DIGITALNA ARHIVA ŠUMARSKOG LISTA
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ages of the CAIh culmination. The differences are even greater in younger stands (more intensive height growth of RU beech compared to this from ŽA), while they decrease gradually with increasing age and the heights of dominant trees from the RU and Ža become almost equalized in the later stages of growth.
Another desirable criterion for the site index model evaluation could be its “behaviour” in terms of the similarity between the results obtained by the model application and the empirical results, in this case with the empirical data of culmination ages and values of maximal CAIh per site class. The obtained results suggest that the site index curves obtained by the CHR function were best fitted into the data obtained by the stem analysis procedure. The models of Korf and Korsun produced relatively accurate results for the best two site classes, but generated unrealistic estimates for the site classes of the lower production capacitates. Therefore, it can be concluded that the CHR model also proved to be the best according to this biological criterion. Bearing in mind all of the above-mentioned, although verification statistics showed that none of the models was completely satisfactory, the CHR function was finally selected for the construction of polymorphic site index curves.
The mentioned biological criterion is very important because site index models may serve as a useful indicator in the quantification of the beginning and the cycle/frequency of thinning. Taking into account that cleaning felling of young stands is not generally performed, the silviculture of beech on sites of high productivity in Serbia traditionally promotes the execution of the first thinning as early as possible, usually in the third decade (Stojanović and Krstić 2005), i.e. at the time of the CAIh culmination (Stajić 2010). The execution of the first thinning in the phase of high increment can be considered a favourable framework for achieving the biological and production optimum in beech stands (Matić et al. 2003, Bobinac 2004) and could be commonly considered more important than frequent intervention (Assman 1970). Additionally, on sites with higher SI values (sites with more vigorous growth) thinning cycle/frequency will be more intensive than on sites with lower SI values, i.e. sites less vigorous growth (Pretzsch 2009). Therefore, the height growth of dominant trees contains two valuable components, the knowledge of which is of great importance for the planning of silvicultural activities: (1) growth of species depending on age and (2) the relationship of tree species to site conditions.
Regarding the values of the CAIh at the culmination age no significant differences were identified among the models (Figure 4). According to this model, the CAIh has approximately the same value for each site index in both areas. Namely, the value of the CAIh at the culmination age ranges from 0.42 m (SI32) to 0.20 m (SI16) for RU and from 0.42 m (SI32) to 0.22 m (SI16) for ŽA.
The modelled data of the CAIh culmination age per site class indicated that beech in the area of RU and ŽA (Table 4), for a given site class, reached the CAIh culmination age later than in Austria, where revealed at the age of 12, 13, 15, 17, and 21 (Kinderman 2004). To compare site index curves of the RU and ŽA with site index curves (site class II, III and IV) from Croatia (Špiranec 1975), based on the Loray’s