Apparent density of wood determined by two different methods in Pinus caribaea var. hondurensis and Pinus tecunumanii

: The aim of this study was to compare the apparent wood density obtained by two different methods in populations of Pinus caribaea var. hondurensis and Pinus tecunumanii, both 50 years old, located in Itirapina, SP. For this purpose, five trees of each species were selected. From each tree, two discs of 7 cm thickness were removed in the diameter at breast height (DBH) region. To determine the apparent density by the gravimetric method, samples were collected at five different positions, from pith to bark, on one of the discs. The other disc was used to obtain the radial profile of apparent wood density, using the X-ray Densitometry method. The results revealed that the apparent density did not differ significantly between the species. Variations in apparent density were observed from pith to bark in the trunks of both species, with a tendency of increasing density towards the bark. Furthermore, positive correlations between radial position and apparent density were observed in both species. The methodologies used to obtain the apparent density showed a high correlation between them, allowing for estimates with good precision. It is worth noting that the apparent density obtained by the gravimetric method showed significantly higher values than the X-ray densitometry method.


Introduction
In Brazil, between 1960 and1970, plantations with fast-growing exotic species of the genus Pinus were expanded to increase wood stocks in order to supply the industrial timber sector (Shimizu and Sebbenn 2008).Several species of this genus were implanted in Brazil, some of them with good silvicultural potential due to their wide edaphoclimatic adaptation and high productivity, providing wood for various uses, such as sawn wood, panels, laminates, cellulose pulp, paper and resin (Braga et al. 2020, Lorenzi et al. 2018).
Pine plantations occupy 1.9 million hectares and are concentrated in Paraná (37%), Santa Catarina (36%) and São Paulo (7.9%) (IBÁ 2022).Among those species with aptitude for the production of wood for industrial use, we have Pinus tecunumanii, which has great potential for commercial plantations due to the good development it already presented (Shimizu and Sebbenn 2008).It comes from Mexico, Guatemala, Belize, Honduras, El Salvador and Nicaragua, and generally grows in regions with altitudes between 600 and 2,600 m (Shimizu and Sebbenn 2008;Nilsson et. al. 2020).In Brazil, this species is cultivated in the Southeast and Midwest regions, in regions above 500 m altitude, where its average annual increment is 15 m 3 /ha (Lorenzi et al. 2018).Pinus tecunumanii is characterized by having a yellowish wood of excellent quality for use in civil construction, cellulose pulp, and it can also be used for landscaping in general (Lorenzi et al. 2018, Kronka et al. 2005, Shimizu and Sebbenn 2008).Its wood normally presents internal variation in density, both in the pith-bark direction and in the longitudinal direction of the trunk; however, this variation is smaller than other species of the genus Pinus (Shimizu and Sebbenn 2008).
Another species within the Pinus genus, exhibiting promising potential for wood production, is Pinus caribaea var.hondurensis.Originating from Belize, Guatemala, Honduras, and Nicaragua, this species typically thrives across altitudes ranging from sea level up to 850 m.In Brazil, it stands as one of the most widely cultivated species, largely owing to the ease of seed procurement (Kronka et al. 2005).It produces white or yellowish wood, it can be used for resin coating, civil construction and landscaping in general.Its wood is of moderate to low density, but of great general utility in the industrial timber sector (Lorenzi et al. 2018, Santos et al. 2018).
Despite the existing information, more studies are still essential, especially those referring to the quality of wood from pine species, with emphasis on the density of the wood, which helps in determining the potential for commercial use.In the production of wood for industrial use, the aim must be to obtain better quality, which implies the incorporation of new parameters for evaluating the suitability of wood for solid use.However, wood density continues to be the main parameter for assessing its quality, as it determines the amount of water mass that is removed during drying, also affecting diffusion through the woody material.High wood density values generally imply slower water loss due to less voids (Latorraca et al. 2000, Yang andLiu 2018).
A fundamental issue to define the quality of wood for industrial use is to know what proportion of juvenile wood exists in a log and how its variation behaves in the pith-bark direction.This is a characteristic that fundamentally depends on the age of the tree, as well as on the environment and management.However, at the same age and under similar environmental and management conditions, it can be seen that there is great variability in this proportion (Malan 1995, Modes et al. 2019).Pine species, with production cycles longer than 30 years, are economically viable and can be an interesting option for log producers who aim to access specific markets, in which the quality of wood for solid and structural purposes could be even better remunerated (Vivian et al. 2022).
One of the main study techniques for the characterization of wood density is the X-ray densitometry methodology, this technique has the advantage of having a permanent radiographic record of the wood samples and high sensitivity and resolution.Allied to data processing speed and interpretation through radial density profiles, it can be very useful in studies that focus on the proportion of juvenile wood and its variations (Tomazello Filho et al. 2005).
Thus, the objective of this work was to characterize the variation in the pith-bark direction of the apparent density obtained by two different methods in 50-year-old Pinus caribaea var.hondurensis and Pinus tecunumanii woods.

