School of Agriculture, Food and Ecosystem Sciences - Research Publications

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    The effects of junction fire development on thermal behaviour at the field scale
    Holyland, B ; Cirulis, B ; Penman, TD ; Filkov, AI (ELSEVIER SCI LTD, 2024-02)
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    Multi-scale investigation of factors influencing moisture thresholds for litter bed flammability
    Burton, JE ; Penman, TD ; Filkov, AI ; Cawson, JG (ELSEVIER, 2023-06-15)
    Fuel moisture is important to flammability. Vegetation communities vary in their moisture thresholds for ignition and fire spread. Different factors, operating at distinct spatial scales (litter vs. vegetation community) may be responsible for these variations in moisture thresholds. The relative importance of these factors at each scale remains unquantified. Our study sought to examine what factors influence moisture thresholds for flammability across two spatial scales (point vs. plot). Litter samples were collected repeatedly over one fire season (2020–21) from selected sites within temperate eucalypt forest along an aridity gradient in south-eastern Australia. Samples were reconstructed then burnt under controlled conditions. At the point-scale (0.05 m²), we quantified flammability as the probability of sustained ignition, flame spread rate and flaming duration. At the plot-scale (400 m²), we quantified flammability as the proportion of sustained ignitions. At the point-scale, moisture thresholds varied with leaf cover on the surface of the litter bed for ignition and leaf size for flame spread rate. At the plot-scale, vapour pressure deficit (VPD) was the best predictor of ignitability and moisture thresholds varied with aridity. Wetter parts of the landscape had a higher VPD threshold for ignition than more arid parts, meaning they were available to burn less often. The relationship between leaf cover and ignitability observed at the point-scale was overwhelmed by the effect of moisture at the plot-scale. Variations in ignitability between vegetation communities were driven by aridity-induced changes in canopy cover and its effect on litter moisture. Ignitability models based on VPD and aridity could be used to predict ignitability now and into the future, given anticipated increases in VPD under climate change.
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    Long-Term Response of Fuel to Mechanical Mastication in South-Eastern Australia
    Pickering, BJ ; Burton, JE ; Penman, TD ; Grant, MA ; Cawson, JG (MDPI, 2022-06)
    Mechanical mastication is a fuel management strategy that modifies vegetation structure to reduce the impact of wildfire. Although past research has quantified immediate changes to fuel post-mastication, few studies consider longer-term fuel trajectories and climatic drivers of this change. Our study sought to quantify changes to fuel loads and structure over time following mastication and as a function of landscape aridity. Measurements were made at 63 sites in Victoria, Australia. All sites had been masticated within the previous 9 years to remove over-abundant shrubs and small trees. We used generalised additive models to explore trends over time and along an aridity gradient. Surface fuel loads were highest immediately post-mastication and in the most arid sites. The surface fine fuel load declined over time, whereas the surface coarse fuel load remained high; these trends occurred irrespective of landscape aridity. Standing fuel (understorey and midstorey vegetation) regenerated consistently, but shrub cover was still substantially low at 9 years post-mastication. Fire managers need to consider the trade-off between a persistently higher surface coarse fuel load and reduced shrub cover to evaluate the efficacy of mastication for fuel management. Coarse fuel may increase soil heating and smoke emissions, but less shrub cover will likely moderate fire behaviour.
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    Spatial Estimates of Future Fire Risk Considering Climate and Fuel Management for Conservation Planning
    Marshall, E ; McColl-Gausden, S ; Collins, L ; Bennett, L ; Penman, TD (MDPI, )
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    The fuel-climate-fire conundrum: How will fire regimes change in temperate eucalypt forests under climate change?
