Aboveground Biomass Allometric Equations and Fuelwood Properties of Six Species Grown in Ethiopia

In Ethiopia, plantations of fast growing species are being established to increase the supply of wood; especially for the biomass fuel consumption. In order to better inventory the supply of biomass, allometric equations and fuel properties need to be determined. The objectives of this study were i) to develop regression equations of tree component from six species ii) to determine fuel properties and fuel value index (FVI) of the six tree species and iii) to compare combustion characteristics of tree component of the six species using thermogravimetric analysis (TGA). The selected exotic species were Eucalyptus globulus, Eucalyptus grandis and Eucalyptus saligna; and the indigenous were Acacia abyssinica, Acacia seyal and Acacia tortilis. For each species, 24 sample trees (20 for regression and four for validation test) were randomly selected from respective forest stands located in central, western and southern part of Ethiopia. Biometrical data were obtained by felling representative trees stratified into diameter at breast height (DBH) classes. The ANOVA and prediction equations were analyzed by using base-10 log-transformed dry weights (kg) of aboveground biomass (AGB) components and their corresponding log-transformed DBH, squared diameter at breast height (DBH2), and DBH2*height (H). For data analysis, Micro-soft Excel 2002 and SPSS 11.5 were used. To determine fuelwood properties of components of the six tree species such as moisture content (MC), basic density (D), volatile matter (VM), fixed carbon (FC), ash (As), gross heat value (GHV) and combustion characteristics, experimental samples were collected by random sampling method. All the test samples were replicated three times. These studies were conducted at the Faculty of Forestry, Universiti Putra Malaysia. The allometric equation with DBH2 as predictor variable showed better results (higher R2 and lower SE) than other growth parameters. The improvement in R2 and SE was very little when H was included as predictor variable (up to 6 and 2.5% respectively). Since DBH can easily be measured with higher accuracy and provided better estimates, it was recommended as an adequate growth parameter for AGB estimation. The GHV of stem wood and branch component did not show significant variation (p>0.05) between species. For foliage biomass the variation between species was highly significant (p<0.001). It was observed that, GHV alone does not indicate the merits of desirable fuelwood qualities; therefore, FVI was calculated considering GHV, MC, D and As. The results revealed that FVI constitutes an adequate criterion for selecting and ranking fuelwood species. The species investigated in current study were ranked according to FVI in descending order of desirable fuelwood properties; i.e. E.globulus, E.grandis, E.saligna, A. tortilis, A.seyal and A. abyssinica. Results from TGA showed that stem wood and branch components in all species exhibited to have similar burning profile; and comparable burning characteristics. Generally, ignition temperature of burning samples increased with increasing volatile matter content. It was also observed that ignition requirement for foliage biomass was lower compared to stem wood and branch component.

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