‘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.

The influence of plant traits on forest fire behaviour has evolutionary, ecological and management implications, but is poorly understood and frequently discounted. We use a process model to quantify that influence and provide validation in a diverse range of eucalypt forests burnt under varying conditions. Measured height of consumption was compared to heights predicted using a surface fuel fire behaviour model, then key aspects of our model were sequentially added to this with and without species-specific information. Our fully specified model had a mean absolute error 3.8 times smaller than the otherwise identical surface fuel model (p < 0.01), and correctly predicted the height of larger (≥1 m) flames 12 times more often (p < 0.001). We conclude that the primary endogenous drivers of fire severity are the species of plants present rather than the surface fuel load, and demonstrate the accuracy and versatility of the model for quantifying this.

Forests of the Australian Alps (SEAustralia) are considered some of the most vulnerable to climate change in the country, with ecosystem collapse considered likely for some due to frequent fire. It is not yet known, however, whether increasing fire frequency may stabilize due to reductions in flammability related to reduced time for fuel accumulation, show no trend, or increase due to positive feedbacks related to vegetation changes. To determine what these trends have been historically, dynamics were measured for 58 years of mapped fire history. The 1.4 million ha forested area was divided into broad formations based on structure and dominant canopy trees, and dynamics were measured for each using flammability ratio, a modification of probability of ignition at a point. Crown fire likelihood was measured for each formation, based on satellite‐derived measurements of the 2003 fire effects across a large part of the area. Contrary to popular perception but consistent with mechanistic expectations, all forests exhibited pronounced positive feedbacks. The strongest response was observed in tall, wet forests dominated by Ash‐type eucalypts, where, despite a short period of low flammability following fire, post‐disturbance stands have been more than eight times as likely to burn than have mature stands. The weakest feedbacks occurred in open forest, although post‐disturbance forests were still 1.5 times as likely to burn as mature forests. Apart from low, dry open woodland where there was insufficient data to detect a trend, all forests were most likely to experience crown fire during their period of regeneration. The implications of this are significant for the Alps, as increasing fire frequency has the potential to accelerate by producing an increasingly flammable landscape. These effects may be semi‐permanent in tall, wet forest, where frequent fire promotes ecosystem collapse into either the more flammable open forest formation, or to heathland.

Forest wildfire impact is widely believed to increase with time since disturbance, presenting a dilemma for the persistence of fire‐sensitive species. However, in south‐western Australia, disturbance has been shown to increase wildfire likelihood for some decades before it again declines. It has been proposed that this trend occurs through ‘ecological controls’ on wildfire such as the self‐thinning of fire‐stimulated understorey growth. Here, we analyse six proposed ecological controls using a surveyed chronosequence of a Eucalyptus jacksonii forest community. We quantify plant growth (growth and self‐pruning) and succession (changing plant traits, self‐thinning), along with consequent changes in surface and suspended litter. We then use a biophysical, mechanistic model to predict the dynamics of flame height and canopy scorch/consumption, along with suppression difficulty during wildfire conditions. To identify the importance of each potential ecological control, we separately manipulate them to grow hypothetical forests from 1 to 100 years; each with one of the controls removed. We then model flame height in each to compare with the original forest that had all controls present. Fire initially promoted dense understorey regeneration, but ecological controls transferred this biomass from fuel (likely to ignite) to overstorey shelter (unlikely to ignite, creating a less flammable microclimate). The effect of these changes was to alter modelled fire behaviour, such that flame dimensions in mature forest were half those in regrowth, canopy damage greatly reduced, and fire suppression opportunities maximised. The primary controls were self‐thinning and self‐pruning. Forest growth and succession explains observed trends in flammability dynamics in south‐western Australian forests, and the persistence of fire‐sensitive species over time. Approaches that cooperate with, rather than disrupt, these processes therefore provide a pathway to mitigate current climatic effects on fire. Read the free Plain Language Summary for this article on the Journal blog.