Insights into growth performance and wood quality of Juvenile Corymbia spp. for metallurgical charcoal production

ABSTRACT

Background:  The steel industry demands raw materials capable of improving charcoal quality and production efficiency. The objective of the study was to identify and quantify the potential advantages of Corymbia wood for charcoal production compared with Eucalyptus and to evaluate the influence of dendrometric, wood and charcoal properties on the differentiation of these genetic materials.

Results:  The Corymbia hybrids exhibited lower height growth than Eucalyptus, while presenting wood that was, on average, 20% denser and contained 44% less heartwood. The resulting charcoal showed, on average, 2% higher gravimetric yield, 15% higher density, and 8% greater mechanical resistance than Eucalyptus charcoal. Basic density was identified as the variable most strongly associated with the differentiation among the evaluated genera and hybrids of C. torelliana x C. citriodora.

Conclusions:  Although Corymbia hybrids showed lower growth, they produced a charcoal mass per hectare similar to that of E. urophylla x E. grandis. These results indicate the potential of the evaluated Corymbia hybrids as an alternative source of raw material for steelmaking charcoal production.

Keywords:
Basic density; bioenergy; carbonization; mechanical resistance; steelmaking charcoal

HIGHLIGHTS

Evaluated Corymbia hybrids produced denser and more resistant charcoal.

Wood basic density was the main trait differentiating Corymbia from Eucalyptus.

Corymbia hybrids showed higher fixed carbon concentration per unit volume.

Juvenile Corymbia wood showed favorable traits for steelmaking charcoal.

INTRODUCTION

The wood used for charcoal production is one of the main factors that influence the properties of this carbonaceous and, consequently, on the adequacy of the product to the market and forest planning projections. The worldwide demand for charcoal in the steel industry is a growing fact, due to the productivity and economy involved in this conversion process and the renewability of the process (Empresa de Pesquisa Energética – EPE, 2025). But what is the challenge in selecting species and genetic materials compatible with charcoal production as a steelmaking bioreducer? The genus Corymbia has been reclassified from the genus Eucalyptus due to phylogenetic and cladistic differences and, mainly, due to the impossibility of hybridization with species of Eucalyptus (Hill and Johnson, 1995; Ladiges et al., 2010). Other characteristics that differentiate the genus include anatomical and structural differences in the wood, such as thicker fiber walls, greater permeability, and variations in vessel area and heartwood–sapwood proportions in Corymbia wood compared with Eucalyptus (Rocha et al., 2024). These characteristics are often associated with denser wood and may influence industrial performance, indicating the potential of Corymbia hybrids for applications where wood quality is a key requirement.

Despite all the information about the species of Corymbia, researches that highlight the main differences between the wood from hybrids of the genera Corymbia and Eucalyptus are scarce. The gains in terms of energy and/or economic efficiency that the new genus correct use provides to the production chain and the quality of charcoal are, so far, little known worldwide. The comparison between the two genus is not the only key to obtaining greater efficiency, but the correct stratification of genetic materials favors the choice of better clones for investment. Thus, the combination of growth and wood quality has shown significant results in the characteristics of the final product (Barros Junior et al., 2026; Melo et al., 2024; Brito et al., 2020).

The higher density of the wood is said to be responsible for the physical and mechanical properties of charcoal, such as its density. More dense charcoal, in general, contributes to higher fixed carbon concentration per volume, and provides lower specific consumption in the reduction of iron ore in the blast furnace, contributing to maximizing pig iron production (Biswas et al., 2011; Assis et al., 2015; Pereira et al., 2025). Studies have highlighted satisfactory gravimetric yield in charcoal and its high density when produced with Corymbia wood (Barros Junior et al., 2026; Peres et al., 2019; Costa et al., 2017). Previous studies have indicated that hybrids of Corymbia torelliana × Corymbia citriodora may represent a promising alternative for steelmaking charcoal production, particularly due to the association between wood density and charcoal density and mechanical strength (Loureiro et al., 2019).

