Flammability of Australian forests

‘Flammability’ means different things to different people. Scientifically, it can be defined through three component variables that describe how well the fuel ignites (ignitibility), how well it burns (combustibility) and how long it burns (sustainability). The ‘fuel’ may be a plant organ, a whole plant or a plant community. While the terms ignitibility, combustibility and sustainability have been developed for laboratory studies, there are conceptual equivalents suited to the field; these are rate of spread, intensity and residence times. Another variable is added for field circumstances—probability of burning at a point. Eucalypt forests can be highly ‘flammable’ even considering all criteria and scales, while Australian forests in general show the whole range of variation from low (‘closed forests’ or ‘rainforests’) to high (e.g. relatively short stringy-barked open forests of Eucalyptus with abundant wiregrass). The expression of flammability depends on the local circumstances. In the field this can be summarised in terms of weather, terrain and ignition. Predicting how much potential forest fuel, and the attributes of that fuel, will be involved at any particular time, and under extreme weather conditions, remains a challenge. How social, climatic and fuel-species' changes will affect flammability, directly and indirectly, in the next 50–100 y is uncertain but potentially very significant.