Exotic plants in the soil-stored seed bank of urban bushland

Native vegetation reserves in urban areas have been invaded by exotic plants, particularly along edges. However, it is not known whether the seeds of exotic plants in the soil-stored seed bank are also largely restricted to edges. Ten urban bushland sites in northern Sydney were randomly selected and the readily germinable, soil-stored seed bank examined to assess whether the number of exotic and native species varied with distance from an urban edge. The similarity of the contents of the seed bank and the composition of the above-ground vegetation was also examined. Soil samples were collected from quadrats 0-10, 20-30 and 50-60 m from the edge at each site, then spread on germination trays in a glasshouse and germinated seedlings identified. Soil samples were collected every 3 months for a year. The number of exotic species in the seed bank was found to be highest near urban edges, as was the total number of species. The above-ground vegetation was found to be a poor indicator of the contents of the seed bank, for both native and exotic species. Most of the exotic species found in the seed bank were not found in the immediately surrounding vegetation. Furthermore, some exotic species were found in the seed bank at sites where no exotic species were present in the vegetation. The results suggest that it is the lack of suitable conditions that is largely restricting invasion of exotic species to edges of bushland. The results also highlight the need for seedbank studies to assess the contents of the soil seed bank in urban bushland in order to prevent further invasions of exotic plants.