Although several studies have reported favorable wood and charcoal characteristics for Corymbia hybrids, information integrating growth performance, wood quality, and charcoal properties of juvenile Corymbia hybrids remains limited. In particular, there is a lack of studies quantifying the potential gains of these materials relative to commercially established Eucalyptus clones and identifying the traits that most contribute to their differentiation for steelmaking charcoal production. Addressing this gap may improve the selection of genetic materials and support decision-making in breeding and industrial deployment programs. Based on the reported association between wood density and charcoal quality, we hypothesized that juvenile Corymbia hybrids would exhibit superior properties than juvenile Eucalyptus clones, despite their early stage of development. Therefore, the objective of this work was to compare growth performance, wood quality, and charcoal properties of Corymbia hybrids and Eucalyptus clones, quantify potential differences between these materials, and identify the dendrometric, wood, and charcoal traits that most contribute to their differentiation.

MATERIAL AND METHODS

Plant material and experimental design

Ten clones were evaluated: one clone of E. urophylla x E. grandis, used as a reference material, and nine hybrids of C. torelliana x C. citriodora. The trees were 3.2 years old and originated from an experimental plantation located in the rural area of Bom Despacho, Minas Gerais, Brazil (-19.691333° S, -45.216105° W; 768 m above sea level). The experiment was established on clayey dystrophic Oxisol. During the study period (2013–2019), the site presented a mean annual precipitation of 1,088 mm and a mean annual temperature of 20.1 °C.

The planting spacing was 4.5 × 2.5 m. The experimental area covered 2.5 ha, with approximately 0.5 ha allocated to each clone. All plots received the same silvicultural treatments and were established under identical site preparation, planting date, and spacing conditions to minimize environmental variation among genetic materials.

Dendrometric data and wood and charcoal characterization

For wood characterization and charcoal production, four trees per clone were sampled, totaling 40 experimental units, including 36 trees of C. torelliana × C. citriodora and four trees of E. urophylla × E. grandis. Dendrometric measurements were obtained for all trees in the experiment and included total height (TH), measured from the base of the stem to the top of the crown; commercial height (CH), measured from the stem base to the point where the stem diameter reached 5 cm; diameter at breast height with bark (DBHwb, cm), measured at 1.3 m above ground level; wood volume with bark (Volwb, m³ ha⁻¹); mean annual increment with bark (MAIwb, m³ ha⁻¹ yr⁻¹); and wood biomass (t ha⁻¹), calculated as the product of wood volume without bark and basic wood density.

For wood characterization, 12 discs were collected per tree at six positions along the stem corresponding to the sections used in Smalian's method: 0%, DBHwb (1.3 m), 25%, 50%, 75%, and 100% of the commercial height (Figure 1). Discs were collected from sections free of visible defects. Basic density (BD, t m⁻³), bark content (BK, %), sapwood proportion (SW, %), heartwood proportion (HW, %), and the heartwood-to-sapwood ratio (H/S) were determined as the average of the six sampling positions.

Basic wood density was determined according to NBR 11941 (ABNT, 2003). The volume of bark-free saturated discs was measured, and the samples were subsequently dried in a forced-air oven at 103 ± 2 °C until constant mass. Bark, sapwood, and heartwood proportions were determined by visually identifying their boundaries on previously sanded discs after water application. When the heartwood boundary was not clearly visible, a solution containing 1 g of dimethyl yellow dissolved in 500 mL of ethanol was applied to enhance the distinction between heartwood and sapwood.

Figure 1
Experimental design and wood sampling procedure used for the evaluation of wood properties in Eucalyptus clone and Corymbia hybrids. (1) Eucalyptus sampling; (2) Corymbia sampling; (3) Preparation of wood discs; (4) Determination of basic wood density; (5) Evaluation of heartwood, sapwood, and bark proportions.

Wood carbonization was carried out in an industrial rectangular AM32 kiln measuring 32 m in length, 5 m in width, and 4 m in wall height, with a dome rise of 1.27 m and a total capacity of 320 m³ of wood with bark.

To enable the identification and recovery of charcoal produced from each clone, the wood samples were placed inside cylindrical metal containers (60 cm in diameter, 85 cm in height, and 200 L capacity) before being loaded into the kiln. Each container held logs obtained from a single tree, cut into 75-cm-long sections, as illustrated in Figure 2. The containers allowed the experimental charcoal to be separated from the remaining charcoal produced during the industrial carbonization process.

The metal containers were distributed among eight wood stacks already loaded in the kiln, with five containers placed on each stack. The central stacks were intentionally excluded because this region is known to exhibit greater temperature variability during carbonization. During kiln loading, the containers were randomly assigned to the selected stacks and positioned parallel to the wood piles, ensuring that the perforations in the containers remained unobstructed throughout the process.

