0236-B4
Reynolds Okai 1
One way of meeting the supply demands of industry is through the reduction of waste in timber harvesting. In this regard, the whole-tree concept, which utilizes stem, branches, stumps, buttress log and so on, may be useful. In Ghana, it has been established that for every tree felled, nearly 50% of the tree volume is left in the forest in the form of branchwood, crownwood and stumps. The annual extraction of logs by the mills, which is now estimated to be 3.7 million m3, is far above the annual allowable cut (AAC) of 1 million m3. Thus nearly 4millionm3 of wood is lost annually in the form of logging residues. The technological properties of branchwood of Aningeria robusta and Terminalia ivorensis were therefore examined to determine if logging residues could be used to help meet the timber demands of the downstream processing sector. The sawing characteristics and strength properties of branchwood of A. robusta and T. ivorensis of diameters ranging from 10 cm to 25 cm were determined. The parameters assessed under sawing characteristics were lumber value and volume yields. Under strength properties, the modulus of rupture, modulus of elasticity, compressive and shear strengths were assessed.
Results obtained for the milling and strength properties of branchwood of A. robusta and T. ivorensis have suggested that logging residues can be used to help meet the timber demands of the downstream processing sector. The mean lumber value and volume yields of the two species were between 20-25% and 32-41% respectively. The modulus of rupture and modulus of elasticity of branchwood of A. robusta and T. ivorensis were found to be less than that of their corresponding stemwood. However, the shear and compression strengths parallel to grain of the branchwood were significantly higher than that of their corresponding stemwood. To demonstrate the potential of utilizing logging residues by the downstream processing sector, prototype furniture was successfully manufactured from branchwood of A. robusta and T. ivorensis. In the light of the above results, it is recommended that amendment of the existing logging rules should be considered to cater for the processing of branches, stumps, buttress logs, etc.
Timber from the natural forest in Ghana is dwindling at a fast rate and this poses a threat to the raw material base. Indiscriminate bushfires, illegal logging operations, farming, and surface mining threaten the timber resource. In order to ensure that timber harvesting in Ghana's natural forest can be sustained into the future, an Annual Allowable Cut (AAC) has been set at one million cubic meters of round log (Foli et al. 1997). Besides, other measures like increased royalties, acquisition of timber utilisation contract (TUC) and ban on chainsaw operations have been put into place to ensure sustainability of Ghana's forest. However, the demand for wood is so alarming that the AAC of one million cubic meters is insufficient to meet the nation's demand for wood. The annual extraction of logs by the mills is now estimated to be nearly 3.7 million m3 (Birikorang et al. 2001). Thus nearly 4 million m3 of wood is lost annually in the form of logging residues.
One way of meeting the supply demands of industry is through the reduction of waste in timber harvesting. In this regard, the whole-tree concept, which utilises, stem, branches, stump, buttress log and so on, may be useful. It has been established that in Ghana for every tree felled, nearly 50% of the tree volume is left in the forest in the form of branchwood, crownwood, and stumps (Adam et al 1993). Poor logging practices, e.g., using heavy machinery (crawler-tractor methods) are very damaging to the forests and often lead to massive environmental damage such as soil erosion and high sedimentation in river systems that reduce water quality. Additionally, the residual crop gets heavily damaged and the overall biodiversity is reduced. Some loggers currently employ ground skidding techniques in the high forest as an alternate solution to the inefficient logging methods using the crawler-tractor method. Logging residues are expected to reduce if efficient machinery and equipment are used for logging. In addition, efficient utilisation of logging residues such as branchwood, stumps and crownwood will increase the output volume and value per unit forest area and that in many situations this is necessary to allow for effective, feasible and sustainable forest management. In this paper the potential of using logging residues as raw material for the downstream processing sector is discussed. In the light of Ghana's development objectives it cannot be over emphasised that efficient utilisation of branchwood will significantly increase revenue and foreign exchange earnings as well as generate employment for a large number of people.
