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SEEFOR 7 (1): 49-55
Article ID: 98
Original scientific paper
Radial Variation in Selected Physical and Anatomical Properties Within and Between Trees of 31 Year Old Pinus caribaea (Morelet) Grown in Plantation in Nigeria
Adewunmi Omobolaji Adenaiya1*, Olukayode Yekin Ogunsanwo1
(1) University of Ibadan, Faculty of Agriculture and Forestry, Department of Forest Resources Management, Ibadan, Nigeria
Citation: ADENAIYA AO, OGUNSANWO OY 2016 Radial Variation in Selected Physical and Anatomical Properties Within and Between Trees of 31 Year Old Pinus caribaea (Morelet) Grown in Plantation in Nigeria. South-east Eur for 7 (1): 49-55. DOI: https://doi.org/10.15177/seefor.16-07
Received: 19 Jan 2016; Revised: 8 Mar 2016; Accepted: 9 Mar 2016; Published online: 7 Apr 2016
Background and Purpose: Variation in wood is a common phenomenon. Eliciting information on the pattern and extent of variation in wood properties is crucial to knowing the end use of wood species. This to a larger extent helps in the efficient and sustainable utilization of wood. Pinus caribaea has been internationally affirmed to be suitable for the manufacturing of pulp and paper. Since its introduction into Nigeria, the possibility for its use for pulp and paper has been grossly undermined because of the non-functional paper mills in Nigeria. Hence, the established plantations of the species in the country have gradually been exploited for timber. In order to ensure the efficient use of the species for structural purposes, it is therefore pertinent to understand the variation pattern in its anatomical and physical properties.
Materials and Methods: Five trees of 31 year old P. caribaea wood were harvested at Shasha Forest Reserve, South-West Nigeria. Bolts of 50 cm in length were obtained from each felled tree at breast height (1.3 m). Discs of 5 cm thickness were obtained from each bolt which were used for the anatomical investigation. The bolts were used for the investigation of the physical properties. The data generated were subjected to ANOVA and Regression Analysis.
Results: The results show that considerable differences exist in the anatomical properties among trees (p<0.05). The physical properties show no marked difference among trees (p>0.05). Both the physical and anatomical properties differ insignificantly within the tree (p>0.05). Tree’s basic density, tracheid wall thickness and lumen diameter only explained 49% of the variation in the preservative absorption within the tree (R2=0.4884, P-value=0.053), re-emphasizing the major role played by bordered pits in the permeability of softwoods.
Conclusions: Due to the high preservative absorption observed using a non-pressure treatment, it is concluded that the species is very porous. Its porosity may pose a disadvantage when moisture absorption is concerned. Thus, it is recommended that the wood be properly treated in order to reduce its potentials in absorbing excessive moisture during service.
Keywords: tracheid, absorption, bordered pits, pit aspiration, density, preservative, structural application
Globally, natural forests are rapidly on the decline. Due to the slow growth of natural forests and hence their inability to cater for the growing demand for wood and wood products, plantation forestry has therefore been practised over the years and as such, it has been the major supplier of wood for the wood industries. Industrial plantations of economic species in Nigeria were established in the 20th century, with concentrations of these plantations in the southern part of the country. The main exotic species under plantations in Nigeria include Tectona grandis, Gmelina arborea and Pinus caribaea, mainly due to their fast growth rate.
Pinus caribaea (Carribean Pine) was primarily introduced to Nigeria for the production of long fibre pulp . Currently, Nigeria imports pulp in order to manufacture her paper and paper products . The inability of the pulp and paper industries in Nigeria to produce pulp, and consequently, depending on its importation led to the tree species established under plantations, which were initially intended to feed these pulp and paper mills outgrowing the stage of their suitability for pulp and paper. As a result, these species have been diverted to other uses. Pinus caribaea, even though a softwood species, has recently started gaining prominence for use as structural timber in Nigeria. As a fast growing species in Nigeria, it is therefore a promising species which can augment other structural timber species in the wake of increasing demand for solid wood in Nigeria. However, understanding its wood properties will provide a better insight into the ideal types of structural purposes it can be used for so as to ensure its efficient utilization.
