Introduction

The cultivation of plantations in the short rotation system (SRC, for its acronym in English for Short Rotation Coppice) has been studied, recently, for dendroenergetic purposes (^{Souza et al., 2015}). Several species of eucalyptus and acacia are considered suitable for the production of large volumes of biomass, when they are grown under that system (^{Camps and Marcos, 2002}). This biomass can partially displace the fossil fuels currently used to produce heat and electricity, which, in turn, helps governments comply with environmental laws and binding commitments, aimed at reducing CO₂ emissions from fossil fuels (^{McKendry, 2002}).

Short-rotation species allow higher planting densities and, therefore, a high biomass yield per unit area (^{Hoogwijk et al., 2005}). Regrowth, normally, is stimulated in the spring and grows again, thereby avoiding replanting costs. When rotation periods are too short for a taxon and variety, shoot growth can be hampered by depletion of stored carbohydrate reserves in the root system, which also affects the structure of wood (^{Al Afas et al., 2008}).

Numerous woody taxa, cultivated in short rotation system have become important in the supply of plant biomass (^{Hoogwijk et al., 2005}). Among the most important dendro-energy species worldwide are the eucalyptus (Eucalyptus globulus Labill, E. nitens H. Deane & Maiden, E. denticulata I. O. Cook & Ladiges) (^{Camps and Marcos, 2002}), willow (Salix spp.), poplar (Populus spp.) and acacias (Acacia dealbata Link, A. melanoxylon R Br., A. retinodes Schltdl.) (^{Ríos et al., 2016}). These record high productivity and can be promoted as fast-growing forest plants (^{Hoogwijk et al., 2005}).

Basic density is one of the most important physical properties of wood, because it expresses the amount of dry woody substance in a given volume of wood, when it has a moisture content equal to or higher than the fiber saturation point (FSP). Therefore, practically all mechanical properties are related to such variable, which is why it is used in the prediction of plant material resistance (^{Cisternas, 1994}).

The detailed analysis of the properties of biomass is essential, due to the need to adapt the nature of wood resources to the specific requirement of modern energy conversion technologies. This type of analysis helps to identify plant components that correlate with the characteristics of wood used as fuel (^{Davis et al., 1984}, ^{Butner et al., 1988}). Although there are studies in which the variability in the properties of biomass was evaluated for this purpose, few consider the differences in the properties of the wood of trees when they are cultivated under the short rotation system.

The future applications of bioenergy will be increasingly dependent on the production of biomass in energy plantations under the short rotation system (SRC). The above is a response to the growing demand for heat and electricity from renewable sources, strict environmental regulations and policies aimed at protecting natural resources (^{Senelwa and Sims, 1999}). The objective of the present study was to determine the basic density of wood and higher heating value in two-year-old shoots from stumps of three wood energy crops.

Materials and Methods

Experimental design

A randomized complete block design with a factorial arrangement was used, which included two factors: plant density factors (5 000, 10 000 and 15 000 trees ha^-1) and three plant species (A. dealbata, E. globulus and E. denticulata); each block was a square of 110 m in each side (12 100 m²), made up of four quadrants, each consisting of nine experimental units of 18 m per side (324 m²). Each unit, in turn, consisted of a buffer zone to avoid the edge effect, and a core of 30 useful stumps) (Figure 1). The tree species were managed under four cutting frequencies. The quadrant corresponds to one of four cutting frequencies (1, 2, 3 and 4 years); the first intervention at the first year of establishment of the crop was made to evaluate the response of the regrowth and the elemental properties of the shoots. This work was carried out in the quadrant harvested in December 2012 corresponding to the two -year frequency (Figure 1).

Each color represents a quadrant and a cutting frequency (1, 2, 3 and 4 years). The letters (A, B, C) refer to the species; and the numbers (5,10 and 15), the density of plantation.

Figure 1 Distribution and design of the experimental units in the study site. 

Results

In the components of the biomass, highly significant differences (p ≤ 0.01) were detected between species for most of the assessed variables and between densities for the number of offshoots (Table 1). The plantation density had a reduced influence on most of the variables included in the study, and only in the number of shoots a significant effect was observed. The interaction species × density showed significance in the HHV and plant height, which was related to the modification of the response of at least one species, when moving from one density to another.

Eucalyptus denticulata recorded the highest basic density of the wood in the three planting densities, with values between 0.46 and 0.49 g cm^-3 (Table 2). The rest of the species showed similarity for the basic density, with values between 0.44 and 0.45 g cm^-3 for E. globulus and A. dealbata. Only in the adventitious shoots of A. dealbata low BD values were obtained, with 0.38 g cm^-3, especially in the highest planting density (15 000 trees ha^-1).

Acacia dealbata recorded the largest number of proventitious shoots in all planting densities, with fluctuations of 7.00 shoots per stump in 5 000 trees ha^-1 and 4.00 shoots per stump in 15 000 trees ha^-1. It was observed that A. dealbata shows a tendency to develop proventitious shoots and, only when its number decreases, then adventitious type shoots emerge. The two species of eucalyptus showed tendency to generate both types of shoots, with some predominance of the adventitious ones, especially in E. denticulata. In addition, negative effects were determined in both types of shoots, especially in E. globulus, in which the number was reduced as plant density increased.

