Interaction between planting spacing and wood properties of Eucalyptus clones grown in short rotation

Abstract

Wood quality results from the interaction between the genotype of trees and the silvicultural conditions to which they were subjected. Based on this interaction, research on the factors that add value to the woody raw material has an impact on the production of various forest products. This study aimed to evaluate the influence of different planting spacings (3×1, 3×2, 3×3 and 3×4 m) on the wood properties of three 4-year-old Eucalyptus clones. The wood of each clone was evaluated based on basic density, dry mass, anatomy, structural chemical composition, ash content and higher heating value. For at least one of the three clones studied, the planting spacing explained the variations in the basic density, dry mass, diameter and frequency of vessels and wall thickness of the wood fibers. Regardless of the eucalyptus clone, the area of vessels, the length and fraction of the fiber wall, as well as the contents of extractives, lignin, ash and the higher heating value of the wood were not influenced by the planting spacing. In summary, the change in the useful area alters the properties of wood in eucalyptus genetic materials in different ways and intensities at 4 years old.

Introduction

Planting spacing is one of the main factors to be considered when planning and implementing forestry projects for the production of wood. The distance between trees represented by the useful area results in economic, silvicultural and technological implications, influencing the cost of forest production, the growth rate and age of trees, forestry practices and the quality of the wood produced ([13], [34], [28], [7], [33], [37]).

The planting spacing can cause changes in wood properties, by interfering in the growth and in the morphology of the trees, depending on the age of their evaluation ([21], [34]). Each species, clone or variety has a peculiar behavior under the growth conditions imposed by the planting spacing. In crops of rapid growth and short cycle of rotation, such as those of the genus Eucalyptus, the study of the properties of wood is even more relevant, due to the higher proportion of juvenile wood in the trunk and its various uses ([23], [11], [30]).

Studies that report the influence of spacing on the properties of Eucalyptus wood concentrate basically on the basic density of the wood ([1], [25], [28]), and the knowledge on information that precisely addresses the impact of planting spacing of young trees on the anatomy and chemical composition of wood is incipient. Although the short rotation periods provide greater volumes of wood, it is essential to guarantee conditions that allow the proper establishment of the forest crop ([33]), and consequently, quality wood.

In view of the need to meet the high demand for pulpwood and energy and the costs involved in maintaining and growing the forest, companies in this sector experiment and invest in dense plantations and in short-cycle rotations (less than 7 years), a characteristic of Brazilian forestry. The objective of this research work was to evaluate the influence of planting spacing on the wood properties of Eucalyptus clones in short-cycle plantations.

Material and methods

Material and sampling

Two clones of Eucalyptus grandis and one clone of Eucalyptus grandis × Eucalyptus urophylla from experimental plantations, located in Paranapanema, São Paulo state, Brazil, were studied (23° 23′ 19″ S, 48° 43′ 22″ W; elevation 610 m a.s.l.). The average annual precipitation during cultivation (2010-2014) was 1300 mm and the average annual temperature was 20 °C. The soil was classified as a Latossolo Vermelho (Oxisol). The eucalyptus clones were planted at four planting spacings (3×1, 3×2, 3×3 and 3×4 m) and 48 trees were felled, 16 of each clone, i.e., four trees per spacing (Tab. 1). The trees were harvested within the plot previously defined in the experimental design to eliminate the edge effects. After cutting, the wood volume of each tree was estimated using the Smalian method. Six 3-cm-thick wooden disks were removed from each tree in the positions: 1.3 m from the ground (DBH) and 0, 25, 50, 75 and 100% of the commercial height (Fig. 1).

Tab. 1 - Experimental plot area, tree diameter and commercial height averages by genetic material and spacing. (N): number of trees in the useful plot (no edge effect); (DBH): diameter at breast height (cm); (CH): commercial height (m).

Useful area (m²)	Plot area (m²)	N	E. grandis (clone A)		E. grandis (clone B)		E. grandis × E. urophylla
			DBH	CH	DBH	CH	DBH	CH
3	300	36	11.8	15.0	12.8	15.7	12.1	12.4
6	600	36	14.6	18.9	15.5	19.2	15.1	17.1
9	900	36	16.1	19.7	18.2	21.9	17.3	18.3
12	1200	36	17.3	19.4	20.8	23.0	20.3	20.5

Fig. 1 - Scheme of tree sampling, with demarcation of the positions of removal of the disks and specifications for the technological analysis.

