Many (particularly annual) plant species which occur on floodplains produce a long-lived soil seed bank as a means of persisting through the temporally unpredictable hydrological regime. To understand the spatial and temporal distribution of plant species on floodplains, the role of hydrology, and the implications of changes to the hydrological regime because of water resource development requires knowledge of the potential species. As such, I investigated the effects of hydrological conditions on the characteristics of the soil seed bank of the floodplain of the lower Lachlan River. A seedling emergence experiment was conducted using soil collected from 25 sites from three typical floodplain habitat types (temporary floodplain lakes, intermittent river red gum swamps, and intermittent black box swamps) on the lower Lachlan River, which were subjected to four experimental treatments (rainfall, waterlogged soil, submerged, and receding). These experimental treatments represented typical conditions on a floodplain in a dry phase and during and following flooding. Soil was collected from 17 sites in May 2016, and a further eight sites in May 2017. Soil was recollected from 15 of the sites from 2016 and 5 of the sites from 2017 again in May 2019. During late 2016 and early 2017, the lower Lachlan River experienced the fourth largest flood event on record, and as such, soil collected in 2017 had been recently inundated.
Soil seed bank density and species richness were compared to the number of floodplain-river connection events and time since flooding using 30 years of historic river flow data and commence to fill thresholds for each site. The density and species richness of seedlings was significantly greater on waterlogged soil than all other treatments. Seed bank density and species richness were significantly greater at sites which were connected less often compared to more regularly connected sites. Comparatively lower numbers of terrestrial species were present at sites collected in 2017 following flooding than sites collected in 2016. The number and density of terrestrial species had increased between 2017 and 2019. The density and species richness in the soil seed bank appears to initially deplete following flooding in response to germination and scouring but are then replenished once germinated plants reproduce and contribute more seeds, and the species that remain viable in the soil seed bank are those most tolerant of flooding conditions.