A CONTRIBUTION TO THE SESSILE OAK SITE AND BEECH SITE DEFINING IN CENTRAL SERBIA

The study was carried out in sessile oak forests and beech forests in the region central Serbia. The stands are classified as pure stands with the percentage of other species up to 10% per tree number, mixed forests of sessile oak with other species, and mixed forests of beech with other species, whose percentage does not exceed 50%. Altogether 257 stands were monitored 202 beech stands and 55 sessile oak stands. By the applied method of defining the local heat potential (Luj i ć , 1960), modified by R a t k n ić et al. (2001) and K r s t i ć (2004, 2008), which represents possibility of soil heating without vegetation, were determined. In this way, a scale of 162 possible combinations of local heat potential was obtained, which explains more precisely the dependence of beech stands and sessile oak stands on the topographic conditions. By applying the weighted values of the thermal co-ordinates of aspect and slope (E) for each altitudinal belt of 100 m, it was concluded that pure stands have the widest ecological range. Pure beech stands occur at the sites with 34 combinations of thermal co-ordinates E.V=4.6 to 8.12. Pure sessile oak stands occur at the sites with 12 combinations of thermal co-ordinates E.V=5.10 to 8.11. The percentage of mixed beech stands with other broadleaf species is the highest at the sites with the co-ordinate V=10-11 (at the altitudes between 700 and 900 m) is about 60 %. Mixed stands of sessile oak and beech are located on the terrains with combinations of thermal co-ordinates E.V=7.9 to 8.12. By using the local heat potential of a region, it can be identified which sites, i.e. which combinations of exposure, slope and altitude belong to the particular tree species. Consequently, a more reliable selection of tree species can be done for the bio-reclamation of barrens and other deforested terrains.


INTRODUCTION
Occurrence and survival of vegetation in a region depends on a great number of ecological factors, i.e. it cannot be explained by one group of factors only, because they are interdependent, conditioned and replaceable.However, one group of factors is always dominant; consequently they become the decisive factors.In this sense, the topographic factors are very significant, because in the conditions of the pronounced relief, with frequent and sudden changes of aspect or slope, the environmental conditions change in a relatively small space.They modify the other ecological factors, primarily the climate, so they become dominant in the spatial distribution of forests of the specific tree species.Aspect and topographic position are important determinants of local temperature conditions (K i m m i n s , 2004).According to O h s a w a (1990), altitudes and temperature conditions at the forest limit, latitudinal shift in the temperature limit for correlation of forest, zonation along latitudinal gradients and mountain vegetation.Altitudinal change in temperatures has a major effect on the distribution of plants and vegetation along an elevational gradient in mountainous areas (Ta n g , Fa n g , 2006).
According to A l l e n et al. (2006), similar thematic was treated in many researches: monthly mean solar radiation for horizontal and inclined surfaces, global daily radiation on any surface, estimating solar radiation on slopes of arbitrary aspect, and modeling solar radiation in steeply sloping terrain.However, there is few data about influence of these parameters on the distribution of forests of certain tree species.
The significance of topographic factors and their undoubted relationship with the alternation of the vegetation types was reported by Leibundgut (1951), Bunuševac (1951), L u j i ć (1960), K o l i ć (1972), C o g b i l l and W h i t e (1991), K r s t i ć (2000), K r s t i ć et al. (2001,2002), S m a i l a g i ć et al. (2002), K i m m i n s ( 2004), Ta n g and Fa n g ( 2006), H a y e s et al. (2007), etc.
Dealing with the effect of topographic factors on the heating up of the treeless area at a locality and the significance of soil temperature on the distribution of individual forest communities, L u j i ć (1960) introduced the term local heat potential.The degree of heat which characterises the combination of exposure and slope was named their thermal co-ordinate denoted by E, and the degree of heat of a specific altitude was named their thermal co-ordinate V, and each was enumerated from 1 to 9. The potential of a concrete locality heating up to the altitude of 1,800 m can be expressed by 81 combinations of the above numbers.The approximate idea on the potential of heating of any locality can be obtained by the comparison of the local thermal factors in a region.The significance of such research was indicated by K r s t i ć (2003,2004,2006), K r s t i ć et al. (2001,2002), S m a i l a g i ć et al. (2002), etc., and it was implemented in the papers by R a t k n i ć et al.
Based on the above, the aim of this paper is to define more closely, in the framework of other ecological factors, the effect of the topographic factors on the occurrence and distribution of sessile oak forests and beech forests in central Serbia, i.e. to express more precisely their correlation.

