Energy production and its characteristics from four tropical trees species planted in short rotation woody systems in Costa Rica

Tenorio, C., Moya, R., Murillo, O., and Loría, J. (2024). “Energy production and its characteristics from four tropical trees species planted in short rotation woody systems in Costa Rica,” BioResources 19(1), 695-715.

Abstract

Short-rotation crop (SRC) systems with woody species have been planted in Costa Rica. However, information about different tree species and spacing is limited. The objective of the present study was to examine biomass production and the physical, energy, and chemical properties of feedstock of four tropical tree species (Cordia alliodora, Dipteryx panamensis, Gmelina arborea, and Tectona grandis) in 34-month-old plants planted at four spacings (0.5×1.0 m, 1.0×1.0 m, 1.0×2.0 m, and 2.0×2.0 m). The highest mortality rate (50%) was found in G. arborea; however, diameter, height, basal area, and biomass production in G. arborea plantations were higher than T. grandis, C. alliodora, and D. panamensis. Spacing effects on diameter, height, basal area, and biomass production were observed in 10-month-olds. Wide spacing presented the highest values in diameter and height, but the highest biomass production was found in the narrow spacing. Also, biomass distribution was different in D. panamensis in relation to other species. Specify gravity, density, and moisture content of biomass showed high variation between species and spacing, but the energy and chemical properties of biomass showed few differences. These results suggest that these four species were uniform in terms of their energy and chemical properties, regardless of the spacing used. Finally, three species (G. arborea, C. alliodora, and T. grandis) presented important potential for use in SRC systems. G. arborea was the species with the highest production but a high mortality rate.

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Energy Production and its Characteristics from Four Tropical Trees Species Planted in Short Rotation Woody Systems in Costa Rica

Carolina Tenorio,^a Roger Moya,^b,* Olman Murillo,^c and Jonathan Loría^d

DOI: 10.15376/biores.19.1.695-715

Keywords: Bioenergy; Energy potential; Firewood; Spacing; SCR; Tropical species

Contact information: a: Instituto Tecnológico de Costa Rica, Escuela de Ingeniería Forestal, P.O. Box: 159-7050 Cartago-Costa Rica. e-mail: ctenorio@itcr.ac.cr; (C.T.) ORCID: 0000-0003-2901-7079; b: Instituto Tecnológico de Costa Rica, Escuela de Ingeniería Forestal, P.O. Box: 159-7050 Cartago-Costa Rica. (R.M.) ORCID: 0000-0002-6201-8383; c: Instituto Tecnológico de Costa Rica, Escuela de Ingeniería Forestal, P.O. Box: 159-7050, Cartago-Costa Rica. e-mail: omurillo@itcr.ac.cr, (O.M.) ORCID: 0000-0003-3213-8867; d: Instituto Tecnológico de Costa Rica, Escuela de Ingeniería Forestal, Email: jonathanlm.cr@gmail.com, P.O. Box: 159-7050 Cartago-Costa Rica. (J.L.); *Corresponding author: rmoya@itcr.ac.cr

GRAPHICAL ABSTRACT

INTRODUCTION

Biomass is one of the main renewable energy resources due to its great potential, economic viability, renewability, and various social and environmental benefits (Amjith and Bavanish 2022). The energy obtained from biomass is called primary energy, which means that the energy comes from perennial plants (arboreal or not) and is used to produce biomass for energy (Giudicianni et al. 2021). Trees produced in short rotation crops (SRC) are an important potential source of biomass, which can be used as solid fuel with different shapes (firewood, wood chips, pellet) or processes, including thermic or thermo-chemical (torrefaction, charcoal, gasification, liquation) (Rodrigues et al. 2021). SRC are a silvicultural system based on short rotation, less than 1 to 15 years (Eisenbies et al. 2021). Genetically superior trees and intensive management such as fertilisation, irrigation, and weed control are implemented in these systems (Johnston et al. 2022).

