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ŠUMARSKI LIST 5-6/2014 str. 52     <-- 52 -->        PDF

Lenoir et al., 2008; Ruiz-Labourdette et al., 2012). However, it seems that all these ecological roles of forests, that act as a stabilizers of a changing climate, depend on a stabile water regime at specific sites. Extensive drought occurrence during growing season could significantly reduce plant acclimation to higher temperatures or increased CO2 levels (Saxe et al., 2001; van Mantgem et al., 2009; Peñuelas et al., 2011).
Most predictions of climate change suggest that some site-specific water and temperature stress will occur more often in the future (Boisvenue and Running, 2006). In Balkan region, an unusual, extensive drought period occurred during the second half of the summer in the year 2011 and 2012. The aim of this paper was to determine the impact of this drought occurrence in 2011 to net photosynthetic assimilation, pigment content and transpiration intensity in forest populations located at four protected mountain areas, which are different and specific in their position and available water supply.
MATERIAL AND METHODS
Materijali i metode
Four localities chosen for survey are situated in protected mountain forest areas of Serbia (Figure 1). All localities were chosen in mature forests, with following species: beech (Fagus sylvatica L.), spruce (Picea abies (L.) Karsten) and fir (Abies alba Mill). Site 1 (Vidlič) is Nature reserve at the slope of the Stara mountain in Eastern Serbia (altitude 1 097 m), with beech was observed as the dominant species. Site 2 (Kopaonik) is in the creek valley between two slopes of the Kopaonik National Park (Southern Serbia, altitude 1447 m), where beech and spruce populations were measured and observed. Site 3 (Tara) is on a humid plain section, between several elevated peaks of the Tara National Park (Western Serbia) on altitude of 1 077 m, where the fir and spruce populations were observed as dominant species. Locality 4 is at the reef edge of Fruška Gora National Park (Norhern Serbia, altitude 473 m) where beech trees as part of the mixture with sessile oak were observed. Site description of investigated localities can be found in table 1. 
All measurements were conducted at three time points during the growing period of 2011. First measurement was between 29th June and 1st July (Term 1), second between 1st and 3rd August (Term 2) and third between 13th and 15th September (Term 3).
Rates of photosynthesis (P) and transpiration (T) were measured using LC pro+ Portable Photosynthesis System, manufactured by ADC BioScientific Ltd. Measurements were performed instantaneously on six 3–5 meters high per site, on three leaves on each tree with three replications. Water use efficiency (WUE) was calculated as ratio between