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ŠUMARSKI LIST 5-6/2016 str. 46     <-- 46 -->        PDF

of deciduous tree species, with changes in the growing and resting periods (Lukić et al. 2003). For example, in the optimal site conditions in Serbia, beech trees can grow more than 45 m in height and 1,5 m in diameter at breast height (Vučković and Stajić 2005). In Croatia, in the best sites conditions beech can reach 37,8 m of mean height at the age of 150 (Špiranec 1975). The distance between the trees and their optimal number, and consequently the optimal size of tree growth space are of great importance for the achievement of the optimal site and stand productivity in beech forests (Lukić 1988; Vučković and Stajić 2003, Zelić 2005).
This species is one of the most productive tree species in Serbia, although its productivity largely depends on silvicultural form and site and stand features. The wide vertical and horizontal distribution of beech on different geological substrates and in various soil evolution stages has caused large differences in the productivity of beech forests (Vučković and Stajić 2005). However, previous studies of pure and mixed stands of beech, as instructed by the mentioned authors, don`t provide enough data for a complete overview and classification of sites and stands according to the actual and potential level of production. Therefore, optimization of the management system of beech forests is extremely important to reach the potential site production and to achieve the most favorable economic and environmental effects (Vučković and Stajić 2003).
Some important drivers of the management system optimization are strongly linked to site and stand productivity issue. It is a well-known fact that some sites support one species and its forest, whilst others support some other species and its forest (Vanclay 1992; Kitikidou et al. 2015). In this sense, site quality assessment represents the evaluation of the natural productive capacity of a forest site for a tree species (Kitikidou et al. 2015). There are various methods of estimating potential site productivity. Generally, tree height is widely used indicator of site productivity. In that context,. Bearing this in mind, the potential productivity of a site has been mostly measured by site index, which is defined as the dominant height of a stand at a base age (Monserud 1984; Sterba and Monserud 1993; Bravo and Montero 2001; Gadow 2002; Skovskaard and Vanclay 2008; Pretzsch, 2009; Zlatanov et al. 2012, Bontemps and Bouriaud 2014; Kitikidou et al. 2015).
Generally, studies on site productivity for even-aged forest have not been carried out very intensively, both in Serbia and partly in the entire Region of former Yugoslavia (except in Slovenia). In Bosnia and Herzegovina, classification of site productivity, for example, has been conducted either by applying the concept of relative-tree height productivity classes, i.e. mean height-diameter relations (Balić et al. 2007) or by relative height site classes, i.e. mean height-age relations (Ibrahimspahić 2004), which were established with simple splitting of tree height variation belt into 5 equal parts. In Croatia, during the process of beech yield table creating, Špiranec (1975) established site classes I-IV based on mean tree height-age relationships for all age classes from the age of 20 to the age of 150. Maunaga (1995) was the first to develop five (I-V) site curves for even-aged spruce stands based on the top stand height-age relations etc.
In the Serbian forest sector, the classification of sites concerning productivity has been carried out using so-called ’’site class height chart’’, prepared for different types of tree species. They represent a fitted dependence of the height on the breast diameter and site class in the form of a height curve. The number of fitted height curves depending on diameter represents the number of site classes, as well as the number of volume arrays in tariffs (Banković and Pantić 2006).
In spite of the great relevance of site productivity estimates for forest management, studies on site productivity assessment regarding site index curves have hardly been performed in Serbia. Some initial results, although not too representative and in part meaningless, were provided by Ratknić (1998). In order to improve the existing system of site classification according to productivity, harmonize it with the dominant mode of site productivity estimation in Europe and create opportunities for the comparison of the obtained results with the results of site index investigations from other countries where beech occurs, it is necessary to establish the site index curves for this tree species. Accordingly, the study was aimed at (1) modeling the beech dominant height−age relationships and (2) constructing anamorphic site index curves for this tree species in the region of Žagubica, Eastern Serbia.
The study was conducted in beech stands in Žagubica region (about 15000 ha of the total forest area), situated in eastern Serbia. The altitude ranges from 650-1250 m. The parent rock of the management unit consists of limestone and amphibolic and clay shales. Soil types include shallow, medium and deep soils on limestone (rendzina over dense limestone and brownized rendzina over dense limestone) and brown acid soils.
The average annual temperature is 9,8 °C. The mean monthly temperature is the highest in July (20,3 °C) and the lowest in January (-1,0 °C). The average annual precipitation for this location is 682 mm. The wettest months are May and June (with an average of 87 mm and 88 mm, respectively) and the driest are February and March (with an average of 36 mm and 40 mm, respectively).
Temporary sample plots were used to obtain data on average age and height of dominant beech trees necessary for