SEDIMENT SEEDBANKS IN GRASSLAND ON THE MAGELA CREEK FLOODPLAIN, NORTHERN AUSTRALIA

The size and species composition of plant populations arising from sediment seedbanks of three communities (Pseudoraphis, Hymenachne and Oryza grasslands) on the Magela Creek floodplain were compared under experimental (moist and flooded) conditions. The mean number of seedlings produced in the moist-treatment ranged from 8000 to 15 400 m-2, and in the flooded-treatment ranged from 3800 to 7100 m-2. Seed from 33 taxa germinated: 22 from the Oryza-, 13 from the Pseudoraphis- and 18 from the Hymenachne-site samples. The Pseudoraphis-site seedbank was closer in composition to that of the Hymenachne-site and Oryza-site seedbank. In terms of the species present the Pseudoraphis-site seedbank closely reflected that of its community on the floodplain, whereas the Hymenachne-site and Oryza-site seedbanks were not as closely related to the composition of their respective floodplain communities. Indices of diversity and evenness point to the relative homogeneity of species seedbank production in the moist-treatment of Pseudoraphis- and Hymenachne-site samples, compared with that in both treatments of the Oryza-site samples and the flooded-treatment of Pseudoraphis- and Hymenachne-site samples. Pseudoraphis spinescens (R.Br.) Vick. with a relatively small, but persistent seed dominated the seedling composition of the Pseudoraphis- and Hymenachne-site samples. © 1990.