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Introduction
Gmelina arborea Roxb. (melina) is an important source of timber production in the Costa Rican market and in other tropical regions of the Americas, Asia, Africa and Oceania (Iwuoha et al., 2021). The species has adaptability to sites of different fertility and versatility of wood uses (Ataguba et al., 2015) such as furniture, pulp production, particleboard or plywood manufacturing (Moya et al., 2019); in addition, it has carbon sequestration potential, an important factor in climate change mitigation (Kaith et al., 2023). In Costa Rica, approximately 65 000 ha have been planted with G. arborea under different ecological, growth and silvicultural management conditions (Pinnschmidt et al., 2023), mainly for the production of logs for sawmilling purposes.
Melina wood has medium density, straight and clear grain, good appearance and durability, good conditions for processing and workability, characteristics that make it very desirable in the markets (Moya et al., 2019). As a result of these reasons and the evolution that the reforestation sector has undergone in recent decades, new techniques have been introduced to produce trees in very short periods using clonal silviculture and genetic selection (Borpuzari & Kachari, 2019), thus increasing the productivity of plantations in Costa Rica (Hernández-Castro et al., 2021a, 2021b). Therefore, tree breeding focuses on increasing plantation production with trees that have adequate diameter development and tree shape factors (Ortega-Ramírez et al., 2022). Melina can be improved rapidly, because it is a species that responds very well to strict selection of high or plus trees (Hernández-Castro et al., 2021a).
In Costa Rica, genetic improvement began in the 1990s, leading to the development of highly productive genetics for timber production on a regional scale (Hernández-Castro et al., 2021a). The most successful melina breeding programs in the region use clonal propagation to establish commercial plantation and thus obtain a reliable bank of propagules that are easy to produce and plant, resulting in fast-growing trees with high productivity (Rocha et al., 2020). Some examples include the genetic improvement study of Avila-Arias et al. (2015) in the South Pacific of Costa Rica, who recorded a selection differential of 19.8 cm, 54 m3∙tree-1 and 54.4 % in diameter, volume and stem quality, respectively, of melina plus trees compared to the base population. In this same region, Avila-Arias et al. (2014) demonstrated that melina genetic material significantly reduced cutting shifts (five to six years) with the consequent production of wood for several uses.
Cloning studies in melina have mainly focused on the analysis of species morphological properties (Li et al. 2017) and not so much on wood properties (Rodríguez-Pérez et al., 2022). For example, Hernández-Castro et al. (2021b) evaluated the coefficients of genetic variation, heritability and genetic correlations in phenotypic properties such as diameter, commercial height, commercial volume and stem quality of 10-, 22- and 34-month-old melena clones. On the other hand, Avila-Arias et al. (2014) evaluated the performance of 27-month-old clones regarding diameter and commercial height. Nunes et al. (2016) and Makouanzi et al. (2018) mention that wood physical, mechanical and chemical properties show high genetic controls that should be used in the selection of the best genotypes, together with lower controls such as growth traits. Makouanzi et al. (2018) explained that wood physical properties are often given by a single biosynthetic pathway, influenced little by the environment. Thus, wood properties show greater genetic control than growth traits, because these are more influenced by the environment (Nunes et al., 2016).
Therefore, the objective of this study is to evaluate tree growth parameters, wood physical properties and heritability to establish a genetic ranking for the selection of 14 three-year-old clones of G. arborea in southern Costa Rica. Knowing these variables will allow us to maximize the potential of the genetic improvement program of this species in commercial reforestation in Costa Rica.
Materials and Methods
Sampling area
The study was carried out in a clonal trial of G. arborea at the Forestry Research Institute of the University of Costa Rica (INISEFOR-UNA), located in the town of Monterrey, Puerto Jimenez district, cantón de Golfito, Puntarenas province, southern Costa Rica, between the geographical coordinates 8.57701° N and 83.39655° W. Mean annual precipitation ranging from 3 500 to 4 000 mm, mean annual temperature of 24 °C to 28 °C and altitude of 20 m. The region is classified as very humid forest-premontane transition to basal forest (Dehaspe et al., 2022).
