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Madera y bosques

versión On-line ISSN 2448-7597versión impresa ISSN 1405-0471

Madera bosques vol.24 no.2 Xalapa ago. 2018  Epub 08-Oct-2018

https://doi.org/10.21829/myb.2018.2421420 

Scientific papers

Effect of planting density on wood anatomy in Eucalyptus and Acacia from Brazil

Efecto de la densidad de plantación en la anatomía de madera de Eucalyptus y Acacia de Brasil

Taiane Oliveira Guedes1  2 

Lidiane Costa Lima1 

Paulo Ricardo Gherardi Hein1  * 

José Reinaldo Moreira da Silva1 

Leonardo David Tuffi Santos2 

1 Universidade Federal de Lavras. Departamento de Ciências Florestais. Ciência e Tecnologia da Madeira. Lavras, Minas Gerais, Brasil.

2 Universidade Federal de Minas Gerais. Instituto de Ciências Agrárias. Montes Claros, Minas Gerais, Brasil.


Abstract

The effect of planting density on the anatomical characteristics of wood is still matter of debate. The aim of the study was to investigate the influence of planting density on the variation of wood anatomical elements in Eucalyptus and Acacia trees cultivated in areas of high planting density (monospecific) and low planting density (mixed) stands. The fiber length, fiber width, fiber lumen diameter, cell wall thickness, vessel diameter and vessel frequency were evaluated. The measurements were made from wood discs taken from five Eucalyptus urophylla x E. grandis trees and five Acacia mangium trees grown in high planting density (monospecific plantations, 1667 trees/ha) and in low planting density (mixed stands, 500 trees/ha), totalizing 20 trees at 4.5 years old. Eucalyptus and Acacia produced wood with shorter fibers and smaller vessels in high density stands, when compared to its equivalent in low density. Acacia presented more frequent vessels and fibers with walls thicker than its equivalent in low planting density. Eucalyptus presented longer and thinner fibers and vessels more frequent than Acacia, regardless of planting density. On the other hand, in high planting density, Acacia presented fibers with walls thicker than Eucalyptus and in low density of planting their vessels were larger.

Keywords: cell wall thickness; fiber length; planting spacing; vessel diameter; vessel frequency

Resumen

El efecto de la densidad de plantación sobre las características anatómicas de la madera aún es tema de debate. El objetivo de este estudio fue investigar la influencia de la densidad de plantación sobre la variación de elementos anatómicos de la madera de árboles de Eucalyptus y Acacia, creciendo en áreas de alta densidad de plantación (monoespecífica) y en rodales de baja densidad de plantación (mixta). Se evaluaron: longitud, ancho y diámetro de lumen de la fibra, grosor de pared celular, diámetro y frecuencia de vasos. Las mediciones se hicieron en rodajas de madera tomadas de cinco árboles de Eucalyptus urophylla x E. grandis y cinco de Acacia mangium, creciendo en alta densidad de plantación (plantaciones monoespecíficas, 1667 árb/ha) y de otro tanto igual de árboles creciendo en baja densidad de plantación (rodales mixtos, 500 árb/ha), lo que hace un total de 20 árboles de 4.5 años de edad. Eucalyptus y Acacia de rodales de alta densidad produjeron madera con fibras más cortas y vasos más pequeños en comparación con sus equivalentes en baja densidad de plantación. Eucalyptus presentó fibras más largas y delgadas y vasos más frecuentes que Acacia, independientemente de la densidad de plantación. Acacia presentó vasos más frecuentes y fibras con paredes más gruesas que su equivalente en plantación de baja densidad. Por otro lado, en alta densidad de plantación, Acacia presentó fibras con paredes más gruesas que Eucalyptus y en baja densidad de plantación sus vasos fueron mayores.

