Modelling diameter distribution for a seven year Eucalypus camaldulensis stand in Afaka Forest Reserve-Nigeria using Weibull function

Abstract

Parameters of the Weibull function were estimated for a diameter distribution modelling of a seven year Eucalyptus (Eucalyptus camaldulensis) plantation in Afaka Forest Reserve. Two fit methods; Cumulative Density Function and Probability Density Function (Weibull-CDF and Weibull-PDF) were used. Twenty plots of 20 x 20 m were randomly selected for measurements. Stump diameter (Dst), diameter at breast height (Dbh), diameter at middle and top positions (Dm and Dt) of trees, and tree height for all the selected trees were measured. Pseudo coefficient of determination (Pseudo R2), Residual Mean Square Error (RMSE), Anderson-Darling (A-D), and Kolmogorov-Smirnov (K-S) goodness-of-fit statistics were considered as model selection criteria. The average Dbh of the stand was 14.64cm, and the mean tree height was 6.02m.Weibull cumulative density function (Weibull-CDF) was the best fit method based on goodness-of-fit test (Pseudo R2 = 0.98, RMSE = 0.0397, A-D = 1.1558, K-S = 0.0668). The diameter distribution models produced are considered only efficient for tree stands of seven years rotation age, while additional re-measurement data sets as well as other site factors are required for better models, hence the need for establishing permanent sample plots (PSP) in order to get additional information from re-measurements of the plots.