School of Ecosystem and Forest Sciences - Theses

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    Predicting future fire regimes and the implications for biodiversity in temperate forest ecosystems
    McColl-Gausden, Sarah Catherine ( 2022)
    Fire regimes are changing around the world. Fire seasons are lengthening, high severity fires are occurring more often and in unexpected places. Relationships among fire, climate, and vegetation are varied, dynamic, and under-examined in many ecosystems. While some studies have explored links between fire, climate, and vegetation such as species distributions or future fire weather under changing climate, relatively few have considered the dynamic interactions among all three simultaneously. In this thesis, I develop and apply modelling approaches to predict future fire regimes in south-eastern Australia and explore the implications for fire-responsive functional plant types. In the first quantitative chapter of my thesis (Chapter 2), I develop a new fuel model for south-eastern Australia. I use edaphic, climatic, and fire variables to build a predictive fuel model that is independent of vegetation classes and their future distributions. In Chapter 3, I use my fuel model in a landscape fire regime simulator, alongside multiple predictions of future climate, to examine the immaturity risk to an obligate seeder tree species (Eucalyptus delegatensis). My simulations indicate that this species will be under increased immaturity risk under future fire regimes, particularly for those stands located on the periphery of the current distribution, closer to roads or surrounded by a drier landscape at lower elevations. In Chapter 4, I expand the application of the above simulation approach to examine the relative importance of future fuel and future climate to changing fire regimes in six case study areas across temperate south-eastern Australia. My results indicate that the direct influence of climate on fire weather will be the principal driver of changes in future fire regimes (most commonly involving increased extent, decreased intervals, and an earlier start to the fire season). The indirect influence of climate on vegetation and therefore fuel was also important, acting synergistically or antagonistically with weather depending on the area and the fire regime attribute. Finally, in my fifth chapter, I consider future climate and fire impacts on plant persistence by combining the landscape fire regime simulator with spatially explicit population viability analyses. Obligate seeder species were at risk of population extinction or reduction in more simulation scenarios than facultative resprouters. However, my approach highlighted that the resilience of facultative resprouters might also be tested by climate related changes in demographic processes and fire regimes. Overall, my research has provided new methods and scientific insights into the changing nature of fire regimes in temperate south-eastern Australia. Some negative impacts on biodiversity from a changing fire regime, particularly on more vulnerable plant functional types like obligate seeders, appear inevitable. Further understanding of the complex interactions among fire, climate, and vegetation will enable improved integration of risks to people, property, and biodiversity into land and fire management planning.
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    Impacts of short-interval wildfires on tree demography and forest structure in temperate Australia
    Fairman, Thomas Alexander ( 2019)
    Fire is a powerful agent of disturbance in terrestrial ecosystems, and it shapes vegetation composition and patterns globally. This is particularly true in south-eastern Australia where forests are dominated by species of the genus Eucalyptus, many of which have the capacity to recover from high-severity fire by resprouting from epicormic or basal buds. Climate change is predicted to yield more severe fire weather and lengthen fire seasons in temperate Australia, leading to increased wildfire frequency in these forests. While increased fire frequency – resulting in wildfire intervals of under a decade – are known to negatively affect fire-sensitive eucalypts (obligate seeders which have a juvenile period of 10 – 15 years) less is known about how such changes impact fire-tolerant, resprouting eucalypts. This Thesis examines the impacts of recent wildfires in south-eastern Australia, where a series of large wildfires burned over four million hectares of land, leading to the burning of different types of fire-tolerant eucalypt forests (basal resprouters, epicormic resprouters) by high-severity wildfires once, twice, and sometimes three times between 2003 and 2013. In the context of this massive natural experiment in the landscape, my overarching aim was to quantify the impacts of short-interval wildfire on eucalypt tree demography and regeneration, to improve understanding of potential fire-related changes to the structure and resilience of fire-tolerant forests. Short-interval high-severity wildfires significantly increased whole-tree mortality and decreased the abundance of both resprouts and seedlings in basally resprouting eucalypt forests. In these sub-alpine forests, dominated by snow gum (E. pauciflora), more frequent wildfire (two and particularly three short-interval fires) also increased the cover of grasses at the expense of shrubs. In mixed-species eucalypt forests, which occupy extensive tracts of low elevation landscapes in south-eastern Australia, resprouting occurs from both basal and epicormic buds. In these forests, the dynamics of both topkill (i.e. stem, but not whole-tree, mortality) and whole-tree mortality have important ramifications for forest structure. After a single high-severity wildfire, small-diameter stems were typically topkilled; after two short-interval wildfires, the diameter of stems topkilled increased. Additionally, the overall likelihood of either basal or epicormic resprouting decreased after two short-interval wildfires. This decline in resprouting capacity indicated that the size class most vulnerable to ‘resprout failure’ after multiple wildfires was intermediate sized stems (in the vicinity of 20 - 30 cm DBHUB), rather than smaller or larger stems. Seedling regeneration also decreased in these forest types after short-interval wildfires, suggesting that, as for fire-sensitive forests, immaturity risk may be a relevant factor for fire-tolerant forests. Short-interval wildfires reduced the total and aboveground carbon stocks of mixed-species forests, while also increasing the proportion of carbon mass in the dead pool, indicating that resprouter forests might not be perpetually secure carbon stocks under emerging fire regimes. My Thesis highlights that fire-tolerant forests may not be as invulnerable to changes in fire frequency as widely assumed, and that management interventions will likely be required to counteract increasing tree mortality and decreasing tree regeneration if predictions of more frequent and severe wildfires in temperate Australia are realised.