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establishment. In the later period they were extensively tended. The first thinnings were usually carried out at the age when economically viable assortments were most likely to obtain. Such a trend has also been characteristic of conifer cultures throughout Europe because late crown thinnings provide higher and more valuable cutting yields (Valsta, 1992; Slodičák et al., 2005). An objective assessment of this tending approach to spruce cultures is most rationally achieved on permanent experimental plots where thinnings have been carried out and periodic measurements and assessments have been conducted using standard procedures (Zeide 2001; Pretzsch 2005). The effects of thinning verified on permanent experimental plots determine the silvicultural treatment of the forest stands in a wider area. They are particularly significant if experimental plots are located in areas where there are adverse exogenous impacts (ice-breaks and snow-breaks). Thinning approaches to spruce cultures in Serbia have been studied by a number of authors. These studies were mostly based on the analysis of the structure of untended stands or on the research results of initial state on experimental plots (Marković and Petrović, 1960; Vučković et al., 1990; Dražić, 1994; Bjelanović and Vukin, 2010). Short-term thinning effects determined on the basis of periodic measurements of the elements of growth conducted on permanent experimental plots in Serbia can be found in Stojanović and Krstić (1984) and Bobinac (2004). Based on the available research results obtained on permanent experimental plots in other study areas in the surrounding regions of Serbia, we can conclude that they have used similar approaches to thinning in spruce cultures. In Bosnia and Herzegovina, there are no data of long-term investigations on the effects of thinning on permanent experimental plots. On the newly established permanent experimental plots in the said country, moderate to light thinning from below was studied on the basis of the analysis of the structure of untended stands (initial measurements of the elements of growth) and the effects of thinning on the stand development may be expected in the future (Govedar, 2007; Govedar et al., 2013; Bodružić et al., 2015). In Croatia, results from a number of permanent experimental plots have been published and provided data only on short-term effects of thinnings from below of light to moderate intensity (Orlić et al., 1991; 1997; Orlić, 1999). Regarding the impact of thinning from below of light to moderate intensity and spontaneous development on the development of spruce cultures, there are some interesting results of long-term monitoring of stand development on a permanent experimental plot, between the ages of 113 and 161 (Dubravac et al., 2006). The approach to thinning in spruce plantations both in Serbia and in neighboring countries, as well as in the whole of Europe, has been under a heavy influence coming from Germany after the publication of the results of several decade long research on the implementation of crown thinning and thinning from below with varying intensity on permanent experimental plots (Assmann, 1970). The most important conclusions of these long-term studies have shown that spruce stands in which thinnings from below of light to moderate intensity were conducted had greater current volume increments per hectare in their early to middle age compared to untended stands. Furthermore, higher increments resulted in the higher overall productivity of these stands in comparison to unthinned stands (control plots). On the other hand, the stands which had crown thinning operations carried out in the second half of the rotation had smaller volume increments compared to the stands with light to moderate thinnings from below. They had lower productivity than unthinned stands and stands with light thinning from below. Recent studies have shown that late crown thinnings produce higher and more valuable cutting yields, but their total productivity is lower (Slodičák et al., 2005). These were some of the main reasons for the absence of crown thinning operations in spruce stands. Due to biological characteristics of spruce, primarily its brittle tip and shallow root system, spruce stands are more susceptible to breakages and windfalls. Such negative phenomena are more pronounced in the cultures established outside of its ecological potential and they are intensified in untended and extensively cultivated stands after each canopy opening over a short period of time. That was another reason for the absence of crown thinning operations in spruce stands (Nilsson et al. 2010; Wallentin and Nilsson, 2014). The absence of thinning at an early age increases the tree slenderness but improves the stem clearness. Thus, it follows that spruce cultures require intensive thinning, i.e. it should be implemented as early as possible, in shorter intervals and the intensity of interventions should be moderate (Mraček and Perez 1986; Cameron, 2002). However, this method of spruce culture management entails higher felling costs and thinner assortments of small economic value. Recent results of several decade long research on permanent experimental plots in spruce stands in which crown selective thinnings were carried out at early ages show that this approach has intensified the growth of trees after the thinning, especially of final crop trees which have been the primary tending objects. This approach further contributed to the rapid increase in their size and thus to the shortening of the production cycle. It also reduced their slenderness and thus increased the stability of the remaining trees after thinning (Slodičák and Novák, 2003; 2010; Nilsson et al., 2010; Štefančik, 2012). Furthermore, the results of long-term research on the effects of thinning in spruce cultures generally indicate that stands respond positively to thinning, regardless of their age at the time of commercial thinning (Makinen and Isomaki, 2004a, 2004b). However, not only do delayed thinnings of heavy |