    McColl-Gausden, SC ; Bennett, LT ; Clarke, HG ; Ababei, DA ; Penman, TD (WILEY, 2022-09)
    Fire regimes are changing across the globe in response to complex interactions between climate, fuel, and fire across space and time. Despite these complex interactions, research into predicting fire regime change is often unidimensional, typically focusing on direct relationships between fire activity and climate, increasing the chances of erroneous fire predictions that have ignored feedbacks with, for example, fuel loads and availability. Here, we quantify the direct and indirect role of climate on fire regime change in eucalypt dominated landscapes using a novel simulation approach that uses a landscape fire modelling framework to simulate fire regimes over decades to centuries. We estimated the relative roles of climate-mediated changes as both direct effects on fire weather and indirect effects on fuel load and structure in a full factorial simulation experiment (present and future weather, present and future fuel) that included six climate ensemble members. We applied this simulation framework to predict changes in fire regimes across six temperate forested landscapes in south-eastern Australia that encompass a broad continuum from climate-limited to fuel-limited. Climate-mediated change in weather and fuel was predicted to intensify fire regimes in all six landscapes by increasing wildfire extent and intensity and decreasing fire interval, potentially led by an earlier start to the fire season. Future weather was the dominant factor influencing changes in all the tested fire regime attributes: area burnt, area burnt at high intensity, fire interval, high-intensity fire interval, and season midpoint. However, effects of future fuel acted synergistically or antagonistically with future weather depending on the landscape and the fire regime attribute. Our results suggest that fire regimes are likely to shift across temperate ecosystems in south-eastern Australia in coming decades, particularly in climate-limited systems where there is the potential for a greater availability of fuels to burn through increased aridity.
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    The 2019-2020 Australian forest fires are a harbinger of decreased prescribed burning effectiveness under rising extreme conditions
    Clarke, H ; Cirulis, B ; Penman, T ; Price, O ; Boer, MM ; Bradstock, R (NATURE PORTFOLIO, 2022-07-13)
    There is an imperative for fire agencies to quantify the potential for prescribed burning to mitigate risk to life, property and environmental values while facing changing climates. The 2019-2020 Black Summer fires in eastern Australia raised questions about the effectiveness of prescribed burning in mitigating risk under unprecedented fire conditions. We performed a simulation experiment to test the effects of different rates of prescribed burning treatment on risks posed by wildfire to life, property and infrastructure. In four forested case study landscapes, we found that the risks posed by wildfire were substantially higher under the fire weather conditions of the 2019-2020 season, compared to the full range of long-term historic weather conditions. For area burnt and house loss, the 2019-2020 conditions resulted in more than a doubling of residual risk across the four landscapes, regardless of treatment rate (mean increase of 230%, range 164-360%). Fire managers must prepare for a higher level of residual risk as climate change increases the likelihood of similar or even more dangerous fire seasons.
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    Warmer and drier conditions have increased the potential for large and severe fire seasons across south-eastern Australia
    Collins, L ; Clarke, H ; Clarke, MF ; McColl Gausden, SC ; Nolan, RH ; Penman, T ; Bradstock, R (WILEY, 2022-10-01)
    Aim: The aims were: (1) to identify the environmental drivers of interannual variation in wildfire extent and severity; (2) to examine temporal trends in climatic potential for large and severe wildfires; and (3) to assess whether environmental conditions experienced during the 2019–2020 mega-fire season were anomalous. Location: South-eastern Australia. Time period: 1953–2020. Major taxa studied: Temperate forests. Methods: We used satellite-derived fire severity mapping from 1988 to 2020 to model the effects of drought, weather and fuels on the annual area burned and the proportion of the area burned that was impacted by high-severity fire across four bioregions. Trends in wildfire extent and severity were then estimated from 1953 to 2020 using these derived models and gridded climate data to assess changes in climatic potential for large and severe wildfires. Estimates of wildfire extent and severity for the 2019–2020 fire season were then assessed against prior seasons (1953–2019). Results: Annual area burned was positively related to the severity of seasonal drought and frequency of fire weather conditions that promote substantial daily fire growth. Wildfire severity was elevated in years with severe fire weather and increased with increasing antecedent drought in years without severe fire weather. Fuels had a lesser effect on wildfire extent and severity than climate. Potential fire extent and severity have increased over time in response to an increased severity of drought and worsening fire weather conditions. Estimates of wildfire extent and severity during the 2019–2020 fire season approached the upper extreme within each bioregion, owing to widespread extreme climatic conditions. Main conclusions: The climatic potential for large and severe forest fires has increased across south-eastern Australia since the 1950s, probably because of anthropogenic climate change. The magnitude and severity of the 2019–2020 fires reflected climatic conditions that are driving an increase in the size and severity of wildfires.