Figure 2
Experimental design and charcoal production used for the evaluation of charcoal properties from Eucalyptus and Corymbia wood.

Carbonization temperature was continuously monitored using a supervisory control system connected to twelve thermocouples installed along the kiln side walls and roof at intervals of 3.5 m. Carbonization was initiated simultaneously by eight ignition points located near the kiln floor. The ignition points were progressively sealed according to the average roof temperature as follows: (a) 50% at 150 °C; (b) 75% at 200 °C; (c) 90% at 250 °C; and (d) 100% at 280 °C.

The average maximum temperatures recorded at the kiln roof, chimney wall, and opposite wall during carbonization were 325 °C, 165 °C, and 150 °C, respectively. The final carbonization temperature was 300 °C, and the kiln was opened when the internal temperature reached 38 °C. The entire charcoal production cycle lasted 368 h, comprising 87 h of carbonization and 281 h of natural cooling.

The charcoal was characterized for gravimetric charcoal yield (GCY, %), charcoal production per hectare (CPH, t ha⁻¹), bulk density (BKD, t m⁻³), fines content (FN, %), mechanical resistance (MR, %), coarse charcoal content (CC, %), fine charcoal content (FC, %), volatile matter content (VMC, %), fixed carbon stock per unit volume (FCV, t m⁻³), ash content (ASH, %), and fixed carbon content (FC, %).

Mechanical resistance was determined as the percentage of charcoal retained after compression under a load of 1 ton without generating fines. Coarse charcoal corresponded to particles larger than 25 mm, whereas fine charcoal corresponded to particles smaller than 25 mm.

Gravimetric charcoal yield was calculated according to Equation 1:

$$GCY=\left(\frac{MC}{MDW}\right)\times 100$$

where MC is the charcoal mass (kg) and MDW is the oven-dry wood mass (kg).

Volatile matter, fixed carbon, and ash contents (proximate analysis) were determined according to NBR 8112 (ABNT, 1986). Charcoal granulometry was evaluated according to NBR 7402 (ABNT, 1982), considering coarse charcoal as the fraction retained on the 63- and 25-mm sieves and fine charcoal as the fraction retained on the 19- to 9.5-mm sieves.

Charcoal mechanical resistance was determined using a manual hydraulic press operating at a compression load of one ton per minute for each particle-size class, according to the methodology proposed by Soares et al. (2021). The final value for each genetic material corresponded to the mean of the evaluated particle-size classes.

Data analysis

Dendrometric, wood, and charcoal data were subjected to the Shapiro–Wilk and Bartlett tests to verify the assumptions of normality and homogeneity of variances, respectively. Subsequently, the data were analyzed using analysis of variance (ANOVA), and means were grouped by the Scott–Knott test at the 5% significance level when appropriate.

To investigate the pattern of dissimilarity between Corymbia hybrids and the reference Eucalyptus clone, canonical variable analysis was performed using the traits that showed significance for at least one source of variation in the ANOVA. Genotype clustering based on canonical variable scores was carried out using the k-means algorithm. Separate canonical variable and cluster analyses were also performed for the dendrometric, wood, and charcoal traits that showed significance for the Corymbia versus Eucalyptus source of variation in the initial ANOVA.

The relative contribution of each trait to genotype differentiation was estimated according to Singh's criterion (1981), using the generalized Mahalanobis distance matrix and considering the 12 traits that were significant for at least one source of variation in the ANOVA.

Correlation network analysis was used to graphically represent the relationships among phenotypic correlations between groups of traits. In the network, the proximity between nodes was proportional to the absolute correlation coefficient. Edge thickness was scaled according to the magnitude of the correlation, and only correlations with |r| ≥ 0.50 were highlighted. Positive correlations were represented by green edges, whereas negative correlations were represented by red edges. All statistical analyses were performed using R software (R Core Team, 2022).

RESULTS

Among the 22 traits evaluated, 12 showed significant differences (p < 0.05), either among the 10 clones, between Corymbia and Eucalyptus, or among the nine clones of C. torelliana × C. citriodora (Figure 3). The partitioning of the sources of variation into treatments (10 clones), Corymbia versus Eucalyptus (mean of the nine Corymbia hybrids versus the Eucalyptus clone), and among Corymbia hybrids revealed significant differences for two of the six dendrometric traits (TH and CH), three of the five wood traits (HW, H/S, and BD), and seven of the eleven charcoal traits (GCY, BKD, MR, CC, FC, FCV, and ASH). Significant differences among C. torelliana × C. citriodora clones were also detected for BD, GCY, and MR (Figure 3).