Results and discussion

The highest values of basic wood density were detected at the largest spacing. Between the 3×1 and 3×4 m spacing, the percentage increments were 2.2% (E. grandis clone A), 7.0% (E. grandis clone B) and 7.5% (E. grandis × E. urophylla - Tab. 2).

Tab. 2 - Physical properties, anatomical structure and chemical composition of wood from Eucalyptus clones at different planting spacings. (BD): basic density; (DM_tree): dry mass per tree; (DM_ha): dry mass per hectare; (FL): fiber length; (FWt): fiber wall thickness; (WF): wall fraction; (VD): Vessel diameter; (VF): Vessel frequency; (VA): Vessel area; (Ext): Extractive content; (Lig): Lignin content; (Ash): Ash content; (HHV): Higher heating value.

Species	E. grandis (clone A)				E. grandis (clone B)				E. grandis × E. urophylla
Useful area (m²)	3	6	9	12	3	6	9	12	3	6	9	12
BD (g cm-3)	0.46	0.49	0.5	0.47	0.43	0.44	0.46	0.46	0.4	0.41	0.42	0.43
DMtree (t)	0.039	0.074	0.094	0.092	0.042	0.075	0.118	0.147	0.029	0.059	0.08	0.112
DMha (t)	131.5	123	104.5	76.5	141.3	124.7	131.5	122.5	96.6	97.8	89.4	93.4
FL (µm)	961	988	973	973	945	932	935	964	948	1010	986	960
FWt (µm)	3.16	3.04	3	2.77	2.52	2.33	2.46	2.31	2.61	2.7	2.66	2.68
WF (%)	36.97	36.97	37.59	33.33	30.06	29.47	29.4	26.48	29.69	29.15	28.27	28.26
VD (µm)	101.12	116.75	109.75	111.04	94.49	99.1	96.98	99.62	100.74	113.22	117.74	115.86
VF (n mm-1)	10.09	8.84	8.98	9.19	12.9	11.89	11.6	10.48	10.28	8.68	8.55	8.54
VA (mm2)	80.87	94.67	85.28	88.39	90.55	91.65	85.62	81.52	81.17	87.33	93.17	89.83
Ext (%)	1.14	1.12	1.13	1.29	1.42	1.29	1.14	1.08	0.79	1.13	0.88	0.94
Lig (%)	27.24	27.4	26.77	27.79	28.6	28.01	28.02	28.35	26.52	28.94	28.04	28.26
Ash (%)	0.45	0.4	0.37	0.45	0.44	0.49	0.47	0.52	0.63	0.59	0.67	0.6
HHV (kcal kg-1)	4406	4320	4417	4380	4423	4409	4427	4466	4337	4389	4421	4442

The increments in the basic density values are due to the different planting spacings, which caused different effects on each of the eucalyptus clones, even under similar silvicultural conditions. However, the difference in useful area from the smallest to the largest spacing is small and the age of evaluation of the material was only 4 years. These differences may be more evident in the evaluation of materials at older ages, when the level of competition between trees in the field has greater effects on growth and, consequently, on the properties of wood.

Density is one of the characteristics of wood that has a high heritability ([25], [14]), which explains the low variation (less than 10%) of this property between the smallest and the largest spacing in the genetic materials evaluated. The absence of variation in the basic density of wood in useful areas from 6 to 16 m² was observed for E. benthamii at the age 6 years ([5]); and in areas from 3 to 4.5 m² for E. grandis at the ages of 1, 3 and 5 years ([13]).

Some references indicate a small increase in density as a function of planting spacing in eucalyptus stands ([15], [34], [28], [39]). However, considering the multiple uses of wood, the increase in density impacts the production and economy of the forest-based sector ([25], [30]). The effect of the useful area of the trees, resulting from the spacing, was significant when analyzing the adjusted models of the basic density of the wood, with a total variation between 39% (E. grandis clone B) and 53% (E. grandis × E. urophylla), as shown in Tab. 3. The relationship of the wood density of trees of the three genetic materials by useful area is shown in Fig. 2A.

Tab. 3 - Statistical parameters of fitted models to estimate basic density, dry mass, vessel diameter, vessel frequency and fiber wall thickness of wood as a function of the useful area per tree for the three Eucalyptus clones. (Bd): Basic density; (UA): useful area; (Dm): Dry mass; (Vd): vessel diameter; (Vf): vessel frequency; (FWt): fiber wall thickness; (ns): not significant; (**): significant at 1% by the F test for regression and t for coefficients of equation.

