STRUCTURE AND PRODUCTIVITY OF MIXED SPRUCE AND FIR FORESTS ON MT. KOPAONIK

: The subject of this research are mixed forests of spruce and fir in the area of NP Kopaonik, which belong to the community of spruce and fir - Abieti-Piceetum abietis Mišić et Popović, 1978. The basis for the study of the structural development and production potential of these forests are data from 12 sample plots, with the average size of 0.18 ha. In terms of coenoecological affiliation all the sample plots belong to the group of ecological units – forests of spruce and fir ( Abieti-Piceetum abietis , Mišić et Popović, 1978 ) on acid brown and brown podzolic soils, which are differentiated into 5 ecological units: Abieti-Piceetum abietis oxalidetosum on brown podzolic soil, Abieti-Piceetum abietis oxalidetosum on acid brown soil, Abieti-Piceetum abietis vac cinietosum on brown podzolic soil, Abieti-Piceetum abietis typicum on brown podzolic soil and Abieti-Piceetum abietis Dr.ymetosum on brown pozolic soil. In structural terms, these forests are characterized by very diverse structural forms, ranging from the structure of even-aged stands to typical multi-storey, unevenaged-aged stands. The form of cumulative curves of tree distribution is in most cases determined by spruce as the dominant species. At the same time, thin and medium-thick trees dominate, while the presence of stems with large dimensions is minimal. The average volume of these forestse is 777 m 3 ·ha –1 , with a mixture ratio of 0.7: 0.3 in favor of spruce. The average value of the current volume increment is 14 m 3 ·ha –1 , with a 68% share of spruce and 32% of fir. The percentage of increment ranges from 1.6% to 2.5% in all sample plots and is somewhat higher for fir. The site potential, stand characteristics and relations among the tree species have resulted in structural complexity, high productivity and ecological stability of these forests. Therefore, future forest management should avoid radical measures and procedures that would violate the established relationships and dynamic processes.

Напомена: Захваљујемо се Министарству просвете, науке и технолошког развоја Републике Србије које је финансијски подржало ова истраживања у оквиру пројекта "Одрживо газдовање укупним потенцијалима шума у Републици Србији"-ЕВБР 37008. Abstract: The subject of this research are mixed forests of spruce and fir in the area of NP Kopaonik, which belong to the community of spruce and fir -Abieti-Piceetum abietis Mišić et Popović, 1978. The basis for the study of the structural development and production potential of these forests are data from 12 sample plots, with the average size of 0.18 ha. In terms of coenoecological affiliation all the sample plots belong to the group of ecological units -forests of spruce and fir (Abieti-Piceetum abietis, Mišić et Popović, 1978) on acid brown and brown podzolic soils, which are differentiated into 5 ecological units: Abieti-Piceetum abietis oxalidetosum on brown podzolic soil, Abieti-Piceetum abietis oxalidetosum on acid brown soil, Abieti-Piceetum abietis vaccinietosum on brown podzolic soil, Abieti-Piceetum abietis typicum on brown podzolic soil and Abieti-Piceetum abietis Dr.ymetosum on brown pozolic soil. In structural terms, these forests are characterized by very diverse structural forms, ranging from the structure of even-aged stands to typical multi-storey, unevenaged-aged stands. The form of cumulative curves of tree distribution is in most cases determined by spruce as the dominant species. At the same time, thin and medium-thick trees dominate, while the presence of stems with large dimensions is minimal. The average volume of these forestse is 777 m 3 ·ha -1 , with a mixture ratio of 0.7: 0.3 in favor of spruce. The average value of the current volume increment is 14 m 3 ·ha -1 , with a 68% share of spruce and 32% of fir. The percentage of increment ranges from 1.6% to 2.5% in all sample plots and is somewhat higher for fir. The site potential, stand characteristics and relations among the tree species have resulted in structural complexity, high productivity and ecological stability of these forests. Therefore, future forest management should avoid radical measures and procedures that would violate the established relationships and dynamic processes.