Figure 3
Mean values of dendrometric, wood, and charcoal traits for nine clones of C. torelliana × C. citriodora (1–9) and one clone of E. urophylla × E. grandis (E). Different letters indicate significant differences among genetic materials according to the Scott–Knott test (p < 0.05). Traits without letters did not show significant differences among genetic materials. CV: coefficient of variation.

For the dendrometric traits TH and CH, the Corymbia hybrids were, on average, 12% and 24% smaller than the Eucalyptus clone, respectively (Figure 3). The Corymbia hybrids also exhibited, on average, 44% lower heartwood content and a heartwood-to-sapwood ratio 47% lower than that observed for E. urophylla × E. grandis. Regarding wood basic density, all Corymbia hybrids presented higher BD values than the Eucalyptus clone, with differences of 25%, 21%, and 15% among the groups identified by the Scott–Knott test (Figure 3).

Three distinct groups were identified for gravimetric charcoal yield (GCY), corresponding to high, intermediate, and low yield classes, with mean values of 38.5%, 36.6%, and 34.4%, respectively. The Eucalyptus clone was grouped with the Corymbia hybrids that exhibited intermediate GCY values (Figure 3). All Corymbia hybrids produced charcoal with higher bulk density than the Eucalyptus clone, with differences of 18% and 11% for groups a and b, respectively (Figure 3).

The charcoal produced from five C. torelliana × C. citriodora clones showed mechanical resistance values 4–11% higher than those observed for the Eucalyptus clone and the remaining Corymbia hybrids (group c). In contrast, clone C8 exhibited mechanical resistance approximately 10% lower than that of group c (Figure 3). Clones C2, C3, and C4, which showed the highest mechanical resistance values, were grouped together with the Eucalyptus clone for coarse charcoal content, presenting approximately 5% more coarse charcoal (63–25 mm) and, consequently, 40% less fine charcoal (25–9 mm) than the remaining clones (Figure 3).

The charcoal produced from clones C3, C4, and C9 presented fixed carbon per unit volume values approximately 12% higher than those observed for the remaining Corymbia hybrids and E. urophylla × E. grandis (Figure 3). Ash content was similar among most Corymbia hybrids and the Eucalyptus clone; however, clone C5 exhibited ash contents approximately 36% higher than those of the other genetic materials (Figure 3).

Overall, the C. torelliana × C. citriodora clones were characterized by lower total and commercial heights, lower heartwood proportion, higher wood basic density, and higher charcoal bulk density than the E. urophylla × E. grandis clone (Figure 3).

The first two canonical variables explained 80.27% of the total variation in the dataset. Based on the canonical variable scores and k-means clustering, all Corymbia hybrids were grouped together, whereas E. urophylla × E. grandis remained isolated (Figure 4A). The analysis of trait contributions for the Corymbia versus Eucalyptus source of variation indicated that BD and H/S were the main contributors to genotype differentiation, whereas TH, CC, and HW contributed least to the observed divergence among genetic materials (Figure 4B).

Figure 4
(A) Dispersion of nine C. torelliana × C. citriodora clones and one E. urophylla × E. grandis clone based on the first two canonical variables. (B) Relative contribution of significant traits to genetic divergence among the evaluated materials. Note: BD = basic density (kg m⁻³), H/S = heartwood/sapwood ratio, MTV = volatile matter (%), BKD = bark density (%), CH = commercial height (m), FC = fixed carbon (%), HT = total height (m), and GCY = gravimetric charcoal yield (%).

Canonical variable analysis and k-means clustering performed separately for the significant dendrometric (TH and CH), wood (HW, H/S, and BD), and charcoal traits (GCY, BKD, MR, CC, FC, FCV, and ASH) also resulted in a clear separation between the C. torelliana × C. citriodora clones and the E. grandis × E. urophylla clone, consistent with the pattern observed when all 12 significant traits were analyzed simultaneously (Figure 5).

Figure 5
K-means clustering of C. torelliana × C. citriodora and E. urophylla × E. grandis based on significant (A) dendrometric, (B) wood, and (C) charcoal characteristics.

