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ŠUMARSKI LIST 3-4/2015 str. 52     <-- 52 -->        PDF

strictly divided by habitats, and observed invasive woody species frequently occure together in the analyzed area of Carska bara.
In the light of the anthropogenic interventions and frequent inundation of these habitats, the competitive characteristics of the analyzed invasive species are highlighted, which explain the establishment of their communities. This led to the formation of "novel ecosystems", which are, in this study, formed due to the occurrence of invasive woody plants, that prevent the growth and regeneration of pre-existing plants and thus reduce the potential for system redevelopment (Hobbs et al., 2006; Hobbs et al., 2009). Acer negundo occupies the flood plain forests of the willow and poplar in disturbed conditions (due to vegetative growth), which prevents their natural regeneration (Rosario, 1988; Künstler, 1999). Possible limiting factor is the frequency and duration of flooding (Mędrzycki, 2011). In contrast, Fraxinus pennsylvanica has tolerance to flooding and is characterized by extremely rapid growth (Gucker, 2005; Kremer et al., 2006), due to which it can successfully suppress the native species from competition, establishing thus the stable community. Acer negundo and Fraxinus pennsylvanica often cenotically associate with different kinds of willow and poplar in their homeland (Csiszár and Bartha, 2008; Udvardy, 2008). Such situation was observed and analyzed in this reserve. As these communities are similar in composition and structure to those in their native conditions, it should not be surprising that analyzed invasive woody species are not only naturalized, but cenotically stabilized in the study area.
Description and classification of the communities of some woody alien species is done by several authors (Jurko, 1963; Hadač and Sofron, 1980; Zerbe, 2003; Exner and Willner, 2004; Sîrbu and Oprea, 2011), argumenting why non-native species can build communities which can be very stable and rich in characteristic species as native communities. Since many ruderal or segetal communities dominated by non-native plants are identified in the literature as valid, there is no reason why non-indigenous communities of woody plants should not be (Hadač and Sofron, 1980), like in the case of two new communities described in this paper.
The question of the syntaxonomical affiliation of two newly described associations is challenging because these communities have not been described previously. As it was already mentioned, there is only one described association in Europe so far, in which species Acer negundo is dominant. It is Sambuco nigrae-Aceretum negundo described by Exner and Willner (2004), within potential broad-leaved woodland areas of Austria. It was classified within the class Robinietea. Accordingly, we have also classified our two newly described communities in the class Robinietea. This class includes anthropogenic tree communities, colonizing disturbed habitats such as deforested lands, selvedges, agricultural and industrial fallows etc. Only two alliances were described within this class. Alliance Chelidonio–Robinion Hadač & Sofron 1980 occurs at loamy, mesic, eutrophic soils, or on solidified screes, usually on Southern, Eastern or Western slopes, and the other one Balloto nigrae–Robinion Hadač & Sofron 1980, develops at relatively poor sandy, dry soils, mainly in lowlands. Since the newly described invasive woody associations were found in anthropogenically disturbed habitats in the plains, it is reasonable to classify them within the class Robinietea. Diagnostic and dominant species of Robinietea class are: Robinia pseudoacacia, Chelidonium majus L., Sambucus nigra L., Poa nemoralis L. and Impatiens parviflora DC. (Hadač and Sofron, 1980; Chytrý and Tichý, 2003). The fact that invasive species is the dominant one (Robinia pseudoacacia), suggests that the class Robinietea includes communities whose occurrence and establishment are conditioned by anthropogenic activities. In compliance with this standpoint and currently available data on higher syntaxonomic categories, we have classified both new invasive communities within the mentioned class. We agree with Chytrý and Tichý (2003) that Robinietea class is insufficiently described in relation to diagnostic species throughout its geographic range. However, the diagnostic species and the habitat types of two newly described communities in this work, suggest that they cannot be classified to neither of the two currently available alliances of Robinietea class. For this purpose, the authors consider that an introduction of new syntaxa at the level of alliance or order, including human-caused azonal communities, could be the possible solution.
Another possible solution is further research, which would determine whether some of the communities currently belonging to Robinietea class, could be assigned to other higher syntaxa in accordance with native distribution of the dominant species, as considered by Exner and Willner (2004).
Considering the fact that described invasive forest communities represent a major threat to indigenous biodiversity an immediate action plan is necessary for minimizing its negative effects on the autochthonous flora and fragile riparian habitats of Ramsar site and Special Nature Reserve Carska bara. These results are extremely alarming, especially within the internationally important and protected area.
Acknowledgment
Zahvala
This study was supported by the Serbian Ministry of Education, Science and Technological development (Grant No. 173030). The authors wish to thank the management team of SNR Carska bara for technical help during field investigation.