Growth and yield modeling for logged-over dipterocarp stands using implicit and average stand models

Six candidate regression equations that were considered test in the Philippines and Indonesia were tested and assessed for statistical validity and accuracy in predicting volume given diameter at breast height and tree height. Model 2, which is a square-root logarithmic model, was found to be the best model and can be applied to predict tree volume of logged over dipterocarp stand in the study site. Of the implicit models, the Weibull probability distribution function (p.d.f.) was selected the best model to predict stand variable such as stand volume, number of trees, and stand basal area in the same year when the model was developed. However, the model is not sufficiently accurate to forecast the stand variable in a future year. The model is good enough if it is based on data from plots established for specific number of years elapsed after logging. Among the models using projection age, Model 6 was the best model for predicting growth and yield for logged-over dipterocarp stand, since the model had the highest coefficient of determination (R square) and correlation (r), and the lowest mean square error (MSE) and residual sum square (RSS). Similarly, among the models using initial and projection ages, Model 10 was the best. Both models 6 and 10 are accurate enough since there are no significant differences in results compared to those using the Chi-square method. Model 10 is chosen because the result of prediction at the initial year is exactly the same as the actual volume, and when predicting for longer periods of time, the results are consistent. This means that there is no more growth of trees. Model 10 is more reliable because the growth of trees follows a sigmoid shape. Financial analysis showed that MTSl [Dr. Maman Sutisma Timber Stand Improvement] is more profitable because it has the biggest PNW [present net worth] of Rp. 586,680.00 for a 30-year cutting cycle. It is no longer profitable if the cutting cycle is changed to 35 or more years. While the PNW, IRR [internal rate of return] and BCR [benefit cost ratio] of NTSI [without timber stand improvement] have the biggest values, in this treatment there was no investment, which means that the amount belongs to the owner of the forest, the government of Indonesia. The IRR of NTSI are 77.43 percent, 63.18 percent, and 53.36 percent and the BCR are 1.47, 1.47, and 1.47 for 30, 35, 40 years cutting cycle, respectively