Woody species planted under SRC systems have been established in many countries around the world, especially in the European Union and North America (Djomo et al. 2015; Ile et al. 2022). However, limited experience in establishing SRC is found in the Latin America area (Moya et al. 2019; Silva et al. 2022); little by little, they have gained importance in countries including Chile (Yáñez et al. 2019), Brazil (Santos and Reichert 2022), and some Central American countries such as Costa Rica (Tenorio et al. 2019a,b). There are different species that can be used in SRC systems (Dickmann 2006). In temperate climates, there are Populus, Salix, Pinus, Robinia, and Eucalyptus in the EU region (Amichev et al. 2014; Djomo et al. 2015; Pleguezuelo et al. 2015), and in Latin America, a great diversity of species in the SRC, between 35 and 40 species (Moya et al. 2019), probably due to the great adaptability of the species to different soil conditions (Rockwood et al. 2019).

Costa Rica, a small country in Central America, developed its energy matrix around renewable sources (Hernández-Chaverri and Buenrostro-Figueroa 2021). Biomass represents 14% of the different energy sources (Popp et al. 2021), and a high percentage of this biomass comes from agricultural waste (Alonso et al. 2017). However, a characteristic of biomass from agricultural waste is that it presents a high moisture content, and in addition, its collection system is expensive (Abuelnuor et al. 2014). While forest biomass has a significant volume (Valverde et al. 2021) and potential as a renewable energy source, its use is limited (Moya et al. 2019), even though woody feedstock is more efficient at conversion to energy than many agricultural residues (Alonso et al. 2017).

Woody biomass currently comes from many sources in Costa Rica, among them residues in the plantation during logging, residues in primary sawmills, and short rotation (SRC) plantation systems (Valverde et al. 2021). The research and knowledge of species used in SRC are recent, and few species have been tested and planted (Moya et al. 2019). Notable species include Gmelina arborea (Salazar-Zeledón 2016; Tenorio et al. 2016, 2018; 2019a; Tenorio et al. 2019b), Eucalyptus saligna and Eucalyptus camaldulensis (Navarro-Camacho et al. 2014), and Eucalyptus tereticornis (Valverde et al. 2022).

For G. arborea in SCR, Tenorio et al. (2016 and 2018) reported that, during the first two years, 0.5×1.0 m and 1.0×1.0 m spacing presented higher growth in diameter and height and biomass production than 2.0×1.0 m spacing. In addition, these authors point out that the site is the main factor in the production of biomass and that the energy characteristics of the trees growing in SRC vary widely from one year to another. In another study carried out on G. arborea in SRC at 24 months of age with three spacings (1.0×1.0 m, 0.75×0.75 m, and 1.0×0.5 m) but using two clones used for wood production, the effect of the clone was found in the mortality, the morphological characteristics of the trees, the leaf area, the biomass production, and a few variations in energy biomass characteristics (Tenorio et al. 2019b).

In other species tested in SRC, such as Dipteryx panamensis, the chemical, physical, and energy properties of the biomass were evaluated at three different spacings (1.0×2.0 m, 1.0×0.5 m, and 1.0×1.0 m) at the age of 3 years (Gaitan-Alvarez et al. 2020). Increasing the spacing increased the plant’s green density and moisture content, and 1.0×1.0 m and 1.0×0.5 m spacings were found to produce the highest biomass (Gaitan-Alvarez et al. 2020). The other two species, Tectona grandis and Cordia alliodora have been used in fast-growing plantations for commercial reforestation in Costa Rica for sawlog production (Moya et al. 2019); therefore, information on the processes and properties of these species is abundant and known (Moya et al. 2019). However, the use of these species in the SRC system lacks information about biomass production and its energy properties (Shukla and Viswanath 2021).

The objective of this study was to determine the growth (mortality, diameter, height, and basal area), biomass production, and physical, energy, and chemical properties of four forest trees species (C. alliodora, D. panamensis, G. arborea, and T. grandis) planted under SRC at the age of 36 months, using four planting spacings (0.5×1.0 m, 1.0×1.0 m, 1.0×2.0 m, and 2.0×2.0 m). Knowledge of the information just mentioned will allow determination of the energy potential and biomass production of four different species according to spacing and establish the species with best volume productions and best energy characteristics.