Greater similarity among Corymbia hybrids was observed for wood characteristics than for dendrometric and charcoal characteristics (Figure 5). Among the evaluated clones, the pairs C1–C6 and C3–C4 showed the greatest similarity for dendrometric and charcoal characteristics. In contrast, clones C7 and C8 were the most distinct from the remaining clones for these traits, although they exhibited values similar to those of the other genetic materials for most wood characteristics (Figure 5).

For the C. torelliana × C. citriodora hybrids, the correlation network revealed weak associations among traits belonging to different groups. In general, traits within the same group, particularly the dendrometric traits, showed stronger correlations with each other than with wood or charcoal traits (Figure 6). In contrast, the E. urophylla × E. grandis clone showed stronger correlations among traits from different groups, particularly between wood biomass, gravimetric charcoal yield, and bark content (Figure 6).

Figure 6
Pearson correlation network among dendrometric, wood, and charcoal traits for (A) C. torelliana x C. citriodora hybrids and (B) E. urophylla x E. grandis. Note: DBH = diameter at breast height (m), HT = total height (m), VOL = volume (m³ ha⁻¹), CH = commercial height (m), MAI = mean annual increment (m³ ha⁻¹ year⁻¹), BD = basic density (kg m⁻³), BM = wood biomass (t ha⁻¹), HW = heartwood (%), SW = sapwood (%), BK = bark content (%), H/S = heartwood/sapwood ratio, GCY = gravimetric charcoal yield (%), MTV = volatile matter (%), FC = fixed carbon (%), ASH = ash content (%), FN = fines content (%), MR = mechanical strength (MPa).

DISCUSSION

The significant difference in tree height between Corymbia and Eucalyptus explains the higher volumetric wood production observed for Eucalyptus, given that stem diameters were similar between the genera. However, the comparable biomass production (t ha⁻¹) observed for both groups suggests that the main differences are related to tree architecture rather than to total wood biomass production. These results suggest differences in biomass allocation patterns between Corymbia and Eucalyptus.

In general, Corymbia hybrids exhibit volumetric wood production (m³ ha⁻¹ yr⁻¹) that is 20–30% lower than that reported for Eucalyptus species (Moura, 2001; Florez et al., 2019). Carbon assimilation and its allocation among tree components influence growth allometry and responses to environmental conditions (Paul et al., 2007). Previous studies have reported differences in biomass partitioning between Eucalyptus and Corymbia species, with average biomass allocation of 22% to branches, 10% to leaves, 10% to bark, and 56% to the stem in Eucalyptus, compared with 26%, 15%, 7%, and 50%, respectively, in Corymbia (Paul et al., 2008; Cernusak et al., 2011). The higher proportion of biomass allocated to branches and leaves reported for Corymbia hybrids may contribute to the growth patterns observed in the present study.

For Eucalyptus lower heartwood content has been associated with reduced stem diameter growth (Almeida et al., 2020). However, this pattern was not observed in the present study, as Corymbia and Eucalyptus clones exhibited similar stem diameters, whereas the C. torelliana × C. citriodora hybrids showed significantly lower heartwood contents. This result suggests differences in the relative proportions of heartwood and sapwood between the evaluated genetic materials. From a technological perspective, the combination of higher wood density and lower heartwood content may be favorable for charcoal production.

Sapwood proportion has been widely used as an indicator of leaf area because it represents the fraction of active conductive tissue responsible for water transport within the stem (Waring et al., 1982; Ribchoud and Mathven, 1992). Therefore, the higher sapwood proportions observed in the Corymbia hybrids are consistent with reports indicating that some Corymbia species may develop larger crowns than Eucalyptus species (Vilas Bôas et al., 2009).

Age and growth rate are among the main factors influencing heartwood formation (Brito et al., 2020). Young plantations generally exhibit lower heartwood contents because of the greater proportion of active vascular tissue required to sustain growth (Gominho and Pereira, 2005; Tarelkina et al., 2023). Although early assessments of heartwood formation may provide useful information regarding future heartwood development, long-term evaluations are required to confirm these trends.

For both genera, the heartwood-to-sapwood ratio was lower than one, indicating that sapwood occupied a larger proportion of the stem than heartwood at approximately three years of age. This pattern was more pronounced in the Corymbia hybrids evaluated in the present study.

