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EFFECTS OF HEAVY THINNINGS ON THE INCREMENT AND STABILITY OF A NORWAY SPRUCE STAND AND ITS TREES BETWEEN THE AGES OF 32 AND 50
UTJECAJ JAKIH PRORJEDA NA PRIRAST I STABILNOST STABALA I SASTOJINE SMREKE U STAROSTI OD 32. DO 50. GODINE
Martin BOBINAC, Siniša ANDRAŠEV, Andrijana BAUER-ŽIVKOVIĆ, Nikola ŠUŠIĆ
Summary
The paper studies effects of two heavy thinnings on the increment and slenderness of various categories of trees and stability of the stand as a whole. The research was conducted on a permanent experimental plot in an Norway spruce (Picea abies /L./ Karst.) monoculture in Serbia. This monoculture was established with 5,000 seedlings per hectare on the site of mountain beech forest and the effects of heavy thinnings were investigated in the 33-40 and 41-50 age periods.
To determine the thinning effects we compared current diameter increments (idt) and current height increments (iht) of dominant trees (D100 and D400) obtained by a detailed analysis of trees and of mean stand dominant trees (D100 and D400). At the stand level, we compared the current diameter (id), basal area (IG) and volume (IV) increments of all trees and of the same collective of aspirants in two periods after the thinnings, between the ages of 33 and 40, and between the ages of 41 and 50.
The first thinning was carried out at the age of 32 when the dominant trees were 15 m tall and the next at the age of 40 when the dominant trees were above 20 m in height. They were both low (qd <0.85) and heavy selective thinnings (34-36% of the volume). A more significant increase in the diameter increment was recorded after the second thinning between the ages of 41 and 50. It amounted to 29.1% in aspirants and 36-42% in dominant trees (D100 and D400) compared to the period after the first thinning, i.e., between the ages of 33 and 40. The thinnings further contributed to the establishment of more favorable relations in diameter and height increments of the trees in the studied culture and thus improved their stability.
Key words: Picea abies /L./ Karst., monoculture, permanent experiment plot, heavy thinning, slenderness
Introduction
UVOD
In Serbia excluding Kosovo conifer cultures have been established on an area of 124,800 ha, where 95% are pine and spruce cultures. Norway spruce (Picea abies /L./ Karst.) cultures cover an area of 32,400 ha (26.0%), (Banković et al., 2009). A large number of conifer cultures in Serbia typically had spontaneous development in the first decades after the

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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

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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.

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For the reconstruction of the growth of dominant trees in height and diameter in the study culture, two dominant trees at the age of 40 were analyzed. The trees belonged to D100 and D400 category of dominant trees and the model of height growth was developed. The height growth model was constructed using the Chapman-Richards function (Pienaar and Turnbull, 1973). The model of dominant tree height growth was in line with the increase in the mean stand height of dominant trees in the period between the ages of 41 and 50 taken from the height curve.
To define the effects of thinning on the growth of trees, we compared the current diameter (idt) and height (iht) increments obtained by a detailed analysis of dominant trees (D100 and D400) in the periods between the age of 25 and 32 and between the age of 33 and 40. At the stand level, we compared the current diameter (idt) and height (iht) increments of dominant trees (D100 and D400) in the periods between the age of 33 and 40 and between the age of 41 and 50. We further compared the current diameter, basal area, and volume increments of all trees and of aspirant trees in the periods between the age of 33 and 40 and 41 and 50.
Statistical data analysis comprised measurement of standard numerical parameters of diameter structure: arithmetic mean (da), standard deviation (sd), coefficient of variation (Cv), variation width (vw), minimum (dmin), maximum (dmax), coefficient of skewness (α3) and kurtosis (α4). The Kolmogorov-Smirnov nonparametric test (|D| statistics) was used for the mutual comparison of  the structures of the slenderness degree (h/d1,3 ratio). The t-test was used to test the differences between the current increments in different periods.
