Nonlinear height-diameter models for Tectona grandis (Linn. F.) in Mbavaa Forest Reserve, Benue State, Nigeria

Abstract

The research was carried out at Mbavaa Forest Reserve in Benue State. Height and diameter at breast height are the most important measures of tree  growth and their relationship is useful in determining site-index, calculating tree volume, evaluating site-quality and predicting future growth of the  stand. The measurement of total tree height is relatively complex, time consuming and expensive. The aim of the study was to develop models for  predicting height of Tectona grandis. The variables measured were; Diameter at base, Diameter at breast height and total tree height. The different  height-diameter models used are Exponential, Weibull, Chapman-Richards, Lundqvist-Korf, Naslund and Modified logistic. The asymptotic coefficient a of  the Exponential model was the highest compared to the five other functions. Modified Logistic had the highest Standard Error (SE) compared to the other  five height-diameter models. Chapman-Richards had the highest R² value. The fit statistics and coefficients estimation indicated that the  ChapmenRichards model is the most suitable out of the six models considered for predicting height-diameter relationships in Mbavaa Forest Reserve.  Height growth normally declines as the trees grow larger in size which was attributed to less competition at the upper level of the stand distribution. It is recommended that stands variables like spacing, crown area, soil fertility density, age and silvicultural practices should be incorporated in order to  improve the accuracy and reliability of height-diameter models. The potential tree height–diameter equations should be validated for their predictive  capabilities (e.g. accuracy, precision, and flexibility) across a range of tree diameters.