Modelling Interactions of Fire and Rainforest

Summary of final report on the Australian Flora Foundation funded project:

John Crockett, Brendan Mackey and Julian Ash
School of Botany and Zoology, Australian National University  
Grant Details    Final Report

This project was undertaken to examine the potential importance of fire in governing the relative distribution of rainforest and sclerophyll forest, and the width and position of the boundary between them. Rainforest patches in south coastal N.S.W. were studied. The aim of the project was to use a general fire model to examine the factors important to suppressing fire in rainforest, that is to determine the effect of rainforest vegetation on certain key variables important to determining fire behaviour. Microclimates in the rainforest, sclerophyll forest and the boundary between were characterised and compared. It was shown that there are consistent differences in the microclimates of the vegetation types studied: the microclimate of the rainforest is more buffered from extremes in external conditions than sclerophyll forest or the boundary vegetation.

A litter moisture model was developed and used to examine the effect of altered microclimatic conditions and canopy cover on fuel drying in the different vegetation types. Modelling litter moisture shows that microclimatic conditions in the rainforest cause leaf litter to retain moisture for longer than litter in the other vegetation types. Work also focused on determining the possible effects of live rainforest and boundary vegetation on fire suppression. A leaf flammability experiment was conducted to test the ignitability of leaves from rainforest and sclerophyll species in a muffle furnace. An experimental burn was conducted to test the effect if vines, common to rainforest boundaries, have an effect on the forward progress of a fire burning through sclerophyll forest. Vegetation surveys were conducted at the line of extinguishment of past fires to examine if there were differences in the structure of the understorey vegetation between rainforest, rainforest boundaries and sclerophyll forest that may influence where fires of different intensities go out. This surveying and experimentation showed that differences in the nature of the live vegetation between rainforest, boundaries and sclerophyll forest directly contribute to the flammability of the vegetation.

By examining what suppresses fire in rainforest, and using the fire model to examine when rainforest is likely to burn, it is possible to gain insights into the importance of fire in determining the relative distribution of rainforest and sclerophyll forest in temperate areas, and in governing the position and width of rainforest boundaries. It was found that rainforest and rainforest boundaries will burn when litter moisture is very low, however, the flammability of rainforest and rainforest boundaries remains well below those of sclerophyll forest even during extreme fire weather and fuel dryness conditions. It is concluded that increasing the frequency of fire may impact upon rainforest distribution or the nature of the boundary. The techniques used in the study may be used for the control or prevention of fire in disturbed rainforest or in mixed vegetation containing rainforest. Information regarding the suppressive effect of vines may be useful to the identification of fire retardant vegetation for use in minimising damage to property and from bushfires