Wood samples of branchwood and stemwood (normal wood) were extracted from the Bobiri Forest Reserve and the Afram Headwaters Forest Reserve at Kubease and Abofour in the Ashanti Region of Ghana respectively. The Bobiri Forest Reserve lies within the moist semi-deciduous forest zone and the Afram Headwaters Forest Reserve lies within the dry semi-deciduous forest zone. Four tree samples each of Terminalia ivorensis and Aningeria robusta were randomly selected and felled at each ecological zone. The felling direction of tree samples of Aningeria robusta was determined for a direction that will cause minimal damage to the residual forests. In the case of tree samples of Terminalia ivorensis, the trees were allowed to take their natural direction of felling. Damages to the residual forests caused by felling methods were also assessed. The girth of each tree at breast height was about 2.5 m, and of average height of 30 m. Samples of branchwood was extracted about 15 cm from the node of the stemwood towards the crown of the branches with each branchwood having a diameter greater than or equal to 10 cm. The butt-end and the top-section of the branches were marked for easy identification. Sixty fairly straight branches of each species were cut to a length of 2.0 m at each ecological zone. Thus a total of 240 branchwood with diameters ranging from 10 cm to 25 cm were extracted from the two ecological zones. For each tree sample of the stemwood, billets each of about 2 m length, were extracted and classified under three categories; the butt end, the middle billet and the top billet.
A Randomized Complete Block Design (RCBD), 2 × 4 factorial with 3 replications was developed for the experiment. The treatment structure was a factorial arrangement with the factors being log diameter classes and sawing pattern. The logs were grouped into four diameter classes, and sawing tests were conducted using the live and cant sawing methods. A total of 18 logs under each log diameter class and replicate were sawn. The machine used for the log breakdown was a horizontal mobile bandmill known as woodmizer. The saw blade (spring set) had the following dimensions: tooth pitch 22 mm, width 30 mm, kerf 1.6 mm, gullet depth 5 mm.
After transporting the wood samples from the forest to the workshop of the Forestry Research Institute of Ghana, the logs were further examined for possible defects such as insect attack or splits at the ends, which could result from improper handling. Logs found to be damaged were discarded. The length and crosswise diameter under bark at the top and butt end of each log was measured and the mean log diameter at the top and butt end of the log determined. The volume Vi of each log was calculated from Equation (1), where L, Ds, and Dl are the length, top end diameter, and butt end diameter of the log respectively. The logs were then randomly picked within each diameter class and grouped separately under each replication, and thereafter, sawn into boards of dimensions 2.5 cm × 7.5 cm × 200 cm using live and cant sawing patterns. Note that under each log diameter class and replicate, equal numbers of logs were sawn using the live sawing and cant sawing patterns. The surfaces of the sawn lumber were examined for woolly or fuzzy grain, knot, pinworm holes and washboards. They were then graded into first and second (FAS) grade boards. The volumes of the boards were computed by measuring the length, width and thickness at three points along the length of the boards. For each log, the lumber yield was computed by expressing the volume of boards obtained as a percentage of the input volume of the log. The lumber volume and value yields for the study were calculated from Equations (2) and (3), respectively where Yvol is volume yield (%), Yval is value yield (%), Vi is log volume (m3), Vo is lumber volume (m3), Co is unit price of the standard lumber (US$/m3),
(1)
(2)
(3)
Ci is unit price of each lumber (US$/m3). The standard lumber is first and second (FAS) grade lumber from stemwood of dimensions 5.0 cm × 10.0 cm × 365 cm at a unit cost of US$150/m3 and US$120/m3 for Aningeria robusta and Terminalia ivorensis respectively. The amount quoted is the price equivalent in dollars of unit cost of lumber in Ghana for the species under consideration.
For each tree sample extracted from the forest, three portions of the billets were used to determine some physical and mechanical properties. Samples of the stemwood used for comparison tests contained portions of the tree near the butt end, the mid-section, and near the top-section or crown of the tree. Similarly, samples of the branchwood contained portions of the tree near the node, the mid-section, and near the crown of the branches.
