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ŠUMARSKI LIST 5-6/2016 str. 49     <-- 49 -->        PDF

management of forest resources (Zelić 2000). Site index curve is the most commonly used indicator for the estimation and reliability of forest site productivity. Construction of site index curves is a fundamental task in site-quality differentiation and stand development prediction, which gives them great importance in planning and implementing sustainable management of forests. Standard procedures for the construction of site index curves are, however, still lacking (Elfing and Kiviste 1997). In general, site index curves can be calculated using anamorphic or polymorphic procedures. In our study, anamorphic beech site index curves were computed by applying the proportional or guide curve method. This type of site index curves is performed by using the data obtained from temporary sample plots. This type of data is occasionally used to fit site index curves (Walters et al. 1989). The main reason for the use of this data source has been the lack of permanent sample plots in beech forests of this area. It is important to note that when temporary sample plots are used as a data set for the construction of site index curves, it is necessary to provide the appropriate distribution of plots with respect to age and site class. In this context, a set of 109 temporary sample plots established in our study can be considered to be representative enough, because for reliable evaluation of height-age relationships ’’… a total of at least of 100 plots is necessary, although more are desirable…’’ (Husch et al. 2003). In addition, the temporary sample plots are relatively quite uniformly distributed per age and site classes (except partly in youth and mostly for SI16) to derive representative height growth beech curves for the studied area.
Bearing in mind the fact that the true functional form cannot be known, it is important that the selection is made after a comparison of many functional forms and several criteria (Nanang and Nunifu 1999). Therefore, seven commonly used growth functions were tested in order to find the best model for approximation of dominant height-age relationships. The first obtained results of the conducted analysis showed that all the applied models had a relatively high R2 values, indicating that the models accounted for more than 65% of the variation in the dominant height, which can be considered as a large effect, according to Cohen’s (1986) instructions. Furthermore, the model by Korsun and Schumacher showed the best overall statistical features. These results are not absolutely surprising, because Shumacher’s model is perhaps one of the most widely used model form in site index curve development (Nanang and Hunifu 1999). However, Korsun’s model was preliminary chosen to calculate site index curves, because of a little better model’s statistical indicators. Nevertheless, according to Todorović (1961), the three-parameter growth functions (Korsun’s one) generally give more realistic performance of relation between a growth element and age than two-parameter functions (Schumacher’s one).
In order to further evaluate the applicability of the Korsun’s function and the other models used, the average curves constructed by applying these models were graphically presented. The obtained results (Figure 1) show that the lines of Korf’s and Schumacher’s functions have a very pronounced S-shape and they come out tangentially and almost parallel to the x-axis in the first 10 years of growth. This means that these models, although they are characterized by solid statistical possibilities, have a graphical representation of an unsatisfactory form and that they cannot be fitted appropriately in the real empirical data. Chapman-