A floristic and ecological study of the Rhine alluvial meadows in the domanial forests of Offendorf and Daubensand (Bas-Rhin, France). Application to their conservation and management | Etude floristique et ecologique des prairies alluviales en forets rhenanes domaniales d'Offendorf et de Daubensand (Bas-Rhin, France). Application a leur gestion conservatoire

The meadows and grasslands taking place in the forests of Offendorf and Daubensand lie in the former Rhine alluvial floodplain. On ca. 2 ha, 140 vascular and 22 bryophyte species were recorded. A principal components analysis showed that the main ecological gradients, shown on the first two axes of the ordination explaining 37/ of the total floristic variance, are linked to the former management practices and the soil humidity. The most ancient grasslands of Daubensand include up to 70 vascular species on ca. 50 ares. They are characterized by a continuous grass layer dominated by Bromus erectus and Festuca rubra, and belong to the calcareous oligotrophic grasslands of the Mesobromion. The meadows in Offendorf are dominated by medium sized or even tall meso-nitrophilous dicotyledones such as Angelica sylvestris, Thalictrum flavum and Symphytum officinale. They possess ca. 60 vascular species. Certain are characteristic for the large Medioeuropean alluvial floodplains and are protected by the law: Viola elatior, Inula britannica. Allium angulosum, Euphorbia palustris. The former agricultural field in Daubensand is characterized by a mostly therophytic flora (Valerianella locusta, Cerastium pumilum subsp. glutinosum, C. glomeratum...) and by annual bryophytes (Riccia subbifurca, Phascum cuspidatum, Pottia truncata...). Since the canalization of the Rhine and the loss of the fluvial dynamics, and since the evolution of the former agricultural practices creating a substitution dynamics, these vegetations evolve. Hence, the third axis of the ordination can be interpreted as a vegetation dynamics gradient, from the fringe communities characterized by Agrimonia eupatoria, Salvia pratensis and Achillea millefolium to the vegetations dominated by shrubs. The ecological and/or management practice history of these vegetations justify their ecological management. The mowing in autumn for the grasslands and end Juny for the mesophilous meadows is suggested to fight against the shrub recolonization and against the xenophytes such as Solidago gigantea spreading in the valley since the canalization of the river. To limit the process of internal eutrophication and to favour the moss layer, the exportation of the organic matter is absolutely necessary.