Setting and Experimental Design
The trial was established with a randomized complete block design. Genetic material of 14 genotypes was used, which had not been evaluated so far, so they are F1 clones. Six blocks composed of six individuals of each clone in pairs were planted, that is, three pairs of each clone, giving an average of 84 trees per block (Figure 1). The initial stock density of the plantation was 816 trees∙ha-1 with a three-year thinning of 50 %, so the stock density at the time of sampling was 408 trees∙ha-1.
Growth parameters
A total of two trees were randomly selected from each clone in each block for a total of 168 trees sampled in the entire trial (two clones x 14 clones x six blocks). The selected trees had straight trunks and were free of disease or pest symptoms. For these trees, diameter at breast height at 1.3 m (DBH, cm), total height (Ht, m) and commercial height (Hc, m) were determined. These data and stock plantation density (N) were used to calculate the basal area (m2) and volume (m3) of each of the individuals was determined using the standing tree volume formula, where the shape factor was 0.65 cm∙m-1 and 10 000 corresponds to the conversion factor from cm2 to m2:
Once the values of volume and basal area of the individuals were determined, in both cases an average per clone was recorded and this value was extrapolated to the stock density of the plantation (408 trees∙ha-1), thus providing a value of volume and basal area per hectare for each clone. These two parameters per unit area were used, as they indicate the use capacity of the site (Vásquez-Bautista et al., 2016).
Wood properties
The selected trees were cut in 3 cm thick transverse sections along the stem: the first disk was cut at DBH, then a 3 cm thick disk was extracted every 2.5 m until completing the total trunk height 3.75 m, 6.25 m, 8.75 m, 8.75 m, 11.75 m and 13.75 m (Figure 2a).
Figure 2 Extraction of the sampled cross section of the stem (a), morphological parameters measured in the cross section (b) and sawing pattern of the cross section for determining the physical properties (c) of Gmelina arborea.
Morphological properties
Tree diameter, bark (thickness and percentage of total area), sapwood (thickness and percentage of total area) and heartwood (diameter and percentage of total area) were determined in each cross section for each height. Two perpendicular lines were drawn crossing the center of each cross section, one in the A-B direction and the other in the C-D direction (Figure 2c). Total diameter, bark-free diameter and heartwood diameter were measured in both directions drawn on the cross section (Figure 2b). The averages of the two cross-sectional measurements were calculated to obtain both total diameter and heartwood diameter. Bark and sapwood thicknesses were calculated as the difference between the total diameter, bark-free diameter (in the case of bark) and heartwood diameter (in the case of sapwood). Diameters were calculated assuming a geometric circle. The percentages of bark, sapwood and heartwood were calculated depending on the area of each tissue and the total cross-section.
Physical properties
Green density, specific gravity and moisture content in green condition were determined in each cross section of the stem and for each height (DBH, 3.75, 6.25 m, 8.75 m, 11.25 m and 12.75 m). Sections 3.0 cm wide were cut from each cross section, including the pith (Figure 2c). These sections were separated by the pith, with two samples obtained from each (Figure 2c). Green density was calculated by green weight/green volume ratio, specific gravity and MC-G according to ASTM D-143 and ASTM D-2395 procedures (ASTM International, 2022a, 2022b).
Statistical Analysis
A general statistical description (average and coefficient of variation) was applied for all wood properties. Prior to the analysis of variance (ANOVA), the normality of the data distribution was verified with the Shapiro-Wilk test and the homogeneity of variances with a homoscedasticity test. Data was analyzed using a two-factor randomized block design corresponding to clone as factor 1 and height as factor 2:
where, Y is the wood property value, μ is the population mean, C is the clone effect, H is the height effect and C i *H j is the effect of the clone*height interaction and ε is the model error.
Subsequently, a comparison of means was made using Tukey's multiple range test (P < 0.05). Data analysis was carried out with the statistical program SAS Studio for academics (Cary, NC, EUA).
Genetic analysis
Data was analyzed with the SELEGEN REML/BLUP software version 2008 (Brasilia, Brazil, Embrapa), extensively explained by Resende (2002, 2016). The statistical model combined fixed and random effects, where the number of replications is a fixed effect because it is decided by a value and the random effects were the genetic effects and the residuals, according to that proposed by Resende (2016) for this type of trials.
where,
Z is the data vector, r is the repetition effect vector ("fixed") and summed to the overall average, W is the vector of individual additive genetic effects ("random"), p is the vector of plot effects ("random") and ε is the vector of residuals ("random").