Research results
Rezultati istraživanja
Stand structure elements – Elementi strukture sastojine
The most important data about the elements of stand structure at the ages of 32, 40 and 50 are shown in Tables 1 and 2. At the age of 32, there were 3,911 trees per hectare, with a basal area of 57.39 m2·ha-1 and the volume of 384,17 m3·ha-1, while the average periodic increment at the age of 32 amounted to 12 m3·ha-1·year-1. Trees were distributed in diameter degrees from 7.5 to 22.5 cm, with the maximum distribution in the diameter degree of 12.5 cm (Graph 1). The quadratic mean diameter was 13.7 cm, Lorey`s mean

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height (hL) 14.0 m, and degree of slenderness (h/d1,3) 102. At the age of 32, the variability of diameter structure amounted to 29.3% , with weak positive skewness (α3 = 0.212) and platykurtic kurtosis (α4 = 2.076). The mean diameter of the collective of 311 aspirant trees per hectare at the age of 32 was 1.38·dg, while the mean height amounted to 1.08·hL. Aspirant trees had characteristically small diameter variation (kv%=10.1%)  and the degree of slenderness of 80.
At the age of 40,  there were 1,978 trees per hectare, with a basal area of 50.25 m2·ha-1 and the volume of 424.75 m3·ha-1. The trees were distributed in diameter degrees from 12.5 to 27.5 cm, with the greatest number of trees in the diameter degree of 17.5 cm (Graph 2). The quadratic mean diameter was 18.0 cm, Lorey`s mean height (hL) was 19.1 m, while the degree of slenderness amounted to 106. At the age of 40, the variability of diameter structure amounted to 25.3%, with weak right skewness (α3 = 0.116) and platykurtic kurtosis (α4 = 2.203).
At the age of 50, there were 1,067 trees per hectare, with a basal area of 52.87 m2·ha-1 and the volume of 539.72 m3·ha-1. The trees were distributed in diameter degrees from 12.5 to 37.5 cm, with the greatest number of trees in the diameter degree of 27.5 cm (Graph 3). The quadratic mean diameter was 25.1 cm, Lorey`s mean height (hL) was 24.3 m, while

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the degree of slenderness amounted to 97. The  trees were characterized by a relatively small variation in diameter (kv%=24.1), and their distribution per diameter degrees was characterized by weak skewness (α3=-0.092) and mild platykurtic kurtosis (α4 = 2.445).
Height curves of the investigated culture at the ages of 32, 40 and 50 are shown in Graph 4. The assessment of the height curve model, expressed through the coefficient of determination (R2) and the standard error of regression (se), shows a satisfactory agreement between the empirically measured heights and the obtained models (Table 3).
At the age of 40, we can notice a more intense shift of height curves towards greater heights compared to the age of 32. At the age of 50, they shifted to the right, towards greater diameters compared to the age of 40. In the period between the ages of 33 and 40, the mean stand height (hL) of the aspirant collective increased by 5.4 m (from 15.1 m to 20.5 m) and  in the period between the 41st and 50th  year of age by 4.9 m (from 20.5 m to 25.4 m) .
The growth of individual trees – Rast pojedinačnih stabala
The height growth determined by the detailed analysis of dominant trees (D100 and D400) before the age of 40 and the model of height growth before the age of 50 are depicted in Graph 5. The model of the height growth of dominant trees before the age of 50 shows satisfactory agreement with the empirical sizes obtained on the basis of the detailed analysis in the period before the age of 40 and the measurement of growth elements conducted at the age of 50 (Table 4).
The height growth of the studied dominant trees was characterized by a gradual rise before the age of 10 and the current height increment culmination at the age of 17 at a height of about 6 m. After the culmination, the current height increment retained high annual values to the observed age of 40. The height growth model points to the culmination of the current height increment at the age of 25, at a height of 11 m (Graph 6 left).
The current diameter increment of dominant trees, obtained from the detailed tree analysis, had a culmination at the age of 15. It retained high values to the age of 21, which was then followed by a significant decline, so that at the age of 31 it had the smallest values (Graph 6 right). The average value of the diameter increment of dominant trees in the period between the ages of 25 and 32 amounted to 4.9 mm∙year-1 in the representative of the 100 thickest trees and

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4.6 mm∙year-1 in the representative of the 400 thickest trees per hectare.
