Recent changes in the distribution and rooting elevation of Schoenoplectus club-rushes in the Scheldt estuary and the consequences for their survival

We aimed to assess the distribution and trends in abundance and rooting elevation in relation to substrate type (soft sediment/riprap) for three Schoenoplectus club-rush species in the Zeeschelde estuary (Belgium). Surveys took place in 1995, 2003 and 2013, following dike fortifications for the SIGMA flood control plan. Compared to 1995 club-rush tufts are now positioned lower in the tidal frame, especially their upper margin. Club-rush cover decreased by 50% in the last time interval. This is linked to marsh succession after a vegetation set-back by SIGMA works and increasing competition with Phragmites and Salix . The lower margin of club-rushes shifted downwards on riprap, but not on soft sediment. This substrate-dependent rooting elevation reach likely indicates that substrate stability can be an important factor for club-rush persistence and growth on the lower marsh margin. This emerged only recently probably because tufts needed time to expand after the SIGMA works and because of the improved water quality. Currently, Schoenoplectus club-rushes on soft sediment in the Zeeschelde suffer from the lack of suitable areas with cyclic natural morphodynamics that maintain pioneer habitat. To maintain club-rush diversity we suggest to manage threatened club-rush populations or to translocate them to newly created restoration sites.