Genetic parameters
The genetic parameters estimated were average heritability of clones (H2clone), coefficient of genotypic variation among clones (CVgi%) and clone genotypic prediction error variance (PEV), assuming complete survival, according to the following equations:
where,
Vp = complete survival, 6 is the number of repetitions (r = 6)
Vad = additive variance
Vplot = within-block clone variance
Ve: variance of error, 6 is the number of trees in each plot after thinning (n = 3), 18 is the total number of effective trees in test.
Precision = √ precision of family selection assuming complete survival.
Selecting the best clone
The best clone was chosen based on a selection index with the results of each of the variables tested: total volume, heartwood percentage, green density (GD), specific gravity (SG) and moisture content under green condition (MC-G), using the following equation:
Each variable was given a relative weight: total volume with 30 %, green density with 25 %, heartwood percentage with 15 % and MC-G with 15 %. After calculating the selection index for each clone, four categories were established. These were obtained by dividing the highest and lowest values recorded by 4. Table 1 shows the range and name of the category.
Table 1 Values and names of the categories established for the classification of 14 Gmelina arborea clones in Costa Rica.
| Category | Range | Name of the category |
|---|---|---|
| 1 | 42.1 – 44.5 | High production clones and acceptable wood properties |
| 2 | 39.5 – 42.0 | Medium-high production clones with medium-high wood quality |
| 3 | 38.0 – 39.4 | Medium to low production clones and medium to low wood quality |
| 4 | 34.0 – 38.0 | Low production clones with unacceptable wood properties |
Results
Growth properties
Table 2 shows the growth parameters of the clones studied. None of the measured variables (DBH, total height, commercial height and basal area) showed statistical differences (P > 0.05) among the 14 clones studied.
Table 2 Growth characteristics of 14 clones of Gmelina arborea from southern Costa Rica.
| Clone | DBH (cm) | Total height (m) | Commercial height (m) | Basal area (m2∙ha-1) | Total volume (m3∙ha-1) |
|---|---|---|---|---|---|
| 1 | 23.15 (23.52) a | 23.15 (59.22) a | 13.35 (39.73) a | 17.73 a | 81.72 |
| 2 | 21.20 (8.00) a | 23.80 (59.22) a | 15.08 (15.71) a | 14.51 a | 78.47 |
| 3 | 23.50 (4.81) a | 24.10 (59.22) a | 16.45 (6.45) a | 17.79 a | 101.40 |
| 4 | 24.60 (2.30) a | 24.95 (59.22) a | 17.30 (1.63) a | 19.49 a | 102.32 |
| 5 | 24.05 (2.65) a | 25.10 (59.22) a | 14.70 (25.98) a | 18.63 a | 99.12 |
| 6 | 24.15 (7.91) a | 24.80 (59.22) a | 17.35 (0.41) a | 18.83 a | 98.99 |
| 7 | 25.25 (1.40) a | 25.00 (59.22) a | 17.60 (3.21) a | 20.53 a | 104.67 |
| 8 | 25.75 (8.51) a | 24.80 (59.22) a | 17.20 (7.40) a | 21.42 a | 109.93 |
| 9 | 22.65 (7.18) a | 24.55 (59.22) a | 16.43 (10.98) a | 16.56 a | 91.81 |
| 10 | 23.90 (2.37) a | 24.90 (59.22) a | 17.00 (0.98) a | 18.39 a | 92.69 |
| 11 | 24.95 (2.55) a | 24.70 (59.22) a | 17.40 (0.81) a | 20.04 a | 99.82 |
| 12 | 24.90 (6.25) a | 25.55 (59.22) a | 18.03 (0.59) a | 20.00 a | 105.37 |
| 13 | 24.05 (0.29) a | 24.55 (59.22) a | 17.25 (2.05) a | 18.62 a | 96.30 |
| 16 | 23.40 (15.71) a | 24.80 (59.22) a | 16.70 (10.16) a | 17.84 a | 95.40 |
Same letters indicate that variables had no significant differences between clones according to Tukey's test (P > 0.05). Values in parentheses correspond to the coefficient of variation (%). DBH: diameter at breast height at 1.3 m.