EXPERIMENTAL

Geographic Location and Plantation Description

Four different plots of SCR system with Cordia alliodora, Dipteryx panamensis, Gmelina arborea, and Tectona grandis were planted in Florencia (Fig. 1a), San Carlos, Alajuela, Costa Rica (N 10 21´ 29.7,” W 84 28´ 3.4”). The site where plots were planted was divided into three blocks. The site presented a slight slope in a west-to-east direction, so the site was separated into three blocks. For each of the species, four different spacings with three plots were established: 0.5×1.0 m equivalent to 20000 N/ha (S1), 1.0×1.0 m, equivalent to 10000 N/ha (S2), 1.0×2.0 m, equivalent to 5000 N/ha (S3), and 2.0×2.0 m, equivalent to 2500 N/ha (S4). A plot was established in each block and 64 trees were planted (8 trees in width and 8 trees in length). A total of 12 plots were randomly established (Fig. 1b) per species (4 spacings x 3 plots = 12 experimental units).

Fig. 1. Geographic location of SCR system in Costa Rica (a) and distribution of plot for each species in different spacing (b)

Determination of Mortality, Diameter, Height, and Basal Area

A plot was established in the center of every experimental unit, considering 36 trees (6×6 trees), and the trees with 2 external rows and 2 external columns of repetitions were not considered. The diameter and height were measured on 36 trees. A 30 cm diameter was measured from the basal part of the tree. These two parameters and mortality were determined at the ages of 6, 12, 19, 22, 29, and 34 months. The irregularity in the measurement age is attributed to the pandemic period between 2020 and 2021. The percentage of mortality and the basal area of each plot per species were determined. The mortality rate was determined based on the percentage relationship between the number of individuals within each plot and the total number of trees (36 trees) within the plot. The basal area of each plot was calculated using Eq. 1. The average value of the spacing was calculated and extrapolated to the hectare level. Then parameters determined (mortality, diameter, height, and basal area) will make it possible to know the behavior of the species in the trials, and in addition, to know the growing of the best species in the different spacing conditions.

(1)

Tree Sampling and Sampling in the Tree

Biomass sampling was conducted at 34 months of age. Three trees were sampled in each plot, and then 144 trees per species were sampled (4 species x 4 spacings x 3 plots x 3 trees = 144 trees). Before cutting the sample tree, the diameter of 30 cm from the basal part of the tree was measured. After being cut at ground level, branches and leaves were separated from the trunk. Each part (leaves, branches, and trunks) was weighed separately. Then, six 10-cm-long cross sections were extracted from three different heights: two samples from the base of the tree, two samples at total height, and two samples at 50% total height. One sample from each height was used for specific gravity and green density determination, and the other sample was used for the determination of the moisture content of the trunk and bark. All this material was packed in plastic bags to keep it dry. The remaining trunk material was ground to obtain chips not greater than 3 mm long.

Biomass Determination and Energy Production

The biomass of the total trunk was estimated using the moisture content of the trunk (MC_trunk) and its weight in green condition (Eq. 2). The weight of all the leaves and branches after drying was used to determine the biomass for these parts of the tree. The calculated biomass of the trunk (biomass_trunk), leaves (biomass_leaves), and branches (biomass_branch) were used to calculate the percentage distribution of biomass for each of these parts of the tree. In addition, the biomass values obtained were projected to estimate biomass per hectare for each type of plantation spacing,

(2)

where the biomass can be the biomass of trunk, biomass of bark, biomass of branch, or biomass of leaves.

Determination of the Moisture Content, Specific Gravity, and Green Density

To calculate the moisture content (MC), leaves (MC_leave) and branches (MC_branch) in green condition were weighed and then placed in an oven at 103 °C for 24 h and reweighed after that period. The MC was calculated using the weight percent ratio of weight after drying and weight before drying (MC= (weight before drying-weight after drying)/weight after drying x 100). Meanwhile, to calculate the MC of the bark (MC_bark) and the trunk (MC_trunk), the 10 cm samples obtained at three different heights were used after removing the bark from the trunk. Both parts were oven-dried at 103 °C for 24 h. The weight before and after drying was measured and used to find the MC with the percent ratio as described above. The specific gravity (SG) was only determined for the trunk, using the trunk cross-section comprising the xylem and the bark. ASTM D-143 (2022) was used to determine the volume of this cross-section based on its weight and water displacement. Then, the cross-section was placed in the oven for 24 h at 103 °C according ASTM D-143 standard (2022). The SG was determined according to the ASTM D-4442 (2020). In the same manner, the trunk density was determined based on the green weight/green volume ratio.