Wood microstructure has long been recognised as an important tool in identifying the suitability of a wood species for a particular purpose  Also, it is unanimously agreed among researchers that wood density is a major factor affecting wood quality [4, 5]. Interestingly, within and between variability studies in wood properties of matured trees of P. caribaea in Nigeria are limited, given its value and the potentials it holds as a promising wood species in the country. Information on wood variation are essential for the improvement of properties which play crucial roles in its use as a raw material for both domestic and industrial uses . Oluwadare  investigated the variation in some selected wood properties among different age classes of P. caribaea grown in Nigeria. Udoakpan  studied within tree variation in density, ring width and anisotropic shrinkage of Nigeria grown P. caribaea and observed little variability in the studied wood properties. However, within and between tree radial variation in the anatomical and physical properties of Nigerian grown P. caribaea within the same age class is unknown, which are also crucial determinants of wood quality .
Known to be a non-durable species, utilizing it for external structural applications therefore makes the treatment of P. caribaea wood with wood preservatives against biodeteriorating agents imperative in order to extend its service life, hence, promoting a more sustainable use of the wood. However, with several studies revealing a significant within-tree variation in wood properties , while variation in wood is expected to affect the uniformity in preservative uptake of the wood [9, 10] - an important index which determines the effectiveness of a wood preservative, it is therefore necessary to investigate the extent to which radial variation in the wood properties of P. caribaea will affect preservative absorption by its wood. Information gained from this will enable the development of an appropriate preservative treatment regime for this species in order to aid its efficient utilization.
In order to contribute to the existing knowledge on the variation in P. caribaea wood, this study aims to quantify radial variation in selected anatomical and physical properties within and among trees of P. caribaea with a view to advancing further knowledge on the species.
MATERIALS AND METHODS
Wood samples and Site Characterization
Wood samples used for this research were obtained from a 31 year old pine plantation grown in Shasha Forest Reserve, South-West Nigeria. The forest reserve is located on latitude 70 and 7030’ N and longitude 40 and 50 E . The total annual rainfall ranges from 887 mm to 2,180 mm, and it has a minimum and maximum annual temperature of 19.50C and 32.50C respectively . Five defect-free trees of P. caribaea with absence of reaction tendencies were harvested. Bolts of 50 cm in length were obtained from the breast height (1.3 m) of the trees. From each of these bolts, discs of 5 cm thickness were collected for use in the anatomical investigation. The breast height was chosen as the point of sampling along the tree bole because of the strong correlation of wood density with whole tree density (r2=0.99) at this point [13, 14]. Thus, it is expected that results obtained from this chosen sample will reflect the wood properties for the whole tree since density is strongly correlated with all other wood properties.
Determination of Basic Density
Each of the remainder bolts (45 cm in length) were partitioned radially from pith to bark into three zones: inner wood (rings 1-10), middle wood (rings 11-20) and outer wood (rings 21-31) based on the number of rings. Ten wood samples of dimensions 6 cm x 1.5 cm x 1.5 cm were obtained from each wood zone and oven-dried at 1030C for 18 hours. Basic density was estimated as the ratio of oven-dried weight of wood to green volume of wood .
Determination of Tracheid Dimensions
The discs were partitioned in a similar pattern as the one described above. From each of the discs, splints were obtained from each ring and macerated in equal volume (ratio 1:1) of 10% acetic acid and 30% hydrogen peroxide . The macerated slivers were then washed; distilled water was then added after which they were shaken vigorously to separate the fibres. The suspension was mounted on a slide with the aid of a rubber teat, stained with safranin and 10 tracheids were measured in swollen condition , using the Rheichert visopan microscope for length (mm), diameter (µm), lumen width (µm) and cell wall thickness (µm). Two slides were prepared for each wood sample.
Determination of Preservative Absorption
The samples used in estimating the basic density for each partitioned wood zone, as described above, were subsequently treated by soaking for 24 hours in a preservative formulated from a mixture of castor oil (Ricinus communis) extracted mechanically from its seeds and kerosene (an organic solvent used as the diluent) in the ratio 3:7 using the volume dilution method . Preservative absorption of the wood samples was determined by using the method employed by Olajuyigbe et al. :
where WPA is weight of preservative absorbed (kg), and V is volume of wood sample (m3).