Since height and diameter increments of dominant trees retain high values in the period after the culmination, up to the age of 21, the period between the age of 15 (17) and 21 can be taken as the optimum time for thinning in the investigated stand. According to the model, the current height increment had a culmination at the age of 25 (when the dominant trees reached a height of 11 m), so if we apply this biological criterion, the first commercial thinning should be carried out not later than the age of 25 (Graph 6).
Thinning characteristics – Karakteristike prorjeda
Thinning in the study culture was primarily aimed at fostering the development of high-quality trees (candidates and aspirants) that belonged to the category of dominant trees. The first thinning was carried out at the age of 32 (when the aspirants reached an average height of 15.1 m) and the second thinning at the age of 40 (when the aspirants reached an average height of 20.5 m).
In the thinning that was conducted in the 32nd year of age, 1,378 trees were cut per hectare (35.2%). Their volume amounted to 115.1 m3∙ha-1. In the period from the age of 32 to the age of 40, 555 trees per hectare were cut down due to mortality and snow-breaks (14.2%). Their volume was 22.0 m3∙ha-1. It follows that out of the initial number of trees, a total of 1,933 trees per hectare were cut down (49.4%) in the 32nd year of age. Their total volume was 137.1 m3∙ha-1 (35.9%). The trees harvested at the age of 32 had diameters at breast height from 7.0 to 20.0 cm and the mean diameter of 12.7 cm (0.93·dg), while the subsequently harvested trees had diameters of 6.6-10.6 cm and the mean diameter of 9.3 cm (0.68·dg) (Tables 1 and 2, Graph 1).
The thinning that was conducted at the age of 40 included 911 trees per hectare (46.1%) with the volume of 142.3 m3·ha-1 (33.7%). The trees harvested at the age of 40 had diameters at breast height from 10.1 to 23.9 cm and the mean diameter of 15.7 cm (0.88·dg) (Tables 1 and 2, Graph 2).
The two thinnings and the subsequently felled thin trees on the experimental plot in the period from the age of 33 to 40 included 2,844 trees per hectare (72.7%) with the volume of 279.4 m3·ha-1. Based on the ratio between the quadratic mean diameter of the trees harvested in the thinning and the remaining trees at the ages of 32 and 40, the thinnings were low (qd=0.84-0.80) and based on the percentage decrease in basal area (35-37%) and volume (34-36%), they were heavy thinnings (Tables 1 and 2).

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In the thinning at the age of 32, out of the total 115.1 m3∙ha-1of the trees marked for felling, almost 100 m3∙ha-1 were in the diameter range of 10 to 20 cm. However, in the thinning performed at the age of 40, out of the total 142.3 m3∙ha-1 of the trees marked for felling, over 110 m3∙ha-1 were found in the diameter range of 10 to 20 cm and about 30 m3∙ha-1 was in the diameter range of 20 to 25 cm.
The effects of thinning on trees and stand increment – Utjecaj prorjeda na prirast stabala i sastojine
In the period after the first thinning, between the ages of 33 and 40, the current annual diameter increment determined by a detailed analysis of the representative of the 100 thickest trees in the stand was higher by an average of 55% compared to the period before the thinning, between the ages of 25 and 32. The differences were highly statistically significant (p<0.001). In the period after the thinning, the current annual height increment of the representative tree was higher by an average of 9% compared to the study period before the thinning, but the differences were not statistically significant. The current annual diameter increment determined by a detailed analysis of the representative of the 400 thickest trees was, in the period between the ages of 33 and 40, higher by an average of 4% compared to the period before the thinning, but the differences were not statistically significant. After the thinning, the current annual height increment of the representative tree was higher by an average of 18% compared to the period before the thinning, and the differences were statistically significant (Table 5, Graph 6).

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At the stand level, the diameter increment of the 100 and 400 thickest trees in the stand in the period between the ages of 41 and 50 was higher by 36 to 42% than the diameter increment in the period between the ages of 33 and 40. The statistical significance was weak (p <0.05) to high (p <0.001). The current annual height increment of the representatives was lower in the period between the ages of 41 and 50, by an average of 26-27%, than in the period between the ages of 33 and 40 and the differences were highly statistically significant (Table 6, Graph 6).