Palabras clave: grosor de pared celular; longitud de fibra; espaciamiento de plantación; diámetro de vaso; frecuencia de vasos

Introduction

Eucalyptus plantations in Brazil have grown at a rate of 36 m³/ha/year, the planted surface occupied 5.6 million hectares in 2015 and the biomass produced has met the demand for wood from industries producing pulp and paper, charcoal and firewood (Indústria Brasileira de Árvore [IBA], 2016). At the same time, the Acacia genus has been devoted to multiple uses, once the trees have the ability to restore damaged areas, produce tannin, fix nitrogen, supply energy and present a yield ranging from 10 to 25 m³/ha/year in Indonesia (Sein and Mitlohner, 2011). Grigoletti et al. (2003) suggest that the use of Acacia in forest systems of consortium can further optimize biomass productivity. Several studies have shown promissory findings when Acacia and Eucalyptus are planted together (Bouillet et al., 2008; Forrester, Theiveyanathan, Collopy and Marcar, 2010; Gonçalves and Lelis, 2012). In general, the planting density in mixed stands is lower (~400 trees/ha - 600 trees/ha, low density) than those adopted in high monocultures (~1000 trees/ha - 1800 trees/ha, high density), especially in Brazil (Associação Brasileira de Produtores de Florestas Plantadas [ABRAF], 2013). The wood products industry is increasingly attentive to the final quality of their products. The mixed stands have been indicated as a more sustainable option of forest production (Bouillet et al., 2008) and, therefore, such raw material has been recommended for a range of applications in which high quality products are required.

In forests, any change that would alter the growth pattern of a tree will likely result in variation in the technological properties of the wood. However, the response of the cambial activity of the tree to these environmental changes, and therefore the resultant wood properties, is not clear and the results are controversial (Zobel, 1992). Most of studies on the relationship of wood properties variation with growth conditions are frequently contrasting, as pointed out by Gonçalves, Stape, Laclau, Smethurst and Gava (2004) in their literature review.

Reduced spacing, such as those adopted in Eucalyptus monocultures, usually induce greater competition for light, water and nutrients between trees, causing variations in height and diameter (Zobel and Van Buijtenen, 1989). Thus, considering that trees planted with larger spacing have a greater availability of water and nutrients, allowing the formation of larger conduits, we hypothesized that the supposed greater amount of assimilates cause variation in the anatomical structure of the timber. Furthermore, lower plantation density is usually adopted in mixed stands (Viera, Schumacher and Liberalesso, 2011), possibly causing variation on the quality of the wood produced and potentially generating considerable impact in their multiple forms of use.

The variation in anatomical characters of this material is important as it influences the final properties of the wood, such as its density and mechanical strength. When the fiber is intended for the manufacture of paper, for example, the fiber length affects the resistance to tear or folding test (Santos, 2005). According Nisgoski, de Muñiz, Trianoski, de Matos and Venson (2012) the lumen and fiber width are also important characteristics because they determine the fiber flexibility coefficient (lumen width / fiber width). The coefficient of flexibility is a parameter used in pulp and paper industry to determine the probability of collapse between fibers and the degree of union between them on a paper sheet.

Many studies have evaluated the effect of growth conditions on the anatomical features of the wood from fast-growing species, as Eucalyptus (Pirralho et al. 2014; Monteoliva, Barotto and Fernandez, 2015) and Acacia (Igartúa and Monteoliva, 2010), however, few articles (Forrester et al., 2010) have deal with mixed stands composed by these two species.

Objectives

The aim of this study was to investigate whether the planting density has an influence on anatomical elements of wood from Eucalyptus urophylla × E. grandis and Acacia mangium.

Material and methods

Characterization of the study area

The experiment was established in January 2009 at the Instituto de Ciências Agrárias of the Federal University of Minas Gerais, in Montes Claros city, Minas Gerais, Brazil (16°40'03.60"S; 43°50'41.52"O; 598 meters above sea level) as described by Silva et al. (2018). The climate is Aw-Tropical Savanna, characterized by high annual temperatures and rainy regime marked by two distinct seasons, rainy summer and dry winter (Köppen and Geiger, 1928). The soil is characterized as a eutrophic red-yellow argisoil (Claessen, Barreto, de Paula and Duarte, 2006).

Experimental design and sampling

The trees were planted in high density and low density stand systems with Eucalyptus urophylla x E. grandis hybrid (hereafter Eucalyptus) and Acacia mangium Willd (hereafter Acacia) as shown in figure 1. The plantation density in the monoculture system (high density) was 2 m × 3 m, while in the mixed stand (low density) the spacing was 10 m × 2 m. The experiment was conducted in a completely randomized design composed by four treatments, represented in this study by five replications. The four treatments were considered as: i) Eucalyptus in high planting density (monospecific plantations, 1667 trees/ha), ii) Eucalyptus in low planting density (mixed stands, 500 trees/ha), iii) Acacia in high planting density (monospecific plantations, 1667 trees/ha), iv) Acacia in low planting density (mixed stands, 500 trees/ha). Ten trees of Acacia and ten of Eucalyptus were selected (5 of each type of planting density) and harvested in August 2013, when the trees achieved four years and six months of age. The selection of these 20 individuals (2 species × 2 planting density × 5 repetitions) was based on the uprightness of the stem and absence of visible diseases.

