Munmarlary revisited: Response of a north Australian Eucalyptus tetrodonta savanna protected from fire for 20 years

A comparison was made between the total density of tree species recorded on three 1 ha plots that have been protected from fire for 20 years, and from three surrounding sites that have been subjected to the ambient fire regime. Both unburnt plots and ambient sites were in a lowland coastal Eucalyptus tetrodonta savanna in Kakadu National Park. Fire protection resulted in a substantial increase in the number of saplings (16 times more than ambient), poles (five times more than ambient) and trees (2.5 times more than ambient), but slightly fewer (7%) sprouts than ambient. Of the 32 species recorded in the six 0. 2 ha samples, only nine species could legitimately be analysed using Chi-squared analysis to test for differences in the density of sprouts and saplings between unburnt and ambient samples; eight of these species had significantly different distributions. Typically the unburnt samples had a greater number of saplings compared to ambient conditions, but fewer sprouts. Eucalyptus miniata showed no significant difference in the density of sprouts and saplings between the unburnt and ambient samples. Chi-squared analyses of the frequency distribution among four size classes (sprouts, saplings, poles and trees) was possible for six species. The results for five of these species mirrored the findings of the comparison between sprouts and saplings. However, the fan palm Livistona humilus, which typically forms a component of the mid-layer in E. tetrodonta savannas, was found to have a large number of sprouts on the unburnt samples and a complete absence of stems in any of the other size classes. Dead L. humilus stems attested to the former occurrence of larger size classes of this species on the unburnt plots. No rainforest species were recorded in the unburnt samples. Minor differences in species composition between unburnt and ambient samples are thought to reflect sampling effects. The results of this study are consistent with the conclusions of an earlier study at the same site which also concluded that rainforest tree species do not readily colonize unburnt Eucalyptus savanna.