The effect of carbonization on the wood anatomy of Sclerolobium paniculatum Vogel

Abstract

Considering the extraordinary diversity of the Brazilian Cerrado and the difficulties related to the inspection of environmental crimes, knowing the wood and charcoal anatomy of widely exploited species is important. Thus, this study aimed to verify the anatomical characteristics of the wood and charcoal of Sclerolobium paniculatum. Therefore, anatomical characterizations of the wood and the charcoal produced were performed in order to compare the characteristics of both materials and observe any possible changes in the anatomical properties after carbonization. The results exposed that the qualitative anatomical characteristics of S. paniculatum wood can be maintained after the carbonization process. However, quantitatively, the carbonization increased the vessel frequency value and height and width of rays, despite reducing the frequency of rays. The diameter of the vessels was not altered by carbonization. This characterization of the species can then serve as a database for future identification of charcoal produced with this wood. In addition, it can encourage increasing the quality of inspection and consequently reducing the illegal exploitation of the species in natural environments.

Full Article

The Effect of Carbonization on the Wood Anatomy of Sclerolobium paniculatum Vogel

Edy Eime Pereira Baraúna,^a,* Patricia Hellenn Stallbaun,^a Thiago Campos Monteiro,^b Thiago Cardoso Silva,^b Talita Baldin,^a Fernando Colen,^a and Marina Donária Chaves Arantes ^c

Considering the extraordinary diversity of the Brazilian Cerrado and the difficulties related to the inspection of environmental crimes, knowing the wood and charcoal anatomy of widely exploited species is important. Thus, this study aimed to verify the anatomical characteristics of the wood and charcoal of Sclerolobium paniculatum. Therefore, anatomical characterizations of the wood and the charcoal produced were performed in order to compare the characteristics of both materials and observe any possible changes in the anatomical properties after carbonization. The results exposed that the qualitative anatomical characteristics of S. paniculatum wood can be maintained after the carbonization process. However, quantitatively, the carbonization increased the vessel frequency value and height and width of rays, despite reducing the frequency of rays. The diameter of the vessels was not altered by carbonization. This characterization of the species can then serve as a database for future identification of charcoal produced with this wood. In addition, it can encourage increasing the quality of inspection and consequently reducing the illegal exploitation of the species in natural environments.

Keywords: Charcoal; Anatomical characteristics; Brazilian Cerrado; Wood fiber

Contact information: a: Universidade Federal de Minas Gerais, Instituto de Ciências Agrárias, Montes Claros, Minas Gerais 39404-547 Brasil; b: Universidade Federal do Paraná, Setor de Ciências Agrárias, Departamento de Engenharia e Tecnologia Florestal, Curitiba, Paraná 80210-170 Brasil; c: Universidade Federal de São João del-Rei, Departamento de Engenharia Florestal, Sete Lagoas, Minas Gerais 35701-970 Brasil; *Corresponding author: ebarauna@ica.ufmg.br

INTRODUCTION

The production of charcoal always causes a discussion in the community that raises ecological awareness about the origin of its raw material. A problem in many countries is the identification of the origin of charcoal, which can come from planting forests or the exploration and management of native forests (Monteiro et al. 2010; Perdigão et al. 2020; Ramalho et al. 2020; Nisgoski et al. 2021). Often, wood from native forests is obtained illegally. In this sense, illegal logging might be hidden or obscured via wood carbonization, since this process makes it more difficult to identify the species used for charcoal production. Therefore, it is important to know what anatomical changes occur during wood carbonization.

Qualitative characteristics of wood, in general, remain unchanged during carbonization, while some variations occur in the biometrics of anatomical elements, e.g., the dimensions and frequencies of vessels and rays (Gonçalves and Scheel-Ybert 2016; Ávila et al. 2017). Thus, based on the anatomy of the wood, it is possible to distinguish the origin of species after its carbonization (Muñiz et al. 2012b). Following this premise, several studies have been carried out on the anatomy of wood and charcoal for some native species of hardwoods in Brazil (Muñiz et al. 2013; Nisgoski et al. 2014; Stange et al. 2018; Perdigão et al. 2020; Stüpp et al. 2021).

Considering the devastation of native forests in Brazil, primarily in the Cerrado biome, and the scarcity of descriptive studies detailing the charcoal of these species, knowing the anatomy of wood and its charcoal is necessary. This biome has undergone major changes in the last 50 years due to agricultural expansion and considerable changes in its landscape, generating irreversible environmental impacts and damage to this ecosystem (Dutra e Silva and Barbosa 2020). However, this biome is an important energy source, primarily to the state of Tocantins, and its knowledge is important for sustainable forest management of the ecosystems (Carrijo et al. 2020).

In this biome, several forest species present silvicultural potential, including Sclerolobium paniculatum Vogel (Fabaceae), which is utilized for its rapid growth, high survival rate, shape, and vigor (Castro et al. 1990; Terra et al. 2019). This species is an excellent carbon fixer, in addition to having medium-density wood with a high calorific value, which helps in selecting it for energy production (Machado Neto et al. 2015).

