TAPER AND VOLUME MODELS FOR Tectona grandis (L.f.) TREES IN AKURE AND ALA FOREST RESERVES, ONDO STATE, NIGERIA

Abstract

Taper refers to the general decrease of stem diameter of a tree with increasing height. It is a very important attribute of the tree because of its influence on volume and utility of the tree. In this study, stem taper of Tectona grandis was assessed and ten different models were fitted to characterise it. Data were collected from Akure and Ala forest reserves where logging permits had been issued for harvesting teak. A total of 209 trees marked for felling were used for the study. Tree measurement carried out before felling was diameter at breast height (dbh). After felling, stump diameter, stump height, total length of the tree, and diameter at 10%, 20%, 30%, 40%,50%,60/%,70%, 80%, 90% and 100% positions along the stem were measured. Of the data for the 209 trees, 80% were selected across all dbh classes and used as estimation data, while the remaining 20% were used for model validation. Out of the ten taper models fitted, the simple taper model with power function performed best having minimum Akaike’s Information Criterion (AIC) value of 159.59 and bias of 0.07. The model is expressed as: 𝑑 = 9.91 × 𝐷 × ( 𝐻−ℎ 𝐻−1.3) 2.48 where d is the diameter at any height position (h) along the stem, D is the diameter at breast height and H is the total height of the tree. Similarly, five different tree volume models were fitted and compared in this study. The best model is expressed as: 𝑉 = 1.83𝐷0.49𝐻2.51 with AIC value of 58.82, bias of 0.10 and Root Mean Square Error value of 0.53. The residual plots of these best models showed random spread below and above the zero line, implying that the assumptions of regression analysis were satisfied.