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(below 5% decrease) of the basal area increment per hectare are also attainable with heavier thinnings performed later in life (Assmann, 1970). Quantification of the `qualitative` research of spruce cultures was conducted by Pretzsch (2005), who created practical models of the current increment of basal area (and volume) depending on the intensity of thinning and site class. However, a large number of authors point to a pronounced variability in the size of the current increment of basal area and volume per hectare depending not only on the applied thinning techniques and intensity, but also on the age and site class. These values can be 25% lower or 40% higher compared to control plots or the surface areas with light thinnings from below (Pretzsch, 2005; Wallentin, 2007).
The first selective thinning of heavy intensity in the studied culture was conducted at the age when economically viable assortments were most likely to achieve. According to the flows of diameter and height increments of dominant trees, it was a late thinning and it resulted in the weak revitalization of tree diameter increments in the period between the ages of 33 and 40.  However, dominant trees, especially the 400 thickest trees per hectare, were in the phase of more pronounced height growth during the thinning, which increased the degree of slenderness and consequently reduced the stability of the stand. According to Abetz and Klädtke (2002), the optimum degree of slenderness in spruce cultures is 80-90, and above that size trees are highly prone to wet snow and wind breakages, so it can be concluded that at the age of 32 only the tending trees fulfilled the criterion (hL/dg=80) and the stand as a whole was highly prone to wet snow and wind breakages (hL/dg =102). Under the influence of the second selective thinning of heavy intensity, at the age of 40, the future trees had a 29% larger diameter increment in the period between the ages of 41 and 50 compared to the period between the ages of 33 and 40, which together with the decreasing trend of height increment caused a smaller degree of slenderness and greater stability of the stand and individual trees. All things considered, it follows that the heavy thinnings carried out at the ages of 32 and 40 contributed to the establishment of favorable relations in the diameter and height growth of trees in the investigated culture and thereby improved their structural stability.
Acknowledgement
Zahvala
This paper was realized as a part of the project “Studying climate change and its influence on the environment: impacts, adaptation and mitigation” (43007) financed by the Ministry of Education and Science of the Republic of Serbia within the framework of integrated and interdisciplinary research for the period 2011-2018.
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