Physical and mechanical wood properties of two varieties of Pinus caribaea

This study aimed to characterize the properties of wood from Pinus caribaea var. hondurensis and Pinus caribaea var bahamensis , in 7-year-old individuals, in the municipality of Batatais, State of São Paulo, Brazil. For this, twelve trees of each species/variety were felled, and a 1 m long log was removed from the base of each tree. Samples were taken from the obtained logs to evaluate the physical-mechanical properties of the wood. It was verified that the woods of Pinus caribaea var. hondurensis and Pinus caribaea var. bahamensis do not differ significantly from each other for: Apparent density, basic density, compression parallel to the grain, shear parallel to grain, modulus of elasticity, volumetric shrinkage, anisotropy coefficient, radial shrinkage, and tangential shrinkage. The wood of the species/variety Pinus caribaea var. bahamensis showed a significantly higher modulus of rupture than the variety Pinus caribaea var. hondurensis . Regarding the use of wood from these species/variety, at a younger age, both showed potential use for different purposes. Since this initial assessment can be used as an indication of pre-selection of wood for the various segments of the timber industry, mainly for uses that do not have many restrictions in structural applications and dimensional variation.


Introduction
The increased demand for wood has led to a search for new species with silvicultural potential.Among the various species that were introduced as alternatives to hardwoods that were excessively exploited, as well as to supply the high volume of wood consumed in the various segments of the timber sector, we have Pinus spp.(Trianoski et al., 2014).The area planted with Pinus in Brazil has been increasing significantly, reaching 1,930,055 ha in 2021 and a productivity of 29.7 m³.ha -1 .year - (Indústria Brasileira de Árvores -IBÁ, 2022).
Among the Pinus species planted in Brazil, we have Pinus caribaea Morelet, this species covers three natural varieties: caribaea, bahamensis and hondurensis (Kronka et al., 2005).Pinus caribaea produces white or yellowish wood, with the possibility of resin extraction (Lorenzi et al., 2018).Pinus caribaea presents volumetric shrinkage around 12.9%, tangential shrinkage 7.8% and radial shrinkage 6.3% (Glass and Zelinka, 2021).In Uganda, Pinus caribaea wood is used for structural purposes, and even woods that come from plantations that have been burned are used in civil construction works if they do not have many restrictions in terms of mechanical resistance (Zziwa et al., 2020).
Pinus caribaea var.hondurensis was one of the most planted species in Brazil, due to the ease of obtaining seeds, the wood has a density classified as being moderate to low and, in general, is very useful for the timber industry (Kronka et al., 2005;Shimizu & Sebbenn, 2008).The initial growth of Pinus caribaea var.hondurensis is positively influenced by the increase in the variables: precipitation, solar radiation, real evapotranspiration, and relative humidity (Lima et al., 2022).
Pinus caribaea var.bahamensis stands out as one of the most important species to produce wood and resin in the southeastern region of Brazil, the wood of this species tends to be denser than the hondurensis variety, having a greater physical-mechanical resistance (Shimizu & Sebbenn, 2008).Some aspects must be considered in the production of pine wood for industrial use, such as variations in the proportion of juvenile wood, production age, volume of wood produced from each genetic variety, silvicultural practices, and population management, all these factors affect productivity and wood quality to be produced (Zanuncio et al., 2022).
Wood at a younger age tends to contain a greater proportion of juvenile wood than mature wood, therefore, structural pieces of wood that contain a greater amount of juvenile wood have lower mechanical strength classes.This being the reason why knowledge of the differences between the properties of juvenile and mature wood is so important for the use of mechanically processed wood.Another relevant question is to know the behavior of the dimensional variations of the wood, in which the juvenile and the adult wood behave, and in general, their behavior is slightly different (Vidaurre et al., 2012).
In general, we found that studies on the quality of Pinus caribaea wood are still scarce, mainly regarding the use of wood at a younger age, and which also aim to maximize wood production and raise the income of the producers who cultivate it.Therefore, the objective of this work was to characterize the physical and mechanical properties of Pinus caribaea wood at a younger age, depending on the variety and to provide basic subsidies for the best industrial use of this wood.

