The distribution and ecology of Schoenoplectus triqueter in the Shannon Estuary

In Ireland, triangular club-rush (Schoenoplectus triqueter) is found only in the upper part of the Shannon Estuary below Limerick City. It forms a large and apparently stable population here, in contrast to Britain, where it has declined precipitously in recent years. S. triqueter is found in three situations in the Shannon Estuary: as the outer zone of vegetation on mud banks of the main channel, on tributary and creek sides, and in lagoons. The S. triqueter population is more extensive than indicated in previous surveys. Three new populations, the principal one in the River Ratty, were located during this study. No evidence of hybridisation was found in the Shannon population and no seed production was observed in 1998 or 1999. S. triqueter is confined to the upper part of the estuary where average soil salinity levels do not exceed 10g l^-1. Salinity appears to be the most important factor limiting the downstream extension of S. triqueter in the main estuary and the River Maigue. S. triqueter is confined to the lowest reaches of the muddy riparian zone, where its rhizomes grow under anoxic conditions and where it endures substantial periods of inundation. The inundation regime may explain the lack of seed production by S. triqueter in the field, through immersion of flowers and deposition of sediment on flowers. Seeds were produced prolifically by S. triqueter in cultivation.