THE IMPACT OF CLIMATE ELEMENTS ON THE DIAMETER INCREMENT OF AUSTRIAN PINE ( Pinus niGRa ARN . ) IN SERBIA

The aims of this paper were the analysis of climate elements and Thornthwaite’s climate index, the study of the development of diameter and current diameter increment of the Austrian pine trees from several artificial stands in the Belgrade area, as well as an analysis of the influences of the pluviometric regime and climate index on the current diameter increment of the Austrian pine over the period 1985-2010. By comparative analysis and statistical data procession, the correlation between the diameter increment and climate characteristics was determined. In spite of the wide-held view that the Austrian pine is a species that cannot make use of the capacity of the best sites where it grows, is to a great extent able to react to increased humidity, one of the most important site factors upon which the growth of basic quantitative tree parameters directly depends.


INTRODUCTION
Since numerous studies on global warming and potential changes of temperature and humidity point to a very wide range of effects and impacts, both on the forest ecosystems in general and on the individual trees (Smith et al., 1989, Andrasko, 1990, Botkin et al., 1992), further knowledge of the interrelations and interactions of plant-climate parameters is of growing importance.
The reforestation and establishment of conifer plantations in Serbia was very intensive during the last decades of the 20th century, and conifer species were used not only for the reforestation of barren soil, but introduced into forests of different tree species and degrees of degradation and sites of different production characteristics.The plantations and artificially established Austrian pine stands (Pinus nigra Arn.) are dominant, and they occupy an area of 86,000 hectares in Serbia (Banković et al., 2009).The studies of the Austrian pine are complex, as it is one of the most important ameliorative and pioneering species in the forestry of Serbia.
The purpose of this study is to assess the climatic sensitivity of the Austrian Pine (Pinus nigra Arn.) in the Belgrade area through the influence of climate on radial growth.This paper is aimed at an analysis of the climate elements and Thornthwaite's climate index, study of the development of diameter and current diameter increment of the Austrian pine trees from several artificial stands in Belgrade area, and analysis of the influences of the pluviometric regime and climate index on the current diameter increment of the Austrian pine over the period 1985-2010.

MATERIALS AND METHODS
The research was conducted in three artificially established Austrian pine stands aged 45 years, at three sites in Belgrade area.
Site I is located in the Forest Management Unit "Avala", in Department 9e, at altitudes ranging from 270 to 280 m.a.s.l., west-exposed, inclination 0-5°, on eutric brown soil.It belongs to the coeno-ecological group of Hungarian oak and Turkey oak forest types (Quercion frainetto Ht. 1954) on brown and loessial soil.Site II is located in the Forest Management Unit "Lipovica", in Department 18e, at altitudes ranging from 260 to 280 m.a.s.l., southeastexposed, inclination 6-15°, on eutric brown soil.It belongs to the coeno-ecological group of Hungarian oak and Turkey oak forest types (Quercion frainetto Ht. 1954) on brown and loessial soil.Site III is located in the Forest Management Unit "Makis"parts of Ada Ciganlija -forests along the highway", in Department 22i, 90 m.a.s.l.level, without clearly expressed exposure, on even terrain, on chernozem.It belongs to the coeno-ecological group of pedunculate oak and European alder forest types (Alno-Quercion roboris Horv.1938.) on semigley and some automorphic soils.
Only by using the multi-annual climate data obtained by meteorological measurements can the current climate conditions be validly estimated.The data collected by the Republic Hydrometeorological Service of Serbia for the period 1985-2010 for the Belgrade Weather Station (situated at latitude 44° 8' N and longitude 20° 47' E), i.e. the arithmetic means of the series of available data, sorted by time and area (Basic Climate Characteristics for the Territory of Serbia, 2010), were used for the analysis of the climate characteristics and climate change in Belgrade.
The artificially established stands are 45 years old and found on three sites in Belgrade.
The pluviometric regime and air temperature were analyzed and the climate index was determined for each year of the observed period by using Thornthwaite's method (Thornthwaite, 1948, Thornthwaite andMather, 1955).By using the complex calculation method, which is now presented by a special computer program, based on the average monthly air temperature and average monthly precipitation, and taking into account the latitude at which an observed site is located and the duration of daylight, the calorific index (i) and the annual calorific index (I) were determined first; then by applying special logarithmic nomograms the uncorrected potential evapotranspiration (PE) was calculated, and by subsequent calculation process, the actual evapotranspiration (SE) and the water loss in the soil (M).The final results refer to the humidity index (Ih), aridity index (I a ), and climate index (I c ), based on which, by using the classification prescribed by this method, the character of the climate for the observed area was determined.The climate index calculated in this way is the result of basic climate factors (temperature and precipitation regime), coupled with basic orographic elements (geographic coordinates of the observed site and duration of daylight).Dendrochronological methods, as described by Fritts (1976) and Cook et al. (1990), were used for sampling the trees, processing and cross-dating the cores.
Dendrometric analysis was done on the dominant trees in the artificially established Austrian pine stands at three sites in Belgrade area.On 20 Austrian pine half-sib lines on established sample plots at each site, the width of the wood increment ring at breast height, i.e. diameter increment per each year of the observed period, was determined.
By comparative analysis and statistical data procession (regression analysis and analysis of variance), the correlation between the diameter increment and climate characteristics was determined.A graphic presentation of the climate indices and quantity of precipitation during a year in the growing season (April-September) and the diameter increment of the Austrian pine in the observed period is given.