Planting area and sampling
The study area is in the municipality of Batatais, São Paulo State, Brazil (20°56′01′′S, 47°36′28′′W, elevation 880m) (Zanata et al., 2015).According to the Köppen Climate Classification system, climatic type is Cwa, a warm subtropical climate with dry winter, with an average annual temperature of 20°C and an average rainfall of 1569 mm year -1 (Alvares et al., 2013).
The Brazilian System of Soil Classification and the Pedological Map of São Paulo State indicate that the area under study has soil of medium texture, such as Latosol Red, Red Yellow (LVA31), Dystrophic, in soft wavy relief and Gleissolos in the alluvial floodplains (Santos et al., 2018;Rossi, 2017).
The trees selected for study were from a commercial plantation of 7-year-old Pinus caribaea of two varieties: Pinus caribaea var.bahamensis and Pinus caribaea var.hondurensis.Trees were planted at a spacing of 3.0 m x 3.5 m, in 22 and 12 hectares, respectively.We selected 24 trees, 12 of each species/variety, trees were selected in the medium DBH (1.3 m from the ground) class, in each species/variety (Table 1).Before trees were felled, the geographic North was marked on each stem to standardize sampling.From the chosen trees, the first log of 1 m in length was taken, properly identified, and marked.Subsequently, these logs were transported to the ESALQ/USP sawmill, Piracicaba, SP, where they were unfolded.
From each of the unfolded logs, a central plank 7 cm thick was taken.A batten measuring 4 cm x 4 cm x 1 m was removed from the region close to the bark, then a study was carried out to determine physical and mechanical properties, which specific specimens were removed for each test, and carried out at the Laboratory of Wood Engineering at ESALQ/USP (Figure 1).Source: Authors.

Wood physical property
Apparent density (ρap) -Samples of 2 x 2 x 3 cm were obtained from the battens and dried until they reached 12% humidity, to obtain the apparent density.The dimensions of the samples were measured with a digital caliper with a sensitivity of 0.01 cm, and the mass of the specimen was obtained on a semi-analytical digital scale, with a sensitivity of 0.01 g, following the NBR 7190 standards (ABNT, 1997).
Basic Density (BD) -To obtain the basic density, samples of (3 x 2 x 5 cm) were used, and the hydrostatic balance method was used according to NBR 11941 (ABNT, 2003).

Wood mechanical properties
All mechanical tests were carried out with dry samples, in an environment with normalized temperature, until they reached 12% humidity, a standard reference condition, in accordance with the recommendation of NBR7190 (Associação Brasileira de Normas Técnicas -ABNT, 1997).
Compression parallel to the grain (fc0) -To obtain the compression parallel to the grain, 2 x 2 x 3 cm specimens were used, obtained from each batten, in a total of 24 units.Compression tests were carried out in a universal testing machine, according to the adapted NBR 7190 standard (ABNT, 1997).
Shear parallel to grain (fv0) -In obtaining the shear parallel to grain, test specimens with nominal dimensions of 2 x 2 x 3 cm, with 4 cm 2 in the shear area, were used according to the NBR 7190 standard (ABNT 1997).
Static bending -For resistance of the wood to static bending, specimens of 2 x 2 x 35 cm obtained from each batten were used.The tests were conducted in a universal test machine with a load application speed of 10 MPa.A combination of the NBR 7190 standard (ABNT 1997) and ASTM D143-94 (ASTM, 1994) was used, with dimensions (b x h) of 2 x 2 cm for cross section and 30 cm free span (L) and the modulus of rupture (MOR) and modulus of elasticity (MOE) in static bending were tested simultaneously.The test was conducted on the same universal machine.

Statistical analysis
In the evaluation of the experiment, the variance homogeneity test was initially performed, and, for this, the Hartley test was used.Subsequently, the F test of analysis of variance was carried out according to a completely randomized experimental design.Tukey's test at 5% probability level was applied whenever a significant difference was observed, in some treatment in the F test.Descriptive statistics was also used in data analysis.The data obtained for each variable were statistically analyzed using the SAS ® statistical program (SAS Institute Inc., 1999).