Properties	Statistical parameters	E. grandis (clone A)	E. grandis (clone A)	E. grandis × E. urophylla
Basic density	Equation	Bd = 0.470 + 0.001 · UA	Bd = 0.424 + 0.003 · UA	Bd = 0.389 + 0.003 · UA
	R²	0.020 ns	0.391 **	0.530 **
	Syx	0.031	0.015	0.011
Dry mass per tree	Equation	Dm = 0.031+0.006 · UA	Dm = 0.006+0.012 · UA	Dm = 0.002+0.009 · UA
	R²	0.714 **	0.978 **	0.964 **
	Syx	0.013	0.006	0.006
Dry mass per hectare	Equation	Dm = 154.8 - 6.12 · UA	Dm = 142.4 - 1.66 · UA	Dm = 98.74 - 0.59 · UA
	R²	0.747 **	0.264 ns	0.040 ns
	Syx	12.79	9.93	10.45
Vessel diameter	Equation	Vd = 104.0 + 0.759 · UA	Vd = 94.2 + 0.442 · UA	Vd = 99.4 + 1.663 · UA
	R²	0.104 ns	0.062 ns	0.433**
	Syx	7.97	6.19	6.82
Vessel frequency	Equation	Vf = 9.91 - 0.085 · UA	Vf = 13.61 - 0.252 · UA	Vf = 10.34 - 0.178 · UA
	R²	0.115 ns	0.501 **	0.230 ns
	Syx	0.85	0.90	1.16
Fiber wall thickness	Equation	FWt = 3.29 - 0.040 · UA	FWt = 2.53 - 0.017 · UA	FWt = 2.62 + 0.005 · UA
	R²	0.450 **	0.060ns	0.015 ns
	Syx	0.16	0.24	0.15

Fig. 2 - Scatterplots of basic density (A), dry mass of the wood per tree (B) and per hectare (C) as a function of the useful area per tree for the three Eucalyptus clones.

The amount of dry wood mass per tree of each eucalyptus clone studied was influenced by the spacing (Tab. 2). Regardless of the clone, wider spacing provided a greater amount of dry mass (Tab. 3, Fig. 2B), as the larger useful area favors the volumetric increase per tree.

There are increases of 53, 105 and 83 kg of dry wood mass per tree, from the 3×1 to the 3×4 m spacing in E. grandis (clone A); E. grandis (clone B) and E. grandis × E. urophylla, respectively (Tab. 2). This highlights the specificity of each genetic material given the variation in planting spacing. The greater planting spacing increases the proportion of wood per tree, contributing to management practices and to the production sectors ([24], [25]). The greater planting spacing (3×4 m) resulted in the largest amount of dry mass per tree, since the largest useful area favors individual growth in diameter and height (Tab. 1). The smallest spacing contributed to the largest amount of dry mass per hectare, due to the greater competition between tree individuals. The useful area resulting from the planting spacing significantly explained approximately 75% of the dry mass production of wood per hectare for E. grandis (clone A - Tab. 3). The largest amount of dry mass per area was concentrated at the denser spacing (Fig. 2C), corroborating with that found for E. grandis × E. urophylla aged approximately 3 years ([40]) and E. grandis at 10 years of age ([19]).

Compared to the other clones, E. grandis (clone A) showed the most marked reduction in the amount of dry mass per area (t ha^-1) as the planting spacing increased (Fig. 2C). This behavior can be associated with the fact that this clone has the highest density values and the lowest values of solid volume of wood, leading to a decreasing trend between the growth rate of trees and the basic density of eucalyptus wood. Despite being an important factor since the 19th century, the influence of the useful area of the planting spacing on the dimensions of the woody anatomical cells, on the angle of the microfibrils and on the density is still poorly understood ([41], [20]), mainly for tropical woods at a young age.

Only the diameter and frequency of vessels and the wall thickness of the fibers, among the anatomical elements, were influenced by the planting spacing in at least one of the three eucalyptus clones evaluated. The vessel area and the length and fraction of the fiber wall (Tab. 2) were not susceptible to variations in spacing in any of the studied clones. The diameter of wood vessels tended to increase as the planting spacing between trees increased. However, the effect was significant only for the E. grandis × E. urophylla clone, with 43% variation (Tab. 3, Fig. 3A), whereas the other two clones showed 15% increase in vessel dimensions from 3×1 m to 3×4 m. Increases in the diameters of the anatomical vessels of E. grandis x E. urophylla with the planting spacing have been reported in the literature: (i) 3.3% from 3×1 to 3×4 m at 4 and 5 years old ([15]); and (ii) 4.8% from 2×3 to 10×2 m at 4 years and 6 months of age ([20]).