Figure 1 Plantation scheme showing high and low density stands with Eucalyptus (white circles) and Acacia (gray circles) trees. 

In regard to the growing characteristics of trees, Eucalyptus presented diameter at breast height (DBH) of 18.92 cm ± 0.93 cm in monospecific plantations and 15.01 cm ± 1.56 cm in mixed stands, while Acacia had DBH of 14.64 cm ± 1.73 cm (monospecific plantation) and 11.11 cm ± 1.09 cm (mixed stands), as reported by Silva, Roldão, Santos, Hein (2018).

Wood discs (thickness: 30 mm) were cut at breast height, properly identified, placed in plastic bags to keep moisture and then divided into four wedges, free from defects. Sixty-eight cubic specimens measuring 20 mm × 20 mm × 20 mm used in this study were cut from the wedges. For each wedge, cubic specimens were cut near the pith, at intermediate radial position and near the cambium.

Anatomical characterization of wood

Histological sections were made from the specimens in order to study the variation in the anatomical features of the wood. For preparing the slides, the wood specimens were immersed in water for 24 hours and then placed in a pressure cooker with water and glycerin during 15 minutes, to soften the wood.

The sections were produced using a sliding microtome with nominal thickness of 30 microns and semi-permanent sections were stored with glycerin. The thin sections were placed in bleach until they become clear and subsequently in distilled water, to remove traces of bleach. The sections were stained with Safranin and passed through an alcoholic series, from lower concentration (20%) to absolute ethanol, in order to remove the dye excess and dehydrate them, as described by Johansen (1945). The slides were mounted with the thin sections, fixed with Entelan and covered with coverslips.

For measuring fiber diameter, wall thickness and fibers length, small fragments of wood were removed from specimens, placed in macerate solution of hydrogen peroxide and 1 N acetic acid at a ratio of 1:1 and conditioned in an oven at temperature of 60 °C for 48 hours, in order to promote fiber individualization, as suggested by Franklin (1945). After this period, the fibers were washed in water, stained with diluted safranin and mounted in histological slides. The temporary mounts were prepared for the measurements on optical microscope equipped with an ocular micrometer and a graduated lamina with 1 mm of scale. The average fiber length and diameter was based on the observation of 30 fibers per lamina using a light microscope with10x objective lens. The cell wall thickness was measured by observing 30 fibers per sample from the macerated material through the 40x objective lens, as experimental procedure adopted by Monteiro et al. (2017).

To measure the frequency of vessels, the mounts were analyzed using a 10x objective lens. Vessel diameter was observed and accounted for twin or multiple vessels; each unit was individually measured in tangential and radial direction and the average values were calculated for each unit, according to an adaptation of International Association of Wood Anatomy [IAWA] (1989). The vessels were randomized including different sizes found in the sections.

The capture of images was performed using a digital camera coupled to an optical microscope (Ken-Vision, TT-1010 model) and the software WinCellPro was used for the anatomical measurements of the wood sections, which followed the IAWA (1989) procedure. The following quantitative anatomical features were evaluated: vessel lumen diameter (VD), vessel frequency (VF), fiber length (FL), fiber width (FW), fiber lumen diameter (LD) and fiber cell wall thickness (CWT). Each slide was composed by three replicates of cross section. Thirty measurements per slide were made for each anatomical feature.

Analysis of results

The statistical software "SPSS v.19" was used to calculate the descriptive statistics, analysis of variance (Oneway), multiple comparison among means (Tukey) and t-test. For the analysis of variance were considered four variation sources: 1) high density plantation of Eucalyptus hybrid; 2) high density plantation of Acacia mangium; 3) low planting density stands of Eucalyptus hybrid and 4) high planting density stands of Acacia mangium. The normality of the residuals and the homogeneity of variances were tested and the averages compared by Tukey test at 5% significance level.