STUDY AREA, MATERIAL AND METHOD
The study was performed in the Župa region in central Serbia.The latitude of the study region is between 43º26' and 43º33' N, and longitude between 18º28' and 18º44' E. As in the study region the difference in latitude is lower than 1°, according to L u j i ć (1960) and K r s t i ć (2004,2008), the deviations in the intensity of radiation are very low, so the degree of the terrain heating up will not be significantly affected.
The study was carried out in sessile oak forests (Quercus petraea (Mat) Liebl.) and beech forests (Fagus moesiaca /Domin, Maly/Czeczott.) in central Serbia.All forests in this study are treated without the definition of plant communities, origin, age, development phase, and stand structure.To identify the dependence of the occurrence and distribution of thus classified stands on the topographic factor, the data were collected on the altitude, exposure, slope and terrain configuration.Special attention is devoted to stand composition.The stands are classified as absolutely pure (without the presence of other species), pure stands with individual percentage of other species, pure beech stands and sessile oak stands with the percentage of other species up to 10% per tree number, and mixed forests of beech and sessile oak with other species, whose percentage does not exceed 50%.Altogether 257 stands were monitored -202 beech stands and 55 sessile oak stands, i.e.: -40.0% absolutely pure sessile oak stands, 31.0%pure stands (with the percentage of other species up to 10%), and 29.0% mixed stands with the percentage of other species below 50%; -40.0%absolutely pure beech stands, 39.7% pure stands (with the percentage of other species up to 10%), and 20.3% mixed stands with the percentage of other species below 50%.
These methods encompass all beech stands and sessile oak stands where they are dominant, and the localities in the "zone of competition" for the site.
As between the absolutely pure stands and pure stands with percentage of other species up to 10% there is no significant difference according to thermal co-ordinate E, in further text they are presented together (Table 1).The data processing and the analysis of results are performed according to L u j i ć's method (1960), modified by K r s t i ć (2004,2008) by defining the local heat potential.The thermal co-ordinates of exposure (aspect) and slope, and the co-ordinates of the altitude are determined for each study stand.The data are grouped according to the local heat potential per altitudes in the belts of 100 m.For the more precise definition of the dependence of sessile oak stands and beech stands on topographic factors, the weighted mean value of thermal co-ordinates of exposure and slope (E) were calculated for each altitudinal belt of 100 m, i.e. thermal co-ordinate (V).
The co-ordinates of exposure and slope (E=1-9) are retained and the co-ordinate of altitude is amended.Instead of Lujić's nine-degree scale (V=1-9, one thermal degree =200 m), the eighteen-degree scale (V=1-18) was applied, where one thermal degree signifies 100 m (K r s t i ć , 2004, 2006, 2008, R a t k n i ć et al., 2001).This resulted in a more sensitive scale of 162 possible combinations of the local heat potential, which explains more precisely the dependence of beech and sessile oak stands on the topographic conditions, because the altitudinal change in temperatures has a major effect on the distribution of plants and vegetation along an elevational gradient in mountainous areas, and mean air temperatures generally colder at the higher elevation site than at the lower site (B u n u š e v a c , 1951, K i m m i n s ,  , 2002).Soil temperatures compared to air temperatures suggest that they were significantly damped relative to air temperatures.

The distribution of stands based on the local heat potential
The distribution of beech stands and sessile oak stands in the study region per altitudes is shown in Table 1, and the percentage of stands, based on the local heat potential (E.V), is shown in Table 2 and 3. Based on the mean values of the thermal co-ordinate of aspect and slope (E), we can see the significant differences in the distribution of sessile oak and beech stands of different composition and mixture.It should be noted that sessile oak stands and beech stands are absent in the combinations of the thermal co-ordinates which represent the cooler sites, e.g. the thermal co-ordinate E=1-3 (Table 2 and 3).These are the northern exposures with the slope above 30°, i.e. northeast and northwest exposures with the slope above 43°.They are also absent in the warmest sites with thermal co-ordinate E=9, i.e. the terrains of the southern exposures, slope 28-47°.