Wood properties and variations
Figure 3 shows the variables measured in the 14 clones of G. arborea depending on stem height. The average diameter of the clones ranged from 23.93 cm to 14.24 cm (Figure 3a), the average heartwood diameter ranged from 16.18 cm to 8.38 cm (Figure 3b) and the heartwood percentage ranged from 51.45 % to 33.94 % (Figure 3e). Heartwood percentage showed wide variation among clones (Figure 3e), while for tree and heartwood diameter, clones 5 and 6 had the lowest values (Figure 3a-b). These three parameters, as expected, decreased with height. As for average bark and sapwood thickness, the first parameter varied between 0.45 cm and 2.89 cm and increased with stem height with little variation among trees (Figure 3d), while sapwood thickness varied between 7.30 cm and 4.99 cm and decreased with height with great variation among clones (Figure 3c), with clone 10 being the thickest. Bark percentage tended to decrease from the base up to a height of 7.5 m; thereafter, it increased up to the highest part sampled, again with significant variation among trees (Figure 3f). The average percentage of sapwood remained in a range of 36.08 % to 52.56 % and tends to increase depending on the height and with no clear trend among clones (Figure 3g).
Regarding physical properties, Figure 3 also shows that the average moisture content in green condition varies between 182.68 % and 112.52 % and decreases with increasing tree height (Figure 3h). In this parameter, a trend is shown among the clones depending on the heights; clone 10 along the entire length of the stem has the lowest moisture content and clone 9 has the highest content. Regarding specific weight, it increases with tree height and the average variation is from 0.37 to 0.42 (Figure 3i). Finally, the average green density ranged from 0.88 g∙cm-3 to 1.04 g∙cm-3 and decreased with tree height (Figure 3j). In this parameter, clone 10 had lower green density and clones 9 and 4 had higher values.
Genetic analysis
According to Table 3, in the analysis of variance by components we observed that clone had no significant effect (P > 0.05) on specific gravity and green moisture, while the properties with the greatest variance were green density with 21.8 %, sapwood thickness with 17.2 % and bark percentage with 15.5 %. Regarding height, this factor had a significant effect on all variables (P < 0.001); those with the greatest variance were bark thickness with 82 %, total diameter with 78.2 %, heartwood percentage with 72.7 % and heartwood diameter with 71.3 %. On the other hand, there was no clone*height interaction, so this relationship had no significant influence on the variables studied and, therefore, the contribution to the variance was very low. In the case of error, the variables with the least contribution to the variance were bark thickness with 6.0 % and total diameter with 6.9 %.
Table 3 Analysis of variance and variance component for wood characteristics of 14 clones of Gmelina arborea.
| Variable | Clone | Height | Clone*Height | Error | ||||
|---|---|---|---|---|---|---|---|---|
| F-value | Variance (%) | F-value | Variance (%) | F-value | Variance (%) | F-value | Variance (%) | |
| Volume | 5.03** | 4.3 | 164.05** | 85.3 | 0.23 ns | 0.0 | - | 10.2 |
| Total diameter | 4.14** | 4.3 | 86.66** | 78.2 | 0.00 ns | 10.5 | - | 6.9 |
| Heartwood diameter | 9.18** | 10.5 | 70.85** | 71.3 | 0.00 ns | 11.2 | - | 7.0 |
| Sapwood thickness | 8.17** | 17.2 | 32.12** | 60.8 | 0.00 ns | 13.5 | - | 8.5 |
| Bark thickness | 445.09** | 1.8 | 23 268.6** | 82.0 | 0.00 ns | 10.1 | - | 6.0 |
| Heartwood percentage | 5.89** | 8.3 | 60.15** | 72.7 | 0.65 ns | 11.5 | - | 7.6 |
| Sapwood percentage | 4.91** | 10.0 | 39.77** | 69.8 | 0.56 ns | 12.1 | - | 8.0 |
| Bark percentage | 2.41** | 15.5 | 9.00** | 54.0 | 1.48 ns | 16.8 | - | 13.7 |
| Specific weight | 1.97ns | 13.9 | 10.12** | 61.7 | 0.41 ns | 13.6 | - | 10.8 |
| Moisture content | 1.85ns | 8.8 | 14.74** | 65.6 | 1.30 ns | 14.8 | - | 10.8 |
| Green density | 382.78** | 21.8 | 165.00** | 21.0 | 0.00 ns | 34.0 | - | 23.1 |
*P < 0.05, ** P < 0.01 and ns: non-significant (P > 0.05).
