Functions for multi-phase assessment of biomass in acacia woodlands of the Rift Valley of Ethiopia

Abstract

In this article, single-tree biomass functions for acacia woodland tree species are presented. The main aim of developing the functions was for utilizing them in a multi-phase sampling design in estimating the woody biomass in acacia woodlands of the Rift Valley of Ethiopia. Eighty-five trees (5–35 cm diameter) were selected randomly from the five dominant tree species: Acacia tortilis (Forsk.) Hayne, Acacia senegal (L) Willd., Acacia seyal Del., Acacia etbaica Schweif. and Balanites aegyptiaca (L) Del. Four categories of functions were developed. These were (i) functions with crown area as independent variable, to be applied using high resolution satellite images or aerial photos; (ii) functions with crown area and tree height as independent variables, to be applied using data from aerial photos; (iii) functions for quick estimation in ground surveys, based on the diameter at 0.8 m; and (iv) functions for thorough estimation in ground surveys, in which the diameter at 0.8 m, the diameter at stump height, and the crown area were included. Also, two sets of functions were developed, one with and one without species indicator variables. In gathering data for developing biomass functions, the selection of sample discs from trees for determining the ratio between dry weight and fresh weight is important. In addition to the functions, a robust method for the disc sampling is proposed.