Potential contributions of the soil seed bank and seed rain for accelerating the restoration of riparian catchments in Australia

The quality and quantity of soil seed bank composition can play a key role in secondary succession restoration efforts such as those involved in this study, which concerned the restoration of damaged native sites in riparian environments in Victoria, Australia. The objective of this study of the restoration work was to determine the role played by naturally existing soil seed bank in the success of natural restoration in these restored and unmanaged riparian areas. In this regard, we sampled the soil seed bank, seed rain, seedlings in the field and vegetation cover from three sub catchments defined by restoration attempts which had been conducted (i) < 3 years, (ii) 4–8 years, and (iii) > 9 years previous to sampling. In addition, (iv) in order to provide comparative data, adjacent sites (iv) chosen for future restoration and (v) with remnant vegetation, were also sampled. From samples, a total of 8858 seedlings were recorded from the seed bank and the vegetation survey showed 170 species, with exotic species being more numerous than native. The seed rain (a total of 1422 seeds) was dominated by exotic species at all sites. When comparing the vegetation distribution and the seed rain composition, it was clear that whilst the seed bank was more promising as a comparative recruitment source of native species, there were still too many sites dominated by exotic species to rely on this as a long-term restoration strategy. However, this study indicated that there were significant variations in restoration potential among the sites, suggesting that some careful prior site selection for investment of restoration efforts is an important issue. As a consequence, we have recommended that a detailed understanding of the soil seed bank and seed rain species composition prior to the restoration is essential, since a positive seed bank composition with a significant relative density of native species seeds, will provide an indication of native species’ resilience and their potential for recovery. We therefore believe that the findings of this study will provide valuable information to natural resource management agencies regarding the strategy for prioritisation of restoration sites, which will be more beneficial than randomly selecting sites for habitat restoration. In addition, with successful sector restoration, it is expected that the increasing usefulness of the seed bank will allow further restoration of adjacent areas with time.