ABSTRACT
The development of allometric models is crucial to assess forest biomass and carbon stocks. However, very few allometric equations have been developed in Ethiopia and as a result, generalized allometric equations are often used for forests developed in other study areas. The general objective of this study, therefore, was to develop allometric equations for estimating aboveground biomass of three selected native woody species by using the branching method.
The three native plants were Dovyalis verrucosa, Ekebergia capensis, and Olivia rochetiana of dry evergreen Afromontane forest of Suba-Sebeta forest in the central highland of Ethiopia. Inventory of woody species was carried out for determining the relative proportion of the three selected woody species in the stand. A total of 36 individual plants (12 from each species) were selected from the diameter categories randomly to the trim branch. Sample trees were climbed, basal diameters of small branches were recorded and three branches per tree were removed for further parameter measurement. Plant biometric parameters such as diameter at stump height (DSH), diameter at breast height (DBH), total height (Ht) and crown area (CA) were measured, Woody density of the three selected species was also determined. The best allometric equations were evaluated on the basis of performance statistics (bias, coefficient of determination (R2), standard error of estimate (SEE), prediction residuals sum of squares (PRESS), index of agreement between measured and predicted biomass values (D)), The results of the study showed that the strongest predictor variables for estimating aboveground biomass for Dovyalis verrucosa were DBH(R = 0.97, p<0.05), Ekebergia capensis DSH (R=0.97, p<0.05) and Olivia rochetiana DBH (R=0.96, p<0.05).The mean wood density for Dovyalis, verrucosa was estimated to 0.483 g cm-3,0.353 g cm-3 for Ekebergia capensis and 0.434 g cm-3 for Olivia rochetiana. The best performing equation for estimating total aboveground biomass of Dovyalis verrucosa explained 98% of the biomass variation (Model 8- AGB = 0.155 x (DSH)1.164 x (DBH)0.778 , R2=0.98, p<0.001), for E.capensis (Model 8- AGB = 0.030 x (DSH)0.953 x (DBH)1.840, R2=0.99, p<0.001) and for O.rochetiana (Model 13- AGB= 0.242 x (DBH) 1.418 x (Ht) 1.085 x (CA) 0.036 x (𝜌̅) 1.562 , R2=0.99, p<0.001). It was revealed that using previously published general allometric equations either overestimated or underestimated the aboveground biomass. It was concluded that the equations developed in this study could better estimate the aboveground biomass of
D. verrucosa, E. capensis, and O. rochetiana in the study region. Furthermore, this study will help to better estimate the three species within the reference of dry evergreen Afromontane forest in the study region and agro-ecologies similar to this study.
Keywords: Allometric equations, Biomass, Ethiopia, native species, predictor variables
