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(1) The abundance of grass species at six sites on each of six different soil groups in grasslands and forests in south-eastern Queensland was determined and a diasporeabundance spectrum was produced for each site by classifying the species into three groups with respect to the morphology of their diaspores. (2) In spite of the considerable taxonomic diversity amongst the grass floras of the thirty-six sites, when the dissimilarity between the diaspore-abundance spectra was assessed, clustering revealed two distinct groups of sites. (3) In one group the predominant species possess hygroscopically-active awns these sites are found exclusively on krasnozems, chocolate softs and black earths. These soils are well structured with a relatively high clay content and their surfaces are either loose and crumby or crack on drying. While such surface conditions might be expected to provide microsites for all three types of diaspores it is suggested that actively-awned species predominate since their diaspores are at a competitive advantage in locating microsites. (4) In the other group the predominant species bear unawned diaspores, these sites are found exclusively on siliceous sands, red podzolic soils and solodic soils. These soils are apedal with a high sand content. Their surfaces were either hard setting, showed a tendency to form surface crusts or were loose and single grained. Such surfaces do not provide microsites suitable for location by diaspores bearing active awns and it is suggested that the preponderance of unawned species over passively-awned species results from the effects of fire. [ABSTRACT FROM AUTHOR]