Availability of seed for recruitment of riparian vegetation: A comparison of a tropical and a temperate river ecosystem in Australia

Processes that are important for the recruitment of plants include aspects of the reproductive phenology, development and release of propagules, dispersal of propagules and the storage of mature seed ready for germination when conditions are suitable. This paper explores the relative importance of these mechanisms by examining the contents of the seedbank in the soil, the reproductive phenology of particular overstorey species, the importance of dispersal by water and the survival and longevity of seed on two contrasting rivers in Western Australia. Examination of the soil seedbank showed that regeneration of vegetation from this source is probably important for annual species of herbs and grasses but of only minor significance for perennial species. This is most likely due to high levels of disturbance and the unstable soils in the riparian zone. Reproductive phenology of the four overstorey species monitored in this study appears to be well-adapted to the hydrological regimes on the respective rivers. For the seed of riparian overstorey species examined, seed longevity was poor and seed predation rates were high. The occurrence of seed in floodwater debris indicated the importance of secondary dispersal of seed by water, particularly for the Ord River. For the two overstorey riparian species examined on the Ord River in the subtropical north of Australia, there is little storage of seed and plants are reliant on favourable conditions prevailing at the time of seed fall. The likelihood of seed finding a safe site for successful germination is enhanced by secondary dispersal in high river flows. For overstorey species on the Blackwood River in the temperate zone of south-western Australia there is some storage of seed in the canopy but dispersal of seed to safe sites is also enhanced by river flow. For riparian vegetation on these rivers, regeneration from seed can occur through several processes. The relative importance of these different mechanism varies for each river, reflecting their vastly different hydrology and climate.