Material and Methods
In this research, wood samples were obtained from experimental planting trees of Pinus caribaea var.hondurensis and Pinus tecunumanii, from the region of Itirapina SP, aged 50 years.The study area is located between latitudes 22°00'S and 22°15'S and longitudes 47°45'W and 48°00'W.The soils are deep quartz sand and hydromorphic (Santos et al. 2018).The climate in the region is tropical with a dry winter (Cwa), featuring an average temperature of 19.3°C.The region boasts an average altitude of 801 meters and an average annual rainfall of 1,377 mm (Alvares et al. 2013).
Samples were collected from five trees of each selected species following the mean diameter at breast height DBH (1.30 m from the ground).From each selected tree, two discs of 7 cm thick at DBH height were removed (Table 1).In one of the discs, a representative sample block was removed from the pith region to the bark.From the sample block, five samples measuring 2 cm x 2 cm x 3 cm were taken, representing positions 0, 25, 50, 75 and 100% of the tree's radius, to study the pith-bark variation in apparent density.

Determination of apparent density gravimetric method by direct measurement
For the determination of the apparent density by the gravimetric method by direct measurement (ADDM), the samples of (2 x 2 x 3) cm were dried until they reached 12% of humidity, to obtain the weight of the dry mass.The dimensions of the samples were measured with a digital caliper with a sensitivity of 0.01 mm, and the dry mass was measured on a semi-analytical scale, with a sensitivity of 0.01 g, NBR 7190-4 (ABNT 2022).

X-ray densitometry of the wood of Pinus caribaea var. hondurensis and Pinus with resin extraction to determine apparent density
The other disc obtained was used for the Xray densitometry study, to obtain the radial profile of the apparent density (ADXR).These procedures were carried out at the Laboratory of Anatomy, Identification and Densitometry of X-rays in Wood of the Escola Superior de Agricultura "Luiz de Queiroz" -University of São Paulo.
To obtain the radial profile of apparent wood density of Pinus caribaea var.hondurensis and Pinus tecunumanii, the cross section of the discs was demarcated and specimens (2.0 cm wide) were cut in diametric alignment (bark-bark).The specimens were glued onto a wooden support and sectioned transversely (1.6 mm thick) using a double parallel circular saw.Then, the wood samples were identified with an ink pen and the resin was extracted using Soxhlet equipment (8 hours in 1:1 alcoholtoluene solution, 8 hours in alcohol, and 8 hours in boiling water).The samples were dried at room temperature and transferred to an acclimatization room and conditioned (20ºC, 60%RH, 24 h) (Amaral and Tomazello Filho 1998).The samples were fitted in a metallic support and transferred to the internal and shielded compartment contained in the equipment, being carried out its calibration and continuous scanning of the radial profile through a collimated X-ray beam, the X-ray values were transformed into apparent density using QMS software.The punctual values of apparent density of the wood were observed together with the image of the cross-section of the wood samples (QMS 1999).After obtaining the values of apparent radial density by the QMS software, an analysis report file was obtained with specific values of apparent density of the wood at every 80 microns, called DAT.Using the Excel software, it was possible to read the DAT file, constructing radial profiles of apparent density of the wood of Pinus caribaea var.hondurensis and Pinus tecunumanii (QMS 1999).
For the purpose of comparing the two methodologies, the average values of the apparent density obtained by X-ray densitometry, corresponding to the positions of samples taken from the disks, these values were considered to obtain the apparent density by the gravimetric method by direct measurement.