The current diameter increment of the aspirants in the specified periods (41-50 and 33-40 years of age) was within the range of the diameter increment of dominant (D100 and D400) trees in the stand (Table 7). The diameter increment of the aspirants was 29.1% higher between the ages of 41 and 50 than between the ages of 33 and 40. In the 400 and 100 thickest trees in the stand, the current diameter increment was 36% or 42% higher in the period between the ages of 41 and 50, compared to the period between the ages of 33 and 40. When calculating the current diameter increment for the collective of all trees, calculation is different because there is a different number of trees in each period, so the current diameter increment in the period between the ages of 41 and 50 was 68% higher compared to the previous period  when the number of trees on the experimental plot was almost twice bigger (Table 7).
In the period between the ages of 33 and 40, the remaining trees (1978 trees per hectare) achieved a total volume increment of 177.68 m3∙ha-1, or an average of 22.21 m3∙ha-1∙year-1, which is 29% higher than the attained cutting yield at the age of 32. In the period between the ages of 41 and 50, the remaining trees (1,067 trees per hectare) achieved a total volume increment of 257.3 m3∙ha--1, or an average of 25.73 m3∙ha-1∙year-1, which is 81% higher than the realized cutting yield at the age of 40. In the period between the ages of 33 and 40, 311 aspirant trees per hectare  (15.7% of the total number of trees) had a share of about 32% in the current increment of basal area and volume, while in the period between the ages of 41 and 50, these 311 aspirant trees per hectare (29.1% of the total number of trees) had a share of 45 to 46% (Table 7). In the period between the ages of 41 and 50, almost half the total number of trees achieved a 15.8% higher volume increment per hectare than in the period from 33 to 40, and the same collective of aspirant trees (311 trees per hectare) achieved a 68.5% higher current volume increment (Table 7).
The total stand volume increment recorded on the experimental plot at the age of 40 amounted to 561.85 m3·ha-1, and the average annual increment was 14.05 m3∙ha-1∙year-1. The share of the previous increment in the stand before the age of 40 amounted to 137. m3·ha-1 or 24.4% of the total volume increment. The total volume increment measured at the age of 50 amounted to 819.1 m3·ha-1, the average annual increment was 16.38 m3∙ha-1∙year-1, and the share of the previous increment amounted to 279.4 m3·ha-1 or 34.1% of the total recorded volume increment.
The effect of thinning on slenderness of trees and stability of stand – Utjecaj prorjeda na stupanj vitkosti stabala i stabilnost sastojine
The degree of slenderness of dominant trees, obtained on the basis of the detailed analysis of dominant trees (D100 and D400), had  minimum values in the period between the ages of 15 and 26 and amounted to 60-70. After this period, the degree of slenderness showed an increase and reached the value of approximately 80 at the age of 32 in both trees. At the age of 40, the degree of slenderness of the tree representing the 100 thickest trees was 80, and 90 of the tree representing the 400 thickest trees. At the age of 50, the average degree of slenderness of dominant trees, obtained based on the measurement of growth elements, corresponded to the degree of slenderness obtained by a detailed analysis of trees at the age of 40 (Graph 7).
The cumulative curves of slenderness for the total number of trees show that at the age of 32 less than 8% of trees had the degree of slenderness below 80, only 2% at the age of 40, and 8% of trees at the age of  50. Furthermore, 53% of trees had the degree of slenderness above 100 at the age of 32, 63% at the age of 40 and 36% at the age of 50 (Graph 8 left).
At  the age of 32, 50% of the aspirant trees had the degree of slenderness below 80, less than 15% at the age of 40, and less than 30% at the age of 50. However, all aspirant trees had the degree of slenderness below 90 at the age of 32, 50%

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of the aspirant trees at the age of 40 and 90% at the age of 50 (Graph 8 right).
According to the non-parametric Kolmogorov-Smirnov test, the cumulative curves of slenderness for all trees show significant differences between the three measurements of growth elements, which is ascribed to the reactions of trees to the conducted thinning and to the reduction in the number of trees due to thinning. However, with the same number of aspirant trees, significant differences were found only in the period between the ages of 32 and 40, i.e. after the first thinning (Table 8).
