Aims: River gravel bars are endangered habitats in Europe. However, classification schemes of their vegetation and habitat types differ among European countries, and they are even ignored in some national schemes. This causes problems in conservation planning, monitoring and management. Hence we aimed at building the first unified vegetation classification for river gravel-bar habitats across European mountain systems. Location: Europe. Methods: In total 4,769 vegetation plot records of river gravel-bar plant communities were collected from national, regional or private databases, digitized from the literature and newly collected in the field. A hierarchical classification expert system with formal definitions of vegetation types was created. The definitions combined the criteria of presence or cover of groups of species with similar ecology or single species narrowly specialized to a particular gravel-bar habitat. The TWINSPAN classification was applied to early-successional vegetation types to check whether the classification based on formal definitions was supported by the results of unsupervised classification. Similarity patterns among vegetation types were visualized using the detrended correspondence analysis (DCA) ordination. Results: Early-successional and scrub gravel-bar vegetation types were respectively classified into two classes: Thlaspietea rotundifolii and Salicetea purpureae. Eleven associations and four alliances (Calamagrostion pseudophragmitae, Epilobion fleischeri, Salicion cantabricae and Salicion eleagno-daphnoidis) were defined formally. Based on a critical revision, some associations or alliances defined in the previous literature were merged or discarded. The main gradient in variability within the gravel-bar vegetation is connected with the altitudinal gradient, biogeographical variation, local hydromorphological processes and various successional changes. Conclusions: The first unified and formalized classification system of the European mountain river gravel-bar vegetation was created, and species composition, ecology 2 of 27 | Applied Vegetation Science KALNÍKOVÁ et AL. 1 | INTRODUC TI ON Gravel bars of mountain stream beds and banks are azonal habitats that host specialized flora and specific vegetation types. They are typical of wandering or braided river systems occurring from glacial river floodplains in the alpine belt to broad floodplains in the piedmonts. They develop on rivers with significant variation in discharge and are maintained by torrents that are bringing new sediments and regularly disturbing or rearranging river banks. Gravel bars occur preferably in places where the strong current suddenly slows down, allowing the deposition of the particles released by bank erosion in the upstream
The setting up of the European network of protected areas (Natura 2000) is nearing completion. Alpine rivers with nearnatural morphology and dynamics have become rare and are highly endangered. Myricaria germanica, an endangered indicator species of Alpine rivers, reflects riverine landscape diversity. The study is based upon an evaluation of literature as well as upon new data. The analysis of the actual distribution of habitat 3230 Alpine rivers and their ligneous vegetation with M. germanica identifies the river Isel and its tributaries Schwarzach, Kalserbach and Tauernbach as an important addition to the Natura 2000 network.5
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