Sessile oak stands
In the studied region, pure sessile oak stands range within the altitudinal belt 700-1,100 m (Table 1).Other species, associate species in pure sessile oak forests, such as Quercus cerris L., Fraxinus ornus L., but also Fagus moesiaca Domyn, Maly/Czeczott, etc., occur individually in 25% of stands.Mixed forests of sessile oak with other species occur throughout the range in this region.Mixed forests of sessile oak and Turkey oak occur at lower altitudes -up to 700 m.It should be emphasised that mixed forests of sessile oak and beech occur at their transient sites -on wider ridges at all exposures, in narrow shaded hollows of southern exposure.
The analysis of the above data on the percentage of sessile oak stands in the study region is presented based on the local heat potential in Table 2 and Figure 1.Stands of sessile oak are the most frequent on the sites with thermal co-ordinate E=8, i.e. the terrains of the southern exposures, slope 8-27º.This is certainly because these sites are somewhat warmer.
Pure sessile oak stands have the widest ecological range.In the studied region pure sessile oak stands range within the altitudinal belt 700-1,100 m at the sites whose thermal co-ordinate E is between 5 and 8.The highest percentage of pure stands is on the sites with thermal co-ordinate E=8, as much as 64.1%, and at the altitude between 700 and 1,100 m (co-ordinate altitude V=8-11).At the altitudes between 700 and 1,000 m, i.e. the thermal co-ordinates of the altitude V=9 to 11, the percentage of pure stands is as much as 87%.Only 12.8% of pure stands occur at the altitudes above 1000 m.
The percentage of mixed sessile oak stands with other broadleaf species is the highest also at the sites with the co-ordinate E=8 with 44% and at the altitudes between 600 and 700 m.They most frequently occur at the sites which are warmer.This is certainly the result of the occurrence of mixed forests with the xerophilous Turkey oak.Above this altitude, mixed sessile oak-beech stands occur at the sites with the thermal co-ordinates E=7-8 and V=9 to 11.That indicates warmer sites of the southern aspect and higher slopes.
Mixed forests of sessile oak and beech occur also at the sites with thermal co-ordinate E=8, or very warm sites, i.e. southern exposures of high slope.

Beech stands
The analysis of the data on the percentage of beech stands in the study region is presented based on the local heat potential in Table 3.
Pure beech stands also have the widest ecological range within the altitudinal belt 400-1,300 m.They occur at the sites with 34 combinations of thermal co-ordinates E.V=4.6-8.12.The highest percentage of the pure stands is at the sites with thermal coordinate E=5, as much as 32.9%, and at the altitude between 700 and 900 m (co-ordinate altitude V=10-11).At the altitudes between 600 and 1,000 m, i.e. the thermal co-ordinates of the altitude V=9 to 12, the percentage of pure stands is 75%.Only 1.9% of pure stands occur at the altitudes above 1,200 m.The percentage of mixed beech stands with other broadleaf species is the highest also at the sites with the co-ordinate V=10-11 (at the altitudes between 700 and 900 m) is about 60%.In the mountain region, on the localities within the "zone of competition" of the sessile oak and beech sites, Sessile oak is dominant on the warmer exposures.Only 5% of mixed beech stands occur at the altitudes above 900 m.
Mean values of the thermal co-ordinate E Thermal co-ordinate E was multiplied with the number of stands for each thermal co-ordinate in the altitudinal belt of the 100 m.Those values are cumulated, and total cumulated values are divided is with total number of stands in the concrete altitudinal belt of the 100 m.Mean value of thermal co-ordinate E was derived that way, and shown in the Table 4.
The average mean values of the thermal co-ordinate E in pure sessile oak stands is 7.41, in mixed stands where it is the dominant species -6.75.
The average mean values of the thermal co-ordinate E in pure beech stands is 5.76, in mixed stands where it is the dominant species -5.7.