The highest value for the diameter at neck height (Dac) was for E. globulus with 60.09 mm, followed by E. denticulata with 49.70 mm and the lowest was obtained by A. dealbata, with 32.45 mm. These values corresponded to the proventitious shoots and the lowest planting density (5 000 trees ha^-1). Dac decreased as plantation density increased (Table 2), which was statistically significant in E. globulus. In this species there was a significant increase of the Dac in the adventitious shoots, as the density of plantation increased, so that with 15 000 trees ha^-1 reached 25.85 mm. The above coincided with the reduction in the number of scions registered in that density of plantation.

Higher height values were present in the proventitious shoots of E. globulus and this increased as the density of the plantation increased, from 7.06 m in 5 000 trees ha^-1, up to 7.96 m with 15 000 trees ha^-1. In the adventitious shoots, an increase in height was observed between the densities of 5 000 trees ha^-1 (2.5 m) and 15 000 trees ha^-1 (3.53 m). E. denticulata showed intermediate values, while A. dealbata the lowest values in the proventitious shoots (3.19 m to 4.24 m) and adventitious (1.66 m). In all the species dominance of the proventitious was observed, which showed higher values in number, diameter and shoot height.

The leaves of the dominant shoots (Prov 1 and Prov 2) registered, in general, the highest calorific value in the three species studied (Figure 2). The highest values reached 5 280 kcal kg^-1 for E. globulus, 5 150 kcal kg^-1 in E. denticulata and 4 927 kcal kg^-1 in A. dealbata. The HHV of the stem and branches showed similarity in A. dealbata with values between 4 573 kcal kg^-1 - 4 691 kcal kg^-1; and E. denticulata 4 508 kcal kg^-1 - 4 558 kcal kg-1. E. globulus had the lowest, with 4 399 kcal kg^-1 for stems and 4 434 kcal kg^-1 in branches. The adventitious shoots showed a caloric decrease of up to 400 kcal kg^-1 in E. globulus (Figure 2).

Poder calorífico superior = Higher heating value; Árboles = Trees; Tipo y número de rebrote = Type and number of shoot

Figure 2 Higher heating value (HHV) per component (stem, branch and leaf) of proventitious (Prov) and adventitious (Adv) shoots in first rotation stumps. 

The calorific value in re-shooted stumps presented differences in their dominant (proventitious) and codominant (adventitious) shoots. The two dominant proventitious scions (Prov 1 and Prov 2, with high values of Dac and height) recorded the highest calorific value in stems, branches and leaves, with a tendency to decrease according to their vigor within the stump (Figure 2). The species with the greatest variation was E. globulus, with a decrease of up to 641.9 kcal kg^-1 HHV in leaves, and 214 kcal kg^-1 in stems and branches (Figure 2). The planting density lacked direct significant influence on the calorific value, under the short rotation system.

Discussion

The results for the basic density of wood in shoots of first rotation agree with the studies realized in plantations of single-stem species, of more advanced age. In some of these works the basic density of Acacia dealbata and the influence of the plantation localities were assessed, with values between 0.32 g cm^-3 and 0.54 g cm^-3 (^{Pinilla and Hernández, 2010}). In the specific case of localities, with edaphoclimatic characteristics similar to those in this study, a density of 0.47 g cm^-3 was obtained in eucalyptus plants at five years old (^{Pinilla and Hernández, 2010}).

In an investigation in which two eucalyptus clones were included, with 57 and 69 months of age, average values similar to those recorded in the present study were documented (0.46 g cm^-3) (^{Protásio et al., 2014}). Evaluations of the basic density in 37 provenances of E. globulus, with seven years of age, variations of 0.43 to 0.49 g cm^-3 were observed (^{Miranda et al., 2001}). In the present study, E. denticulata had the highest basic density of wood under the short rotation system. The results are similar to what was recorded for nine-year-old plantations, in which the wood characteristics of some Eucalyptus species in Brazil were assessed; an average basic density of 0.48 g cm^-3 is indicated for E. denticulata, at the age of 4.5 years (^{Duarte et al., 2000}).

The short rotation system showed BD similar to that observed in traditional monofustal plantations, and the shoots maintain the properties of the wood, mainly those of the dominant type (proventitious). Based on the results, it was confirmed that the basic density is an important characteristic of wood and should be considered as a criterion for the selection of biomass sources, since it is directly related to the production of energy per unit volume (^{Protásio et al., 2014}).

