SITE INDEX CURVES FOR SITE QUALITY ASSESSMENT OF <em>NAUCLEA</em> <em>DIDERRICHII</em> MONOCULTURE PLANTATIONS IN OMO FOREST RESERVE, NIGERIA

Abstract

Timber production potential of a site is measured by site index, which is defined as the dominant height of a stand at a base age. A site index equation for site quality determination in Nauclea diderrichii (Opepe) stands in Omo Forest Reserve, Nigeria was developed using a base age of 30 years. Dominant height data were collected from 30 temporary sample plots of 25 × 25 m in plantations of five to 30 years. Linear and non-linear models widely used in site index studies in forestry were fitted to dominant height-age data. Adjusted coefficient of determination (R²_Adj), root mean square error (RMSE) and residual plots were used in choosing the ‘best model’. The best model was H_d = 36.29 (1 – exp
(-0.027 Age))^0.542. The proportional or guide curve method was adopted in constructing the sites curves, with the chosen model as the average guide curve. Based on this curve, the plantation was divided into five site quality classes, A to E, with class A representing the best site while class E, the poorest. Result showed that a 30-year-old Opepe plantation in the study area attained average dominant height of about 30, 26 and 22 m on site quality classes A, C and E respectively. The site index curve was used as a guide to recommend the time of first and subsequent thinning operations in Opepe plantations as well as suitable sites for future plantations.