Abstract: Declining availability of the prime economic species in timber market has led to the introduction of Lesser-Used-Species (LUS). Their acceptability demands information on their wood technical properties. This study investigates wood properties of Gliricidia sepium to determine its potential for timber in Nigeria. Test specimens obtained from three trees at breast height were prepared according to BS 373 standard and Poku et al. (2001). Hatt-Turner impact machine and Hounsfield Tensometer were used to determine mechanical properties. Mean Moisture Content (MC) of 8.62% was insignificantly lower in the wood nearest the pith. Wood Basic Density (WBD) was significantly (p<0.05) heavier (1062.7 kg m-3) nearest the pith than nearest the bark (987.8 kg m-3) with a mean of 1025.3 kg m-3. Tangential (TS) and Volumetric Shrinkage (VS) were significantly higher in the wood nearest the bark with mean of 4.3 and 6.9%, respectively. Mean impact bending was 0.65 m with no significant difference between the wood samples. Modulus Of Rupture (MOR) and elasticity (MOE) were significantly higher (153.8, 7678.1 N mm-2) nearest the pith than 85.8 and 5580.21 N mm-2 for wood nearest the bark, respectively. Significant correlation exists between MC vs WBD, r = -0.82; RS vs TS, r = 0.91 and VS, r = 0.97; TS vs VS, r = 0.97; IB vs MOR/MOE, r = -0.83 and MOR, r = 0.89; MOR vs MOR/MOE, r = 0.90; MOR/MOE vs RS, r = -0.91 and MOR/MOE vs VS, r = -0.86. G. sepium has potential to be a major LUS timber and could substitute some economic species in Nigeria’s timber market.
INTRODUCTION
Wood as a natural lingo-fibrous material produced by tree has been one of the earliest structural materials discovered by man. It exhibits a lot of variations in properties in terms of durability, strength, figure, density and grain (George, 1975). Due to this diversity in nature and character of wood, exploitation of trees for structural and construction purposes was selective and limited to the very strong and durable species such as Milicia excelsa, Khaya sp., Afzelia africana, Nauclea diderrichii, Triplochiton scleroxylon, Terminalia sp. and Cordia millenii (Ifebueme, 1977; Onilude and Ogunsanwo, 2002). The over exploitation of these species has led to their scarcity in the timber market. This has given way to the introduction of Lesser-Used Species (LUS) in the timber market as demand for wood and its products for building and construction purposes are increasing daily.
Plantation forestry has become the practicable tool for meeting the wood requirements of many nations and mechanical properties of wood aid in the selection of any species for building and construction (FPL, 1987). Gliricidia sepium is an exotic species introduced into Nigeria as a source of fuelwood. However, its use has been mainly for fodder, life-fencing, manure and shades (Glover, 1989; Stewart and Simons, 1996). It can grow to a height of 15 m if not pollarded, tolerate dry season and fire and can be propagated easily from seeds and branches (Lavini, 1996; Simons, 1996). It has a distinctly dark brown heartwood and yellowish band of sapwood which could be mistaken for Milicia excelsa, Afzelia africana and Nauclea diderichii planks. The heartwood is resistant, very hard and heavy, strong, coarse-textured, with an irregular grain. It seasons well and takes high polish (Elevitch and Francis, 2006).
It has been mentioned by Quintanar et al. (1997), Elevitch and Francis (2006) and Greijmans (2007) that the wood is good for building, construction and furniture making however there are no quantitative data to support this claims. The provision of such data on the mechanical properties of G. sepium will be of great significance to knowing the uses that this species could be put to. Also, there have been a lot of studies on the chemical and livestock uses of this species (Bennison and Paterson, 1993; Elevitch and Francis, 2006; Greijmans, 2007) however, information on the strength properties is virtually lacking. The dearth of information on the technical properties of this species perhaps is responsible for its non availability in the timber market in Nigeria.
Wood structures that are well constructed without adequate knowledge of the strength properties of the wood used performed poorly when compared with engineered structures (Soltis, 1994). The success of LUS in the timber market requires technical information that relates to utilization about the species (Poku et al., 2001). It is therefore imperative that the provision of technical information on this species is necessary as this will enhance its success and acceptability in the timber market. This study investigates selected indices of strength properties of Gliricidia sepium wood as a preliminary step to unveil its technical properties for building and construction purposes. The linear relationships between these properties were equally discussed in the study.
