Above-ground biomass production and biomass equations for even-aged Gmelina arborea (ROXB) plantations in south-western Nigeria

Above-ground biomass production was estimated for Gmelina arborea (Roxb) plantations (ages 5–21 years) in Oluwa Forest Reserve, Nigeria. A total of 120 trees were harvested and weighed to provide data for the estimation of biomass as well as for developing biomass equations. The results showed that Gmelina has high biomass yield, ranging from 83.2 tha−1 (5 years) to 394.9 tha−1 (21 years). Mean annual biomass increment varied from 16.2 to 20.9 tha−1 yr−1. Average biomass allocations to different components of the tree were as follows: Stem about 84%; branch 13% and foliage 3%. The high biomass values were attributed to fast growth, high stand density (range: 837–1275 stemsha−1) and good site conditions in the study area. Biomass values of the various tree components (stem, branch and foliage) as well as total above-ground biomass (TAGB) were regressed with diameter at breast height (dbh) on the one hand (equation form 1), and with dbh and total height (in the form of D2H) on the other (equation form 2), with the aim of choosing the most suitable equation form. All equations generated had very high adjusted R2 and very low standard error of estimate. Residuals for both equation forms were normally and randomly distributed. Only little improvement in the ability of the equations to explain the variations in biomass was achieved by including the variable of total height. Considering the ease and accuracy in obtaining dbh data and the enormous time, errors and difficulty in obtaining height data, the equation form involving only dbh data as independent variable was recommended for use in estimating biomass for Gmelina in Oluwa forest reserve.