Statistical analysis
In the evaluation of the experiment, the variance homogeneity test was initially applied using the Hartley test.Subsequently, analysis of variance was performed and, when a significant difference was observed, Tukey's test and Student's t test were applied for each specific situation to compare means.The F test was used to determine the means that differed from each other, whenever a significant difference was observed at the 5% probability level of the treatments, and Student's t test was applied at the 95% significance level.
A study of relationships between the evaluated properties was carried out using regression analysis, at a significance level of 95%.The results obtained from the apparent densities were statistically analyzed with the aid of the statistical procedure PROC GLM and PROC REG of the SAS statistical program (SAS 1999).

Densitometry of Pinus tecunumanii and Pinus caribaea var. hondurensis
Wood images of the densitometric profile of Pinus tecunumanii and Pinus caribaea var.hondurensis, allowed the characterization of the radial variation of the apparent density (Figures 1  and 2).According to the apparent density determination, the densitometric profile of the wood of Pinus tecunumanii and Pinus caribaea var.hondurensis was characteristic and revealed a reduction in the density values that coincided with the demarcation of the limits of the growth rings, which is an indication of variations in the anatomical structure of the wood, showing distinct growth rings, constituted by early and late wood, this being of higher apparent density.It can also be seen a trend towards an increase in the specific density value in the pith-bark direction (Figures 1 and 2).Apparent density values of the order of 02-05 g.cm -3 and 08-1.2 g.cm -3 , for the early and late woods of the growth rings of Pinus caribaea var.hondurensis, were obtained by Belini et al. (2011), whose values were similar to those observed in Figure 2.
The average values of apparent density obtained by the X-ray densitometry method, for each species, are shown in Figure 3.In the analysis of the average apparent density obtained by X-ray, no significant difference was observed between the Pinus species (Figure 3).

