SEED POPULATION-DYNAMICS OF ONOPORDUM OVER 1 YEAR IN SOUTHERN NEW-SOUTH-WALES

1. Seed input, changes in the seed bank and seedling recruitment of Onopordum were monitored over 1 year at three contrasting sites (pasture, oat crop and hillside) within an 80-hectare paddock at Galong, New South Wales, about 100 km NW of Canberra. Each site was c. 2000 m(2). Time constraints precluded a longer study and repetition of the sampling procedures at similar sites in other areas. 2. In the pasture there were few Onopordum plants but a seed bank of c. 5000 seeds m(-2). During the year the seed bank decreased by more than half, but the most dormant component remained constant at c. 800 seeds m(-2). 3. In an oat crop nearby there were no flowering Onopordum plants and there was no direct seed input. Nevertheless, the seed bank of c. 700 seeds m(-2) did not decrease. 4. At the third site, a grazed hillside, Onopordum density was high (19 plants m(-2)). The seed input of 1788 seeds m(-2) caused a temporary small increase in the seed bank in summer but there was no change over the 1-year period either in the total seed bank (c. 3000 seeds m(-2)) or in its most dormant component (c. 750 seeds m(-2)). 5. Seedlings appeared in all seasons, invariably after rain. More than 95% of seedlings appeared in late summer or autumn, but different management practices changed the pattern of winter establishment. 6. For ecological studies of weeds, the population dynamics of the seeds in the soil and of the plants above ground are equally significant. This study shows that the existence of a small pool of labile seeds (c. 15% minimum) in the seed bank that can be recruited opportunistically, together with a large, strongly persistent component, will make any form of short-term control difficult.