The seed bank provides a mechanism for the survival of wetland plants during unfavorable conditions and a pool of propagules for establishment during favorable conditions. Unregulated arid and semi-arid Australian wetlands are subject to highly variable hydrology; however, the lower River Murray is a highly regulated system with largely stable water levels and wetlands that were historically temporary now permanently inundated. Hydrological restoration of these wetlands involves structures that isolate the wetland from the main channel to reinstate drying cycles. Monitoring of wetlands where drying cycles have been reinstated often showed a lack of response in the vegetation and it was proposed that this was due to a depauperate seed bank. The aims of this study were to determine whether the poor response of the vegetation was due to depauperate seed banks in managed wetlands and assess the seed banks of wetlands where structures are being built.
The seed banks of 15 lower River Murray wetlands were assessed using the seedling emergence technique and seed separation. Sediment samples were collected from below normal pool level (the area permanently inundated due to river regulation) at ten sites with managed wetting and drying cycles (managed wetlands) and five sites where structures were under construction (unmanaged wetlands).
Seven out of the ten managed wetlands, had depauperate seed banks with seed densities <10,000 seeds m-2 and two or fewer species detected with no germination was observed in three wetlands. All of the unmanaged wetlands had depauperate seed banks with no germination in two wetlands. Seed separation detected no seed in wetlands where no germination was observed.
Data from this study showed that the observed poor response of the vegetation in managed wetlands was likely due to a depauperate seed bank, and suggests that the unmanaged wetlands will respond in a similar manner when drying cycles are introduced. However, reinstatement of a more “natural” hydrological regime is fundamental in wetland restoration as without the appropriate hydrology, many species will not recruit and a diverse seed bank will not develop.