Climate elements
Heat and humidity, along with other ecological factors, directly influence tree height, diameter and volume increment, as well as the quality of wood volume.If the environmental conditions are the same, the quantities of the wood volume that can be produced by the forest vegetation, and its technical value, depend upon the variation of some basic climate factors (Bunushevac, 1951).

Air temperature over the period 1985-2010
Air temperature is one of the most important climate parameters.Based on the processed data collected by the Republic Hydrometeorological Service of the Republic of Serbia, the data on air temperature in the observed period (from 1985 to 2010) for the Belgrade Weather Station are presented in Table 1.The average annual air temperature over the observed period is 12.6°C, and the average air temperature during the growing season is 19.4°C.In the hottest month of the year, July, the average air temperature is 23.2°C, while in the coldest month of the year, January, it is 1.4°C.The average annual temperature in the observed period ranges from 11.2°C (in 1985) to 14°C (in 2007 and 2008).

Precipitation regime over the period 1985-2010
The data on the pluviometric regime are presented in Table 2.The annual quantity of precipitation in the Belgrade area over the observed period was 694.5 mm of rainfall, ranging from 367.7 mm in 2000 to 1,051.2 mm in 1999.During the growing season (April-September), the average quantity of precipitation was 395.9 mm, which accounts for 53.0% of the total annual precipitation.The lowest percent of precipitation during the vegetation period, in comparison with the annual quantity, 39.8%, was reported in 2009, and the highest percent, 72.9%, was reported in 2001.The month with the greatest number of rainy days was June, with 102.7 mm of rainfall, and the month with the lowest number of rainy days was February, with 41.4 mm of rainfall.

Hydrological balance determined by using Thornthwaite's method
The climate (hydrological) index according to Thornthwaite's method is one of the most comprehensive climate summaries of some forest areas.It is one of the indirect calculation methods for determination of hydrological balance and climate type.This method is frequently applied in forestry science studies since it gives the greatest number of data on plant life, such as the quantity of spare water in the soil R (mm), during the season when there is an excess of it V (mm), and during the season when there is a lack of it M (mm).Thornthwaite introduced the term potential evapotranspiration PE (Thornthwaite, 1948, Thornthwaite and Mather, 1955, Bunushevac, 1951).
The uncorrected potential evapotranspiration (PE) refers to the quantity of water which would evaporate from the soil under certain temperature conditions when it reaches the optimal humidity, i.e. 100 mm of water up to 100 cm depth throughout a whole year, i.e. 100 l•m -2 .It was 643 mm over the observed period 1985-2010.Since evapotranspiration depends on not only on the energy-temperature characteristics, but on the latitude of a certain area as well, the uncorrected potential evapotranspiration is corrected, and thereby the second parameter, the corrected potential evapotranspiration PE, is determined.It was 766 mm in the Belgrade area.
The actual evapotranspiration SE is the real quantity of water that is released by transpiration from the soil or evaporates from the soil when it reaches optimal humidity.It depends on both the energy-temperature conditions and the quantity of precipitation P, and in this instance, it was 600 mm.
The humidity excess occured not only at the beginning of the year, in January, February and March, but in December as well, and was equal to 95 mm (as it does not occur during the growing season, it is of lesser importance to the plant).The lack of water, which was equal to 166 mm, occured in July, August and September, and had an adverse effect on plant increment (Table 3, Graph 1).

Climate index determined by using Thornthwaite's method
The humidity index Ih, aridity index Ia and climate index Ic in the Belgrade area, as well as climate types determined using Thornthwaite's classification (Thornthwaite, 1948) in the Belgrade area in the observed period are shown in Table 4.These data are important for comparative analysis with the trends of the current diameter increment of Austrian pine.
The average general climate index in Belgrade area over the period 1985-2010 was -0.6328, and the climate type was dry sub-humid (C1).The average general index ranged from -47.451 (in 2000) to 37.8388 (in 1999), i.e. the climate ranged from arid (E), to mild humid (B1).
The graphical representation of the climate index (Im) per year over the observed period (Graph 2) points to the periodical variations of the dry and humid periods, with some greater or smaller oscillations.