As observed for heartwood formation, basic wood density is strongly influenced by tree age and may vary considerably from approximately three years onward (Brito et al., 2020; Santos et al., 2024). Nevertheless, the hybrids evaluated in the present study exhibited marked differences in density even at a juvenile age, particularly between the two genera. Wood density is recognized as a trait with relatively high heritability (Tan et al., 2018), which may contribute to the consistent differences observed between the evaluated Corymbia hybrids and the Eucalyptus clone.

The basic density of Eucalyptus wood typically ranges from 0.350 to 0.650 g cm⁻³ (Barbosa et al., 2019; Almeida et al., 2020; Costa et al., 2020). In contrast, Corymbia species generally exhibit wood densities approximately 20% higher than those of Eucalyptus, ranging from 0.500 to 0.800 g cm⁻³ up to 15 years of age (Garcia et al., 2014; Glencross et al., 2014; Hung et al., 2016; Medeiros et al., 2016; Florez et al., 2019).

In the present study, the C. torelliana × C. citriodora hybrids were classified into three density groups: low (0.55 t m⁻³), intermediate (0.61 t m⁻³), and high (0.64 t m⁻³). However, all hybrids exhibited higher wood density than the E. grandis × E. urophylla clone, reinforcing the distinct density pattern commonly reported for Corymbia materials.

Trueman et al. (2012), evaluating C. torelliana × C. citriodora hybrids at three sites in Queensland, Australia, reported higher wood basic density and lower heartwood proportion in trees growing under conditions of lower rainfall and reduced growth. The authors also observed radial variation in wood density, with mean values of 574, 523, and 562 kg m⁻³ for sapwood, heartwood, and pith, respectively, resulting in an overall mean density of 553 kg m⁻³. Combined with heartwood and sapwood proportions of 11.2% and 41.2%, respectively, these characteristics contributed to wood densities approximately 17% higher than those reported for E. cloeziana clones at 13.5 years of age.

Hybrid traits are often intermediate between those of their parental species (Rosenthal et al., 2002). Trueman et al. (2012) reported heartwood and sapwood proportions of 18.2% and 43.2%, respectively, as well as a wood basic density of 547 kg m⁻³ for C. torelliana. In the present study, the C. torelliana × C. citriodora hybrids exhibited higher heartwood and sapwood proportions, as well as higher wood density, than those reported for C. torelliana by Trueman et al. (2012). These differences may be associated with the contribution of C. citriodora to the hybrid phenotype.

Several studies have highlighted wood basic density as an important trait for charcoal quality, particularly for steelmaking applications (Pereira et al., 2013; Couto et al., 2015; Healey et al., 2016; Santos et al., 2016). In the present study, the C. torelliana × C. citriodora hybrids exhibited higher wood basic density and lower heartwood content than the Eucalyptus clone, indicating characteristics commonly associated with favorable charcoal properties.

The densest wood was not always associated with the densest charcoal, as observed for clones C7 and C8, suggesting that the relationship between wood density and charcoal properties is influenced by additional factors. Nevertheless, the Corymbia hybrids generally produced charcoal with higher bulk density and greater fixed carbon per unit volume than the Eucalyptus clone. Charcoal with higher density is often preferred for steelmaking applications because it can increase the amount of fixed carbon supplied per unit volume and may improve blast furnace occupancy (Pereira et al., 2025).

Several studies conducted in Brazil have investigated the potential use of Corymbia wood as an alternative to Eucalyptus for steelmaking charcoal production (Costa et al., 2017; Loureiro et al., 2019; Peres et al., 2019). The intrinsic characteristics of wood, particularly its density, are widely recognized as important factors influencing charcoal density (Moutinho et al., 2017; Dufourny et al., 2018). Relationships between wood basic density and fixed carbon content have been reported in previous studies (Loureiro et al., 2019; Ramos et al., 2019). However, the fixed carbon content of charcoal is influenced not only by wood characteristics but also by carbonization conditions, particularly residence time, heating rate, and final carbonization temperature (Couto et al., 2015; Pereira et al., 2025). Previous studies have shown that dense woods producing charcoal with high gravimetric yield and elevated volatile matter content may indicate incomplete carbonization (Trugilho and Silva, 2001). This occurs because charcoal quality is strongly affected by the thermal degradation process and the extent of biomass conversion during carbonization. Fixed carbon content and gravimetric charcoal yield are often characterized by a trade-off relationship, since the loss of volatile compounds and other solid fractions during carbonization tends to increase the relative concentration of fixed carbon in the remaining charcoal (Elyounssi et al., 2010).