Distribution of sessile oak stands and beech stands by sites based on the local heat potential
Taking into account the uneven percentage of the sites with the local heat potential on which pure or mixed stands are located, to calculate the combination of thermal co-ordinates on which their percentage is the highest, we calculated the weighted average percentage of each combination for the stands of each of the above groups by composition and by mixture.Based on the above, Table 5 presents the combinations of the local heat potential with the percentage of each group of stands by mixture occurring in the percentage close to the average or above it.This reflects more really the dependence of the occurrence of sessile oak stands and Beech stands on topographic factors defined by the local heat potential.So such localities can be considered as the sites of sessile oak, sites of beech, and the sites of sessile oak with beech., i.e., the sites of beech with sessile oak.
The data shown in Table 5 indicates that the ecological range of pure sessile oak stands in this region is rather narrowed.It ranges from the combination of the thermal coordinates E.V=8.9 to 8.10.Sessile oak stands are absent at the cooler sites with the thermal co-ordinate E=1-5, and warmer sites with the thermal co-ordinate E=9.The mixed stands of sessile oak and beech (where oak is the dominant species) are located at the sites of thermal co-ordinates E.V=6.12, 8.10 and 8.12.
The data shown in Table 5 also indicates that pure beech stands in this region range from the combination of the thermal co-ordinates E.V=4.8 to 8.11.Beech stands are absent at the cooler sites with the thermal co-ordinate E=1-3.Mixed stands of beech and sessile oak (where beech is the dominant species) are located at the sites of thermal co-ordinates E.V=4.10-8.11.These thermal co-ordinates indicate the sessile oak site, where it is the dominant species, beech site, where it is the dominant species, and their transient site.

DISCUSSION
Aspect and topographic position are important determinants of local temperature conditions.According to K i m m i n s ( 2004) and Wa n g et al. (2004), both latitude and altitude influence the radiation budget, while the altitude of alpine timberline changes significantly along latitudinal and longitudinal gradients.Zonation along latitudinal gradients and mountain vegetation at the forest limit depend on their latitude -differ with the latitude in which mountain is located (O h s a w a , 1990).According to Vu k i ć e v i ć (1991), extreme conditions of relief, parent rock, soil and micro-climate, and consequently by very much degraded forests which are, over a large area, changed into bare lands and rocky grounds.Site defining, which is adequate for the concrete tree species, will be establish plant communities which will use maximally production site potential, and great impact on ecological conditions (S t a m e n k o v i ć et al., 2000).The nature of site for aforestation is criteria decisive for the choice of nursery stock production technology (I s a j e v et al., 2000).
Due to the change of the climate factors, growing season, floristic and the forest limit with the change of the altitude and the difference between the northern and southern slopes was more pronounced at lower elevations and less pronounced at higher elevations (B u n u š e v a c , 1951, Ta n g , Fa n g , 2006, O h s a w a , 1990, K i m m i n s , 2004, Fa n g , Yo d a , 1988, etc.).
By the applied method of defining the local heat potential, which represents possibility of soil heating without vegetation, a scale of 162 possible combinations of local heat potential was obtained, which explains more precisely the dependence of beech stands and sessile oak stands on the topographic conditions.The significance of such research was indicated by K r s t i ć (2003,2004,2006), K r s t i ć et al. (2001,2002), S m a i l a g i ć et al. (2002), etc., and it was implemented in the papers by R a t k n i ć et al. ( 2001) and K r s t i ć (2004,2008).
The results of numerous measurements and researches of solar radiation, light, temperature and evaporation at different exposures and slopes in Serbia by B u n u š e v a c (1951), S o l j a n i k (1960), L u j i ć (1960), K o l i ć (1972), K r s t i ć (1986,2004,2008) -the intensity of the above phenomena, starting from the southern exposure, via eastern and western exposures, decreases and the minimum is attained on the northern exposure; -the warmest combination of exposure and slope in our region is the southern exposure with the slope of 40-50°; -the difference between the warmest and the coolest point in a region depends on the combination of the terrain configuration elements; -the thermal difference between southern and northern exposures with gentle slopes is insignificant, so the division into warm and cool exposures is unacceptable if the slope is not also taken into account, which means that there is only a "warm and cool combination" of exposures and slopes.In the studied region, pure sessile oak stands range within the altitudinal belt 700-1,100 m at the sites whose thermal co-ordinate E is between 5 and 8.According to K r s t i ć (2008), in the region of eastern Serbia, they range between E=3 and 9.In the region of Grdelička Klisura, between 6 and 9 (L u j i ć , 1960), and in the region of western Serbia, between 4 and 9 (R a t k n i ć et al., 2001).