Fig. 3 - Scatterplots of vessel diameter (A), vessel frequency (B) and fiber wall thickness (C) as a function of the useful area per tree for the three Eucalyptus clones.

Conclusion

The wood properties of eucalyptus genetic materials in short rotation were altered by planting spacing in different ways, but with little intensity. Density, a property that affects different uses of wood, was influenced by the different useful areas, even in young wood.

Acknowledgments

We thank the company Suzano S.A., São Paulo, Brazil for its assistance in conducting the research, to the Espírito Santo Research Support Foundation (FAPES) and the Coordination for the Improvement of Higher Education Personnel (CAPES) for granting the scholarships.

References

(1) Alcorn PJ, Pyttel P, Bauhus J, Smith RGB, Thomas D, James R, Nicotra A (2007). Effects of initial planting density on branch development in 4-year-old plantation grown Eucalyptus pilularis and Eucalyptus cloeziana trees. Forest Ecology and Management 252: 41-51.

(2) ABNT (1983). NBR 8112: Carvão vegetal - análise imediata [NBR 8112: Charcoal - immediate analysis]. Associação Brasileira de Normas Técnicas, Rio de Janeiro, Brazil, pp. 1-5. [in Portuguese] Gscholar

(3) ABNT (1984). NBR 8633: Carvão vegetal - determinação do poder calorífico [NBR 8633: Charcoal - determination of heating value]. Associação Brasileira de Normas Técnicas, Rio de Janeiro, Brazil, pp. 1-13. [in Portuguese] Gscholar

(4) ABNT (2003). NBR 11941: Madeira - determinação da densidade básica [NBR 11941: Wood - Determination of basic density]. Associação Brasileira de Normas Técnicas, Rio de Janeiro, Brazil, pp. 1-6. [in Portuguese] Gscholar

(5) Benin CC, Watzlawick LF, Hillig E (2017). Propriedades físicas e mecnicas da madeira de Eucalyptus benthamii sob efeito do espaçamento de plantio [Physical and mechanical properties of Eucalyptus benthamii wood under the effect of planting spacing]. Ciência Florestal 27 (4): 1375-1384. [in Portuguese]

(6) Brand MA, Muniz GIB (2010). Influência da época de colheita da biomassa florestal sobre sua qualidade para a geração de energia [Influence of the harvest season over the quality of wood fuel]. Scientia Forestalis 38 (88): 619-628. [in Portuguese] Gscholar

(7) Brito AS, Vidaurre GB, Oliveira JTS, Silva JGM, Rodrigues BP, Carneiro ACO (2019). Effect of planting spacing in production and permeability of heartwood and sapwood of eucalyptus wood. Floresta e Ambiente 26 (1): e20180378.

(8) Carneiro ACO, Vital BR, Frederico PGU, Figueiró CG, Fialho LF, Silva CMS (2017). Caracterização energética das madeiras de clones de Eucalyptus cultivados em diferentes localidades [Energetic characterization of the wood from Eucalyptus clones grown in different localities]. Brazilian Journal of Wood Science 8 (3): 127-135. [in Portuguese]

(9) Castro AFNM, Castro RVO, Carneiro ACO, Lima JE, Santos RC, Pereira BLC, Alves ICN (2013). Análise multivariada para seleção de clones de eucalipto destinados à produção de carvão vegetal [Multivariate analysis for the selection of eucalyptus clones destined for charcoal production]. Pesquisa Agropecuária Brasileira 48 (6): 627-635. [in Portuguese]

(10) COPANT (1974). Descripción de características generales, macroscópicas de las maderas angiospermas dicotiledôneas [Description of general, macroscopic characteristics of dicotyledonous angiosperm woods]. Comisión Panamericana de Normas Técnicas, La Paz, Bolivia, pp. 1-19. [in Spanish] Gscholar

(11) Eisenbies MH, Volk TA, Espinoza J, Gantz C, Himes A, Posselius J, Shuren R, Stanton B, Summers B (2017). Biomass, spacing and planting design influence cut-and-chip harvesting in hybrid poplar. Biomass and Bioenergy 106: 182-190.