Determination of the Chemical and Energy Characteristics

The chipped material from the trunks (wood and bark) of the three trees in each experimental unit was combined into one sample. This sample was air-dried to 12% MC. Following this, the material was sieved through 0.25 mm and 0.42 mm meshes (40 and 60 meshes, respectively). The same procedure was used with the branches of the three trees sampled. As for the chemical characteristics, the percentage of carbon (C), nitrogen (N), hydrogen (H), carbon-nitrogen ratio (C/N), and carbon-hydrogen ratio (C/H) was determined with the Elemental Analyzer, Vario Macro Cube model. Then C, N, H, C/N, and C/H content was measured for trunk and branch and named C_trunk, C_branch, H_trunk, H_branch, C/H_trunk, and C/H_branch.

Regarding the energy characteristics, the gross calorific value (GCV) and ash content of the trunk and branch were determined: GCV in the trunk and branch (GCV_trunk and GCV_branch), and ash content in the trunk and branch (Ash_trunk and Ash_branch). In order to determine GCV, a portion (approximately 10 grams) of the sieved material from the trunk (wood and bark) and the branches was dried at 103 °C for 24 h. To find GCV, Parr’s calorific test was used according to the ASTM D-5865 (2019), using five samples weighing 2 g each. As for the ash content, 10 g of the sieved material of each experimental unit (trunk and branches) was used, and the procedure followed ASTM D-1102 (2021).

Again, all characteristics of biomass (production, moisture content, specific gravity and green density, and chemical and energy properties) make it possible to determine which species and different spacing conditions with better energy properties.

Data and Statistical Analysis

Compliance of the variables measured with the assumptions of a normal distribution and homogeneity of the variances, as well as the presence of outliers, were verified. A variance analysis was applied to verify the effect of spacing on each species. Spacing was the independent variable of the model and the variables measured (SG, density, carbon and hydrogen percentage and its ratio, GCV, ash content, and biomass) were the response variables. The Tukey test was used to determine statistical differences between the averages of the variables measured. The variance analysis and the Tukey tests were conducted using SAS software (SAS Institute Inc., Cary, NC).

RESULTS AND DISCUSSION

Mortality and Growth in Diameter and Height

According to Fig. 2, mortality rates are presented based on species and spacing. The S2 spacing presented the highest mortality percentage of all species. C. alliodora had a mortality of around 40% for S2 spacing at 34 months, which is attributed to the loss of a plot after the establishment of the plantation (Fig. 2a). Mortality rates of 22% and 15%, respectively, were observed at S1 and S4 spacings (Fig. 2a). The S3 spacing presented a low mortality rate, less than 8% (Fig. 2a). In D. panamensis, the S2 spacing presented a mortality of 28% at 34 months, followed by the S1, S3, and S4 spacings (Fig. 2b). G. arborea presented the highest mortality in all spacings tested (Fig. 2c). The S2, S3, and S4 spacings showed the lowest values before 12 months; however, an increase in mortality was observed up to 34 months (Fig. 2c). The S1 spacing presented values higher than 15% from the sixth month, up to 44% at 34 months (Fig. 2c). Finally, T. grandis presented the highest mortality (34%) at 34 months at S1 and S2 spacings, followed by S4 and S3 spacings (Fig. 2d).

Fig. 2. Mortality by age and spacing of four tropical species grown in a short rotation system: (a) C. alliodora, (b) D. panamensis, (c) G. arborea, and (d) T. grandis.

The trees’ height and diameter increased with age (Figs. 3 and 4). The spacing effect on the height and diameter was observed after 10 months; the wider spacing showed the highest values of height and diameter (Figs. 3 and 4). Another aspect to highlight is that the S3 and S4 spacings had the most effective development in height and diameter at the end of 34 months in all the species, and between these two spacings, there was little difference in height but a greater difference in diameter (Figs. 3 and 4).

Fig. 3. Average height by age and spacing of four tropical species grown in a short rotation system: (a) C. alliodora, (b) D. panamensis, (c) G, arborea and (d) T. grandis

Fig. 4. Average diameter by age and spacing of four tropical species grown in a short rotation system: (a) C. alliodora, (b) D. panamensis, (c) G. arborea and (d) T. grandis

In general, G. arborea trees presented the highest values of height and diameter, followed by those of T. grandis, C. alliodora, and D. panamensis (Figs. 3 and 4). G. arborea presented variation across age: between 6 and 12 months, the S1 spacing presented the highest average height, but this spacing presented the lowest average after 12 months in relation to the other spacings (Fig. 3c).