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    Independent effects of drought and shade on growth, biomass allocation and leaf morphology of a flammable perennial grass Tetrarrhena juncea R.Br
    Cadiz, GO ; Cawson, JG ; Duff, TJ ; Penman, TD ; York, A ; Farrell, C (SPRINGER, 2021-08)
    Knowing the abundance of different plant species provides insights into the properties of vegetation communities, such as flammability. Therefore, a fundamental goal in ecology is identifying environmental conditions affecting the abundance of plant species across landscapes. Water and light are important environmental moderators of plant growth, and by extension, abundance. In the context of understanding forest flammability, the abundance of a flammable plant species in terms of its cover or biomass can shape the flammability of the whole vegetation community. We conducted a glasshouse experiment to determine the impact of drought and shade on growth, biomass allocation and leaf morphology of forest wiregrass Tetrarrhena juncea R.Br., a rhizomatous perennial grass. When it is abundant, this species is known to contribute substantially to the flammability of eucalypt forest understories (via both ignitability and combustibility). Contrasting hypotheses in the literature predict that drought can have a weaker, stronger, or independent (uncoupled) impact on plant growth when light is limiting. We used a randomized complete block design with ten treatments from the combination of two water levels (drought, well-watered) and five light levels (100%, 80%, 60%, 40%, 20%). Drought and shade were found to have independent effects on wiregrass growth, biomass allocation, and leaf morphology, supporting the uncoupled hypothesis. Growth showed greater plasticity in response to drought, while biomass allocation and leaf morphology showed greater plasticity in response to shade. Our results suggest that wiregrass is more likely to be abundant in terms of its cover and biomass when water is not limiting. Under these conditions, the increased wiregrass abundance could create a window of increased flammability for the forest ecosystem.
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    Determinants of growth of the flammable grass, Triodia scariosa: Consequences for fuel dynamics under climate change in the Mediterranean region of South Eastern Australia
    Gibson, RK ; Bradstock, RA ; Penman, T ; Keith, DA ; Driscoll, DA (WILEY-BLACKWELL, 2016-09)
    Environmental conditions may influence the presence and strength of competitive interactions between different life forms, thereby shaping community composition and structure, and corresponding fuel dynamics. Woodland and shrubland communities of the Mediterranean climate region of South Eastern Australia contain a varied mixture of herbaceous and woody plants. The ratio of herbaceous to woody plants changes along gradients of temperature, moisture and soil fertility. This study aimed to experimentally examine the relative importance of, and interactions between environmental controls (moisture and soil fertility) on the balance of dominant herbaceous (Triodia scariosa) and woody plants (e.g. Acacia ligulata and Leptospermum coriaceum) and their ultimate effects on fuel and fire regimes. The results suggest that environmental determinants of the growth of T. scariosa are likely to be more important than interactions with shrubs in controlling the distribution of T. scariosa. The growth of T. scariosa was consistently higher under hot temperatures and on the less fertile yellow sands, which dominate the south of the region. The results suggest that there is strong potential for the distribution and abundance of T. scariosa to be altered in the future with changes in temperature associated with climate change. The distribution of soil types across the Mediterranean climate region of South Eastern Australia may be predisposed to favour the southerly expansion of T. scariosa‐dominated communities in the future under a warmer climate.