(12) Elerati TL (2017). Espaçamento, genótipo, idade e a produção de biomassa em povoamentos de eucalipto [Spacing, genotype, age and biomass production in eucalypt stands]. Dissertação, Departamento de Engenharia Florestal, Universidade Federal de Viçosa, Viçosa, MG, Brazil, pp. 90. [in Portuguese] Gscholar

(13) Eloy E, Silva DA, Schmidt D, Trevisan R, Caron BO, Elli EF (2016). Effect of planting age and spacing on energy properties of Eucalyptus grandis W. Hill EX Maiden. Revista Árvore 40 (4): 749-758.

(14) Fernandes DE, Gomide JL, Colodette JL, Ferreira MZ (2011). Influência da produtividade de clones híbridos de eucalipto na densidade da madeira e na polpação Kraft [Influence of eucalypt hybrid clones productivity on wood basic density and kraft pulping yield]. Scientia Forestalis 39 (90): 143-150. [in Portuguese] Gscholar

(15) Feuchard LD (2015). Influência do espaçamento de plantio e idade de colheita na qualidade da madeira de eucalipto para celulose [Influence of planting space and harvest age on eucalypt wood quality for pulpwood]. Dissertação, Departamento de Ciências Florestais e da Madeira, Universidade Federal do Espírito Santo, Jerônimo Monteiro, ES, Brazil, pp. 42. [in Portuguese] Gscholar

(16) Goldschimid O (1971). Ultraviolet spectra. In: “Lignins” (Sarkanen KV, Ludwing CH eds). Wiley Interscience, New York, USA, pp. 241-266. Gscholar

(17) Gomide JL, Demuner BJ (1986). Determinação do teor de lignina em material lenhoso: método Klason modificado [Determination of lignin content in woody material: modified Klason method]. O Papel 47 (8): 36-38. [in Portuguese] Gscholar

(18) Gort J, Gerendiain AZ, Peltola H, Pulkkinen P, Routa J, Jaatinen R (2009). Differences in fibre properties in Scots pine (Pinus sylvestris L.) genetic entries grown at different spacing and sites. Silva Fennica 43 (3): 355-368.

(19) Goulart M, Haselein R, Hoppe JM, Farias JA, Pauleski DT (2003). Massa específica básica e massa seca de madeira de Eucalyptus grandis sob o efeito do espaçamento de plantio e da posição axial no tronco [Basic density and dry mass of wood of Eucalyptus grandis as affected by tree spacing and trunk position]. Ciência Florestal 13 (2): 167-175. [in Portuguese]

(20) Guedes TO, Lima LC, Hein PRG, Silva JRM, Santos LDT (2018). Effect of planting density on wood anatomy in Eucalyptus and Acacia from Brazil. Madera y Bosques 24 (2): e2421420.

(21) Hébert F, Krause C, Plourde PY, Achim A, Prégent G, Ménétrier J (2016). Effect of tree spacing on tree level volume growth, morphology, and wood properties in a 25-year-old Pinus banksiana plantation in the boreal forest of Quebec. Forests 7 (276): 1-16.

(22) Jardim JM, Gomes FJB, Colodette JL, Brahim BP (2017). Avaliação da qualidade e desempenho de clones de eucalipto na produção de celulose [Evaluation of the quality and performance of eucalyptus clones in cellulose production]. O Papel 78 (11): 122-129. [in Portuguese] Gscholar

(23) Karlsson L, Mörling T, Bergsten U (2013). Influence of silvicultural regimes on the volume and proportion of juvenile and mature wood in boreal Scots pine. Silva Fennica 47: 1-17.

(24) Lasserre JP, Mason EG, Watt MS (2005). The effects of genotype and spacing on Pinus radiata [D. Don] corewood stiffness in an 11-year old experiment. Forest Ecology and Management 205: 375-383.

(25) Lasserre JP, Mason EG, Watt MS, Moore JR (2009). Influence of initial planting spacing and genotype on microfibril angle, wood density, fibre properties and modulus of elasticity in Pinus radiata D. Don corewood. Forest Ecology and Management 258: 1924-1931.

(26) Moore J, Achim A, Lyon A, Mochan S, Gardiner B (2009). Effects of early re-spacing on the physical and mechanical properties of Sitka spruce structural timber. Forest Ecology and Management 258: 1174-1180.