STRUCTURE AND PRODUCTIVITY OF MIXED SPRUCE AND FIR FORESTS ON MT. KOPAONIK
Key words: Kopaonik, forests of spruce and fir, structural development, productivity

INTRODUCTION
Mixed stands (regardless of their of origin) occupy 728.800 ha (32.3%) of the total forest area in Serbia. Of these, 129.600 ha (17.8%) are high natural mixed stands, which include three mixture categories -mixed broadleaf stands (70.7%), mixed broadleaf and conifer stands (22.2%) and mixed conifer stands which cover an area of only 9.200 ha (7.1%) . Mixed conifer stands, mainly of fir and spruce, have high productivity with an average volume of 336 m 3 ·ha -1 , the current volume increment of 9,4 m 3 ·ha -1 and the icrement percentage of 2.8%.
Spruce and fir forests most commonly occur as climate regional forests at the junction between mixed forests of spruce, pine and beech and pure spruce forests. In these forests are equally represented the species of ground flora from both beech-fir forests and spruce forests (Tomić, 2004). The community of spruce and fir in Serbia, Abieti-Piceetum abietis Mišić et Popović, 1978. belongs -Bl. et al., 1939, which includes mixed forests of spruce and fir, spruce-fir-beech forests and monodominant spruce forests at lower altitudes ranging from 1,000 to 1,600 m (Tomić, Rakonjac, 2013). This community was described on Mt. Kopaonik (Mišić, Popović, 1954), Mt. Stara Planina (Mišić et al., 1978), Mt. Zlatar (Mišić et al., 1985;Obratov, 1992) and on other mountains.
In terms of coenoecological affiliation (Banković, , mixed forests of spruce and fir belong to: • the complex (belt) of mesophilic beech and beech-conifer forest types (at 4), • coenoecological group of spruce, fir and beech forest types (Abieti-Piceenion, Br. -Bl.,1939) on humus-sillicate soils, rendzinas, black soils, eutric and dystric brown soils, brown and illimerised soils on limestone in the contact of limestone and silicate rocks and on brown podzolic soils 1 (at 47).
In the context of the present coenoecological groups of forest types in NP Kopaonik spruce and fir forests are represented by two groups of ecological units (ecotypes) (Knežević Cvjetićanin, 2003): 1. spruce and fir forests (Abieti-Piceetum abietis, Mišić et Popović, 1978) on eutric humus-silicate soils and eutric brown soils, 2. spruce and fir forests (Abieti-Piceetum abietis, Mišić et Popović, 1978) on acid brown and brown podzolic soils. Mt. Kopaonik belongs to the few mountains in Serbia where remarkable regularity of vegetation change can be observed in the vertical profile. In the upper parts of Mt.Kopaonik, at the transition from the beech belt to the spruce altitudinal belt, in addition to the mixed broadleaved and coniferous forests of beech and fir, beech-firspruce forests and spruce-beech forests, mixed forests of spruce and fir also occur on an area of 536.10 ha, which accounts for 7.6% of the forest area in this NP. In terms of yield, a number of authors have classified mixed forests of spruce and fir among the most productive forests (Matić, 1959;Stamenković et al., 1990;Matović, 2006;Šljukić, 2015). In addition to high productivity, these forests have expressed environmental and functional values and stability.
Modern understanding of the importance and role of forests gives priority to natural and heterogeneous stand forms in structural, species and spatial terms. It is believed that such forests pro-duce a larger amount of wood and biomass in general, that larger amounts of carbon are deposited in them, that they have higher stability, and environmental and functional value. In this regard, mixed forest, and especially mixed forests of broadleaved and coniferous trees and mixed forests of conifers, represent a preferred silvicultural form in modern forestry. High mixed coniferous forests (mainly spruce and fir forests) occupy only 0.4% of forests in Serbia. Therefore, their conservation and increase of their areas is one of the long-term strategic goals of forest management. These studies, as a phase in the achievement of the stated objective, are focused on the study of the structural development and the productivity of spruce and fir forests at a particular site, as elements that are significant for defining the silvicultural and forest management measures in the process of forest management.

