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, dispe rsal of propagules and the storage of mature seed ready for germination whe n conditions are suitable. This paper explores the relative importance of t hese 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 contrastin g rivers in Western Australia. Examination of the soil seedbank showed that regeneration of vegetation from this source is probably important for annu al species of herbs and grasses but of only minor significance for perennia l species. This is most likely due to high levels of disturbance and the un stable soils in the riparian zone. Reproductive phenology of the four overs torey species monitored in this study appears to be well-adapted to the hyd rological regimes on the respective rivers. For the seed of riparian overst orey species examined, seed longevity was poor and seed predation rates wer e high. The occurrence of seed in floodwater debris indicated the importanc e 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 su btropical north of Australia, there is little storage of seed and plants ar e reliant on favourable conditions prevailing at the time of seed fall. The likelihood of seed finding a safe site for successful germination is enhan ced by secondary dispersal in high river flows. For overstorey species on t he Blackwood River in the temperate zone of south-western Australia there i s 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, rege neration from seed can occur through several processes. The relative import ance of these different mechanism varies for each river, reflecting their v astly different hydrology and climate.