Determination of self-thinning line for Gmelina arborea Roxb stands in Nigeria

Abstract

Self-thinning line defined the site occupation of species and it is an important tool for manipulation of stand density and simulating thinning regime. Few studies have defined the self-thinning line of Gmelina arborea especially in Nigeria. Therefore, in this study, the self-thinning line of G. arborea in Oluwa Forest Reserve was determined. Data were obtained from twenty-four temporary sample plots (TSPs) of 20 x 20 m size established in the G. arborea stands. Three methodologies were used to define the self-thinning line including quantile regression (QR), stochastic frontier function with half-normal and stochastic frontier function with truncated-normal. Root means squared error (RMSE), Akaike information criterion (AIC) and Bayesian information criterion were used to assess the methods. The results showed that the three methods performed relatively well in describing the self-thinning line of the stand. Stochastic frontier with halfnormal with minima AIC and BIC of -5.321 and -0.609, respectively was more suitable. It predicted maximum density of 2630, 1537 and 1079 N/ha at quadratic mean diameter of 15, 25 and 35 cm, respectively. This information would help in the manipulation of the growth condition and determination of thinning schedule of the G. arborea stands.

Keywords: Quantile regression, stand density, stochastic frontier regression