette, IN 479071. Evans, P . D. (1991). The strength properties of clear wood materials forum 15, 231 – 244. Food and Agriculture Organization of the United Nations (FAO), (1985). Coconut wood processing and use. FAO paper 57, Rome, 64. Gerard, J., & Louppe, D. (2011). Afzelia africana Sm. ex Pers. In: Lemmens, R. H. M. J., Louppe, D. & Oteng-Amoako, A.A. (Edi- tors). PROTA (Plant Resources of Tropical Africa/Ressources- végétales de l’ Afriquetropicale), Wageningen. URL: https:// www.feedipedia.org/sites/default/files/public/gerard_2011. pdf (29.7.2022) Haygreen, J. G., & Bowyer, J. L. (1996). Forest product and wood science. An introduction. Third Edition. IOWA State University Press/ AMES 232. Hossain, S. N., Khan, M. A., & Hossain, M. (1991). Moisture auditor water absorption characteristic of Leucea naleucocephala of different ages and some physical parameters of the woods. Le- uceana Research Report. A publication of the Nitrogen Fixation Association. Wiaimanalo, USA, 12, 19. Jane, F. W. (1970). The Structure of wood. 2nd Edition, London: Adam and Charless Black Limited, 487. Keay, R. W. J. (1989). Trees of Nigeria. A revised version of “Nigerian trees“ (Keay et al., 1964). Oxford: Clarendon Press. Kellog, R. M. (1981). Physical properties in wood. In: Wangaard, FF (1981). Wood, its Structure and Properties. Pub. University USA, 195 – 223. Ken Fern (2022). Tropical Plants Database, tropical.theferns. info. 2022-07-29. URL: (29.7.2022) Kiaei, M., & Samariha, A. (2011). Wood density and shrinkage of Ulmus glabra in northwestern of Iran. American-Eurasian J. Agric. & Environ. Sci., 11(2), 257-260. Lausberg, M. J. F., Cown, D. J., MacConchie, J., & Skipmith, H. (1995). Variation in some wood properties of Pseudotsugamenziensii. Provenances grown in New Zealand. New Zealand Journal of Forestry Science, 25(2), 133- 146. 43 Les/Wood, Vol. 71, No. 2, December 2022 Ojo, A. R., Ogutuga, S. O., & Aguda, L. O.: Correlation between dry density and shrinkage in eight tropical hardwood species Ogunsanwo, O. Y . (2000). Characterization of wood properties of plantation grown obeche (Triplochiton sceleroxylon K. Schum) in Omo Forest Reserve. Ogun State, Nigeria, Ph.D Thesis in the department of Forest Resources Management. Ibadan: Uni- versity of Ibadan, 253. Ohene-Coffie, F. (2008). Mansonia altissima (A.Chev.) A. Chev. In: Lo - uppe, D., Oteng-Amoako, A., A., & Brink, M. (Editors). PROTA (Plant Resources of Tropical Africa / Ressourcesvégétales de l’ Afriquetropicale), Wageningen, Netherlands. URL: https:// brill.com/view/journals/afoc/22/2/article-p105_16.xml (29.7.2022) Ojo, A. R., Adejoba, O. R., Adesope, A. S., & Ogutuga, S. O. (2016). Evaluation of dimensional stability of Bambusa vulgaris Schrad Ex J. C. Wendl. Culm along the three orthotropic axes growing in Nigeria. Journal for Worldwide Holistic Sustainable Develo- pment. JWHSD, 2(2), 24-30. Orwa, C., Mutua, A., Kindt, R., Jamnadass, R., & Anthony, S. (2009). Agroforestree Database: A tree reference and selection guide version 4.0 URL: https://www.scirp.org/(S(czeh2t- fqyw2orz553k1w0r45))/reference/referencespapers.aspx?re- ferenceid=1957749 (29.7. 2022) Osadare, A. O. (2001). Basic wood and pulp properties of Nigeria grown Caribbean pine (Pinus caribea Movelet) and their rela- tionships with tree growth indices. Ph. D Thesis. Ibadan: Uni- versity of Ibadan, 384. Oyen, L. P . A. (2005). Nesogordonia kabingaensis (K. Schum.) Ca- puron ex R.Germ. In: Louppe, D., Oteng-Amoako, A. A., & Brink, M. (Editors). PROTA (Plant Resources of Tropical Africa / Ressourcesvégétales de l’ Afriquetropicale), Wageningen, Netherlands. URL: https://tropical.theferns.info/viewtropical. php?id=Nesogordonia+kabingaensis (30.7.2022) Panshin, A. J., & de Zeeuw, C. (1980).Textbook of wood technology. 4th Ed. New York: MacGraw-Hill Book Company, 722. Pilura, A., Yu, Q., & Zhang, S. Y . (2005). Variation in wood density and shrinkage and their relationship to growth of selected young poplar hybrid crosses. Forest Science, 51(5), 472-482. Poku, K., Wu, Q., & Viosky, R. (2001). Wood properties and their va- riations with the tree stem of lesser-used species of tropical hardwood from Ghana. Wood and Fibre Science, 33(22), 284- 291. Protabase (2010). Plant Resources of Tropical Africa – Your gui- de to the use of African plants. URL: https://www.prota4u. org/database/protav8.asp?g=psk&p=Nauclea%20diderrichii (30.7.2022) Protabase, Plant Resources of Tropical Africa (2022). URL: http:// www.prota.org (30.7.2022) Shrivastava, M. B. (1997). Wood technology. New Delhi: Vikas Publi- shing House, 181. SSR VINA CO., LTD (2022) URL: https://ssr.vn/wenge-wood-senna-si- amea-wood-things-to-know/ (30.7.2022) TEDB (1994). The Tropical Timbers of Ghana. Timber Export Develo- pment Board, Takoradi. A report: 1-87. Torelli, N., & Čufar, K. (1995). Mexican tropical hardwoods. Compara- tive study of ash and silica content. Holz als Roh und Werkstoff, 53(1), 61-62. Upton, D. A. J., & Attah, A. (2003). Commercial timbers of Ghana – The potential for lesser used species. Accra: Forestry Com- mission of Ghana, 56. Usta, I., & Guray, A. (2000). Comparison of the shrinkage and swelling and shrinkage characteristics of Corcisonpine (Pinus nigra var. mantima). Turk. Agric. For. J., 24, 461-464. Walker, J. C. F. (2006). Primary wood processing: Principles and pra- ctices. 2nded., Dordrecht: Springer, 596. Wood database (2018). The Wood Dictionary. A new old book. URL: https://www.wood-database.com/the-wood-dictionary/ (30.7.2022) 44 Les/Wood, Vol. 71, No. 2, December 2022