MODELING DIAMETER DISTRIBUTION AND HEIGHT-DIAMETER RELATIONSHIP FOR Gmelina arborea Roxb AND Tectona grandis L. F. PLANTATIONS IN OYINMO FOREST RESERVE, NIGERIA

Abstract

The most important elements of forest structure are tree diameter and height, they are the most commonly measured variables for estimating tree volume, site index, and other important variables in forest growth and yield, succession, and carbon budget models. Intensive measurement of these variables is difficult, time consuming and very expensive, with all its importance in forest management and record keeping, no intensive documentation of forest inventory at Oyinmo Forest Reserve describing the diameter distribution of the trees and other stand conditions. This study is the first of its kind at Oyinmo Forest Reserve to model the diameter distribution and height-diameter relationship of stands in the reserve. Therefore, the overall objective of this study is to develop models for tree stem diameter distribution and height-diameter relationship for Gmelina arborea and Tectona grandis in Oyinmo Forest Reserve, Ondo State, Nigeria. It is located between Latitudes 6°11'11" and 6°15'59"N; Longitudes 5°42'42" and 5°39'10"E, along Owo-Abuja expressway north of the road in Akoko South-West Local Government Area, Ondo state, Nigeria. Sample plots were allocated based on stand ages; 19, 26 and 27 years old Tectona grandis stand and 23 years old Gmelina arborea stand. An area of 100 m x 100 m was demarcated in each of the compartments, within which eleven temporary sample plots of 20 m x 20 m were selected based on the stand ages. Measurements were carried out on Db (cm), Dbh (cm), Mth (m) and Th (m). A total of 1567 trees were observed in 44 temporary sample plots, 1160 trees were observed in 33 sampled plots of Tectona grandis stand while 407 trees were observed in 11 sampled plots of Gmelina arborea stand. The data was subjected to descriptive statistics, diameter and height-diameter relationship models. It was observed that highest number of trees concentrated in the diameter class of 10-14.9 while few trees fall within the diameter class of 45 and above. Johnson SB distributions was adjudged to be the best among the tested functions for modeling Tectona grandis while General Paretor was adjudged to be the best in modeling diameter distribution of Gmelina arborea in Oyinmo Forest Reserve because their calculated D-values were less than the tabulated at p<0.05 meaning that the data followed the specified distribution. However; Logistics Power was outstanding in describing the relationship between the diameter at the base and merchantable height while Gompertz Relation was considered the most suitable model for describing the diameter-height relationship in Oyinmo Forest Reserve because they both have the lowest AIC value of 2150.48 and 2370.08 respectively and standard error.