Restoration of Scirpus lacustris and Scirpus maritimus stands in a former tidal area

After the damming of estuaries in the Netherlands, Scirpus lacustris L. and Scirpus maritimus L. stands deteriorated due to the reduction of tidal range. Experiments were conducted in the field to investigate the effects of soil fertility, water depth and waterfowl grazing on the establishment of both species in a non-tidal waterbody. Fertilisation using a slow- release fertiliser (pellets of 7.5 g Osmocote per plant, containing 17% N, 1.6% P2O5, and 8.7% K) did not affect dry matter production of either species. Scirpus lacustris became established in both shallow (5-15 cm) and moderately deep (30- 40 cm) water, whereas S. maritimus only became well established in shallow water. A high plant density (12 plants m(-2)) had a negative effect on the growth of individual S. lacustris plants at both 5 and 30 cm water depth. Growth of individual plants of S. maritimus was negatively affected by a high plant density (20 plants m(-2)) at 5 cm water depth but not at 30 cm. Support of shoots, by wire-netting, increased total plant dry weight of S. lacustris but not that of S. maritimus. Summer grazing occurred mainly by mute swans (Cygnus olor L.). Grazing during three subsequent growing seasons resulted in the complete disappearance of S. lacustris. In contrast, a 3-year-old S. lacustris stand was able to recover after being grazed several times within 1 year. It was concluded that in former tidal areas Scirpus can be re-established by planting. Waterfowl grazing may, however, seriously limit growth of Scirpus. [KEYWORDS: shore protection; emergent macrophytes; re-introduction; soil fertility; water depth; plant density; waterfowl grazing Var-paludosus; l palla; ecology; netherlands; macrophytes; recovery; wetlands; marshes; growth; lake