449 5 th Symposium Conference Volume for Research in Protected Areas pages 449 - 452 10 to 12 June 2013, Mittersill The Impact of Alien Plant Species on the Conservation Success of a Protected Natura 2000 Area within the Ecological Restoration of the River Traisen, Lower Austria Katharina Lapin & Karl-Georg Bernhardt Institute of Botany, Department of Integrative Biology and Biodiversity Research, University of Natural Resources and Applied Life Sciences, Vienna Abstract The invasion of alien plant species into protected areas has a serious negative impact on the abundance and diversity of the native flora as well as on natural ecosystem processes. For protected areas in particular, the ongoing distribution of invasive species has become a major issue. The objective of this study was to examine the development of invasive species in a riparian ecosystem, namely the planned floodplain restoration area at the mouth of the river Traisen in Lower Austria. The territory for the study lies within a Natura 2000 area, where the regulated riverbed will be revitalized as part of the LIFE+ project “Traisen”. Nine invasive plant species were recorded. During the last two years of study the distribution of the species Bunias orientalis, Impatiens parviflora, Impatiens glandulifera, and Solidago gigantea increased massively. The development was evaluated using a grid of 142 recording sites. In the riparian forests in particular, the proportion of invasive species to native species in the overall species composition increased during the recording period. The Shannon diversity index showed different developments for specific riparian plant communities. Furthermore, the seed bank and phenology were analysed in order to create a monitoring concept for the planned ecological restoration of the floodplain. The goal of this monitoring is to reduce the occurrence of invasive species in protected riparian areas focused on the protection of endangered native species. Keywords invasive alien species, riparian, restoration, biological invasion; Introduction Riparian areas are important habitats for various ecological functions and European plant diversity (GREGORY et al. 1991, NAIMAN & DECAMPS 1997, PFADENHAUER 1997, HOOD & NAIMAN 2000). In the past decades, European riparian zones have been strongly affected by biological invasion (PYŠEK et al. 1994, ESSL et al. 2002, SCHMITZ & LÖSCH 2005). The increasing number of scientific studies in this area emphasizes the importance of the ongoing ecological changes and consequences of biological invasions (LOHMEYER & SUKOPP 1992, RICHARDSON & PYŠEK 2006). The issue of invasive alien species is regularly discussed in several international conventions and nature conservancy programs (CLOUT & WILLIAMS 2009). The entire area was regulated in the 19 th century in order to reduce the number of floodings and increase the available land area for cultivation. During the construction of the Danube power plant Altenwörth in the 1970s, the outfall of the river Traisen was relocated. Although the relocation had a negative influence on the ecological situation, the species richness of the project area was calculated to be very high. The occurrence of species of the fauna-flora-habitat directives is similarly high. Therefore the area was incorporated into the EU-wide network of Natura 2000 sites, a network of nature protection areas established under the Habitats Directive and Birds Directive (1979). Since 2004, the area has been part of the Natura 2000 FFH site 16 “Tullnerfelder Donau-Auen” (www.noe.gv.at). The site covers a total area of 19,483 ha (ELLMAUER et al. 1999). One of main foci of this Natura 2000 site is the protection and support of riparian vegetation. The ecological function of riparian forests, broadleaved alder forests, transitory phytocoenoses and dry grasslands of the riparian areas in particular are among the important protected and supported entities in the “Tullnerfelder Donau-Auen” (www.noe.gv.at/natura2000). The aim of the Habitats Directive (1992) is to assure the long-term survival of Europe's most valuable and threatened species and habitats (ec.europe.eu). Human activities are not excluded from the management of Natura 2000 sites, as long as the management is ecologically and economically sustainable (SHARPSTON 2010). The Natura 2000 site “Tullnerfelder Donau-Auen” constitutes one of the largest connected riparian ecosystems in Austria. But the ecological structure and function of the riparian area are strongly endangered due to the construction of the Danube power plants Altenwörth and Greifenstein. The installation of these power plants caused noticeable changes in the Danube’s hydrology. Most of its abandoned meanders were disconnected from the main stream; but without these important cross-links to the main stream, the ecosystem is highly endangered. Furthermore, the reduction of river flooding leads to massive disturbances within gallery forests (www.noe.gv.at/natura2000). The ecological restoration of the river Traisen is now planned. ©Hohe Tauern National Park; download unter www.biologiezentrum.at