Research object
The research was conducted in the management units "Gobeljska Reka" and "Samokovska Reka", which are an integral part of NP Kopaonik. A total of 12 sample plots (SP) were established, and their areas ranged from 0.12 to 0.32 ha, i.e. on average 0.18 ha.
FMU "Gobeljska Reka" is located between 20 ○ 44' and 20 ○ 51' east longitude and 43 ○ 19' and 43 ○ 23' north latitude in the catchment area of the Gobeljska River and its tributaries. The lowest ground level of this FMU is 720 m, and the highest 1,934 m above sea level, which indicates a significant difference in terms of conditions for the growth of certain tree species and the overall ecological differences of hilly, low-mountain and high-mountain sites and the accompanying forest sites. The general exposure of the management unit is northern, although due to the developed relief both eastern and western exposures occur, as well as numerous interexposures. The main types of soils formed on silicate rocks are acid brown soils (dystric cambisol) and brown podzolic soils. Acid brown soils occur on a smaller surface than brown podzolic soils.
The FMU "Samokovska Reka" is located between 20 ○ 43' and 20 ○ 49' east longitude and 43 ○ 16' and 43 ○ 22' north latitude in the catchment area of the Samokovska River, which is limited with its watersheds. The altitude ranges from 680 m to 2,170 m above sea level. The general exposure of the area is northern, although due to the exceptional relief differentiation all other exposures also occur. The largest part of the area of this FMU is mild to moderately steep slope, and a small part is steep to rocky mountain slope.
When climatic zoning of Serbia is concerned, Rakičević (1980) singled out the areas of Mt. Kopaonik as a special climate region, characterized by the coldest and longest winters in Serbia (with negative monthly temperatures from December to April), the lowest average annual air temperature and long average duration of snow cover. At the lowest border of the vertical forest zone on Mt. Kopaonik (750-1,950 m) the average annual air temperature ranges from 6.8 0 C in southern exposures to 7.1 0 C in northern exposures (in the vegetation period it is around 13 0 C) and at the upper border it is 1.8 0 C. The average annual rainfall at the lower border of the zone is below 776 mm in southern exposures, 861 mm in northern exposures, while at the upper border of the zone it is above 955 mm. Climate classification according to Thornthwaite shows that the following climate types occur in the above altitudinal forest zone: moderately himid (B1) at the lower limit excessively humid climate type (B4), at altitudes in the range from 1,500 to 1,700 m and perhumid (A) climate at altitudes above 1,700 m (Krstić et al., 2014). The main wind directions on Mt. Kopaonik are southwest and northeast (Smailagić, 1995).
All the sample plots belong to the group of ecological units (ecotypes) -spruce and fir forests (Abieti-Piceetum abietis, Mišić et Popović 1978) on acid brown and podzolic brown soils, and are located at locations which are regularly managed for basic purposes: landscape of exceptional value and productive-protective forests under the 2 nd degree of protection (2009). The exception is the sample plot of compartment 104 in FMU "Gobeljska Reka", which is located in the nature reserve "Vučak" under the 1 st degree protection regime.
Spatial and ecological characteristics of SPs are shown in Table 1.

Data collection and processing
Pedological and phytocoenological research was previously conducted in the above sites and ecological units were defined on the basis of that research. In these ecological units, in preserved places with good canopy closure and stand quality 12 SPs were established for the purpose of this research. In each SP diameters at breast height and heights were measured for all trees over the forest measurement border.
The collected data were processed by combining commercial software (Excel and Statgraphics).
A variety of functions were tested for the modeling of height curves, and the selection of the final model was carried out based on the quality of statistical parameters of the regression and correlation analysis, and based on the degree of coincidence of fitted and empirical data. Legend: FMU А -"Gobeljska Reka". FMU -B "Samokovska Reka", Е -Еxposure of volume tables is used for calculating the volume, and double-entry volume tables were used for the fir and spruce (Banković et al., 2003(Banković et al., , 2003a. Current volume increment was determined according to the increment percentage, and the increment percentage was obtained on the basis of the regression models that express its dependence on the number of trees per area unit, the share of specific species in the mixture, the diameter and height of the mean stand tree by cross section . The measured and calculated numerical values were grouped into diameter classes -diameter structure, distribution of basal area and volume, i.e. into height classes -height structure, which enables a better analysis of the structure of these forests.