Discussion and conclusions
Rasprava i zaključci
A comparison of the growth elements of the investigated culture with the elements of growth in other young to middle-aged spruce cultures in Serbia (Stojanović and Banković,1981; Stojanović and Krstić, 1984; Vučković et al., 1990; Tomanić, 1990; Dražić, 1994; Koprivica and Ratknić, 1996; Koprivica et al., 1998; Krstić, 1998; Ratknić, 1994; Ratknić and Vučković, 1999; Ratknić et al., 2001) shows that the  investigated culture is in the category of the best cultures in Serbia. Another conclusion we can make from the comparison of the elements of growth of the investigated culture and the spruce cultures in other areas is that the investigated culture has high productivity (Assmann,1970; Maunaga, 1999; Pretzsch, 2005; Orlić, 1987; 1994; 1999; Orlić et al., 1997, Oršanić, 1995; Slodičak and Novak, 2003) or good site quality (Halaj et al., 1987, Maunaga, 2001).
The first thinning at the age of 32 and the height of  dominant trees of 15 m was  low (qd <0.85) and heavy (36% of the volume) selective thinning. The age at which the thinning was carried out does not significantly deviate from the age at which the first “commercial” thinnings are performed in Europe (Slodičák and Novak, 2003). However, the flows of the current diameter increment determined by the detailed analysis of dominant trees (D100 and D400) show that the thinning at the age of 32 was performed late, while the height growth (increment) model shows that the culmination was reached at the age of 25 (when the dominant trees reached the height of 11 m), which means that the first commercial thinning should not be performed later than the age of 25. The adoption of this biological criterion for the determination of the optimum timing for thinning would, according to (Vučković, 1991; Kotar, 2005), produce the best effects of thinning on the increment and stability of trees and stands. Following the recommendations of Assmann (1970), it has been stated that when applying selective thinning after Schädelin (1934),

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tending candidates should be selected from the dominant trees that are 8 to 12 m tall.
The initial large number of trees with high volume per hectare and heavy thinning at the ages of 32 and 40 have caused high values of the cutting yield, 115.13 m3∙ha-1 and 142.28 m3∙ha-1, which are in the category of the highest cutting yields achieved on individual experimental plots of young stands (Stojanović and Krstić, 1984; Vučković et al., 1990; Orlić 1999; Orlić et al., 1991). The applied thinnings had the highest intensity broadly recommended in the literature for spruce cultures of similar age (Halaj et al., 1986; Maunaga, 2001).
In the period between the ages of 33 and 40, total of 555 trees per hectare were cut down due to mortality and snow-breaks (14.2%). Their volume was 22.0 m3∙ha-1. Thus it follows that out of the initial number of trees,  a total of 1,933 trees was cut down (49.4%) in the 32nd year of age. Their total volume was 137.1 m3∙ha-1 (35.9%). The research of Valinger and Petersson (1996) carried out on permanent experimental plots established in 24-45-year-old spruce cultures suggests that the greatest relative number of trees damaged by snow and wind was recorded on  control plots and in the early period after extremely heavy thinnings (over 40% of basal area). According to these results, the increased risk of damage caused by snow and wind is greatly affected by the unfavourable position of the culture and high culture density, which is consistent with our research.
In the period between the ages of 33 and 40, the reaction in the diameter increment of the remaining trees was weak. Compared to the results of Stojanović and Krstić (1984), the current diameter increment of the remaining trees after the thinning, between the ages of 33 and 40 (0.31 cm∙year.-1, Table 7), was similar to the value of the diameter increment on the control area with a 30% higher number of trees per hectare in the two five-year age periods between the stand ages of 32 and 42 (0.31 to 0.34 cm∙year-1). The current diameter increment of the future trees in the investigated culture (0.54 cm∙year-1, Table 7) was within the range of the increment of remaining trees on the experimental plots with the thinnings of light to moderate intensity stated by Stojanović and Krstić (1984). The size of the diameter increment of the aspirant trees in the researched culture between the ages of 33 and 40 was significantly lower than the sizes stated by Štefančík (2012) for the thinning procedures first implemented at the stand age of 20. In the period after thinning was performed in the investigated culture, between the ages of 33 and 40, the current volume increment was 22.15 m3ha-1∙year-1 and it was similar to the increment found by Orlić (1999) in 41-year-old spruce cultures with about 40% smaller number of trees after a thinning at the age of 32. Compared to the sizes of the current increment of basal area and volume stated by Stojanović and Krstić (1984), which were achieved at a similar age and with the application of  light to moderate thinning from below, the increments in the investigated culture were 35% smaller. Compared to data provided by Slodičák and Novak (2003), who conducted light selective thinnings at a similar age on a series of experimental areas in spruce stands of approximately the same age, basal area increments per hectare in the investigated culture were higher by 13-166%.
