COMPOSITION AND SEASONAL FLUX OF THE SOIL SEED BANK OF SPECIES-RICH THEMEDA-TRIANDRA GRASSLANDS IN RELATION TO BURNING HISTORY

Most species-rich grasslands dominated by Themeda triandra in southeas tern Australia have been ungrazed and frequently burnt for decades. Th e seedling emergence technique was used to determine the size and taxo nomic composition of the soil seed bank of five grasslands that had di fferent fire histories (i.e. burnt at 1 yr, 3 yr and greater than or e qual to 10 yr intervals) and this was compared to the standing vegetat ion at each sire. A nested sampling design (subplot, plot, site) was u sed to determine the effect of spatial scale on the patterns observed in both the vegetation and the seed bank. Temporal variation in the se ed bank was assessed by repeated soil sampling over a two year period. 61 native and 30 exotic species were recorded in the vegetation. Rich ness varied more between sites than within sites. Sites were therefore internally homogeneous for species richness. However, no correlation between burning frequency and richness was found. DCA ordination separ ated the sites into distinct groups, but sites with similar fire histo ry did not necessarily group closely. 60 taxa germinated from the soil seed bank, comprising 32 native and 28 exotic species; 11 species, mo stly therophytes, were restricted to the seed bank. The richness of th e seed bank was significantly lower than the vegetation at all spatial scales. No correlation between seed bank richness and fire history wa s found. The seed bank of species-rich grasslands is dominated by a li mited number of widespread, highly clumped, annual, native and exotic monocots. Most native hemicryptophytes, and perennials in general, wer e represented in the soil by a transient seed bank. Only 12 % of study species, all therophytes, were considered to form large, persistent s eed banks, the size of which was greater in unburnt grasslands at all times of the year. The distinct floristic patterns observed in the veg etation are less clearly represented in the seed bank. The seed bank r epresents a floristically distinct (and less variable) component of th e vegetation when compared to the standing flora. The size of the long -term. soil seed bank suggests that it has little functional importanc e for many native species and probably contributes little to seedling regeneration processes following disturbance. Altering established fir e regimes is likely to only change the composition and small-scale ric hness of the existing site vegetation and will not re-integrate specie s previously lost from the vegetation due to past management. It is su ggested that the maintenance of vegetation remnants and processes that encompass a range of long-term burning histories will be necessary if the flora is to be conserved in situ. Restoration of degraded grassla nds cannot rely on the soil seed bank but rather, will be dependent on the reintroduction of propagules.