There are several references for the basic density of wood and the higher calorific value in the literature, which have been evaluated in the first rotation and in traditional single-stem plantations (^{Vargas et al., 2005}; Espina, 2006; ^{Peredo et al., 2007}; ^{Igartúa and Monteoliva, 2009}; ^{Igartúa et al., 2015}; ^{Batista et al., 2016}). However, studies of the relationship between the basic density and the calorific value of biomass in short rotation crops are relatively recent in Chile, which explains why a greater number of evaluations is required. The results of the joint study of both variables can be used to define the criteria for the selection of species with higher yield, quality and productive stability of the biomass for a given location and density of plantation.

A. dealbata showed the tendency to develop proventitious shoots in low densities of plantation (5 000 trees ha^-1) and, only, when the dominance of these decreases, then the adventitious emerge. This response is attributed to the competition for space and nutrients inter to intra stumps, as well as to the number of shoots, initial growth and characteristics of the species (^{Ríos et al., 2017}). The two species of eucalyptus recorded a tendency to generate both types of shoots, with some predominance of the adventitious ones, especially in E. denticulata. In addition, negative effects were determined in both types of shoots, in particular in E. globulus, in which the number was reduced as the density of planting increased. This reaction was related to the tendency to natural elimination of weak shoots, which reduce their growth due to competition for space and solar radiation (^{Ríos et al., 2017}).

E. globulus grew more in Dac and height, in contrast to the other two species. The results agree with that reported in other studies that evaluated the vigor of shoots of E. globulus, E. viminalis and E. regnans, whose highest values correspond to the diameter and height in the shoots of E. globulus (^{Geldres et al., 2004}). Although several authors have emphasized the adaptability of Acacia species in unfavorable environmental conditions (^{McKinnell, 1990}; ^{Hussain and Gul, 1991}; ^{Sandoval et al., 2012}), A. dealbata showed a low level of growth, due to the moment of cutting. and extraction of biomass from the first rotation. Likewise, it evidenced a high number of shoots, so it is advisable to adjust the management of thinnings and prunings to make the use of solar radiation more efficient and facilitate the obtaining of high quality wood (^{Pinilla and Navarrete, 2010}).

The variations of the HHV registered between species and components (stem, branch and leaves) were related to the differences in the chemical composition of the biomass (^{Senelwa and Sims, 1999}). The two species of Eucalyptus showed similar elementary properties for the energy potential of their biomass (Figure 2). In assessments of the characteristics of the fuels from E. globulus and A. dealbata, managed with the short rotation system, a calorific power higher than 4 705 kcal g^-1 was obtained in trees with three years old (^{Senelwa and Sims 1999}), similar to the results documented with two-year-old shoots.

In monostem A. dealbata plantations with ages of four to six years, the stems registered low levels of HHV in different localities of Chile, with values of 3 909 kcal kg^-1 to 4 288 kcal kg^-1 (^{Pinilla and Navarrete, 2010}). The components stem, branches and leaves showed differences in their calorific value, which agrees with what is documented in literature, in which quotes high values of calorific power in the leaves of E. globulus, with values of 5 730 kcal kg^{- 1} (^{Senelwa and Sims, 1999}). The above, surpasses the results of the present study for the three species, in which the leaves in the proventitious shoots had a calorific power between 5 100 and 5 300 kcal kg^-1.

The number of shoots and their level of dominance influenced the HHV of wood in all the analyzed taxa. There was a tendency to decrease HHV in stumps with a high number of shoots and when they were less dominant, growth and lignification was lower, with respect to the dominant ones. This was most evident in A. dealbata, which recorded the highest number of offspring, and this negatively influenced its growth, lignification and HHV.

The variation in the HHV was more noticeable in branches and leaves, while the stems showed greater stability, in relation to the rest of the organs. The proventitious shoots of E. denticulata lose up to 28 % of their calorific value in the component leaves, branches and stems (Figure 2), especially with a planting density of 10 000 trees ha^-1, in which it was observed, also that the adventitious offspring reduced their HHV due to the level of dominance.

A. dealbata had a higher proportion of proventitious shoots that maintained their HHV (Figure 2), which was considered a typical property of most acacias, due to their type of cespitose growth. Only in the density of 10 000 trees ha^-1 was gradual reduction of the HHV as the level of stem dominance was decreased, which was more noticeable in the leaves and branches. The results show that priority must be given to the management of proventitious shoots, leaving two or three per stump, which will obtain high values of calorific value, supported by the chemical composition of the biomass (^{Susott et al., 1975}; ^{Murphey and Masters, 1978}).

Conclusions

In the three studied taxa under the short rotation system (SRC) considerable variation was observed in the basic density of wood and higher heating value. The variation between species is related to the density of plantation, which influences the response of each taxon in terms of the number of shoots and the growth of these, measured in Dac and height. The high values from the number of shoots negatively affect the basic density and the calorific value; therefore, it is advisable to carry out thinnings and prunings. The short rotation system favors the reduction of harvest turns, which increases the biomass per unit area without losing its elementary properties, especially when two stems of the proventitious type are used per stump. The two eucalyptus species evaluated, and to a lesser extent, A. dealbata, have characteristics that favor their use in the production of biomass, from shoots, for wood energy purposes.