The second thinning at the age of 40 and at the height of dominant trees over 20 m was low (qd <0.85) heavy (34% of the volume) selective thinning. The heavy thinning at the age of 40 with the thinning volume of 142.28 m3∙ha-1 caused a greater diameter increment, i.e. better reaction in the diameter increment of the remaining trees, compared to the thinning at the age of 32. The future trees had a 28% higher diameter increment in the age period between 41 and 50 years compared to the age period between 33 and 40.
The difference in the diameter increment size between the two study periods in the investigated stand is in line with other authors` results that spruce stands on favourable sites and with the number of trees that is not too large for the given age react positively to thinning even if they are performed later in life (Makinen and Isomaki, 2004a; 2004b; Preuhsler and Schmidt, 1989). According to the results (Preushler and Schmidt, 1989), late moderate to heavy thinnings from below in densely closed spruce stands, aged 48 and 58 years, exhibit a slower reaction in the volume increment in the early period after the cutting (5-7 years) compared to the following five-year period, which is consistent with our research. According to the results of Stojanović and Krstić (1984) that refer to the stand age between 32 and 42, larger diameter (11%) and volume (20%) increments in the second five-year period than in the first were identified only on the heavily thinned experimental plots. On the experimental plots with light to moderate first thinning and light second thinning, the mentioned autors recorded smaller diameter, basal area and volume increments in the second than in the first observed (five-year) period. The higher increment in the later age period, between the ages of 50 and 70 years, is related to a small number of trees per hectare in spruce monocultures. Slodičák and Novak (2003) noted increment increase in the later period, after heavy thinning from below, while Štefančík (2012) stresses that heavy thinning, particularly selective thinning, increases the diameter increment of target trees. In the case of heavy and very heavy thinning from below, larger increase in diameter increment was also observed after the second compared to the period after the first thinning (Nilsson et al., 2010).
On several series of experimental plots in several decades of research (over 100 years) in spruce stands, it has been found that if the stands are young and if we apply light to moderate thinning from below in a short period of time, they will achieve higher increments of basal area (and volume) per hectare than control plots. Further, similar values

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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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Sažetak
Istraživanja su obavljena u kulturi smreke (Picea abies /L./ Karst.), koja je osnovana sa 5.000 sadnica po hektaru na staništu planinske šume bukve na nadmorskoj visini 870 m. Na osnovi usporedbe elemenata rasta istraživane kulture s elementima rasta u drugim mladim do srednjedobnim kulturama smreke, istraživana kultura je u kategoriji visokoproizvodnih kultura u Srbiji i šire u Europi.
Na trajnoj pokusnoj plohi, u dobro sklopljenom dijelu istraživane smrekove kulture u starosti 32 godine, izmjereno je 3.911 stabala po hektaru, s volumenom od 384,17 m3·ha-1, a ukupno evidentiran prirast drvne zalihe sastojine do 40. godine iznosio je 561,85 m3·ha-1, a do 50. godine 819,1 m3·ha-1. Na osnovi dvije prorjede, u 32. i 40. godini i naknadno posječenih tanjih stabala u razdoblju od 33. do 40. god., na pokusnoj plohi ukupno je posječeno 2.844 stabla po hektaru (72,7%) volumena 279,4 m3·ha-1 (tablica 1). Numerički pokazatelji strukture stabala na pokusnoj plohi u 32., 40. i 50. godini prikazani su u tablici 2, a debljinska struktura na grafikonima 2, 3 i 4.