Despite being mostly found in natural environments, planting of S. paniculatum is encouraged in degraded areas, given its potential for environmental recovery. Besides, its wood can be managed after this period since it has energy characteristics similar to those of Eucalyptus (Souza et al. 2004). However, large-scale plantations are not yet a reality for forest production of this species in Brazil. In this sense, considering the extraordinary diversity of the Brazilian Cerrado and the difficulties related to the inspection of environmental crimes, knowing the wood and charcoal anatomy of widely exploited species is important. Thus, this study aimed to verify the anatomical characteristics of the wood and charcoal of S. paniculatum.

EXPERIMENTAL

Wood Collection and Preparation

To perform the study, nine Sclerolobium paniculatum Vogel trees were collected from the municipalities of Dueré (11°20’46”S and 49°16’14”W) and Cariri do Tocantins (11°53’25”S and 49°09’25”W), in the state of Tocantins, Brazil. The areas are classified by the Köppen system as tropical climate with a dry season (Aw) and are phytogeographically inserted in a Cerrado environment (Gonçalves et al. 2016).

As a criterion, the trees were selected according to their diameter at breast height (DBH), ranging from 20 to 25 cm, and their phytosanitary aspect. For each tree, a thick disc of 10 cm was obtained at the height of DBH, which was separated into two opposite wedges. From these wedges, 2 cm x 2 cm x 2 cm pieces were made, which were taken from the transition region between heartwood and sapwood. These pieces were free from defects. Altogether, for each tree, two representative samples of each evaluated wedge were obtained, being one allocated for anatomical analysis of the wood and the other submitted to the carbonization process.

Carbonization Process

The pieces of wood allocated for the carbonization process were taken to an acclimatized room to stabilize humidity to 12%. Then, the samples were wrapped in aluminum foil and carbonized in a muffle, with a final temperature of 450 °C (2 h at the final temperature) and a heating rate of 1.66 °C·min^-1, totaling 8 h of carbonization, according to the methodology described by Muniz et al. (2012) and Stüpp et al. (2021).

Anatomical Analyses

The anatomical characterization of S. paniculatum wood was performed following the IAWA Committee description standard outlined in Wheeler et al. (1989). Thirty measurements were performed for the diameter and frequency of the vessels, as well as for the frequency, height, and width of the rays (in μm).

For the wood fiber individualization, the method described by Franklin (1945) and modified by Berlyn et al. (1976) was used. Small longitudinal wood samples were placed in a mixture of hydrogen peroxide and acetic acid (in a 1 to 1 ratio) and left in a kiln at a temperature of 60 °C for 24 to 48 h. Then, the dissociated material was washed in running water and stained with hydroalcoholic safranin. In order to observe the dissociated cellular elements, microscope slides were made, on which 30 measurements were performed for each of the parameters, i.e., fiber length, fiber diameter, lumen diameter, wall thickness, and fiber wall fraction. The fiber cell wall thickness was mathematically determined by the difference between the fiber and lumen diameter. The wall fraction was estimated according to Eq. 1,

(1)

where FWF is the fiber wall fraction (%), WT is the wall thickness (μm), and FW is the fiber width (μm) (Melo et al. 2016; Monteiro et al. 2017).

The anatomical characterization of charcoal was performed by taking measurements of the diameter and frequency of the vessels. The observations and measurements of the anatomical characteristics were performed with a Zeiss microscope with reflected light. The charcoal samples were manually broken according to the three fundamental planes of the wood. The measurements of the constituent elements were performed with Axio Vision Rel. software (version 4.7, Carl Zeiss AG, Jena, Germany) using the description outlined in Wheeler et al. (1989).

Statistical Analyses

Statistical analysis was performed according to the types of material analyzed, i.e., wood or charcoal. In a completely randomized experiment, the statistical differences between the anatomy of the wood and the charcoal were evaluated by the F test, at a 95% probability level, using the Sisvar software (Ferreira 2019).

RESULTS AND DISCUSSION

Sclerolobium paniculatum wood is characterized by presenting growth layers irregularly demarcated by fibrous zones, vessels with diffuse porosity, predominant solitary vessels, fibers with simple pits, and axial parenchyma being sparse paratracheal vasicentric with two to five cells per series (Fig. 1A). After carbonization, it was observed that the exposure of the wood to high temperatures caused cracks between rays (Fig. 1B). Despite this, it was possible to identify the maintenance of the cell arrangement when viewing the transversal sections of the species, revealing the conservation of some anatomical elements after the adopted carbonization regime.

The anatomy of pits in the vessels of the studied wood may be associated with the dry conditions of the Cerrado, since this biome is characterized by strong seasonal rainfall with cold and dry winter, as well as compact and dry soil for the first few meters down (Ferri 1979; Rizzini 1997; Franco 2002; Oliveira and Marquis 2002; Machado et al. 2007). These environmental factors can affect the microstructure of the wood, changing its anatomical properties. Thus, some identification parameters may be of low significance in the identification of charcoals (Gonçalves et al. 2011).