MATERIALS AND METHODS
The study site is University of Ibadan Campus, it is situated at about 5 km North of Ibadan city on Latitude 7°27’N and Longitude 3°54’E at an altitude of 200 m. The mean annual rainfall is 1200 mm with distinct wet (April-October) and dry (November-March) seasons. Mean monthly temperature is between 26- 28°C with February being the hottest month.
Wood samples of G. sepium were obtained within University of Ibadan Campus. They were those planted as life fencing poles which have been pollarded severally. A bolt of 45 cm from three trees obtained at breast height was cut and later sawn to obtain wood specimens for characterization according to BS 373 (BSI, 1989; Poku et al., 2001).
Determination of Physical and Mechanical Properties
Wood bolts sawn and cut into 20x20x300 mm dimension according to British
standard specification BS 373 (BSI, 1989; Poku et al., 2001) were prepared.
A cube specimen measuring 20x20x20 mm was cut from failed test specimens to
study moisture content, wood basic density and shrinkage for woods nearest the
pith and bark, respectively. All the test specimens from each tree were pooled
together to determine the mechanical properties namely impact bending, Modulus
Of Rupture (MOR) and elasticity (MOE). The Hatt-Turner impact Testing machine
and Hounsfield Tensometer machine were used to determine the tests. Basic wood
density was determined by the oven-dry weight to volume measurements.
Statistical Analysis
ANOVA was used to test the significance of the treatment means (wood nearest
the pith and wood nearest the bark) at p<0.05 probability level. Correlation
analysis was used to test the relative association among the parameters selected.
RESULTS AND DISCUSSION
Physical Properties
Table 1 shows the physical properties measured. Moisture
content was low with a range of 8.44 to 8.80% and a mean of 8.62%. It is more
in the wood nearest the bark than in the heartwood zone. The mean value is less
than 12.0% often reported in literatures for most hardwood species (Ogunsanwo,
2000). Quintanar et al. (1997) opined that the abundance of crystals
in the heartwood zone may be responsible for a low hygroscopic capacity of the
wood from this zone. This may be one of the reasons for its resistance to termite
and decay. Wood with high moisture content is highly susceptible to greater
degree of decay unless it is well seasoned. The Wood Basic Density (WBD) was
significantly different (p<0.05) between the wood zones. The wood (mainly
heartwood) nearest the pith had higher value of 1062.7 kg m-3 than
those nearest the bark (987.8 kg m-3).The mean of 1025.3 kg m-3
is higher than most economic species in the timber market in Nigeria and West
African coast (Brown, 1978; Poku et al., 2001; Ogunsanwo and Onilude,
2002; Amartey et al., 2004) but within the range of 0.2 to 1.2 kg m-3
for hardwoods (Valkomies, 2006). This result supports the remarks made by Quintanar
et al. (1997), Elevitch and Francis (2006) and Greijmans (2007), that
the wood is heavy. The greater value of the wood from the pith zone may be due
to deposition of extraneous materials (crystals) in conjuction with occulusion
of the cell lumina and wall arising from growth of tyloses. These with cell
wall thickness which has been reported to be thicker in this species (Quintanar
et al., 1997) to greater extent could have influenced the density of
the wood; this calls for further investigation. The wood of G. sepium could
be compared with those of Dalbergia melanoxylon, Diospyrus sp.
and Lophira alata (Brown, 1978; Ogunsanwo and Onilude, 2002). Density
has been shown to be a good indicator for selection of wood for use (Poku et
al., 2001). It means that the wood of G. sepium could find use in
areas where heavy wood is required for construction. Therefore, the wood of
G. sepium could be a major LUS in the timber market and could substitute
those economic species which are already scarce in the timber market. Since
density of wood has been used as a single indicator to express wood quality
and utilization potential of any species for different end-uses, it may be interesting
that timber from Gliricidia may find more uses for different building and construction
purposes.
In Table 1, radial shrinkage showed no significant difference between the wood zones. However, tangential shrinkage was higher in the wood nearest the bark (4.6%) than wood nearest the pith (3.9%). The average value of 4.3% was moderate for a denser wood like G. sepium compared with 6.9% obtained for the wood of Petersianthus macrocarpus with a specific gravity of 0.69 (Poku et al., 2001). It is most likely that the wood of G. sepium will be stable in service as depicted by the ratio of tangential shrinkage to radial shrinkage in Table 1. This may be one of the reasons why the wood of Gliricidia seasons very well with little or no defect as wood which are low in tangential shrinkage tend to have low movement in service. The wood could be useful for exterior purposes as claimed by Quintanar et al. (1997), Elevitch and Francis (2006) and Greijmans (2007).
