Experimental fire regimes and understory species composition in a tropical savanna in northern Australia

Native grass species in northern Australia can be distributed on a local scale as a consequence of fire regimes. A field site in tropical savanna vegetation in northern Australia was used to assess how the species composition of the understorey changed over 4 years of burning in different seasons and at different frequencies. To account for the observed changes, experimentation examined the soil seed bank, reproductive allocation of dominant grass species, factors limiting grass seedling recruitment, and the mortality of adult grasses. The application of fire at the study site changed the species composition of understorey plants, but not always in the first growing season after fire and not always because of fire alone. The changes in species composition were influenced most strongly by modifications to the physical environment on the ground (such as the removal of leaf litter), rather than by the biotic factors that were investigated. Meanwhile, sown seeds of the grass Sarga intrans were able to establish in a range of burnt and unburnt microsites despite the plant not occurring at the site naturally. Its absence is therefore determined by a lack of seed dispersal and not through a lack of suitable microsites created by infrequent burning which has predominated at the study site. Understorey plants provide most of the fuel for fires in northern Australia’s tropical savannas, so changes to the understorey species composition could have implications for fire behaviour and fire regimes in this region.