Vasicentric axial parenchyma, biseriate and uniseriate, and homocellular rays are characteristic of the genus Sclerolobium (SUDAM 1981; Mainieri et al. 1983; Mainieri and Chimelo 1989), which were also observed in this study (Fig. 1C). After the transformation into charcoal, some anatomical structures remained, and some cracks appeared following the paratracheal parenchyma line, causing an expansion in the tangential direction (Fig. 1D). This phenomenon also occurred in some rays (in the radial direction).

The rays were characterized as being exclusively uniseriate (99%) and homogeneously formed by procumbent cells (as shown in Fig. 1E). The same characteristics were found by Pires and Marcati (2005) in a study with two other varieties of the species, i.e., S. paniculatum var. subvelutinum and S. paniculatum var. rubiginosum. Therefore, it is worth noting that this can be an important characteristic for identifying the wood of this species.

Fig. 1. Images of the anatomical characterization of Sclerolobium paniculatum in the transversal section: (A) wood and (B) charcoal, where the arrows indicate splits in rays; in the longitudinal tangential section: (C) wood and (D) charcoal

The height and width of the rays underwent significant changes during the carbonization process, registering an increase in their values (Table 1). In addition, the carbonization process reduced the frequency of rays, and the charcoal presented a higher vessel frequency.

Table 1. Anatomical Characteristics of the Wood and Charcoal of S. paniculatum

When evaluating two tropical forest species of angiosperms belonging to the Fabaceae family, Muñiz et al. (2012a) discovered an increase in radius width in Cedrelinga catenaeformis and a decrease in radius width in Enterolobium schomburgkii. As such, Muñiz et al. (2012a) and Nigoski et al. (2014) stated that the behavior in relation to the width of the rays does not follow a pattern, which indicated that, in this case, the species is the relevant factor, observing that the width of the rays can be increased in some wood species, consequently decreasing the frequency. The same conclusion was reported by Gonçalves et al. (2012) and Gonçalves and Scheel-Ybert (2016) when verifying an increase in the height of the rays for the group of analyzed species from the Cerrado biome of São Paulo.

The descriptive statistics of the averages obtained via the quantitative anatomical characterization of the fibers are shown in Table 2. The fiber length presented average values of approximately 765 µm while the fiber wall thickness exhibited average values of approximately 6.5 µm. They were classified as thin to thick walls, according to the classification of the IAWA Committee (Wheeler et al. 1989).

Table 2. Anatomical Characterization of Fibers of S. paniculatum Wood

The thickness of the fiber wall, the frequency and diameter of the vessels, the amount of parenchymal tissue, in addition to other factors, e.g., environmental variables, can affect the density of the wood, and consequently its energy properties (Kollmann and Côté Jr. 1968; Panshin and De Zeeuw 1980; Zobel and Buijtenen 1989; Oliveira et al. 2021). Thus, these anatomical characteristics make S. paniculatum wood present a medium density, i.e., approximately 700 kg·cm^-3 (Machado Neto et al. 2015).

The wall fraction presented an average value of 70%. Woods characterized by the presence of fibers with a high wall fraction value have the potential for production of charcoal, presenting a greater mass to support the thermal decomposition of wood, favoring the yield and quality of charcoal (Paula 2005). Thus, these characteristics increase the demand for S. paniculatum wood for the production of energy via direct combustion or the production of charcoal.

As seen in other studies, qualitatively, the anatomical structures did not change, and important characteristics for species identification can be observed in the transversal face, even in charcoal (Muñiz et al. 2012a; Muñiz et al. 2013). Thus, this characterization of S. paniculatum can serve as a database for future identification of charcoal produced with its wood. In addition, it can encourage increasing the quality of inspection and consequently reducing the illegal exploitation of the species in natural environments.

CONCLUSIONS

1. The qualitative anatomical characteristics of Sclerolobium paniculatum wood are maintained after the carbonization process.
2. Quantitatively, the carbonization of S. paniculatum increased the frequency of the vessels and the height and width of the rays, despite reducing the frequency of the rays. The diameter of the vessels was not altered via carbonization.
3. This characterization of S. paniculatum can serve as a database for future identification of charcoal produced with its wood. In addition, it can encourage increasing the quality of inspection and consequently reducing the illegal exploitation of the species in natural environments.

ACKNOWLEDGMENTS

The authors thank the Research Development Foundation – FUNDEP (Project 27506) from Universidade Federal de Minas Gerais and the Universidade Federal do Paraná for the financial support. This study was performed with the support of the Coordination for the Improvement of Higher Education Personnel – (CAPES) – Brazil – Financing Code 001.

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Article submitted: August 4, 2021; Peer review completed: September 11, 2021; Revised version received and accepted: September 29, 2021; Published: October 6, 2021.

DOI: 10.15376/biores.16.4.7846-7854