The physical properties determined were the nominal specific gravity at 12% moisture content and the moisture content at green state for both sapwood and heartwood. The strength properties determined for both sapwood and heartwood were modulus of elasticity, modulus of rupture, compression strength parallel to grain, and shear strength parallel to grain. Test results for the physical and mechanical properties are presented for wood samples from the moist semi-deciduous forest zone. For each tests of the branchwood, 10 samples each from the butt billet, the middle billet and the top billet from each tree were used. Thus a total of 120 samples from four trees were used to perform each test. Similarly, for the stemwood, 10 samples each from the butt billet, the middle billet and the top billet from each tree were used. Thus a total 120 samples from four trees were used to perform each test. Strength tests were conducted after conditioning the sapwood and heartwood to 12% moisture content. The preparation of small clear test specimens and methods for determining the physical and mechanical strength properties can be found in the literature for methods of testing small clear specimens of timber (Gwendoline 1983, British Standard Institution, BS 373: 1957). A 50 Ton Avery testing machine was used to determine the strength properties.
Assessment of damages to residual forest by felling methods revealed that when the direction of felling was pre-determined, only two residual trees were damaged when four trees were felled. On the other hand when the trees were felled arbitrary without taking the felling directions into consideration, about five residual trees were damaged when four trees were felled.
3.2.1 Tension wood and fuzzy grain
Tension wood is associated with eccentric growth rings and a higher density than the surrounding normal wood. It is also characterised by a higher longitudinal shrinkage and the presence of woolly or fuzzy grain on surfaces sawn in the longitudinal direction (Tsoumis 1991). Furthermore, tension wood is characterised by difficulty in nailing, sawing and drying. Besides, saw blade becomes overheated during sawing of tension wood (Tsoumis 1991). However, examination of the cross-section of branchwood of Terminalia ivorensis and Aningeria robusta did not show the presence of eccentric growth rings. Besides, fuzzy grains were not found on the surfaces of the sawn lumber and this suggests the absence of tension wood in the branches of Terminalia ivorensis and Aningeria robusta.
3.2.2 Blunting effect of branchwood on saw blades
It was observed in the study that branchwood of Terminalia ivorensis was easy to saw compared to branchwood of Aningeria robusta, which exhibited severe blunting effect on the saw blades. When processing Aningeria robusta, the saw blades had to be changed frequently. The severe blunting effect of Aningeria robusta on saws was due to the fibrous or spongy nature of its tissues. In particular, tissues that were fibrous tended to pull the saw blade resulting in friction and consequently overheating of the saw blades. The relative ease in sawing the branchwood of Terminalia ivorensis suggests the absence of tension wood since tension woods are characterised by difficulty in sawing.
3.2.3 Lumber yields
Lumber yield is an index to the processing efficiency of a mill. Some of the factors that affect lumber yield are log diameter, sawing pattern, log shape, defects in the log, operator's skill, type of machinery, and best opening face. In practice, big diameter logs generate higher lumber yield than small diameter logs. Since branchwood can be classified as a small diameter log, its lumber yield is expected to be low and any attempt to use branchwood as a raw material for downstream processing should begin by first looking at its lumber value and volume yields. Figure 1 shows the lumber value and volume yield for branchwood of Aningeria robusta and Terminalia ivorensis at log diameter ranging from 10 to 25 cm. The mean lumber value yields for first and second (FAS) grade boards of Aningeria robusta and Terminalia ivorensis are 24.9% and 20.1% respectively, and the mean lumber volume yield is 41.2% for Aningeria robusta and 32.2% for Terminalia ivorensis. The combined lumber value yield for branchwood of Aningeria robusta and Terminalia ivorensis is 22.5%, and the corresponding combined lumber volume yield is 36.7% The low lumber value and volume yield for Terminalia ivorensis compared to the higher lumber value and volume yields for Aningeria robusta was due to the presence of high proportion of sapwood in Terminalia ivorensis and the formation of "washboard".
