Comparison of Tree Volume Estimates Obtained by Different Methods for Gmelina Arborea in Omo Forest Reserve, Southwest Nigeria

Abstract

Volume estimates obtainedfrom four different methods oftree volume estimation were compared with a view to determining the most precise method. The major difference between the methods lies in the position where the diameter used in the volume estimation was obtained. The methods include the “Bolt method” which requires dividing the entire stem into “bolts ” so that diameter measurements are made at both ends of each bolt. Other methods are the Huber s method which requires diameter measurement at the middle position along the stem, Smalian’s method where diameter measurements are made at the base and top of the stem, and Newton’s method which takes diameter at base, middle and top positions along the stem. Data for the study were collectedfrom six different age series (13, 15, 19, 20, 24, and 25 years old) of Gmelina arborea plantations in Omo forest reserve, Nigeria. The determination of the volume of trees using the “Bolt method” requires tree felling so that the stem of each tree could be sectioned into bolts for detailed measurement. Due to the destructive nature of the procedure, only five trees per age series were selected. Prior to felling, the diameter of each tree at base, middle and top positions along the bole and the merchantable height were measured. Each sample tree was then felled and cross-cut into bolts whose length and diameter were measured for volume computation. The volume of the bolts that made up a bole and the stump volume were added to obtain the merchantable volume for the tree. For each felled tree, Huber, Smalian and Newton’s formulae were also applied to the tree data obtained before felling in order to obtain volume estimates. One-way analysis of variance (ANOVA) was used to test for significant differences in tree volumes obtained by the four methods. The results indicate that there were . significant differences in the volume obtained using the four methods. Bolt s method of tree volume estimation produced the most accurate tree volume in this study since it accommodates all irregularities along the stems of the trees.