The higher fixed carbon stock per unit volume observed for the C. torelliana × C. citriodora hybrids suggests that, although the fixed carbon content (%) was similar to that of the Eucalyptus clone, the greater charcoal density contributed to a higher amount of fixed carbon per unit volume. This characteristic may be relevant for steelmaking applications, where charcoal density is often associated with operational performance. Previous studies have reported that higher charcoal bulk density is associated with lower specific charcoal consumption during pig iron production, possibly due to reduced reactivity and improved blast furnace occupancy (Lima et al., 2022a; Lima et al., 2022b).

The fact that clones C3 and C4 produced denser charcoal with greater mechanical resistance was accompanied by a higher proportion of coarse charcoal particles, indicating an association among charcoal density, mechanical resistance, and granulometry in the evaluated C. torelliana × C. citriodora hybrids. Higher mechanical resistance is generally considered advantageous because it reduces charcoal fragmentation during handling and industrial use, contributing to the maintenance of particle size distribution under operational conditions (Assis et al., 2020).

Among the evaluated trait groups, wood properties showed lower variability among the Corymbia hybrids than dendrometric and charcoal traits. This pattern suggests that wood properties were more homogeneous among the evaluated genetic materials, whereas growth and charcoal characteristics exhibited greater variability. Such variability may reflect differences among genotypes as well as responses to management and carbonization conditions.

The clear separation between C. torelliana × C. citriodora hybrids and the E. grandis × E. urophylla clone, both when trait groups were analyzed separately and when all significant traits were considered together, highlights the ability of the evaluated variables to discriminate among genetic materials. These results reinforce the importance of jointly evaluating dendrometric, wood, and charcoal traits, since charcoal quality cannot be fully characterized by a single property.

The weak correlations observed among trait groups for the C. torelliana × C. citriodora hybrids (Figure 6) suggest that indirect selection based on a limited number of traits may be less effective for these materials. Consequently, more comprehensive evaluations incorporating multiple wood and charcoal properties may be required to support decision-making in breeding and selection programs.

Understanding the relationships among growth, wood, and charcoal traits can contribute to the selection of genetic materials with characteristics desirable for charcoal production. Previous studies have demonstrated that information on wood productivity and quality can assist in predicting charcoal performance and support the selection of materials for steelmaking applications (Protásio et al., 2014; Silva et al., 2018; Costa et al., 2020). In addition, such information may support forest planning and contribute to the evaluation of potential industrial applications of different genetic materials.

The results should be interpreted considering some limitations of experimental design. The comparison involved nine Corymbia hybrids and a single commercially established Eucalyptus clone used as a reference material. In addition, four trees were sampled per clone, which is consistent with operational wood-quality studies but may limit broader inferences regarding the variability within each genetic material. Furthermore, although carbonization was conducted under industrial conditions with temperature monitoring and randomized sample allocation, some thermal heterogeneity is inherent to large-scale kilns. Therefore, the conclusions should be restricted to the evaluated genetic materials and carbonization conditions.

CONCLUSIONS

The evaluated hybrids of C. torelliana × C. citriodora exhibited higher wood basic density, charcoal density, mechanical resistance, and fixed carbon per unit volume than the E. grandis × E. urophylla clone used as a reference material. These characteristics indicate the potential of the evaluated Corymbia hybrids for steelmaking charcoal production.

Although the Corymbia hybrids showed lower growth than E. grandis × E. urophylla, the differences in wood and charcoal properties suggest that these materials may represent promising alternatives for charcoal production under the conditions evaluated in this study.

Clear differences were observed between the C. torelliana × C. citriodora hybrids and the E. grandis × E. urophylla clone regarding dendrometric, wood, and charcoal characteristics. Among the Corymbia hybrids, the greatest similarity was observed for wood properties, whereas greater variability was found for growth and charcoal traits.

Indirect selection based on a limited set of traits may not be effective for C. torelliana × C. citriodora hybrids, given the low correlations observed among different groups of variables.

ACKNOWLEDGEMENTS

The authors thank the supporting institutions and companies involved in the execution of this project.

Data availability:

The datasets supporting the conclusions are included in the article.

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Scientific Editor:

Paulo Ricardo Gherardi Hein

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[e-mail]

Publication Dates

• Publication in this collection
  12 Oct 2026
• Date of issue
  2026

History

• Received
  18 May 2025
• Accepted
  02 June 2026