(27) Moulin JC, Arantes MDC, Vidaurre GB, Paes JB, Carneiro ACO (2015). Efeito do espaçamento, da idade e da irrigação nos componentes químicos da madeira de eucalipto [Effect of spacing, age and irrigation on chemical components of Eucalyptus wood]. Revista Árvore 39 (1): 199-208. [in Portuguese]

(28) Moulin JC, Rocha MFV, Arantes MDC, Boschetti WTN, Jesus MS, Trugilho PF (2017). Influência do espaçamento de plantio e irrigação na densidade e na massa seca em espécies de Eucalyptus [The influence of plant spacing and irrigation on density and dry mass on Eucalyptus species]. Nativa 5: 367-371. [in Portuguese]

(29) Nath CD, Munoz F, Pélissier R, Burslem DFRP, Muthusankar G (2016). Growth rings in tropical trees: role of functional traits, environment, and phylogeny. Trees 30: 2153-2175.

(30) Pachas ANA, Sakanphet S, Soukkhy O, Lao M, Savathvong S, Newby JC, Souliyasack B, Keoboualapha B, Dieters MJ (2019). Initial spacing of teak (Tectona grandis) in northern Lao PDR: impacts on the growth of teak and companion crops. Forest Ecology and Management 435: 77-88.

(31) Protásio TP, Goulart SL, Neves TA, Trugilho PF, Ramalho FMG, Queiroz LMSB (2014). Qualidade da madeira e do carvão vegetal oriundos de floresta plantada em Minas Gerais [Wood and charcoal quality from planted forest in Minas Gerais State, Brazil]. Pesquisa Florestal Brasileira 34 (78): 111-123. [in Portuguese]

(32) Ramalho RS (1987). O uso de macerado no estudo anatômico de madeira [The use of macerate in the anatomical study of wood]. UFV, Viçosa, Brazil, pp. 4. [in Portuguese] Gscholar

(33) Resquin F, Navarro-Cerrillo RM, Carrasco-Letelier L, Casnati CR (2019). Influence of contrasting stocking densities on the dynamics of above-ground biomass and wood density of Eucalyptus benthamii, Eucalyptus dunnii, and Eucalyptus grandis for bioenergy in Uruguay. Forest Ecology and Management 438: 63-74.

(34) Rocha MFV, Vital BR, Carneiro ACO, Carvalho AMML, Cardoso MT, Hein PRG (2016). Effects of plant spacing on the physical, chemical and energy properties of Eucalyptus wood and bark. Journal of Tropical Forest Science 28 (3): 243-248. Online | Gscholar

(35) Santos RC, Carneiro ACO, Vital BR, Castro RVO, Vidaurre GB, Trugilho PF, Castro AFNM (2016). Influência das propriedades químicas e da relação siringil/guaiacil da madeira de eucalipto na produção de carvão vegetal [Effect of chemical properties and syringyl/guaiacyl ratio of wood clones of Eucalyptus on the production of charcoal]. Ciência Florestal 26 (2): 657-669. [in Portuguese]

(36) Silva JC, Tomazello Filho M, Oliveira JTS, Castro VR (2007). Influência da idade e da posição radial nas dimensões das fibras e dos vasos da madeira de E. grandis Hill ex. Maiden [Influence of age and radial position on fiber and vessel dimensions of E. grandis Hill ex. Maiden wood]. Revista Árvore 31 (6): 1081-1090. [in Portuguese]

(37) Souza CO, Silva JGM, Arantes MDC, Vidaurre GB, Dias Júnior AF, Oliveira MP (2020). Pyrolysis of Anadenanthera peregrina wood grown in different spacings from a forest plantation in Brazil aiming at the energy production. Environment, Development and Sustainability 22: 5153-5168.

(38) TAPPI (2006). T280 pm-99 standard: acetone extractives of wood and pulp. Technical Association of the Pulp and Paper Industry, Atlanta, USA, pp. 1-3. Gscholar

(39) Teixeira EC, Lima APL, Lima SF, Soratto DN, Ramos KV, Silva WG (2020). Potencial energético da madeira de clones de eucalipto em diferentes espaçamentos [Energetic potential of eucalyptus clones wood at different planting spacings]. Research, Society and Development 9 (7): e02973733. [in Portuguese]

(40) Torres CMME, Oliveira AC, Pereira BLC, Jacovine LAG, Neto SNO, Carneiro ACO, Torres CMME (2016). Estimativas da produção e propriedades da madeira de eucalipto em sistemas agroflorestais [Estimates of production and properties of eucalyptus wood in agroforestry systems]. Scientia Forestalis 44 (109): 137-148. [in Portuguese]

(41) Watson P, Garner C, Robertson R, Reath S, Gee W, Hunt K (2003). The effects of initial tree spacing on the fibre properties of plantation-grown coastal western hemlock. Canadian Journal of Forest Research 33 (12): 2460-2468.