Equations for estimating bark thickness of <i>Gmelina arborea</i> (roxb) trees in Omo Forest Reserve, Nigeria

Abstract

The measurement of bark thickness is an important factor for computing inside bark volume of a standing tree or log. Bark thickness at breast height can easily be measured. However, when bark thickness at relative height of a standing tree is required, the application of equations becomes imperative. In this study, equations were developed for estimating bark thickness at relative height and at breast height. Stratified random sampling was used to establish 50 sample plots of 0.04 ha size across 10 age series in the Gmelina arborea plantation in Omo Forest Reserve, Nigeria. Eight equations for estimating bark thickness as function of diameter inside bark (dib) and Relative Bark Thickness (RBT) were developed. Equation was also developed for predicting absolute bark thickness at breast height. The equations were assess based on Root Mean Square Error (RMSE), Mean Absolute Bias (MAB), Akaike Information Criterion (AIC) and Shapiro-Wilk test of normality. The results showed that six out of the nine equations performed relatively well in estimating bark thickness. The best equation for estimating bark thickness as function of dib had RMSE, MAB and AIC values of 0.065, 0.049 and -125.989, respectively. The best equation for RBT had 0.109, 0.079 and -75.577, respectively. The equation for absolute bark thickness at breast height had 0.204, 0.152 and -12.697. The equations did not violate the assumption of normality as revealed by normality test (p > 0.05). With these equations, any analytic volume equation can be used to compute the inside bark volume of the standing trees. The relative back thickness and diameter inside back functions developed in this study were found to be satisfactory based on the various criteria used for their assessment. Thus, they are recommended for use in estimating the back thickness and diameter inside bark of Gmelina arborea stands in similar ecosystem.