Comparison of methodologies for obtaining Apparent Density
Table 2 shows the analysis of variance performed for apparent density of Pinus tecunumanii and Pinus caribaea var.hondurensis.According to the observed results, there were no significant differences, at a 5% probability level, between the species.However, differences were found between the radial positions on the tree trunk (Table 2).When comparing the methodology, a significant difference was found between the adopted methods (Table 2).It was also observed that the effects of species x methodology x radial position interaction were not significant for apparent density (Table 2).So, there is no dependence between these factors, which demonstrates that the apparent density variation pattern is the same for all treatments (Table 2).11.04 n.s.: not significant *: significant at the 5% significance level and CVe: coefficient of experimental variation.
It was verified that there was no significant variation for apparent density between the two species under study according to the average values presented for this property (Table 3).Trianoski et al. (2013) observed values of 0.58 g.cm -3 and 0.50 g.cm 3 , respectively, in Pinus tecunumanii and Pinus caribaea var.hondurensis, aged 18 years, with these values being higher than those observed in this study at the age of 50 years.However, there are cases in which the interaction of age with environmental factors significantly influenced the wood density values of Pinus taeda, with wood density increasing with age (Melo 2015).Gravimetric method by direct measurement 0.53 a X-Ray densitometry methodology 0.49 b Means followed by different letters in the same column differ from each other (at the 5% significance level).
We can consider that these species produce light wood (Zenid 2007).According to the consulted literature, it was to be expected that Pinus tecunumanii would present higher density values in relation to Pinus caribaea (Trianoski et al. 2013).However, this difference between the species was not significant to the point of being statistically different (Table 3).One possible explanation would be that genetic factors were not influential enough to differentiate the apparent density of wood among the species (Moura et al., 2004).
From the analysis of Table 3, it is possible to observe the variations of the apparent density in function of the radial position.We can see that the values tend to increase from pith to bark, where the lowest values are close to the pith and the highest near the bark.In general, according to the literature, Pinus species show an increase in wood density in the pith-bark direction (Souza et al. 2007 andRios et al. 2018).
The apparent density values obtained by the two methodologies differed significantly, where the values obtained by the gravimetric method by direct measurement were higher than those obtained by the X-Ray densitometry methodology (Table 3).Similar results were obtained in OSB panels of Pinus taeda and Pinus elliottii var.elliottii × Pinus caribaea var.hondurensis, where the apparent density obtained by the gravimetric method showed significantly higher results than the densitometric method (Surdi et. al. 2014).One of the reasons for the difference between the methodologies would be the fact that in order to determine the apparent density using the X-Ray densitometry methodology, the samples had to go through the process of removing the resin and this could be the cause of the reduction in apparent density values (Surdi et. al. 2014).
In the regression study in order to verify if there is a relationship between the methodologies for obtaining the apparent density separately by species and methodology, it showed that both for Pinus caribaea and for Pinus tecunumanii the methodologies for obtaining the apparent density have a high relationship between them, being possible to estimate with good precision the apparent density determined by the X-ray densitometry method with the apparent density obtained by the gravimetric method by direct measurement (Figures 4 A and B).To better explain this tendency of apparent density to increase from pith towards bark, a regression analysis was also performed between radial position (in cm) and apparent density separately by species and methodology (Figures 5A  and B).For both species and methodologies for obtaining the apparent density, it can be seen that there was an increase in the pith-bark direction (Figures 5A, 4B).This can be explained by the proportional increase of mature wood in relation to juvenile wood.The one with low density may be associated with the large presence of juvenile wood (Modes et al. 2019).
For Pinus taeda, wood density also increased in the pith-bark direction (Ballarin and Palma 2003;Souza et al. 2007).It has also been found that the density of wood in the radial direction, from the inside to the outside, increased in Pinus patula (Rios et al., 2018).This same trend was observed in Pinus elliottii, where the radial position significantly influenced the density values, with wood density increasing in the pith-bark direction (Melo et al. 2013).For Pinus caribaea var.hondurensis density increases in the pith-bark direction, and this occurs due to the decrease in the thickness of the growth rings in the pith-bark direction (Gonçalez et al. 2018).The apparent density of Pinus caribaea var.hondurensis tends to differentiate over the years, but after a certain age there is a stabilization (Correa and Bellote 2011).
The exponential regression model best represented the relationships between apparent density and radial position (Figures 5A and 4B).As can be seen, we have a strong dependence between radial position and apparent density for both species (Figures 5A and 4B).According to the obtained equations, the apparent density obtained by the two methodologies can be estimated as a function of the radial position.

Conclusions
According to the results presented, it can be concluded that: Apparent density does not differ significantly between Pinus tecunumanii and Pinus caribaea var.

hondurensis;
The apparent density varies from the pith to the bark of the trunks; For Pinus caribaea var.hondurensis there are positive correlations between radial position and apparent density; In relation to Pinus tecunumanii there is also a positive correlation: radial position with apparent density; For Pinus tecunumanii and Pinus caribaea var.hondurensis the apparent density shows a tendency to increase from pith to bark; The methodologies for obtaining apparent density have a high relationship between them, making it possible to estimate them with good precision, and The apparent density obtained by the gravimetric method presents significantly higher values than the X-ray densitometry method.

Figure 1 .
Figure 1.Diametral densitometric profile of the disc of the 50-year-old Pinus tecunumanii tree chosen randomly among those sampled.

Figure 3 .
Figure 3.Comparison between the apparent wood density of 50-year-old Pinus tecunumanii and Pinus caribaea var.hondurensis obtained by X-ray densitometry of tree wood.

Figure 5 .
Figure 5. Relationship between radial position (Radius in cm) and apparent density as a function of methodology for 50-year-old Pinus caribaea var.hondurensis (A) Pinus tecunumanii (B).

Table 1 .
Average values of diameters at breast height (DBH) and total heights (TH) of trees sampled in populations of 50-year-old Pinus caribaea var.hondurensis and Pinus tecunumanii.

Table 2 .
Summary of analysis of variance for apparent density (ADDM) of 50-year-old Pinus caribaea var.hondurensis and Pinus tecunumanii.