The strength properties of Terminalia ivorensis and Aningeria robusta at 12% moisture content are presented in Table 1. It can be seen that under static bending, the overall modulus of rupture and modulus of elasticity of branchwood of Aningeria robusta and Terminalia ivorensis are smaller than that of their corresponding stemwood. However, the overall shear strength parallel to grain and compression strength parallel to grain of branchwood of the two species are bigger than that of their corresponding stemwood. When one considers the overall strength values of branchwood and stemwood for the two species under investigation, it can also be observed from Table 1 that the specific gravity of branchwood are generally higher than the specific gravity of stemwood. However, the strength values are not always related to the specific gravities. Thus it can be concluded that not all strength properties are influenced by specific gravity.
Table 1. Specific gravity and strength properties of Terminalia ivorensis and Aningeria robusta at 12% moisture content
Figure 2 shows a set of garden furniture manufactured from branchwood of Aningeria robusta at 12% moisture content. It was observed that turning, planning, boring, sanding, and gluing of various components of the products were satisfactory. Several other products ranging from shoe stand to refrigerator stands were also manufactured from branchwood.
Fig. 2. A set of garden furniture manufactured from branchwood of Aningeria robusta
The current annual allowable cut (AAC) of 1 million cubic metres of round logs is insufficient to meet the nation's demand for wood considering the fact that nearly 4 million cubic metres of round logs are consumed annually. Unfortunately, additional 4 million cubic metres of wood are lost annually in the form of logging residues. The present study has demonstrated that there is the potential to utilize logging residues as raw material for the downstream processing sector. It is recommended that amendment of the existing logging rules should be considered to cater for the processing of branches, stumps, buttress logs, etc. This will ensure a sustainable forest management in Ghana.
In Ghana, the average lumber recovery from logs of average diameter of 70 cm using the conventional bandmill for processing is about 42%. The present study has shown that the mean lumber volume yield of branchwood of diameters ranging from 10 to 25 cm is about 37% cm. The study has also shown that the strength properties of branchwood are comparable to the strength properties of the corresponding stemwood. Thus judging from the success achieved in the manufacture of prototype furniture, it is crystal clear that the study on the milling and strength properties of branchwood of Aningeria robusta and Terminalia ivorensis has suggested that logging residues can be used to help meet timber demands of the downstream processing sector. The intense pressure on the forest for timber species could be reduced if logging residues are efficiently utilised. An amendment to the existing logging rules is therefore recommended to ensure the sustainability of Ghana's forest.
Adam, A. R.; Ofosu-Asiedu, A.; Dei Amoah, C.; Asante Asiamah A.; 1993. Wood waste and logging damage in Akuse and Afram Headwaters Forest Reserve. Report of ITTO Project PD 74/90. Better utilization of tropical timber resources in order to improve sustainability and reduce negative ecological impact, FORIG, Kumasi, pp. 46-51.
Birikorang G; Okai, R; Asenso-Okyere, K; Afrane, S; Robinson, G; 2001. Ghana wood industry and log export ban study. Report submitted to the Ministry of Lands and Forestry, Accra, Ghana.
British Standard Institution.; 1957. British Standard 373:1957. Testing small clear specimens of timber. BSI, London.
Foli, E; Adade, K; Agyeman, V; 1997. Collaborative forest management systems for off-reserve areas in southern Ghana. Proceedings of ITTO/FD seminar on sustainable timber production from outside forest reserves. FORIG, Kumasi, Ghana.
Gwendoline, M. L. 1983. The strength properties of timber. Building research establishment report. pp. 60.
Tsoumis, G.; 1991. Science and technology of wood. Structure, properties and utilisation. Van Nostrand Reinhold. pp 95
1 Research Scientist and Deputy Coordinator, IUFRO Division 5.04.08, Forestry Research Institute of Ghana University, PO Box 63, Kumasi, Ghana. [email protected]