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ŠUMARSKI LIST 5-6/2014 str. 56     <-- 56 -->        PDF

significant decrease of CO2 assimilation, suggesting that beech population was successful in saving water. At Vidlič, low precipitation in September caused significant disturbances in water regime thus significantly reducing WUE and photosynthesis. On humid localities, at Kopaonik (beech) and Tara (fir), WUE decrease was correlated with high transpiration rates. We speculate that additional water availability at these localities, apart from precipitation, along with elevated temperatures, provided sufficient capacity for stabile transpiration flow.
Extremely reduced amount of rain fall during the vegetation season, as a consequence of disturbed climate, certainly has a direct impact reflected in reduced CO2 assimilation and therefore potentially smaller bioproduction. Our results indicate that influence of such climate change depends on terrain configuration and plant species. It has been reported that the impact of global warming to plants is greater at mountain regions than at low altitudes (Beniston 2006). Strong evidence already exist that forest species (both herbs and woody plants) have already started to migrate upwards in response to climate change (Ruiz-Labourdette et al., 2012; Lenoir et al., 2008). Our results suggest that negative impact of drought at mountains will be more evident in tree populations located on ridges and higher areas of mountain slopes, where water runoff and leaching is faster. Since these localities are mostly positioned at higher relative altitudes, drought periods will slow down shift of forest covers toward sites at higher mountain slopes, which could be the consequence of temperature increase. Suggested acclimation of plants to elevated temperatures (Shen et al., 2009), will be limited by insufficient supply of water. As a consequence, during seasons with extensive drought occurrence, such as period analyzed during 2011 in Serbia, plant water use efficiency will decrease. Since the frequency of extreme temperatures and precipitation regimes is predicted to increase (Schär et al., 2004), consequence could be a decreased amount of wood development, and tendency for reduction of mountain forest covers in such specific sites. These results are in agreement with some previous studies which state that drought appears as a main limitation factor which suppresses the acclimation role of temperate forests as a carbon sink created by elevated temperatures and increased CO2 atmospheric level (Saxe et al., 2001; van Mantgem et al., 2009; Peñuelas et al., 2011).
The results have indicated that reduced precipitation during the second part of the vegetation season singinficantly limited CO2 assimilation of beech (Fagus sylvatica L.), common spruce (Picea abies (L.) Karsten) and silver fir (Abies alba Mill). These reductions of photosynthetic and transpiration activity that resulted in reduced water use efficiency, were particularly evident at ridges and high mountain slopes, where water supply mostly depends on rainfall. Therefore, the acclimation of investigated woody species to altered climatic conditions could be limited by locality specific soil humidity conditions. Water deficiency must be considered as a determinant ecological parameter of forest population productivity and distribution.
This paper was realized as a part of the project "Biosensing Technologies and Global System for Long-Term Research and Integrated Management of Ecosystems" (43002) financed by the Ministry of Education and Science of the Republic of Serbia within the framework of integrated and interdisciplinary research for the period 2011–2014.
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