The data obtained were analyzed using descriptive statistics and two-way analysis of variance (ANOVA) in order to evaluate the variation in the wood species’ anatomical and physical properties among trees (tree effect) and within tree (radial effect). A multivariate regression analysis was used to model the preservative absorption as a function of the basic density and anatomical properties investigated. During the model development process, predictor variables which contributed insignificantly to preservative absorption were eliminated.
Between-Tree Variation in Wood Properties
The pattern of radial variation in the tracheid biometry and physical properties investigated among trees are presented graphically in Figures 1-6. The figures show no consistent pattern of variation from pith to bark in the physical and anatomical properties among the trees. The standard errors for each wood property among the trees greatly differ (Table 1), further indicating heterogeneity among the trees.
FIGURE 1. Radial variation in wood density among trees of P. caribaea
FIGURE 2. Radial variation in tracheid length among trees of P. caribaea
FIGURE 3. Radial variation in tracheid wall thickness among trees of P. caribaea
FIGURE 4. Radial variation in tracheid lumen width among trees of P. caribaea
FIGURE 5. Radial variation in tracheid diameter among trees of P. caribaea
FIGURE 6. Radial variation in preservative absorption among trees of P. caribaea
Values of wood properties for individual trees are presented in Table 1. The mean range of tracheid length is 1.85-2.68 mm, with the wood having an average tracheid length of 2.31 mm. Tracheid diameter ranged between 45.37-60.72 µm, with an overall average of 53.25 µm. Mean values of 34.86 µm and 9.2 µm were recorded for tracheid lumen diameter and cell wall thickness respectively. However, it was observed that all the anatomical properties differed significantly (p<0.05) among trees (Table 3).
For the physical properties, basic wood density ranged between 413.85-536.95 kg·m-3, while preservative absorption of the wood ranged between 110.17-158.93 kg·m-3. Mean preservative absorption and basic density of the wood were 126.28 kg·m-3 and 500.41 kgm-3 respectively. Both the wood basic density and preservative absorption were not statistically significant (p>0.05) among trees (Table 3).
TABLE 1. Physical and anatomical properties of P. caribaea wood
Within-Tree Variation in Wood Properties
The within-tree radial variation in the wood properties of P. caribaea from pith to bark is shown in Table 2 below. Tracheid length steadily increased from the core (2.16 mm) to the periphery (2.45 mm). Tracheid diameter and lumen width, as observed in this study decreased from the core outwards. However, an inconsistent pattern of variation was observed for the cell wall thickness, with a gradual increase from the core wood (8.65 µm) to the middle wood (9.54 µm), and then further declining in the outer wood (9.38 µm). The physical properties investigated (wood basic density and preservative absorption) both showed a consistent increase from pith to bark. Within-tree differences observed for all the wood properties from pith to bark were however not statistically significant (p>0.05) (Table 3).
TABLE 2. Radial variation in wood properties of P. caribaea wood
TABLE 3. The analysis of variation among and within trees of P. caribaea (radial)
The Relationship Between Wood Properties and Preservative Absorption
The model presented in Table 4 reveals the key variables that significantly influenced preservative absorption in the wood of P. caribaea. A positive relationship was observed between tracheid diameter and absorption. Wood density was positively related to preservative absorption, while an inverse relationship was observed between cell wall thickness and preservative absorption. From the model, tracheid diameter, wall thickness and wood density only explained 49% of the variation in preservative absorption (R2=0.488) by the wood of P. caribaea, with the developed model being significant at 94% probability level (p-value=0.053).
TABLE 4. The relationship between wood properties and preservative absorption in P. caribaea wood
Generally, this study shows that the anatomical properties of the wood varied significantly from the inter-tree comparison, while no marked differences were observed in the physical properties. Furthermore, the within-tree analysis revealed that individual trees were uniform in both their physical and anatomical properties. Ideally, anatomical properties of a particular wood should reflect its physical attributes [18, 19]; however, this is in contrast to the findings of this study. The non-association between both wood properties (anatomical and physical) among the trees suggests that differences in other anatomical properties and extrinsic factors such as extractive content, number and distribution of resin canals, earlywood/latewood ratio within rings, etc which were not examined in this study may have contributed greatly to the differing variation patterns between the anatomical and physical properties observed among the P. caribaea trees [18, 19]. The haphazard pattern of variation observed among trees for each wood property may be a result of the differences in the genotype of individual trees . Nonetheless, the noticeable between-tree variation observed in the anatomical properties on the other hand may prove advantageous in tree breeding programmes, giving way for future improvement on its anatomical attributes.
