Growth functions of Gmelina arborea (ROXB) for sustainable management of an even aged forest in Ukpon river forest reserve, Obubra, cross-river state, Nigeria

Abstract

This study involved developments of growth functions for predicting future individual tree characteristics, stem volumes and yield using stand age, stocking, diameter at breast height (dbh) and total height as predictor variables for Gmelina arborea plantation in Ukpon river forest reserve, in Obubra local government area of Cross River State, Nigeria. Data were collected from sample plots within five (5) age series (3, 5, 7, 9, and 11 years) in Gmelina plantation. Five sample plots of 0.04 hectare each were randomly selected from each age bracket. Diameter at breast height (dbh) of all trees in each plot was measured using diameter tape and grouped into 10cm diameter classes. Two trees with mean diameter in each class were randomly selected and measured for total height using Sunnto altimeter. Four dominant trees were randomly selected and measured for dominant diameter at breast height and dominant total height in each plot. Over bark diameter at the hohenadls positions (0.1, 0.3, 0.5 and 0.9) along the free bole for the two trees with mean dbh were measured using Haga altimeter for volume and form factor computation. Seven growth models were fitted to age, stocking diameter, and height using linear and non-linear regression techniques. The model selection criteria were based on high coefficient of determination (R²), low goodness of fit, high significant of variance ratio (F) and least residual mean square error (MSE). The results showed that Gmelina species had the fastest initial height growth of 3.6m per year within the first five years of growth. The mean diameter at breast height increased from 13.31cm at age 3 to 6.99cm at age 11 with mean diameter increment of 4.4cm per year in the first five years and 2.3cm per year thereafter between ages of 7 and 11 years. Mean total height increased from 13.3m to 25m from age 3 to 11 years with corresponding dominant height of 17.28m to 33.88m respectively. The average stands form factor of 0.412 was obtained. The mean basal area increased from 15.44m²/ha to 33.18m²/ha for ages 3 to 11 years respectively while the mean annual increment (MAI) declined from 27.46m²/ha/yr at age 3 to 21.34m³/ha/yr at age 9 and increased slightly to 28.69m³/ha/yr at age 11 years. This study serves as a useful tool for proper evaluation and prediction of future stand growth for sustainable management of forest plantation under similar environmental conditions.