Diameter increment
The average current diameter increments of half-sib lines on Site I ranged from 1.052 mm in 2003 to 2.283 mm in 1985, and the average annual current diameter increment was 1.571 mm.The lowest current diameter increment of the half-sib lines from Site II was 1.075 mm (in 1993), the highest current diameter increment was 1.849 mm (in 2005), and the average current diameter increment regarding   the observed period was 1.482 mm.On Site III the lowest current diameter increment was reported in 1993 (0.857 mm), while the highest current diameter increment was reported in 1999 (2.271 mm), and the average current diameter increment was 1.641 mm (Table 5).
The variance analysis shows that there was no statistically significant difference among the current diameter increments of the trees from three sites (Tables 5, 6, 7).The average current annual diameter increment of the stands from the three sites was 1.565 mm, since the current annual diameter ranged from 0.857 mm to 2.283 mm.
The development of diameter and current diameter increment of the half-sib lines of Austrian pine from the three observed sites, as well as the average value of these parameters over the period 1985-2010, is presented in Graph 3.
Analysis of the trends of the average current diameter increment of all half-sib lines per site and per year over the observed period showed not only the expected growth, but the significant oscillations in the years when the climate conditions were the most important site factor directly influencing the variations of the elements of growth.

The impact of climate elements on diameter increment
The correlation coefficients, presented in the Correlation matrix (Table 8), show that there was a strong correlation between the trends of the current diameter increment and climate parameters (annual sum of precipitation, sum of precipitation in the growing season, and climate index).
Based on this, it can be concluded that the climate conditions were the significant site factor directly influencing the changes in the current diameter increment.
The trends of the diameter increment, quantity of precipitation (annual and during the growing sea-   The increase in the quantity of precipitation over a year, and particularly during the growing season, leads to an increase in the diameter increment, and vice versa. Sufficient rainfall with good distribution during the year also has an excellent influence on the radial growth of Austrian pine (Pinus nigra Arn.).Based on results of this work, it is not recommended to introduce this species to other habitats at a lower altitude, with higher average annual temperatures and lower annual rainfall because this will affect the quantity and quality of the woody biomass.Harmful non-biofactors will influence biofactors as well, which will negatively affect the condition not only of some trees, but also of the whole plantation.
Similar conclusions were presented by other researchers, such as Tokar and Krekulova (2005) in Slovakia, Koprivica and Matovic (2004) in the Ibarska gorge in Serbia, Vukin and Isajev (2006) in the area of Jelova Gora in Serbia, in studies of Austrian pine (Pinus nigra Arn.); Zafirov (2005) in the area of Vitosha Mountain, Bulgaria, Kilgore and Telewski (2004) in Northern Michigan, North America, in research into the Scots pine (Pinus sylvestris L.); Wimmer and Grabner (2000) in the Eastern Ore mountains in Germany, Kilgore and Telewski (2004) in Northern Michigan, North America, with Norway Spruce (Picea abies L.), etc. CONCLUSION All activities regarding the monitoring and study of climate change and its influence should be designed in order to provide long-term systematic, comprehensive and continuous monitoring and forecasting of the conditions of the local and regional climate.Based on the results of research into the impact of climate elements on the current diameter increment of Austrian pine trees from three artificially established stands at three sites in the Belgrade area in Serbia, the following conclusions can be made: -there is a strong correlation between the trends of the current diameter increments and annual sums of precipitation, and between the sums of precipitation in the growing season and climate indices; the increase in the quantity of precipitation over a year, particularly during the growing season, leads to an increase in the diameter increment, and vice versa; -climate conditions are an important site factor, which directly influenced the trend and changes in the current diameter increment; -the Austrian pine is a species which is to a great extent capable of reacting to increased humidity, one of the most important site factors upon which the growth of the basic quantitative tree parameters directly depends.
The complexity of forest ecosystems and the numerous ways in which society uses and misuses them, as well as the fact that the forest ecosystems in Serbia are endangered by different factors, requires the ap- plication of a range of different protective measures (Bunushevac, 1951).
The use of dendrochronological techniques to compare the impact of climate elements on the diameter increment of Austrian pine (Pinus nigra Arn.) may elucidate ecological tolerances, or range limits, and consequently adaptability, in particular environments.

Graph 2 .
-Annual climate index in Belgrade area, over the period 1985-2010.
The development of diameter (a) and current diameter increment (b) of Austrian pine trees over the period 1985-2010.
son) and annual climate indices presented in Graph 4, clearly show that the curve of the current diameter increment follows the trend of the annual sums of precipitation, precipitation during the growing season, and the climate index per year of the observed period.

Graph 4 .
-The correlation between the current diameter increment and annual sum of precipitation and the quantity of precipitation during the growing season, and climate index.

Table 1 -
The average monthly and annual air temperatures in Belgrade area (°C), over the period1985-2010.

Table 2 .
-Monthly and annual sums of precipitation in Belgrade area (mm) over the period

Table 4 .
-Humidity index, aridity index, climate indices and climate types determined by using Thornthwaite's method regarding Belgrade Weather Station, over the period 1985-2010.