RESULTS
A necessary prerequisite of quality management planning at the stand level, among others, is the existence of reliable information on the state of forests and permanent monitoring of the situation. In this regard, the analysis of site characteristics, health status, structural development, species interactions and evaluation of productivity are of critical importance. The quality of analysis and decision-making in the planning process are improved with long-term comparative analy-ses (the principle of dynamic inventory) and by determining the degree of disproportion between the current and targeted (optimal) situation.
Numerous authors emphasized the importance of study and knowledge of structure (internal structure) and productivity of stands (Miletić, 1930;Klepac, 1956;Pretzsch, 1997;Kint et al., 2004;Govedar, 2005;Medarević, 2006;Stojanović et al., 2008;Stajić, 2010;Pantić et аl., 2011;Matović, 2012;Čavlović, 2013) in order to define their condition, select the best ways of management, analyze the effects and plan future management measures for the establishment and preservation of biodiversity, and thus sustainable management of forests and forest ecosystems. Since today one of the most important general objectives of European forestry is increase in the mixture and structural diversity of forest stands, the knowledge of structure becomes even more important. This statement is completely true for spruce and fir forests in Serbia, due to their small area, lack of research, the need for preservation and possible increase of their share in the growing stock.
The number of trees is the element that shows horizontal stand structure. The most variable element is the internal structure of stands, depending on the site quality and applied silvicultural measures, and in mixed forests of spruce and fir on Mt. Kopaonik it varies in a very wide range (Figures 1 and 2). Height distribution of trees ( Figure 3) is more stable than in their diameter distribution, and is a more reliable indicator for the structural definition of the stands.
The stochastic ratio of height to diameter of trees is modeled as a height curve (Figures 4 and  5), which is a prerequisite for calculating the volume and volume increment of a stand. In addition, the position in the coordinate system (in relation to the axis) of the height curves indicates the ratio of height growth dynamics to the site quality as well as the relations among the tree species in mixed and multi-storey stands.
The total produced volume and volume increment, as quantitative (numerical) parameters of forest productivity are the result of the impact of a complex of site factors, within stand relations, implemented management measures and their interactions. The average values of these and other analyzed parameters of the study stands are given in Table 2.

