Mechanisms by which growth and succession limit the impact of fire in a south-western Australian forested ecosystem

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.