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ŠUMARSKI LIST 1-2/2018 str. 37     <-- 37 -->        PDF

intensity produce weak reactions of the remaining trees and consequently significantly smaller volume increments per hectare, but they also increase the instability of stands several years after thinning (Nilsson et al., 2010).
The paper studies two thinnings carried out  on a permanent experimental plot in a spruce monoculture. They were characterized as selective thinnings and based on the overall structure of the felled trees they were heavy to very heavy thinnings from below. The research has made it possible to assess the effects of thinning on the increment and slenderness of different categories of trees and stability of the stand in two stand age periods - from 33 to 40 and from 41 to 50 years of age.
Materials and methods
Materijali i metode
The experiment – Pokusni objekt
The research was conducted in a spruce culture on Velika Brezovica of the Kučaj mountain range in north-eastern Serbia (MU Bogovina I, compartment 87a) at 870 m above sea level with southern and southwestern aspects and an inclination of 5º. According to the Base Geological Map (1968) and its description (1970), the culture was established on the soil developed on proluviums, a material made mainly from crystalline schist and Paleozoic sediments, sandstone, argilo schist, lydite and, less frequently, Mesozoic limestone. On the study area, at 900 m above sea level, the mean annual air temperature is 7.5°C and the mean annual rainfall amounts to 840 mm. According to Thornthwaite climate classification, the study area has a continental  humid climate, type B2. Spruce is not a part of the natural composition of the forest communities in the study area, which is dominated by mountain beech forest. The culture was established by afforestation of pasture on the site which is not suitable for spruce due to periodic ice-breaks or snow-breaks. It was established by dense planting (2×1 m), and according to available data from the management records it hadn`t been thinned before the  age of 32. At the end of 1994 (culture aged 32 years), a permanent experimental plot (15×30 m) was established in a densely closed part of the stand where the diameter of 176 trees was measured and the first thinning conducted (Bobinac, 2004). The second tree diameter measurement and the second thinning were carried out on the permanent experimental plot at the age of 40, and the third measurement was conducted at the end of 2012 when the culture was 50 years old. A total of 48 trees had the diameter measured on this occasion.
Treatment – Tretman
Investigated culture is characterized by high productivity, which causes that the production of sawmill roundwood can be set as a management goal together with the attainment of the stand stability in short rotations.
Two heavy selective thinnings were carried out using the procedure described by Schädelin (1934). At the age of 32, the initial number of 556 candidates per hectare for tending were selected in the upper storey, and it was approximated from yield tables of Schwapach for the stand age of 80 years and site class I (Nikolić i Bankovic, 2009). Based on this number of candidates, the first selective thinning was done at the stand age of 32 (Bobinac, 2004). At the age of 40, of the 556 candidates, 311 future trees per hectare (55.9%) were selected, 67 trees (12.1%) were felled and 178 trees (32.0%) were declared indifferent. This method of tree selection was applied because of frequent snow and ice breaks in the study area which break tree tips and crowns, thus preventing the achievement of long-term goals of stand tending.
The selected candidates and future trees had one of the strongest rivals in the category of dominant trees cut down. Dying, damaged and tapering trees were also cut down.
Measurement and data analysis of growth elements – Izmjera i analiza podataka elemenata rasta
All trees on the permanent experimental plot had two cross diameters measured with an accuracy of 1 mm. In order to construct height curves at the stated ages of the culture, we measured the heights of the selected candidates, at the age of 32, or aspirants, at the age of 40. Minimum five heights in each diameter degree of 5 cm were measured with Blume-Leis and Vertex III hypsometers, at the age of 32 and 40, respectively. Some trees had their lengths measured during the thinning with a diameter tape made of steel, in order to check measurements given by hypsometers. Height curves were smoothed using Michailoff`s function, h=ae-b/d+1.30. Tree volume was determined from spruce volume tables by Baur (1890) with the following analytical form: v=0,00007∙d1,32,05363∙h0,70952 (1971). The measurements of diameters at breast height and heights are presented for the culture ages of 32, 40 and 50. The volume of the trees that were damaged or died between two thinnings was determined based on their diameters and heights at the beginning of the study period.
For the numerical characterization of the thinnings, the ratio of the quadratic mean diameter of the trees marked for felling to the mean diameter of the trees that remained after the thinning, i.e. qd ratio was used (Pretzsch, 2005). The degree of slenderness (h/d1,3 ratio) according to the classification used by (Slodičák and Novak, 2006) was used as an indicator of the stability of the stand and the trees. For the distribution of the degree of slenderness at the stand level in the 32nd, 40th and 50th year of age we used the tree heights from the height curve.