Regarding the degree of genetic control, high individual heritability values were determined for heartwood percentage (H2clon greater than 0.10), while no significant values were found for other wood properties (Table 4). In addition to these low heritability values, high values of coefficient of genetic variation (CV > 10 %) were found for most properties (Table 4).
Table 4 Heritability (H2clon) and coefficient of genetic variation (CVgi%) of wood characteristics of 14 clones of Gmelina arborea.
| Variables | H2clone | CVgi% |
|---|---|---|
| Volume | 0.004313 ± 0.0379 | 9.1374 |
| Total diameter | 0.007449 ± 0.0997 | 6.2058 |
| Heartwood diameter | 0.00657 ± 0.2789 | 67.3451 |
| Sapwood thickness | 0.070518 ± 0.3066 | 8.6029 |
| Bark thickness | 0.010773 ± 0.1199 | 29.7299 |
| Heartwood percentage | 0.102310 ± 0.369336 | 171.5681 |
| Sapwood percentage | 0.012807 ± 0.1307 | 20.9607 |
| Bark percentage | 0.00215 ± 0.0452 | 5.5260 |
| Specific weight | 0.008974 ± 0.1094 | 92.7106 |
| Moisture content | 0.043746 ± 0.2415 | 13.8030 |
| Green density | 0.033646 ± 0.2118 | 347.0333 |
Figure 4 shows the analysis by ranking and selection index; according to this, the best clones were number 1, 7, 12 and 13, which were positioned in selection category 1, due to the top quality they reflected in their properties compared to the rest of the clones. On the other hand, the worst clone, according to the selection index, was number 16, entering in category 4, while the largest number of clones (3, 4, 5, 6, 8, 10 and 11) were grouped in category 2 (medium performance clones).
Discussion
Growth properties
DBH, total height, basal area and volume of G. arborea clones (Table 2) are comparable with other studies. Among them, Patil et al. (2017) reported an average DBH of 11.76 cm in 18-month-old clones; Kumar (2007) indicates DBH and average height of 7.2 cm and 5.18 m, respectively, in 24-month-old clones; and Hernández-Castro et al. (2021a, 2021b) recorded DBH values of 21 cm and commercial height of 9.63 m at 34 months. In the present study, slightly lower values of 24 cm DBH and 16.6 m in commercial height were obtained (Table 2) for 36-month-old clones. These comparisons indicate that the growth of the genotypes evaluated are higher or comparable to other studies in Costa Rica. Another important fact is that the clones evaluated showed no statistical differences (P > 0.05), which indicates that all the genotypes studied have optimal growth characteristics at 36 months of age.
Wood properties and variations
Decreasing total diameter, heartwood diameter and heartwood percentage when increasing the stem height of the clones is an intrinsic mechanism of tree growth (Taylor Gartner & Morrell, 2002). At the same time, it is important to note that the values of total diameter (23.93 to 14.24 cm), heartwood diameter (16.18 to 8.38 cm) and heartwood percentage (51.45 to 33.94 %) (Figure 3 a-c) are higher than those reported in the literature. For example, Hidayati et al. (2017) report diameters of 12.5 to 20.8 cm in five-year-old melina trees; Moya and Tomazello (2007) noted average diameters of 22.15 cm in nine- to 12-year-old trees. Moya (2004) reported 50 to 70 % heartwood in 12-year-old trees, while in the present study, the percentage of heartwood remained between 40 and 50 % with early-aged clones (three years old). Thus, Moya and Tomazello (2007) demonstrated that cloned trees have a significant amount of heartwood at an early age and that, probably, at the end of the rotation, the percentage of heartwood is higher, since it increases with tree age (Rodríguez-Pérez et al., 2022).
The behavior of the sapwood, which increases in thickness and percentage with increasing tree height (Figure 3b and 3e), is typical in trees. The sapwood is the part with living cells that transpires and contains starch as an energy reserve (Taylor et al., 2002).
Bark, on the other hand, has the function of protection in the tree and decreases in relation to stem height (Paine et al., 2010; Wilson & Witkowski, 2003), as was the case in the melina clones in this study (Figure 3f). This behavior is consistent with other reports for several species in 13-year-old plantation trees (Tenorio et al., 2016). However, this variation along the stem was minimal, which is explained by Wilson and Witkowski (2003), who mention that bark variation is low during the first years of growth before flowering. This suggests no need for the tree to invest a lot of energy in creating a thick tissue in the first years of growth (Wilson & Witkowski, 2003).
