Ephemeral wetland vegetation in irregularly flooded arable fields along the northern Upper Rhine:: the importance of persistent seedbanks

Along the northern Upper Rhine ephemeral wetland vegetation is not confined to primary habitats such as the fringes of backwaters and river banks, but is also found in irregularly flooded arable fields. Within these highly variable environments, where disturbance is not only by flooding but also by agricultural management, we assessed the role of seedbanks for the persistence of ephemeral vegetation. In order to achieve this, we analysed the floristic composition of the above-ground vegetation and the corresponding soil seedbank of arable fields after a long-lasting spring and early summer flood in 2001. Moreover, we compared our vegetation data with other phytosociological studies from primary and secondary habitats along the northern Upper Rhine. The comparison between primary and secondary habitats of mudflat species revealed differences in species composition. While primary habitats were characterised by the predominance of species of Isoeto-Nanojuncetea and Bidentetea, secondary habitats contained additionally species of the classes Agrostietea and Stellarietea. The studied soil seedbanks were dominated by species typical of both, mudflat and agricultural habitats. Most species found in the seedbank were characterised by short life cycles and the ability to rapidly exploit periods of favourable conditions for germination and growth. The flooding treatments that were applied prior to the third season of seedbank analysis almost generally resulted in a high proportion of additional germination of mudflat species, even two years after the start of the analysis. The emergence of some mudflat species such as Gnaphalium uliginosum, Veronica peregrina and semi-aquatic helophytes such as Alisma lanceolatum and A. plantago-aquatica was actually confined to the flooding treatment. Seed densities were exceptionally high and increased with the duration and frequency of inundation at the sampling sites. Juncus bufonius, Ranunculus sceleratus and Veronica catenata were the most abundant species. For juncus bufonius we found a maximum seed density of 707,072 seeds m(-2), which is to our knowledge the highest seed concentration that was ever found in a higher plant. The large persistent soil seedbanks proved to be of outstanding importance for the emergence of ephemeral wetland vegetation after flood disturbances in arable fields. This was also reflected by the relatively high similarity between the seedbank and the established vegetation directly after flooding. Our study highlights the importance of irregularly flooded arable fields as a secondary habitat for the conservation of ephemeral wetland species. At these sites, not only improved drainage and landfill threatens the occurrence of mudflat vegetation but also the replenishment of the seedbank is often prevented by early re-ploughing of the fields.