Number of trees and diameter structure
In the study of spruce and fir forests on Mt. Zlatar Matović (2016) found that the number of trees ranged from 390 to 987 pieces·ha -1 , and Stojanović et al. (2008) 565 pcs·ha -1 . In mixed spruce and fir forests in different sites of the Republic of Srpska Govedar (2005) found from 386 to 727 pcs·ha -1 . In uneven-aged, two-storeyed spruce and fir stands at the site Paunovica (FMU "Cvrcka", BiH) Čuković (2005) found that the number of trees was 401 pcs·ha -1 . The research Stamenković et al. (1990) conducted on Mt. Zlatar in uneven-aged spruce and fir stands, with an 80% share of fir found the number of trees was 802 pcs·ha -1 and on Mt. Tara in structurally even-aged mixed forest of fir and spruce with a 91 % share of fir, the determined number of trees was 763 pcs·ha -1 .
In mixed forests of spruce and fir on Mt. Kopaonik, the number of stems ranges from 440 pcs·ha -1 as has been registered in SP12 to 931 pcs·ha -1 pieces in SP1, i.e. an average of 645 pcs·ha -1 (Table 2). Such a number of trees means    Legend: N -number of trees; G -basal area; dg -diameter of mean stand tree at the cross section; dg max -mean diamater of the 20% thickest stems in the stand; hg, hg max -heights corresponding with the listed mean diameters; V -volume; Iv -current volume increment; p iv -percentage of volume increment high forest cover percentage of the study stands indicating their high stability and multifunctionality (ecological, protective and socio-economic value). Most of the SPs (other than SP 6-9) are dominated by spruce in the total number of trees ( Figure 1). The average share of this species accounted for 59.9% (from 34.9% in SP7 to 94.4% to in SP2), while the average share in the total number of fir trees is 40.1% (from 5.6% in SP2 to 65.1% in SP7). Only in SP9 and SP10 (diameters only) fir achieves slightly larger tree dimensions compared to spruce. In all other investigated cases larger dimensions (often significantly) of diameter and height were measured for spruce trees. The dg of fir ranges in the interval from 19.6 to 38.2 cm, and for spruce in the interval from 26.0 to 46.7 cm, with corresponding heights ranging from 17.4 to 30.3 m and from 20.9 to 30.7 m, respectively. In comparison to fir, the dg of spruce is bigger by 7.4 cm and hg by 4.3 m. Dimensional superiority of spruce can also be seen in diameters and heights of 20% of the thickest trees in the stand. The dg max of fir is 28.8-59.3 cm, and of spruce 35.3-66.6 cm. The corresponding heights for these diameters are from 21.3 to 34.3 m fir, and for spruce 23.9 to 36.0 m. In comparison to fir, dg max of spruce is by 5.4 cm longer, and hg max by 2.5 m. As mean trees are statistically representative of stands, it is possible to conclude that spruce is dominant in comparison to fir in terms of dimensions. It can also be concluded that the variation in diameter and height, i.e. dimensional diversity is high for both tree species in their mixed forests on Mt. Kopaonik.
In terms of diameter structure ( Figure 2) the investigated stands are characterized by a great variety of structural forms, ranging from structures associated with even-aged stands (SPs 2, 3 and 7) and two-storey stands (SPs 5,8,9 and 10) to typical multi-storey, uneven-aged stands (SPs 4, 6, 11 and 12). The flow of cummulative curves of distribution is in most cases caused by the structure of spruce, which dominates in the total number of trees. Marked structural heterogeneity is an important element of the value of mixed forests of spruce and fir on Mt.Kopaonik. The distribution is dominated by thin and medium thick trees, with a minimum participation of trees with large dimensions. A 47.1% share of the total number of trees are in diameter classes of up to 30 cm. The 31-50 cm diameter classes account for 44.6%, while for spruce trees this category accounts for 66.4%, and 33.6% for fir. The share of 8.3% of the total number of trees are in the largest classes (over 50 cm). In this part of the inventory spruce is also prevalent (78.1%) compared to fir (21.9%).

Height structure and height curves
The cumulative curves of height structure in the study stands ( Figure 3) are characterized by an irregular flow with one, two or more maximums of the number of trees represented at different height classes. The height structure of SPs 1, 2, 7 and 8 are flared in shape, (more or less) asymmetrical to the right, with a clear peak at the class of 22,5 m (SP1 and SP2), and in the class of 28.5 m (SP7 and SP8).
The position of the maximum and basic flow of the cumulative curve of distribution are primarily the consequence of spruce distribution into height classes. The height structure of SPs 3, 4, 5, 6, 9, 10, 11 and 12 is characterized by multiple notching of the curves of distribution of trees and marked variation width, which confirms their uneven-aged structure. The appearance of two peaks in the height classes of 7.5 m (10.5 m) and a more pronounced peak at 31.5 m (28.5 m) in these sample plots clearly indicate the dominance of short to medium-high trees (correlation with the diameter structure-domination of thin and medium thick trees). The height structure of mixed forests of spruce on Mt. Kopaonik classifies them into structurally heterogeneous forests with a wide range of forms -from even-aged to uneven-aged stands (in the broadest sense). In terms of multi-functionality, and ecological stability and value, this is an important element of the value of these forests.
The height curves of spruce and fir (Figures 4  and 5) in SPs 1 and 2 were much flatter and less steep than the curves of other SPs, which is the result of the minimum share of trees with medium-large dimensions, a complete lack of trees of large dimensions and certain dynamic disturbanc-es in these two stands. In the other stands, the curves of both tree species are grouped, with marked steepness indicating good growth in height and a positive competitive ratio of spruce and fir in these sites. Slightly steeper height curves of fir compared to spruce clearly indicate a different dependance of this species on light.

