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

Abstract

1. 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.2. 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. 3. 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 dynamicsin 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.

What did it used to look like? A case study from tall, wet mainland Mountain Ash forests prior to British invasion

“…Evidence of multi‐aged forest present at the time of wildfire in 1939, likely generated by low‐severity fire or partial stand replacing disturbance within old‐growth stands, is consistent with work on ecological control theory in tall, wet eastern Australian forests (Lindenmayer & Zylstra, 2024) and south‐west Australian forests (Zylstra et al., 2023). Ecological control theory posits that plant communities exert ecological controls on fire through the processes of growth and succession over time (Zylstra et al., 2023).…”

“…For example, First Nations' cultural traditions describe a complex and nuanced, context‐specific approach to the use of fire (Prober et al., 2016; Rodrigues et al., 2022), where fire exclusion was considered as much a tool of management as burning (Steffensen, 2020). The intentional exclusion of fire was applied widely in tall, wet forests (Wardell‐Johnson et al., 2018; Zylstra et al., 2023) and some semi‐arid woodland communities (Prober et al., 2016). In contrast, reports and popular books have interpreted records kept by early British invaders as indicating that extensive areas of forest were kept open and park‐like through frequent burning by First Nations Peoples (Gammage, 2011; Ryan et al., 1995).…”

What environmental and climatic factors influence multidecadal fire frequency?

“…The earlier that a fire is detected, the smaller the fire will be when fire crews first arrive, and the greater the likelihood that the fire can be successfully contained and suppressed at this early stage of fire development (Plucinski, 2012). These are consistent with "ecologically cooperative" approaches to fire that reinforce natural controls on flammability to create less flammable and more fire-resilient forest landscapes (Zylstra et al, 2022(Zylstra et al, , 2023.…”

“…Recent clear-cut logged sites have been observed to lead to increase fire severity, particularly under extreme weather conditions (Lindenmayer et al, 2021;Taylor et al, 2014). Previous planned burns may increase fire severity depending on the time since the planned burn (Zylstra et al, 2023). The exclusion of disturbed sites by clear-cut logging and/or planned burns resulted in 19,563 points for analysis across the study area (Table 2).…”

Pyrogeography in flux: Reorganization of Australian fire regimes in a hotter world

“…Hysteresis in climate-plant-fire relationships or changes in human fire management may either help resist or exacerbate state change (Bowman et al, 2022). Such emerging management strategies include reintroducing herbivores to reduce fuels (Johnson et al, 2018), using low-flammability plants as green fire breaks (Cui et al, 2019), mechanically removing woody fuels (Furlaud et al, 2023), letting fires burn under appropriate conditions (Boisramé et al, 2017), reseeding populations that collapsed under frequent fire (Bassett et al, 2015), rapidly deploying fire-fighting teams to protect sensitive biodiversity (Hankin et al, 2023;Kelly et al, 2020), and assisting forests to reach lower-flammability mature states (Zylstra et al, 2022(Zylstra et al, , 2023. Crucially, fire management will likely benefit from the ancient wisdom of Indigenous-led fire management, involving fine-tuned application of fire when each vegetation type requires it (Bowman & Sharples, 2023;Steffensen, 2020).…”