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Yield Production Models for Pinus Merkusii Plantations in Java, Indonesia
Dissertation Abstract:
This study sought to find an appropriate formulation and specification of a yield prediction model and to construct yield tables for Pinus merkusii plantations in Java, Indonesia. The study aimed to: 1) explore the use of the simultaneous equations model for yield prediction and to compare the results of the estimation procedures used, namely: ordinary least squares (OLS) and two-stage least squares (2 SLS); 2) compare the simultaneous equations model with single equations model; and 3) compare the results obtained from the model developed with the results obtained by Ferguson in 1954.
The coefficient of the simultaneous equations model were estimated using the OLS and 2 SLS. For the single equations model, the coefficients were estimated through the OLS method.
In this study, the model was developed for both main and whole stands. Data were obtained from a sample of 545 observations as an experimental series taken from 114 permanent rectangular plots established in West and East Java.
With the genetic variation and management regime held constant, yield was found to be adequately affected by stand age, site index, original stand spacing, thinning intensity, and stand basal area.
The results obtained from specifying the model using 2SLS and OLS were equivalent in some respects. First, the components equations of the model, basal area, and yield equations provided highly significant values of the coefficients of determination (R2). Second, both methods provided similar yield curve being S-shaped and satisfied the differentiation properties with respect to site, time, and stand density. However, they differed in prediction accuracy.
Using the chi-square test, the level of accuracy attained by the 2SLS model for the main and wholestands, respectively, was within 21.0 and 21.7 percent of the true values at 0.05 level of significance. This meant that the prediction error in percent was 21.0 and 21.7 when compared with the accuracy of the simultaneous equations model derived by the OLS methods, 2.7 and 3.0 percent increase in the error of prediction for the main and whole stands, respectively, were observed. Conversely, comparing with the single equations model, 0.5 and 0.3 percent decrease in the error of prediction for both stands, were observed.
However, this single equations model was not adopted as the yield model since it lacked stand basal area as an independent variable for yield which was needed for timber management purposes, particularly in specifying thinning regime. Likewise, the OLS method theoretically provides inconsistent and asymptotically efficient estimators of the system parameters. This property was desirable in yield prediction because if more samples were taken, the expected variance of the predicted value was very small. In fact, the predicted value was expected to be very close to the actual value when samples were taken. Hence, the findings obtained by the 2SLS method were used to develop the yield prediction model for both main and whole stands of Pinus merkusii plantations.
The model used was compared with the results of Ferguson's table. It appeared that the results found by Ferguson were significantly greater than the results derived by the yield prediction model. The Ferguson table model was constructed using the graphical method and the data were obtained from 28 permanent sample plots only. The significant difference was probably caused by the use of a small sample size as well as the method applied.