Regarding the basal area, G. arborea presented the highest average in each spacing, followed by C. alliodora, T. grandis, and D. panamensis, except for the S1 spacing of T. grandis, which presented an average basal area greater than C. alliodora (Fig. 5). The basal area decreased with spacing in all species. S4 spacing (narrow spacing) presented the lowest average basal area, while S1 spacing presented the highest average (Fig. 5). For G. arborea, the spacings S4 and S3 presented similar basal areas in different tree ages (Fig. 5c). In T. grandis, during the first 8 months, no differences were observed in the basal area at four spacings (Fig. 5d). In addition, after 34 months of measurement for both G. arborea and T. grandis, in the S2 spacing, the basal area decreased (Figs. 5c and d).

Fig. 5. Average basal area by age and spacing of four tropical species growing in a short rotation system: (a) C. alliodora, (b) D. panamensis, (c) G. arborea, and (d) T. grandis

Biomass Distribution

Table 1 presents the biomass averages for the different parts of the tree. The highest averages of Biomass_trunk and Biomass_branch occurred in G. arborea, followed by C. alliodora, T. grandis, and D. panamensis. Regarding Biomass_bark, T. grandis presented the highest average, followed by G. arborea, C. alliodora, and D. panamensis. In Biomass_leaves, D. panamensis presented the highest average, followed by T. grandis, C. alliodora, and G. arborea (Table 1). Statistical differences were observed only between spacings in the biomass_bark of C. alliodora and D. panamensis. S1 presented the statistically highest value and S4 the lowest for C. alliodora and D. panamensis. S1 presented the statistically highest value, and no differences were observed between S2, S3, or S4 (Table 1). In the case of G. arborea, no differences were observed between spacings for any of the different types of biomasses (Table 1). In the case of T. grandis, no differences were observed in biomass_trunk, biomass_branch, and biomass_leaves, the biomass_bark, biosmass_trunk+bark, and biomass_{trunk+bark+branch} had the highest statistical value of S1 spacing, while S4 presented the lowest values of these biomasses (Table 1).

Table 1. Average Biomass at 34 Months per Tree Part and Spacing for Four Tropical Species Growing in a Short Rotation System

Different letters in values of biomass mean statistical differences at 95% in different spacings.

Figure 6 presents the percentage distribution of biomass for the different parts of the tree at 34 months. A different biomass distribution was observed for different types of biomasses for four species, and as expected, the biomass_trunk showed the highest percentage. For C. alliodora, the spacings S1 and S2 presented a similar biomass distribution; the highest percentage corresponded to the biomass_trunk, followed by the biomass_bark, biomass_branch and the lowest percentage was for biomass_leaves. Among these species, the biomass distribution was different in S3 and S4 spacing; biomass_trunk, accounted for the highest percentage, followed by biomass_branch, biomass_bark, and biomass_leaves (Fig. 6a). The biomass distribution of D. panamensis was similar in S1, S2, and S3 spacing; the highest percentage was for biomass_trunk, followed by biomass_leaves, biomass_bark, and the lowest percentage was for biomass_branch. For S4 spacing, the biomass distribution was different; the highest percentage was found in biomass_trunk, followed by biomass_leaves, biomass_branch, and biomass_bark presented the lowest percentage (Fig. 6b). For G. arborea, the highest percentage was in biomass_trunk at four spacings, followed by biomass_branch, biomass_bark, and biomass_leaves (Fig. 6c). T. grandis exhibited the highest biomass_trunk, followed by biomass_bark, biomass_leaves, and biomass_branch at all spacings (Fig. 6d).

Fig. 6. Biomass distribution at 34 months per tree part and spacing for four tropical species grown in a short rotation system: (a) C. alliodora, (b) D. panamensis, (c) G. arborea and (d) T. grandis

Tree Sampling and Sampling in the Tree

Table 2 presents biomass physical properties. The MC of branches presented the highest values, followed by the bark and trunk + bark.

Table 2. Physical Properties of Biomass at 34 Months for Four Tropical Species Growing in a Short Rotation System