DIGITALNA ARHIVA ŠUMARSKOG LISTA
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four weeks before each measurement had a sudden drop during late August and first half of September. During this severe drought period, at Vidlič and Kopaonik, there was no rainfall 4 weeks prior to measurement date while on Tara and Fruška gora it had minimal values of total 0.2 mm and 4.4 mm, during four weeks. Joined correlation for all localities and species was positive and significant between the rate of photosynthesis (P) and WUE (Table 6). The WUE was in a significant positive correlation with the four week precipitation, but it was negatively correlated with the transpiration rate. Precipitation was in significant negative correlation with the four week average temperature. DISCUSSION Rasprava The correlation between low amount of rainfall, and reduced photosynthetic activity was most obvious on two localities which are positioned on the upper slope or top ridge of the mountain (Vidlič and Fruška gora). Due to their elevated position, water runoff and water leaching from these localities is significant, and soil water saturation highly depends on temperature and atmospheric precipitation. Beech populations showed their maximum net photosynthetic rate during the highest precipitation levels in the vegetation season, at the end of June on Vidlič and Fruška gora. The lowest level of photosynthesis was observed in September, after a period of low rainfall, in spite of more optimal temperatures. At Tara, a humid site, similar results were obtained for spruce, but not for fir, related to sustained soil humidity, achieved by additional water drainage from surrounding hills. At Kopaonik, where beech and spruce population were located on the slope of the mountain, in a small creek valley, where water supply also is not highly dependent on recent rain fall, rate of photosynthesis was not significantly reduced in September, when precipitation was at its lowest point. These photosynthetic reductions influenced by drought occurrence were certainly expected and previously confirmed in many studies (Daly et al., 2003; Guo, 2010; Liu et al., 2010). Correlations of environmental conditions with CO2 assimilation, seasonal dynamic also can have significant influence, depending on plant species and climatic region. However, the same species (beech) had different photosynthetic alterations during the season at three different localities (Vidlič, Kopaonik and Fruška gora). On Kopaonik, where gradual increase of photosynthesis during the season was measured on beech population and more stabile photosynthetic rates were measured on spruce population, long drought could not affect these plants in such significant extent, since this locality had additional water source from a running creek. However, highest temperature during measurements determined in September, could have important positive influence. Similar results were observed in relation to temperature in the work of Frolking et al. (1995), on spruce population. Content of photosynthetic pigments decreased at the end of the vegetation season at all localities, (except in the case of Chl b on Kopaonik and for the fir on Tara and in the case of Chl a+b on Kopaonik for spruce) although this decrease had statistical significance on the beech population at Vidlič, and on the population of spruce, in the case of Chl a at Tata. Similar results were obtained in the experiments of Arunyanark et al. (2008) on peanuts, where significant effects of seasonal period were determined in relation to the chlorophyll content. Reduced pigment content in September could therefore be related with the onset of the seasonal decline of pigment contents occurring as the end of vegetation approaches. Reduction of photosynthetic pigment contents is also often related with drought (Liu et all 2010, Nikolaeva et all 2010, Guo et al. 2010). Number of researchers have confirmed positive correlations between photosynthetic rates and photosynthetic pigment concentrations in woody species (Berveiller et al. 2007; Reis et al., 2009; Saxe et al., 2001; Waring and Landsberg, 2011). However, in this paper, similar correlations were not statistically significant, due to specific properties of each site. Kopaonik and Tara localities were not strongly dependent with recent rain fall for water supply, thus holding high levels of photosynthetic assimilation during the reduced precipitation period, in spite of the small decline in pigment concentrations. WUE was in general positively correlated with the precipitation and the rate of photosynthesis, but in negatively correlated with temperature, proving that water usage is significantly disturbed by intense drought occurrence which was followed by increased temperatures. Insufficient water supply causes stomata to be closed early during the day. The intensity and duration of drought, along with temperature levels, determines if the water loss will be lower or higher than the reduced carbon availability and fixation (Daly et al., 2003). It seems that plants more tolerant to drought have the ability to increase WUE in drought to some extent (Edwards et al., 2012). At Fruška gora, WUE remained at the same level during the observed period, in spite of |