Germinable soil seedbanks of central Queensland rangelands invaded by the exotic weed Parthenium hysterophorus L.

The germinable soil seedbank was determined at two sites in central Queensland on four separate occasions between February 1995 and October 1996. These sites were infested with parthenium weed (Parthenium hysterophorus L.), a serious invasive exotic weed. During this period, the seedbank varied between 3282 and 5094 seeds m at the Clermont site, and between 20 599 and 44 639 seeds m at the Moolayember Creek site. Parthenium hysterophorus exhibited a very abundant and persistent seedbank, accounting for 47-73% of the seedbank at Clermont and 65-87% of the seedbank at Moolayember Creek. The species richness and species diversity of the seedbank, and the seed abundance of many species, was lower at Moolayember Creek during spring (the time of year when the most dense infestations of the weed originate). Parthenium hysterophorus seedlings also emerged more rapidly from the soil samples than did those of all other species. Hence, it seems that various aspects of the weed's seed ecology, including abundance and the persistence of its seedbank and the rapid emergence of its seedlings, are major factors contributing to its aggressiveness in semiarid rangeland communities in central Queensland. The domination by P. hysterophorus of the seedbanks of these sites suggests that the weed is having a substantial negative impact on the ecology of these plant communities. The diversity of these seedbanks was found to be lower in comparison with that observed in other grassland communities that were not dominated by an invasive weed species. Hence, the prolonged presence of P. hysterophorus may have substantially reduced the diversity of these seedbanks, thereby reducing the ability of some of the native species to regenerate in the future