Forest productivity
As a consequence of a high forest cover percentage and significant share of trees with medium-large diameter dimensions, the average value of basal area in spruce and fir forests (Table 2) is 58.4 m 2 ·ha -1 , with a 68.1% share of spruce and 31.9% share of fir. The volumes are in the range from 450 to 1,340 m 3 ·ha -1 and indicate high productivity of the study stands, with the average volume of 776.9 m 3 ·ha -1 and the spruce: fir mixture ratio of 69.3%: 30.7%. The current volume increment ranges from 11.2 to 21.9 m 3 ·ha -1 , with an average of 14.2 m 3 ·ha -1 , and its major part (68.1%) belongs to spruce, while fir accounts for 31.9 % of the value of this element. The absolute value of the current volume increment and the percentage increment ranging from 1.6% to 2.5% (on average 1.9%) are slightly higher in fir, which confirms the already stated high productivity of these forests. In mixed forests of spruce and fir with a small share of white pine on Mt.Zlatar, Stojanović et al. (2008) found an average volume of 470.5 m 3 ·ha -1 , a current volume increment of 13.9 m 3 ·ha -1 , and the increment percentage of 2.95%. In the research (Govedar, 2005) in the Republic of Srpska, where these forests occupy significant areas, the values of current volume increment range from 8.9 to 16.6 m 3 ·ha -1 in uneven-aged two-storey stands (with an average volume 389.0-716.3 m 3 ·ha -1 ) and from 10.9 to 11.8 m 3 ·ha -1 in uneven-aged multi-storey stands (average volume of 440-507 m 3 ·ha -1 ). In the uneven-aged, mixed forests of spruce and fir in the eastern Alps in Italy (Veneto region) the average annual increment is 6.0 m 3 ·ha -1 and an average volume is 350 m 3 ·ha -1 (Bagnaresi et al., 2002;Grassi et al., 2003).
On the whole, all the study stands are characterized by ecological stability and vitality, which, tohether with the site potentials resulted in high productivity. The altitude of the site (1,390-1,600 m above sea level) and its features described in chapter 2.1. are suitable for the growth of both these species of trees as typical sciofita. The upper limit of fir distribution and powerful anthropogenic impacts on this type of trees exercised through past management procedures influenced spruce as the current holder of the productivity of these forests.

CONCLUSIONS
Mixed spruce and fir forests are most commonly climate regional forests located at the transition between mixed three-dominant forests of spruce, pine and beech, and pure spruce forests. Although their share in forest resources Serbia is modest, because of the high productivity, environmental stability and structural complexity, they represent its significant part. A prerequisite for the conservation of these forests, among other things, is good knowledge of their structural and production characteristics, tree species-interactions among themselves and with the site, which serves as the basis for correct planning and subsequent approach to the management of these forests. That was the main motive of these studies, that reached the following conclusions: 1. The forest cover percentage of these forests is high, and the spruce dominates in the total number of stems with 59.9%, while the share of fir is 40.1%; 2. Thin and medium-thick trees dominate, with minimal involvement of large dimension trees, and dimensional superiority of spruce. In addition, both species showed pronounced dimensional diversity; 3. Mixed forests of spruce and fir are characterized by a great variety of structural forms, from structures associated with even-aged stands to two-storey and typical multi-storey uneven-aged stands; 4. The average volume of 776.9 m 3 ·ha -1 and the mixture ratio spruce: fir = 69.3%: 30.7%. The average value of the current volume increment is 14.2 m 3 ·ha -1 , and the increment percentage ranges in the interval 1.6% -2.5% (1.9% on average), which indicates high productivity of these forests; 5. As a consequence of strong anthropogenic pressures on fir in the past (the primary cause), and the fact that in the studied sites on Mt. Kopaonik it is near its upper limit of vertical distribution, spruce is the current holder of productivity in these forests; 6. Because of its small share in the growing stock of Serbia, spruce can be considered a very rare and valuable tree species, which enriches forest types with its presence and indicates the preservation of natural potentials in general, especially in the case of relict communities. Therefore, the need to increase the share, conservation and protection of fir among the borders of its current distribution is emphasised, which represents a long-term task of management in these forests; 7. The main long-term tasks in the management of spruce and fir forests on Mt.Kopaonik are as follows: the conservation of structural variety and stability, the maintenance of good forest cover percentage and mixture, and increase in the share of fir in the existing growing stock (when filling gaps in these forests fir should be the priority species).