Mechanical Properties
In Table 2, impact bending was insignificantly different between
the wood zones with a mean of 0.65 m, though it is higher in wood nearest the
bark. This may be due to the presence of sapwood in some of the specimens from
this zone. The mean value is similar to that of Triplochiton scleroxylon
(Ogunsanwo, 2000). However, MOR was significantly (p<0.05) higher in the
wood nearest the pith (153.85 N mm-2), than in the wood at the periphery
(85.8 N mm-2). The mean value of 119.8 N mm-2 is almost
twice the value (61.9 N mm-2) obtained for Triplochiton scleroxylon
(Ogunsanwo and Onilude, 2002), but less than 136.1 N mm-2 obtained
for wood of teak from slow growing provenance in India (Bhat and Priya, 2004).
Similarly, MOE was significantly higher in the wood closest to the pith (7678.1
N mm-2) compared with 5580.2 N mm-2 for the wood nearest
the bark.
| Table 1: | Physical properties of Gliricidia sepium wood |
| *: Significance at p<0.05, ns = not significance, Means with the same letter(s) within row are not significantly different LSD p<0.05 | |
| Table 2: | Mechanical properties of Gliricidia sepium wood |
| *: Significance at p<0.05, ns = not significance, Means with the same letter(s) within row are not significantly different LSD p<0.05 | |
| Table 3: | Simple correlation coefficients indicating linear relationships among selected physical and anatomical properties |
| *: Significant relationship at p<0.05 | |
The mean value of 6629.2 N mm-2 is higher than 6239.4 N mm-2 obtained for Triplochiton scleroxylon plantation wood. However, the wood is inferior to that of Teak (Sanwo, 1986; Bhat and Priya, 2004), the ratio of MOR: MOE further stressed this as shown in Table 2. The higher values obtained for both MOR and MOE for wood specimens nearest the pith may not be unconnected with the higher wood density obtained within this zone. In Ghana and Nigeria, Triplochiton scleroxylon, Milicia excelsa, Khaya sp. and Tectona grandis are major economic timbers for export and for various end-uses locally. However, Poku et al. (2001), Ogunsanwo (2002) and Amartey et al. (2004) argued that these species are fast declining in the timber market, there is need to find suitable substitute for them. The results obtained in this study may eventually provide wider avenues to explore the technical characteristics of Gliricidia wood rather than limiting it to the traditional uses of life fencing, fodder crops mulch and agro-forestry.
Correlation of Wood Properties
The simple correlation coefficients in Table 3 elucidate
the extent of linear relationships among the wood properties. Moisture content
significantly and negatively correlated with wood basic density (r = -0.82)
and ratio of MOR/MOE (r = -0.78). On the contrary wood basic density had weak
and negative correlation with all the wood properties with MOR having the highest
correlation (r = -0.47). The shrinkage directions (radial and tangential) were
highly correlated (r = 0.91 and 0.97). Of the three mechanical properties, impact
bending was more correlated with radial shrinkage (r = 0.77), ratio of MOR/MOE
(r = -0.83) and MOR (r = 0.89). MOR had greater correlation with moisture content
(r = 0.74) and MOE (r = 0.79). The linear relationships among the indices measured
may be used as indicator of their predictive power one from the other.
CONCLUSIONS
This preliminary study has shown that the wood of Gliricidia sepium is a potential LUS that could find a major value in the timber market. The density of the wood is high and could be put into useful purposes where heavy wood is needed for construction. The mechanical properties were comparable and superior to other hardwood species and thus make it a potential substitute to those over-exploited species that are fast disappearing from the timber market. The results obtained in this study has provided quantitative information on the mechanical properties of Gliricidia which hitherto has not been provided to support claims that the wood is strong, heavy and useful for building and construction.
With this preliminary study, there may be need to consider G. sepium as a plantation species for timber production.
More trees will be sampled for axial and radial variation. This should be done in both pollarded and non-pollarded trees.
There would be need to investigate the anatomical and chemical properties of the wood and how this relate to strength properties.
ACKNOWLEDGMENTS
The authors wish to thank the Campus Tree Management Committee of the University of Ibadan for providing the wood. Also the assistance of Dr. Omole, O.A. and Mr. Olajuyigbe S.O. during the mechanical testing of the wood specimens is appreciated.