Survival, Tree height and Biomass
The growth performance and productivity of trees can be determined by tree genetic qualities, edaphic conditions, environmental situations and management factors (Lugo et al., 1988). A. abyssinica and E. camaldunesis had the highest survival rates than other species. .Acacia polyacantha also showed comparable survival rates. A. decurrens, A. lophantha, A. melanoxylon and E. globulus had shown lowest survival rates in this study. Animal grazing could be another reason for low survival of A. decurrens and A. lophantha. These two species were browsed and damaged by animals especially after 15 months of planting. The poorest performance of the rest of tree species may be related to low moisture in the experimental site. E. globulus also occurs mostly in wetter and high rainfall areas (Coppen, 2003) especially above 2000 m (Maundu and Tengnas, 2005). A. decurrens tolerates acidity and grows better in light, deep and drained fertile soils (Ruskin, 1980), but the experimental site was poor in fertility.
Height was different among tested species. A. decurrens, A. polyacantha, E. camaldulensis, and E. globulus showed better performance in terms of height which is in line with the study of Tesfaye et al.(2015). This study reported that E. globulus had highest height soil management i.e. mulching compared to other species. Similarly, Mekonnen et al. (2006) reported that E. globulus and A. decurrens scored the highest growth in 64m on the sold as compared to other indigenous and exotic trees and shrubs species. However, A. abyssinica showed the lowest growth performance. Biomass is one of the most important criteria for fuelwood production. Our study showed also a variation in tree biomass between tree species. A. polyacantha and A. decurrens had the highest biomass relative to other species. However, the result of this study was lower than Mekonnen et al. (2006) reported A. decurrens provided 29.9 kg/tree and E. globulus with 10.4 kg/tree. However, this study showed that A. decurrens and E. globulus provided 3.8 kg/tree and 3.1 kg/tree, respectively. The low biomass in our study was related to poor fertility, soil moisture and tree management.
Wood moisture, density, and ash
Acacia decurrens had the lowest moisture content compared to the rest of the studied tree species. Lower moisture content increases the net calorific value of wood for heating (Senelwa and Sims, 1999). High moisture in wood decreases the heating value of fuelwood (Demirbas, and Demirbas, 2009). When wood fuel has moisture, it absorbs high energy during combustion and this resulted in heat escaping (Ince, 1979). One of the limiting issues in fuelwood combustion is moisture because of its effect on heating value. Because the combustion reaction is exothermic, but water evaporation is highly end other moisture content (Jenkins et al., 1998). Self-supporting combustion for the majority of biomass–based fuels is about 65% moisture content on a wet basis (Jenkins et al., 1998). Moreover, most of the energy is used to evaporate water rather than combustion (Jenkins et al., 1998). The results in our study showed that moisture content was below 65% on a fresh basis for all tree species. In biomass burning some of the energy is used to change water to vapor, this reduces the availability of energy for heating. Therefore, moisture content determines the energy value of biomass (Rosillo-Calle, 2012).
Wood density varied among the tested tree species. A. decurrens has the highest density and the lowest density was recorded in A. abyssinica. The density of A. decurrens in our study was 0.76 g/cm3 and this is higher than Searle and Owen (2005) report i.e., 0.64 g/cm3. High-density characteristics make A. decurrens an excellent for firewood (National Research Council (US), 1980). The species suitable for fuelwood have a density of 0.55-0.58 g/cm3 as indicated by Kumar et al. (2010). The density in E. globulus (0.49 g/cm3) and E. camaldunesis (0.48 g/cm3) in our study was less than Kumar et al. (2010). Higher basic density is an important parameter to fuelwood characteristics. Trees with the highest density can generate large energy per unit volume (Groves and Chivuya, 1989).
Ash content is one of the important factors which directly affect the heating value of firewood (Demirbas, 2003). The average ash content observed in this study was 4% which is higher than Alemu (1997) reported 1.86% for Acacia abyssinica, Acacia seyal, Albizia gummifera, Croton macrostachyus, Grevillea robusta, and Millettia ferruginea. The result of our study was also higher than Friedl et al. (2005) who reported 2.9% for wood materials. The lowest ash content was observed in A.decurrens and E.globulus in our study. Ash value in A.decurrens was 2.2%, which is lower than 2.7% in Hybrid poplar reported by Jenkins et al. (1998). High ash results may be related to wood bark. Ash contents in wood bark are high compared to ash contents in wood tissues (Werkelin et al., 2005). Woods with less than 1% ash have high heating values when ash increases there is a decrease in heating value, even though some elements in the ash may catalyze thermal decomposition (Jenkins et al., 1998). On the ash-free basis, almost all non-woody plant biomass has more or less the same energy value (Rosillo-Calle, 2012).
Lignin and cellulose content
The overall mean value of lignin content in this study was 22.1%. This value was smaller than Demirbas (2001) with an overall average value of 32.0 % for different conifer and broad leaf trees and some crop husks. The overall lignin content was ranged from 14 to 28% in our study and this is different from Rowell (2005) reported that the content of lignin in wood ranges between 24-35%. Our results showed E. globulus has the highest lignin (25.8%) and cellulose content (46.1%) compared to the remaining species and this is supported in other studies. Eucalyptus woods have higher lignin content compared to Acacia woods (Kumar et al., 1992). However, higher lignin content in E. globulus in our study was not in agreement with Rencoret et al. (2007) study which presented the lowest lignin content of E. globulus compared with other studied eucalyptus species. Still the lignin and cellulose content of E. globulus in this study was higher than Stackpole et al. (2011) reported 20.5% and 43.4% for lignin and cellulose, respectively. The lowest lignin and cellulose content was found in A. abyssinica in this study. Lignin content mostly and strongly correlated with a higher heating value of wood (White, 2007; Demirbas, 2001; Demirbas, 2002). The higher heating value increases with the increase in lignin and cellulose content. However, the thermal degradation process is complicated and affected by temperature and moisture content. Lignin contains high hydrogen and carbon and high calorific values compared to cellulose and hemi celluloses (Brebu and Vasile, 2010). There is also a correlation between extractive contents and the high heating value of woodfuels (Telmo and Lousada, 2011). Therefore, it is not only lignins that affecting the heating value or calorific value of woodfuels but, also extractives like phenolic compounds and resins have a greater effect (Moya and Tenorio, 2013). High extractive contents directly influence the heating value of the fuelwood material and these extractives also varied from species to species (Demirbas, 2003; Yang and Jaakkola, 2011). Ligno cellulose materials with high extractive contents are preferred for fuelwood because of extractives increase the heating value of the fuelwood (Demirbas, 2002; Telmo and Lousada, 2011). Kataki and Konwer (2001) described heating value affected directly by the content of extractives present in fuelwood. Extractives in Eucalyptus grandis reach up to 4.1% (Poletto et al., 2012) and in E.globulus reaches up to 4.7% (Stackpole et al., 2011). Acacia decurrens contain 74% tannin which is an extractive and part of polyphenol compound (Reid et al., 2013). Extractive content in Acacia meamsii is higher than Eucalyptus. This is due to the presence of tannin and gum substance in Acacia meamsii (Foelkel, 2008). E. globules have 0.26% dichloro methane lipophilic extractives which are lower than A.mangium 1.32% (Neto et al., 2006). In general, higher calorific value due to high lignin content by itself could not be the only criteria to select fuel wood. The low moisture content and higher basic density of wood compensate for low carbon presence in fuelwood (Foelkel, 2008).