Za definiranje utjecaja prorjeda na prirast stabala uspoređen je tečajni debljinski (idt) i visinski (iht) prirast u razdoblju od 25. do 32. godine i 33.-40. godine kod detaljno analiziranih dominantnih stabala (D100 i D400), a na sastojinskoj razini uspoređen je tečajni debljinski (idt) i visinski (iht) prirast u razdoblju od 33. do 40. godine i 41.do 50. godine kod dominantnih stabala (D100 i D400). Testiranje razlika između tečajnih prirasta u različitim razdobljima obavljeno je uz pomoć t-testa. Na razini sastojine uspoređen je i tečajni prirast promjera, temeljnice i volumena svih stabala i stabala budućnosti (311 kom.∙ha-1) u razdoblju od 33.-40. godine i u razdoblju od 41.-50. godine. Neparametrijski Kolmogorov-Smirnov test korišten je za međusobnu usporedbu struktura stupnja vitkosti (odnos h/d1,3). Istraživanja su omogućila da se definira utjecaj prorjeda na prirast i stupanj vitkosti različitih kategorija stabla i sastojine u dva starosna razdoblja sastojine, 33.-40. i 41.-50. godine.
Prva prorjeda u 32. godini, pri visini dominantnih stabala 15 m, imala je karakter selektivne prorjede, bila je niska (qd<0,85) i jaka (jačina prorjede je 36% volumena). Prorjeda je obavljena u starosti sastojine koja značajnije ne odstupa od razdoblja kada se izvode prve „komercijalne“ prorjede u Europi. Međutim, tokovi tečajnog debljinskog prirasta detaljno analiziranih dominantnih stabala (D100 i D400)  pokazali su da je prekasno izvršiti prorjedu u 32. godini, a model rasta (prirasta) visina је pokazao kulminaciju u 25. godini (kada su dominantna stabla postigla visinu 11 m), pa bi se 25. godina mogla označiti i kao godina u kojoj je najkasnije trebalo izvršiti prvu komercijalnu prorjedu (grafikon 7). Zatečeni velik broj stabala, s velikom drvnom zalihom po hektaru, u 32. godini i jaka prorjeda uvjetovali su  visoki iznos prorjednog etata, od 115,13 m3∙ha-1, što je u kategoriji najviših iznosa volumena koji se okvirno preporučuju u literaturi za sličnu starost smrekovih kultura. U razdoblju od 33.-40 godine debljinski prirast je iznosio kod preostalih stabala 0,31 cm∙god-1, a kod stabala budućnosti 0,54 cm∙god.-1 (tablica 7). Dominantna stabla, posebno 400 najdebljih stabala po hektaru, nalazila su se u fazi velikog visinskog prirasta u vrijeme i poslije prorjede u 32. godini (tablica 5 i 6), što je za posljedicu imalo povećanje stupnja vitkosti stabala, odnosno povećanje nestabilnosti sastojine (grafikon 8).
Druga prorjeda u 40. godini, pri visini dominantnih stabala preko 20 m, imala je karakter selektivne prorjede, bila je niska (qd<0,85) i jaka (jačina prorjede je 34% volumena). Jaka prorjeda u 40. godini, s prorjednim etatom 142,28 m3∙ha-1, uvjetovala je veći debljinski prirast, odnosno bolju reakciju debljinskog prirasta na preostalim stablima, u odnosu na prorjedu u 32. godini. Stabla budućnosti imali su za 29%, a dominantna stabala (D100 i D400) za 36-42%, veći debljinski prirast u starosnom razdoblju od 41.-50. godine, u odnosu na starosno razdoblje 33.-40. godine, što je uz opadajući trend visinskog prirasta uvjetovalo i manji stupanj vitkosti, odnosno veću stabilnost stabala i sastojine (grafikon 8).
Na osnovi navedenog, proizlazi da su provedene jake prorjede doprinijele uspostavljanju povoljnih odnosa u debljinskom i visinskom rastu stabala u istraživanoj kulturi i time poboljšale njihovu stabilnost.
Ključne riječi: Picea abies /L./ Karst., monokultura, trajna pokusna ploha, jake prorjede, stupanj vitkosti