Results and Discussion
According to the presented results, it was verified that the apparent density, basic density, compression parallel to the grain, shear parallel to grain and MOE did not differ significantly between the varieties of Pinus caribaea, but for MOR, wood from the variety Pinus caribaea var.bahamensis showed significantly greater resistance than the variety Pinus caribaea var.
For the volumetric shrinkage, coefficient of anisotropy, radial and tangential shrinkage, there were no significant differences between the varieties of Pinus caribaea (Table 2 and Figures 2c-f).The mean values obtained for volumetric shrinkage are lower than those observed by Loiola et. al. (2021) and Lima et al. (2020), however, these authors evaluated wood from Pinus caribaea var.hondurensis aged 14 and 50 years, respectively (Table 2 and Figure 2c).The volumetric retraction values are considered as low, that is, little retraction as specified by Carvalho (1996).In the case of the anisotropy coefficient, the mean values obtained for volumetric shrinkage are higher than those observed by Loiola et al. (2021) (Table 2, Figure 2d).
These values are considered high according to Carvalho (1996).
Radial and tangential retractions are considered low by Carvalho (1996).Being lower than those observed by Loiola et. al. (2021) in Pinus caribaea wood, aged 14 years (Table 2, Figures 2e-f).Therefore, the values of volumetric, radial, and tangential retraction are considered low.However, the anisotropy coefficient is high, indicating that Pinus caribaea var.
hondurensis and Pinus caribaea var.bahamensis, at the age of 7 years, may be susceptible to drying defects.This same trend was observed in 13-year-old Cryptomeria japonica wood by (Vivian et al., 2023).3 and Figure 3a).For shear parallel to the grain, the values obtained (Table 3 and Figure 3b) are lower than the values of 10.38 MPa and 10.64 MPa for Pinus caribaea var.hondurensis and Pinus caribaea var.bahamensis 14-year-old obtained by Trianoski et. al. (2014).These values are also lower than the 10.49MPa of Pinus caribaea var.bahamensis obtained for 50-year-old wood by Lima et al. (2020).hondurensis aged 20 years (Table 3, Figure 3c).For the MOE, the values obtained (Table 3, Figure 3d) were lower than 7187 MPa and 7106 MPa for Pinus caribaea var.hondurensis and Pinus caribaea var.bahamensis, respectively, obtained from 14-year-old wood by Trianoski et. al. (2014) and those obtained by Gonçalez et. al. (2018) for Pinus caribaea var.hondurensis 20 years old.There is a statistically significant difference (P = <0.050).Source: Authors.
The results mentioned above are in line with those of several studies in South Africa with pine wood, aged less than 20 years, and they have repeatedly shown that the main limitation of using pine wood for the structural market is its lack of rigidity.While the flexural strength seems to be sufficient, however, a new approach is needed for the standardization of the structural use of wood produced from younger age pine (Nel et al., 2018).Another factor to be considered would be the fact that the mechanical strength of Pine wood has a high positive relationship with the proportion of late wood, which in the case of woods at a younger age, this proportion of late wood is lower (Winck et al., 2023).
In general, according to the results obtained by Pinus caribaea for the two varieties at 7 years of age, we can consider that they were very similar to the wood of Pinus caribaea var.hondurensis aged 50 years evaluated by (Lima et al., 2020), which may be an indication that these physical-mechanical properties of this species begin to stabilize at a young age.The wood of 50-year-old Pinus caribaea var.hondurensis, confirmed its potential for various uses, such as: the manufacture of panels, light structural use in civil construction and sawmills, which could then be an alternative to supply the demand for raw materials in the industrial sector (Lima et al., 2020).In comparison with Pinus taeda wood, the Pinus caribaea var.
hondurensis and Pinus caribaea var.bahamensis demonstrated a high potential for industrial use, and, in most cases, excellent properties were found, and many times they were superior to the reference species.This wood of these species can then be used for various purposes as a complementary or substitute raw material in forest-based industries, this was also confirmed by (Trianoski et al., 2014).
To better explain the relationships between the physical-mechanical properties of the wood of Pinus caribaea var.The highest positive correlation was observed between apparent density and compression parallel to the grain; basic density and compression parallel to the grain; apparent density and basic density; volumetric shrinkage and radial shrinkage; volumetric shrinkage and tangential shrinkage; tangential shrinkage and anisotropy coefficient from the wood of Pinus caribaea var.bahamensis (Figure 4A).For the wood of Pinus caribaea var.hondurensis, the highest positive correlation was observed between apparent density and compression parallel to the grain; volumetric shrinkage and compression parallel to the grain; volumetric shrinkage and radial shrinkage; volumetric shrinkage and tangential; modulus of elasticity and radial shrinkage; modulus of rupture and modulus of elasticity (Figure 4B).
The highest values of correlation coefficients were obtained in the relationship between apparent density and compressive strength, for both species (Figures 4A and 4B).This demonstrates that there is a strong relationship between these properties, indicating that the apparent density can be used as a parameter for estimating the compressive strength.These results are consistent with those observed by Trianoski et al. (2014) for Pinus caribaea var.bahamensis and Pinus caribaea var.hondurensis.This same tendency was observed by Mustefago et al. (2019), for Pinus patula and Pinus taeda.However, a high positive correlation between apparent density and modulus of elasticity was obtained for Pinus elliotti by Wen et. al. (2023).
Therefore, we can consider that the varieties of Pinus caribeae presented similar physical and mechanical characteristics.Although young pine wood is considered to be of low quality, we can confirm that these varieties have good potential for use in the wood industry.However, it is an abundant material source that is underutilized because its mechanical properties are still not well understood.Even though Pine wood at young ages has often been rejected, it has some good mechanical properties that can be combined with structural applications that do not need high strength (Cherry et al., 2022).Zanuncio et al., (2022) also consider that tropical Pinus species at a younger age can be considered promising for plantations on a commercial scale, increasing the supply of wood for the various segments of the timber industry, and even, in some cases, they can produce wood with little dimensional variation and of good mechanical resistance.
Information on the properties of Pinus wood at a younger age, as well as the proportion of juvenile wood, can impact forest management, wood classification, decisions about the rotation cycle and the wood processing to be adopted at the sawmill (Rosa et al., 2023).One of the advantages of studying the characteristics of wood at a younger age would be the fact of being able to make an early selection of these materials and use these results as a selection criterion to be adopted in improvement programs of Pinus species to produce cellulose and paper, high quality lumber for sawmills and other uses (Wen et al., 2023).
New studies are recommended for Pinus species at a younger age, such as the evaluation of wood quality related to climate.According to Da Ros et al., (2021) this research can be essential for genetic improvement programs that seek to use hereditary and economically important traits to select material with adaptability to climate change.

