A comparison was made of the dimorphic subterranean and aerial achenes of the annual Emex spinosa (L.) Campd. Subterranean achenes were less dormant, had a higher percentage of viability, germinated at a faster rate, were less temperature-dependent in germination and produced larger seedlings than aerial achenes. Subterranean achenes germinated, equally well in light or dark but aerial achenes had a strong light requirement. There was a linear relationship between numbers of weight of aerial achenes and plant size in the field. More resources were allocated to reproduction in plants restricted in size because of environmental conditions. Subterranean achene production was less affected than aerial by density, nitrogen levels, varying field conditions and interference between plants grown, from subterranean and aerial achenes. In mixtures of such plants, those from subterranean achenes had larger leaf area and stems and more aerial achenes. These differences were not apparent in monocultures by the time of harvest. Seed dimorphism is more likely in short-lived fugitive species and the different roles associated with dimorphism (persistence in situ versus dispersal in E. spinosa) are seen as an ecological advantage in such species.

The seed production, germination and regeneration characteristics of two native species (Acacia Iongifolia var. sophorae and Banksia integrifolia) and two invasive subspecies (Chrysanthemoides monilifera ssp. monilifera and ssp. rotundata) were compared.C. monilifera ssp. rotundata was similar in seed characteristics and regeneration to B. integrifolia in that: (1) seed production was over an extended period each year; (2) seed longevity was low (0-2% viable after burial for 3 years), but increased with depth of burial; and (3) re generation from 26 to 28% of mature plants occurred after fire.By contrast, C. monilifera ssp. monilifera was similar to A. Iongifolia var. sophorae in that; (1) seed production was over a short period each year;(2) ofthe total buried, 6-13% of seeds were viable after 3 years, and again numbers increased with depth of burial (percentages of missing seeds of K. longifolia were high but all remaining seeds were viable after 3 years); and (3) regeneration after fire occurred only from seed, hence their seed longevity is an important factor for long-term survival.However, growth habit and niche occupation by ssp. rotundata is similar to that of A. longifolia and the above differences may contribute to the observed displacement of the native by the invasive species. The distribution ofthe two invasive subspecies is at present largely in separate areas in Australia and it is predicted this situation will continue because of their differing climatic preferences, morphology and regeneration from fire.

It has often been assumed that the introduced species Chrysanthemoides monilifera is displacing native species in invaded coastal communities in Australia but no direct evidence has been presented that this displacement is occurring and. if so. to what extent. At an invaded site near Moruva. New South Wales, we found strong negative correlations between the presence (measured as cover, frequency or volume of shrub) of C. monilifera and Acacia longifolia, the hitherto dominant native species, and ofC. monilifera and Lomandra longifo]ia. The negative correlations are partly accounted for by differences in the amounts of each species on different zones on the dune, but the negative correlations remain even within these zones. Also a comparison with uninvaded sites indicates that C. monilifera is displacing A. longifolia from the fore-dune and mid-dune but is having less effect in the swale. In addition, there is a trend towards reduced seed production of A. longifolia when growing in proximity to C. monilifera.There is no significant difference in the presence ofL. longifolia in the uninvaded and invaded sites. In invaded sites, significantly fewer C. monilifera seedlings occur in the presence of L. longifolia. Thus it appears that the observed negative associations arise from different causes. Chrysanthemoides monilifera is displacing the structurally similar, and previously dominant A. longifolia, while the tussock-forming L. longifolia is locally hindering the invasion.