Results and discussion

Anatomical variability

The mean, values of the anatomical features observed in Eucalyptus grandis x E. urophylla hybrid and Acacia mangium grown in high and low density stands are shown in table 1. The Eucalyptus trees grown in low or high plantation density did not present significant differences between values of fiber width (FW), fiber lumen diameter (LD) and cell wall thickness (CWT). However, there was a significant effect of planting density on vessel diameter (VD), frequency (VF) and fiber length (FL) of Eucalyptus wood cells.

Table 1 Descriptive statistics of the growing and anatomical characteristics of Eucalyptus grandis x E. urophylla and Acacia mangium woods produced in high and low density plantations. 

VD (µm) VF (mm-²) FL (µm) FW (µm) LD (µm) CWT (µm)
Eucalyptus high density 120.1C ±23.2 15.2B 864.6B 16.,13B 9.31C 3.18B
Eucalyptus low density 125.9B ±26.1 16.1A 906.9A 16.48B 10.09C 3.22B
Acacia high density 119.2C 11.08C 744.3D 19.19A 12.32B 3.42A
Acacia low density 133.7A 9.28D 789.9C 19.88A 13.56A 3.14B

VD - vessel diameter, VF - vessel frequency, FL - fiber length, FW - fiber width, LD - fiber lumen diameter and CWT - cell wall thickness. The anatomical characteristics were compared among treatments by the Tukey test at ρ = 0.01 threshold (means followed by same letter do not significantly differ).

Acacia trees grown in low density stand (spacing: 10 m × 2 m) presented higher values in most anatomical parameters, but the CWT and VF were higher when the Acacia was grown in high density plantations.

Effect of planting density on vessels diameter and frequency

The two species produced wood with larger vessel diameters when cultivated in low density plantations (Table 1). The diameter of vessels found in this study for 4.5 years-old Eucalyptus wood in low density stand (125.9µm) was significantly higher than in high density plantation (120.1µm). The vessel diameters of these woods were slightly higher than those reported by Alzate (2004), who found an average vessel diameter of 103.63 µm for Eucalyptus grandis x E. urophylla with 8 years old, and Lima de Oliveira et al. (2012) who found average values of 99.21 µm in several species of Eucalyptus with 64 months of age. Here, the average vessel diameter of Acacia wood was 133.7 µm and 119.2 µm for the low density stand and high density (Table 1), respectively (the VD of Acacia wood were significantly different by t-test). These values are consistent with those reported by Antunes (2009), who found vessels with diameters ranging from 120 microns to 160 microns in Acacia crassicarpa and Acacia mangium wood.

The variation in vessel frequency of the Eucalyptus and Acacia wood grown in high and in low density stands are shown in figure 2.

Figure 2 Variation in vessel frequency of Eucalyptus grandis x E. urophylla and Acacia mangium grown in high and low density stands. 

In regard to the vessels frequency, there was a significant difference between treatments (Table 1). Figure 2 reveals there was effect within each density system. Eucalyptus wood planted in low density system produces significantly more vessels per unit area than the wood produced in high density. In Acacia plantations, the reverse trend was observed.

The frequency of vessels observed by Alzate (2004) was of 8-13 vessels/mm2 in 8-year-old Eucalyptus grandis x E. urophylla hybrids. Monteiro et al. (2017) reported frequency of 16 vessels per mm2 and Evangelista, Silva, Valle and Xavier (2010) found 9.9 vessels/mm2 in E. urophylla wood. Antunes (2009) reported vessels frequency from 4 to 9 vessels/mm2in A. mangium wood.

Sousa Junior (2004) has evaluated the differences in diameter and frequency of vessels from E. urophylla wood grown in distinct spacing plantations (6 m × 6 m and 3 m × 2 m), reporting lower vessel frequency (10,89 mm-²) and higher vessel diameter (105,85 µm) in the wood produced in lower planting density than the wood from the higher planting density (VF of 11,37 mm-², VD of 96,09 µm). However, the trees growing at different planting density had different ages and provenance: trees planted at 6 m × 6 m had 15 years old and came from Paraopeba city, while the other trees had 25 years old and were cultivated at Turmalina city, Brazil. To our knowledge, the study conducted by Sousa Junior (2004) is the closer one to our findings on Eucalytptus, but it is difficult to make comparisons because of such differences in provenance and age.