Conclusion
Wood properties: apparent density, basic density, compression parallel to the grain, shear parallel to grain, modulus of elasticity, volumetric shrinkage, radial shrinkage, tangential shrinkage, and anisotropy coefficient, evaluated in Pinus caribaea var.hondurensis and Pinus caribaea var.bahamensis did not differ significantly from each other.
The wood of the species/variety Pinus caribaea var.bahamensis has significantly higher resistance to modulus of rupture than the Pinus caribaea var.hondurensis.
For both Pinus caribaea var.hondurensis and Pinus caribaea var.bahamensis, the highest correlation obtained was between apparent density and compressive strength.
Regarding the use of wood from Pinus caribaea var.hondurensis and Pinus caribaea var.bahamensis, at a younger age, both have the potential to be used for different purposes that do not have many restrictions in structural applications.
The values obtained for the properties of 7-year-old Pinus caribaea var.hondurensis and Pinus caribaea var.
bahamensis, can be used for comparative purposes with the species currently used in the timber industry in Brazil for various uses.

Figure 1 -
Figure 1 -Schematic illustration of wood samples for analysis of physical-mechanical properties.
Mean basic density values (Table2, Figure2b) were lower than those observed byTrianoski et al. (2013),Zanuncio et al. (2022),Gonçalez et al. (2018) andLoiola et al. (2021), however, these authors evaluated Pinus caribaea wood at higher ages, that is, 17, 20, 20 and 25 years old, respectively.However, for wood from Pinus caribaea var.hondurensis aged 50 years, values very close to those obtained were verified byLima et al. (2020) for basic density.The apparent density also showed a lower value (Table2 and Figure2a) than that observed byTrianoski et al. (2014), for 17-year-old Pinus caribaea wood.Therefore, these results obtained for density are probably due to the ages of the youngest of the woods under study.

Figure 3 -
Figure 3 -Compression parallel to the grain (a), Shear parallel to grain (b), Modulus of rupture (c) and Modulus of elasticity (d) in 7-year-old Pinus caribaea two varieties.

Table 2 -
Analysis of variance of physical properties for Pinus caribaea var.bahamensis and Pinus caribaea var.hondurensis for 7 year-old wood.shrinkage (βr) n.s.= not significant by test F; CVe= coefficient of experimental variation and DF= degrees of freedom.Source: Authors Considering the presented values of density and mechanical resistance of the woods, the species of Pinus caribaea var.bahamensis and Pinus caribaea var.hondurensis, can be classified as resistance class 20 (C20) NBR 7190 (Associação Brasileira de Normas Técnicas, 1997).
Compression parallel to the grain showed values of 20.64 MPa and 19.50 MPa for Pinus caribaea var.hondurensis and Pinus caribaea var.bahamensis, respectively.However, these values were lower than those observed byTrianoski et.al.