Modelling the recovery of an annual savanna grass following a fire-inducedcrash

The native annual Sorghum populations of the Australian wet-dry tropics are highly resilient to dry season fires. During the early wet season, however , fires that occur after the new grass population has emerged can cause cat astrophic population crashes. We examined savanna plots that had been burnt in this way, and compared them with adjacent unburnt plots. We found that Sorghum densities in the burnt plots were lower on average by a factor of 1 0, but that some fires had reduced the density only to one-third of the unb urnt plots. It is not clear whether these differences relate directly to si te or seasonal factors, or to differences in the way the burning was carrie d out. Other vegetation components responded to the fires differently: forb s (dicotyledonous herbs) increased in cover, while perennial grasses, woody plants, and overall species richness, were not significantly affected. The amount of leaf litter declined. A population model for Sorghum based on th e demography of unburnt populations predicted that they should recover from a wet season burn, taking 7-16 years to return to normal densities. Howeve r, the actual held populations did not seem to be recovering, suggesting th at wet season fires not only lower densities, but may also fundamentally ch ange population processes in these annual grasses.