The anatomical features investigated show a mixed result in comparison with existing literature. The radial pattern of variation in anatomical features has been extensively reported by several studies [14, 16, 21]. While the mean values obtained in this study for tracheid diameter and cell wall thickness were consistent with the results reported by Oluwadare , tracheid length and lumen width were, on the other hand, considerably low compared to his findings. The difference observed in the lumen width may possibly be due to differences in site conditions, silvicultural treatments, tree age, or tree genetic make-up [14, 18, 22]. The wood tracheid length, apart from being considerably lesser to that reported by Oluwadare , is also low when compared to some other pine species [14, 23] and to the average tracheid lengths of 3-5 mm generally reported for softwoods . Possible reasons for this outcome may be due to erroneous measurement of some broken tracheids or, perhaps, due to the reduced number of sampled tracheids in the study when compared to other similar studies [e.g. 1], thus making the sampled tracheids poor representatives of the entire tracheid population of the trees.
The increasing trend from pith to bark observed for the physical properties is consistent with that reported for some softwood species [16, 18, 19]. The mean basic density for the wood observed in this study (500.41 kg·m-3) is similar to that reported by Oluwadare  and Hashemi and Kord  for P. caribaea grown in Nigeria, although a little bit higher than that reported in Sri Lanka . This contrasting result with that obtained in Sri Lanka may be due to differences in tree age or silvicultural treatments [22, 26]. The increase in preservative absorption from pith to bark can be explained by the reduced number of aspirated pits from pith to bark, consequent of the increasing proportion of latewood percentage from the inner wood to the outer wood 
Apparently, the moderate basic density of the wood shows that it will be suitable for light construction works. However, the relatively uniform density of the wood observed in this study makes it ideal for veneer production by peeling or slicing . More so, the preservative absorption values of the wood indicate that sufficient absorption levels can be obtained through treatment by non-pressure methods rather than the use of the higher energy-consuming pressure treatments. It is worthy to note that even though the average tracheid lumen width and basic density are comparable to some other pine species [e.g. 14], it may pose a problem in joinery works due to the high preservative absorption observed in this study. The implication of this, by extension, is that the wood will permit excessive moisture absorption during fluctuations in atmospheric moisture which may result in high swelling and shrinkage.
The observed relationship between the wood properties and preservative absorption was weak due to the low predictive power of the wood properties. The positive relationship observed for both diameter and density with preservative absorption implies that wider tracheids and thick-walled tracheids with fewer aspirated pits enhanced absorption. This finding further reinforces the assertion that bordered pits play a prominent role in the flow of liquids in softwoods [27, 29]. Nonetheless, the observed positive relationship between density and absorption in this study is in contrast with observations of Ahmed et al. . This disparity possibly is as a result of the wood type since permeability in hardwoods is primarily determined by wider vessel lumen and, by extension, a lower wood density. The inverse relationship between wall thickness and preservative absorption suggests that higher pressure is needed to achieve better preservative penetration into the tracheid cell walls, consequent of the type of preservative treatment method employed in this study. This observation however strongly suggests that radial variation in earlywood/latewood proportion significantly influenced absorption given that pit aspiration, and hence lateral fluid permeability within the wood along the radial direction was quite pronounced.
It can be concluded from this study that inter-tree variation in the anatomical properties was considerable while the trees exhibited uniformity in their physical properties. Within-tree variation on the other hand was insignificant. Interestingly, the wood can be described as being very porous, which is evident from the high preservative absorption recorded under a non-pressure method of treatment. This shows that the wood does not belong to the group of refractive wood species, since most softwoods are observed to be due to the pit aspiration phenomena. While the easy treatability of the wood might be an advantage, the possibility of absorbing excessive moisture which can result into pronounced wood movement needs to be addressed through proper treatments to reduce its moisture absorption capacity and thus, to improve its dimensional stability. Furthermore, this study clearly affirms that bordered pits largely determine permeability in softwoods. Although a high preservative absorption was observed, the depth of penetration and uniformity of the preservative absorption may largely be impaired by the extent of pit aspiration in the wood.
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© 2016 by the Croatian Forest Research Institute. This is an Open Access paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0).