Regarding the physical properties of wood, it is well known that specific weight is one of the most important in terms of quality (Moya, 2004). However, there are few studies that relate this parameter to the genetic origin of trees in broadleaf species (Rodríguez-Pérez et al., 2022). Studies in tropical species report that specific weight decreases with stem height (Pande & Singh, 2005; Rodríguez-Pérez et al., 2022; Weber & Sotelo-Montes, 2008), a situation that differs from this study where it was observed that values increase with stem height (Figure 3g). Zobel and Van Buijtenen (1989) indicate that the decrease in specific weight with height is not clearly established, since there is significant variation among species and an important genetic effect that can be attributed to the origin of the materials.
Regarding moisture and green density, both variables decreased with increasing stem height (Figure 3f and 3h), which differs with that reported by Rodríguez-Pérez et al. (2022) and Tenorio et al. (2016) in fast-growing tropical species. However, Arguedas et al. (2018) report that changes in moisture contents depend on factors such as time and site of sampling and tree age, so a fixed variation of moisture content along the stem is not established. Moisture appears to be mainly concentrated in the lower part of the stem in this study.
Genetic analysis
Genetic analysis showed that green density, sapwood thickness and bark thickness are the significant variables with the highest variance in the clone factor (Table 3). Zobel and Jett (1995) and Zobel and Van Buijtenen (1989) mentioned that the inner tissues of the tree (bark, sapwood and heartwood) are usually affected by the environment and growing conditions; for example, water availability affects tree density, in some cases increasing it and in others decreasing it. Heartwood percentage showed the highest heritability value (Table 4), confirming that this property has the highest heritability in many species (Ioannidis & Koropouli, 2023), a value classified as high according to Nunes et al. (2016). This high heartwood heritability is important in species used in reforestation, because this type of wood affects mechanical properties, aesthetic quality and durability (Li et al., 2017). Thus, the development of trees with adequate values of these properties at the early stages of improvement programs, as in the case of the present study, will increase the potential of the species for wood marketing (Abarca-Alvarado et al., 2023).
Another important value is the coefficient of clonal variation, which, according to Resende (2002), allows us to know the genetic variability that exists within the material. In this study, heartwood percentage, specific gravity and green density (Table 4) had the highest coefficients of variation, which according to Hernandez-Castro et al. (2021b) is because the environmental factor is low or not very visible in these properties at early ages.
Heritability and genetic coefficients of variation for wood properties showed moderate to very high values (Table 4). These results allow inferring, from relatively early ages, that it is possible to start with the selection process to have trees with adequate properties in the future; however, this should be taken with caution, because genetic control increases with age (Gion et al., 2011; Hernández-Castro et al. 2021a). Therefore, it is necessary to continue with measurements in these genetic trials to determine the optimal age for selection.
Regarding genetic ranking, four genetic categories were identified (Figure 4). This is very important because, as mentioned by Hernández-Castro et al. (2021b) , the practical usefulness of genetic ranking, in addition to allowing the projection of potential genetic gains, indicates when to carry out evaluations in order to verify the elite clones with obvious quality of properties and thus support genetic improvement programs of the species with greater certainty. Also, this classification allows to know fast, medium and slow growing clones that address the objectives of forest production (Makouanzi et al., 2018). Applying this to the present study, clones 7, 12, 13 and 1 of G. arborea showed higher growth than the other clones studied at 36 months of age (Figure 4). Therefore, these clones show potential for further genetic improvement of this species in reforestation programs based on the properties of the wood.
Conclusions
The 36-month-old Gmelina arborea clones had slightly greater diameter, height and volume than other trees from plantations under different conditions. Wood properties, for the most part, were little affected by clone selection; specifically, clones had no effect on specific gravity and moisture content. The results showed that wood properties have low heritability, a situation attributed to the early age of the trial, in which the genetics of the tree have not yet been fully developed. Nevertheless, the heartwood already shows heritability; this represents an advantage, since this property, of importance in industrial processes, can increase with age. Lastly, although the analysis was carried out at an early age, clones 7, 12, 13 and 1 were higher regarding growth and some wood properties; these clones show potential for further genetic improvement of G. arborea in reforestation programs.
