As climatic changes continue to drive increases in the frequency and severity of forest fires, it is critical to understand all of the factors influencing the risk of forest fire. Using a spatial dataset of areas burnt over a 65 year period in a 528 343 ha study area, we examined three possible drivers of flammability dynamics. These were: that forests became more flammable as fine biomass (fuel) returned following disturbance (H1), that disturbance increased flammability by initiating dense understorey growth that later self-thinned (H2), and that climatic effects were more important than either of these internal dynamics (H3). We found that forests were unlikely to burn for a short ‘young’ period (5–7 years) following fire, very likely to burn as the regrowing understorey became taller and denser (regrowth period), then after a total post-disturbance period of 43–56 years (young + regrowth periods), fire became unlikely and continued to decrease in likelihood (mature period). This trend did not change as the climate warmed, although increases in synoptic variability (mean changes in synoptic systems per season) had a pronounced effect on wildfire likelihood overall. Young forest and regrowth forest became increasingly likely to burn in years of greater synoptic variability and the time taken for forests to mature increased, but in years with the most severe synoptic variability, mature forests were the least likely to burn. Our findings offer an explanation for fire behaviour in numerous long-term studies in diverse forest types globally and indicate that, even in the face of a warming climate, ‘ecologically-cooperative’ approaches may be employed that reinforce rather than disrupt natural ecological controls on forest fire. These range from traditional indigenous fire knowledge, to modern targeting of suppression resources to capitalise on the benefits of self-thinning, and minimise the extent of dense regrowth in the landscape.

Self-thinning forest understoreys reduce wildfire risk, even in a warming climate

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. 

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1111/1365-2435.14305

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

\n OCBIL theory is a multi-hypothesis formulation aimed towards an understanding of the evolution, ecology and conservation of biological and cultural diversity on old, climatically buffered, infertile landscapes (OCBILs). OCBILs have been in existence contemporaneously with rainforest since Gondwanan times. Such landscapes are common in areas of eucalypt species richness embraced by Australia’s two Global Biodiversity Hotspots, the Southwest Australian Floristic Region and the Forests of East Australia. Here, I summarize evidence pertaining to the eucalypts in the context of a recent reformulation of OCBIL theory into 12 evolutionary, ecological and cultural hypotheses and ten conservation management hypotheses. A compelling argument emerges for a new interpretation of the eucalypts evolving out of the OCBILs, rather than out of the rainforests as traditionally interpreted. This calls for a significant reinterpretation of best conservation management of the eucalypts. For example, traditional ideas on application of fire in eucalypt communities regarded as well adapted to this disturbance need to give way to a more nuanced and cautious view. This review of eucalypts seen as evolving out of the OCBILs helps in understanding the group from several new perspectives. Interpretation of other sedentary plant and animal groups as out of the OCBILs is commended for further study.

Out of the OCBILs: new hypotheses for the evolution, ecology and conservation of the eucalypts

Abstract Wildfire risk-management needs to consider interrelated factors that influence fire regimes, including changing climate and sometimes conflicting stakeholder priorities. With wildfires increasing in size and intensity over recent decades, wildfire risk management is becoming more important and more complex. For southwest Australia, wildfire risk-management is predicated on a longitudinal study of the relationship between prescribed burns and wildfires from 1953–2004 over a subset of this biodiverse region. Our study replicates the methodology of the longitudinal study, applying it to the wider region and extending the analysis to 2021. We found the extrapolation of the longitudinal study’s findings to the wider region invalid, as was extrapolation beyond 2004. In particular, the area of prescribed burns generally had negligible influence on wildfire area. However, more spatially complex fire history was strongly correlated with lower probability of large wildfires (independent of area burned). This highlights the limitation of extrapolating wildfire risk-management policies to areas of differing vegetation and/or climate, including changing climate over time. The potential of indigenous-led practices for wildfire risk and biodiversity conservation, particularly for areas with high spatial variability, is apparent as is the need for alternative strategies to prescribed burning as the primary tool in wildfire-risk mitigation.

https://www.tandfonline.com/doi/pdf/10.1080/19475705.2022.2119891?needAccess=true

Wildfire risk management across diverse bioregions in a changing climate

Abstract Protection of biodiversity, human assets, and cultural heritage pose significant challenges to contemporary planning of bushfire mitigation activities. Current mitigation approaches are not always appropriate, and mismanagement is a source of distress for Indigenous peoples. Increased understanding of Indigenous fire knowledge and increased Indigenous participation may provide insight into more appropriate and inclusive land management for fire mitigation. We analysed contemporary Noongar and Western fire practitioner approaches within an Indigenous fire knowledge (IFK) framework to explore knowledge and aspirations for small reserves in the Southwest Australian Floristic Region (SWAFR) global biodiversity hotspot. We recorded an extensive knowledge base, characterised by a highly nuanced approach to burning, held by the Noongar coauthors. We explore potential approaches to applying this knowledge to build collaborative fire mitigation strategies with mutually beneficial outcomes for biodiversity, cultural heritage, and human assets. 

/pdf/indigenous-knowledge-aspiration-and-potential-application-in-w98uqogh.pdf

Indigenous Knowledge, Aspiration, and Potential Application in Contemporary Fire Mitigation in Southwest Australia

\n Occurring across all southern hemisphere continents except Antarctica, old, climatically buffered, infertile landscapes (OCBILs) are centres of biological richness, often in biodiversity hotspots. Among a matrix of young, often disturbed, fertile landscapes (YODFELs), OCBILs are centres of endemism and diversity in the exceptionally rich flora of the south-west Australian global biodiversity hotspot, home to Noongar peoples for ≥ 48 000 years. We analysed contemporary traditional Noongar knowledge of adjacent OCBILs (e.g. granite outcrops) and YODFELs (e.g. creekline fringes) both at a single site and in two larger areas to test whether patterns of disturbance dictated by Noongar custom align with OCBIL theory. We found that Noongar traditional knowledge reflects a regime of concentrated YODFEL rather than OCBIL disturbance—a pattern which aligns with maximal biodiversity preservation. SIMPER testing found traditional Noongar OCBIL and YODFEL activities are 64–75% dissimilar, whereas Pearson’s chi-square tests revealed camping, burning, travelling through country and hunting as primarily YODFEL rather than OCBIL activities. We found that Noongar activities usually avoid OCBIL disturbance. This combined with high floristic diversity following enduring First Peoples’ presence, suggests that traditional Noongar knowledge is valuable and necessary for south-west Australian biodiversity conservation. Similar cultural investigations in other OCBIL-dominated global biodiversity hotspots may prove profitable.

The Noongar of south-western Australia: a case study of long-term biodiversity conservation in a matrix of old and young landscapes

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The Noongar of south-western Australia: a case study of long-term biodiversity conservation in a matrix of old and young landscapes