Fiber biometric changes according to planting density

The results shown in Table 1 indicate that both Eucalyptus and Acacia trees planted in low density system produce significantly longer fibers. This finding agrees with those found by Teago (2012), who studied Acacia mangium and Eucalyptus woods and found that in monoculture these trees produce wood with shorter fiber, besides having greater radial heterogeneity for the fiber width, lumen diameter and wall thickness of the fiber.

The length of Eucalyptus fibers found in this study are in agreement with the literature (Alzate, 2004; Alencar, Barrichelo and Silva 2002; Brisolal and Demarco 2011). But the Acacia fibers are longer than those reported in similar studies. Alencar et al. (2002) reported fibers 1.18 mm long in 4 year-old E. grandis x E. urophylla wood while Brisolal and Demarco (2011) analyzed 6 year-old E. grandis x E. urophylla with fiber length of 1.1 mm. Tomazello-Filho (1983) reported 10 year-old Eucalyptus grandis with fiber length of 1.03 mm. Regarding the fiber length of Acacia wood, Alencar et al. (2002) found fibers of 0.89 mm and Rossi, Azevedo and Souza (2003) found fibers measuring from 1.0 mm to 1.2 mm, a little longer than the fibers of the wood investigated in this study.

The fibers width does not seem to be sensitive to planting space. Table 1 shows no significant difference between the planting density for both species. However, there was a species effect on the variation of this fiber feature. The Acacia wood has wider fibers (~ 20 µm) than Eucalyptus (~ 16 µm). The fibers width of the E. grandis x E. urophylla hybrid is in agreement with the literature: Alencar et al. (2002) found fibers with 19.95 µm in width in 4-year-old E. grandis x E. urophylla; Brisolal and Demarco (2011) reported fibers of 20 µm in width and Alzate (2004) evaluated wide fibers, with average of 19.7µm. In Acacia, the literature indicates a larger variation between fiber widths: 24.7 µm in Alencar et al. (2002) and 16 mm in Antunes (2009).

The lumen diameter of Eucalyptus fiber was not influenced by planting density while there was a significant difference between the mean lumen diameters of the A. mangium samples. The Acacia mangium wood produced in low density stand had fibers with bigger lumen diameter when compared to fibers produced in high density plantation (Table 1). This characteristic is within the range found in the literature for Eucalyptus (Alencar et al., 2002; Brisolal and Demarco, 2011; Alzate, 2004) and Acacia (Alencar et al., 2002; Antunes, 2009).

There was no variation in cell wall thickness of the fibers of Eucalyptus trees grown in high and low density stands. However, when Acacia is planted in high density, the cell wall thickness of the fibers is higher than when it grows in low density stands (Table 1).

The cell wall thickness varies greatly in Eucalyptus literature: 4.31 mm in Alencar et al. (2002); 6.1 µm in Brisolal and Demarco (2011); and 5.01 µm in Alzate (2004). The cell wall thickness of the Eucalyptus wood in this study had lower values (~3.2 µm). For Acacia, the cell wall thicknesses found in this study (3.1 µm - 3.4 µm) were close to the literature: 3.59 µm in Alencar et al. (2002) and 3.2 µm in Antunes (2009). Cell wall thickness is important because it is highly correlated to the density of the wood and also influences its hardness (Cutler, Botha and Stevenson (2008).

Conclusions

The findings of this study indicate that the planting density significantly influences the anatomical features of the Eucalyptus and Acacia wood.

Within species: Eucalyptus and Acacia produced wood with shorter fibers and smaller vessels in high density stands, when compared to its equivalent in low density. However, Acacia presented more frequent vessels and fibers with walls thicker than its equivalent in low planting density.

Among species: Eucalyptus presented longer and thinner fibers and vessels more frequent than Acacia, regardless of planting density. On the other hand, in high planting density, Acacia presented fibers with walls thicker than Eucalyptus and in low density of planting their vessels were larger.

Acknowledgements

The authors express their thanks to the Instituto de Ciências Agrárias, Federal University of Minas Gerais (UFMG, Brazil) for supporting the experimental work. Special thanks to the FAPEMIG, CAPES; and CNPq for granting Productivity Research fellowships for LD Tuffi Santos (Process 304355/2010).

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Received: March 17, 2016; Accepted: December 15, 2017

*Corresponding autor. paulo.hein@dcf.ufla.br

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