Pure beech stands range within the altitudinal belt 400-1,300 m.They occur at the sites with 34 combinations of thermal co-ordinates E.V=4.6-8.12.They are the sites whose thermal co-ordinate E is 4-8.In the region of Grdelička Klisura, according to L u j i ć (1960), they range between 2 and 9, in the region of eastern Serbia between 3 and 9 (K r s t i ć , 2004), andin the region of western Serbia, according to R a t k n i ć et al. (2001), between 3 and 8.
Mixed forests of sessile oak and beech occur also at the very warm sites with thermal co-ordinate E=8, i.e. southern exposures of the high slope.The reason for this is the effect of the fourth topographic factor -terrain configuration.Namely, because of the shading of narrow valleys by the surrounding hills, regardless of the exposure, such localities are cooler and moister, so that beech can grow there, i.e. there is a well-known inversion of vegetation.This is consistent with R i c h a r d s o n et al., (2004) conclusion that the microclimatology of mountain landscapes is dependent on the latitude, continentality and topography.Although the effect of elevation on climate has been long recognized, long-term and systematic in situ measurements are very limited in mountainous areas.
In the mountain region, on the localities within the "zone of competition" of sessile oak and beech sites, sessile oak is dominant on the warmer exposures.Only 5% of mixed Beech stands occur at the altitudes above 900 m.This is consistent with Pe n d r y and P r o c t o r 's (1996) conclusion that changes in forest structure and species composition are in correlation with the changes of temperature with altitude.
The average mean values of the thermal co-ordinate E in pure sessile oak stands is 7.41, in mixed stands where it is the dominant species -6.75.According to L u j i ć (1960), in sessile oak forests in southeastern Serbia, the average value of the thermal co-ordinate E amounts to 6.85.In sessile oak forests in the region of northeastern Serbia, the average mean values of the thermal co-ordinate E in pure sessile oak stands is 7.42, in mixed stands where it is the dominant species -7.24.In the mixed stands of sessile oak and beech, mean values of the thermal co-ordinate E is 7.12, in stands with Turkey oak 7.0 to 8.0.The above data indicate clearly the xerothermisation of the sites with the higher value of the co-ordinate E, which is manifested by the gradual occurrence of all the more thermophilous species in sessile oak forests, to the occurrence of mixed forests of xeromesophilous sessile oak and xerophilous Turkey oak, i.e. their distribution on the increasingly warmer combinations of exposure and slope (K r s t i ć , 2008).
The average mean values of the thermal co-ordinate E in pure beech stands is 5.76, in mixed stands where it is the dominant species -5.7.In Beech forests in northeastern Serbia, the average value of the thermal co-ordinate E amounts 6.12 to 7.24 (K r s t i ć , 2004), which indicates that in studied region beech sites are more mesophilous.In beech forests in southeastern Serbia, the average value of the thermal co-ordinate E amounts to 5.12 (L u j i ć , 1960).
The thermal co-ordinates indicate the sessile oak site, where it is the dominant species, beech site, where it is the dominant species, and their transient site.
By using the local heat potential of a region, it can be identified which sites, i.e. which combinations of exposure, slope and altitude belong to the particular tree species.Consequently, a more reliable selection of tree species can be done for the bio-reclamation of barrens and other deforested terrains.

CONCLUSIONS
Topographic factors, for a specific altitude and the combination of aspect and slope, characterise the potential heating up of the soil of each locality.A closer definition of the effect of topographic factors on the distribution of sessile oak forests and beech forests in Župa in central Serbia, i.e. a more precise account of their dependence, was accomplished by the determination of the local heat potential and the local thermal factor.
Sessile oak stands in this region occur at the altitudinal belt of 700-1,100 m a.s.l.The percentage of pure stands is the highest -64%, at the sites with thermal co-ordinate of aspect and slope E=8.Mixed forests of sessile oak with other broadleaf species, at their transient site, have the highest percentage at the sites with these co-ordinates also E= 8. Pure sessile oak stands occur at the sites with 12 combinations of thermal co-ordinates  (2004,2008), који представља могућност загревања земљишта без вегетације.
2004, K r s t i ć , 2005, 2007, K r s t i ć et al., 2005, Ta n g , Fa n g , 2006, Fr i e d l a n d et al., 2003, S m a i l a g i ć et al.

Table 1 .
Distribution of the study stands by mixture and altitudes

Table 5 .
Distribution of Sessile oak and Beech stands by sites in Župa region in Serbia