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2004). This can lead to the homogenization of ecosystems and decrease in autochthonous species diversity (D’Antonio and Vitousek, 1992). In the final outcome, this can results in "novel ecosystems" (Hobbs et al., 2006; Hobbs et al., 2009). A large number of invasive plants are the trees (Binggeli, 1996). Twenty three alien tree species have been identified in forest ecosystems in Serbia (Medarević et al., 2008), 17 of which are invasive (Grbić et al., 2007). Due to their aggressive expansion abilities, some of them represent a serious threat to the natural regeneration and survival of the native trees. This fact especially refers to Robinia pseudoacacia L., Acer negundo L., Ailanthus altissima (Miller) Swingle and shrub Amorpha fruticosa L. However, more precise data on the distribution of the invasive woody species in Serbia still do not exist, particularly the ones related to protected areas. Except for the few recent literature sources on the topic of alien or invasive plants or communities in Serbia (Vrbničanin et al., 2004; Jarić et al. 2011; Lazarević et al., 2012; Anačkov et al., 2013), the data related to the protected Ramsar areas are still lacking (Panjković and Stojšić, 2001; Čavlović et al., 2011). In the light of the fragility and vulnerability, as well as the rarity of the wetlands (Smart, 1997), alarmingly large presence of highly invasive species was observed, some of which are highly naturalized, such as the North American trees: Acer negundo L. and Fraxinus pennsylvanica Marshall. The species Acer negundo was introduced to Europe in 17th century (Mędzycki, 2011), and Fraxinus pennsylvanica was introduced in 18th century (Csiszár and Bartha, 2008). Since then, they have spread across the Europe, especially in the riparian habitats (DAISIE, 2013). Since the literature sources for our teritorry provided no data on the exact time two analyzed invasive species have arrived to Serbia we looked up for these information in neighboring Hungary, which covers the biggest part of the Pannonian plain, where our study area belongs. After its introduction, A. negundo became quite a popular garden tree and in the second half of 19th century it was widely recommended for planting, as a wind-break and shelter-belt tree across the Western, Central and Eastern parts of Europe (Mędzycki, 2011). In Hungary, it has been known since the second half of the 19th century. It was widely planted on flood areas of the Great Plains from where it escaped and established on riparian forests and black locust plantations, mainly along the rivers (lower Danube valley), marshy areas near Lake Balaton, and also on dry sandy soils of the Pannonian Plain (Udvardy, 2008). A. negundo inhabit wet places, such as our study area in the northern Serbia. F. pennsylvanica was known since the very beginning of the 20th century in Hungary, when there were attempts of converting willow-poplar gallery forests to hardwood stands by using the green ash. Nowadays, this species is widely present, and concentrated in lowland river valleys and marshy or saline areas in Hungary, some of which are close, or bordering Serbia (Tiborcz et al., 2011). F. pennsylvanica inhabit the same wet places, such as our study area in the northern Serbia. According to Török et al. (2003) Hungary represents the gateway for invasions into the rest of the Central and Eastern Europe, and one of the corridors for invasions are precisely the wet habitats (gallery forests, disturbed bogs and marshes). Since our study area is a floodplain, suitable for A. negundo and F. pennsylvanica, we consider that analyzed invasive tree species could spread from Hungary to the northern Serbia across these corridors. Despite all the negative impacts the invasive plant species can exibit, paradoxically, the level of the exploration of invasive plant communities, especially those dominated by woody species, is small. However, except for the ass. Sambuco nigrae–Aceretum negundo Exner 2004, described in the potential broad-leaved woodland areas of Austria (Exner and Willner, 2004), there are no other literature data on the communities built up by the two observed invasive woody species. Invasive species Acer negundo and Fraxinus pennsylvanica have established stable communities in the Special Nature Reserve Carska bara (Vojvodina, Serbia), which are analyzed and described in this paper. Materials and methods Materijali i metode Study area – Područje istraživanja The investigated area Carska bara is located on the alluvial plain between the rivers Tisa and Begej, in the central Banat (Vojvodina, Serbia), southwest of Zrenjanin city. It belongs to the UTM Grid zone 34T, UTM square 10x10 km DR50 and DR51 (Figure 1), presenting one of the preserved floodplains in this part of Serbia. According to the pedological map of Vojvodina (Naugebauer et al., 1971) and ArcGIS 10.2 Software, the alluvial saline soil is mostly present within the study area. The area belongs to the temperate climate zone with emphasized continental characteristics (Stevanović and Stevanović, 1995; Kovačev, 2010). Habitats are presented as the mosaic of the wetlands, ponds, (salt) marshes, wet meadows, steppes and forests, intersected by rivers, canals and dikes. Over 230 bird species (including all European heron species and cormorants), 17 of which are internationally important were recorded here (Puzović et al., 2009). Total of 277 taxa of the higher plants (on species and subspecies level) are noticed in the study area, two of which are the Pannonian endemics – Aster tripolium L. subsp. pannonicus (Jacq.) Soó and Cirsium brachycephalum Juratzka. Also some relict species of marshy flora and of |