School of Agriculture, Food and Ecosystem Sciences - Theses

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    The effects of fire and landscape structure on animal communities, species, and connectivity
    Zylinski, Simeon ( 2023-04)
    The loss and fragmentation of habitat associated with land use change is the primary driver of global biodiversity declines. Changes to fire regimes that alter habitat suitability also threaten a range of animal taxa. Fire has been increasingly recognised as an important ecological process and is now used to manage fire-prone landscapes around the world, but important questions remain about the effects of fire regimes on animals, especially in fragmented landscapes. The aim of this thesis is to determine the influence of landscape structure (the composition and configuration of landscape elements) on animals in heathy woodlands in southern Australia in terms of fire, fragmentation, and interactions between them. First, I explored post-fire growth stage and land use together as components of landscape structure and assessed their relative and interacting effects on mammal communities. I used camera traps to collect mammal presence-absence data in 2019-20 and analysed it using ordination and linear modelling. I found that land cover composition was the primary influence on community composition. The composition of the fire mosaic had a secondary, weaker effect and one that may change depending on land cover composition. Second, I explored habitat structure as a mechanism by which fire regimes may affect mammal species, using a species activity index derived from the same camera trap data. Post-fire growth stage (a categorical representation of time since fire) was not a direct predictor of any species’ activity levels, but some habitat structure attributes were linked to certain growth stages and were therefore a mediating influence on animals. Finally, I assessed how the growth stages influence functional connectivity for a litter-dwelling skink. I used genetic data, landscape resistance modelling, and circuit theory-based mapping to find the relative connectivity of land use types and growth stages. Functional connectivity varied little with growth stage, with the primary influence on connectivity being the matrix of pasture and forestry plantation. Overall, I did not find direct effects of growth stage on animal communities, species, or connectivity. However, less obvious effects such as the composition of the fire mosaic beyond the site-scale and indirect effects through habitat structure are important to consider in future fire management. The extent of native heathy woodland was also vital for native mammal communities and functional connectivity; remaining native vegetation must be protected and expanded for the best outcomes for native diversity and species persistence.
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    Landscape-scale disturbance history and the dynamics of the conifer taxa of Bidoup–Nuiba National Park, Vietnam
    Nguyen, Thiet Van ( 2023-03)
    Tropical forests are complex and poorly understood ecosystems. In recent decades large-scale, long-term forest dynamics plots have been providing important insights into basic demographic processes such as recruitment, growth, and mortality. However, this information offers little insight into long-term forest dynamics and the role of infrequent disturbances that may occur at the scale of decades to centuries. Understanding how species-rich tropical forests will respond to global change requires understanding these long-term dynamics. Dendroecology, the study of tree rings, has provided the foundation for understanding forest dynamics in temperate forests in the northern hemisphere and parts of the southern hemisphere. It has been little applied in the tropics where most tree species do not have annual growth rings. The overarching aim of this study was to better understand the ecology and dynamics of Vietnam’s Central Highlands, and, in particular, the rich conifer taxa that are found there. To address this, my thesis involves the application of dendroecological approaches to: (1) reconstruct historical fire regimes in the Central Highlands of Vietnam; (2) understand the historical dynamics of Pinus kesiya forests and (3) the spatial and temporal patterns of recruitment and growth of the endemic conifer taxa in mixed conifer-angiosperm forests, and (4) identify angiosperm species with significant potential for future tree-ring based studies. A detailed reconstruction of how disturbances have varied across the Central Highlands landscape over recent centuries will provide important insights into the response of these forests to past disturbances and climate variability, as well as potential future trajectories of development under a changing climate. My thesis presents one of the most comprehensive reconstructions of fire in tropical forests using tree rings of Pinus kesiya in the Bidoup Nuiba National Park (BNNP) in the Central Highlands of Vietnam. A tree ring-based fire reconstruction showed that fire occurrence increased after 1900 and was highly correlated with climate indices (ENSO and PDSI) during the period 1900 – 1960. However, after 1964 the relationship between climate and fire disappeared due to the overwhelming pressure of human ignition sources, which have effectively eliminated climate as a factor limiting fire in these landscapes. While climate change is seen as a potential driver of changing fire regimes in many parts of the world, the role of local human populations may be equally or more important in shaping them. A detailed history of fire in these forests provides a baseline against which recent and future changes can be assessed. Dendrochronological analysis also indicated that extreme droughts, fire, and canopy disturbance have been associated with tree regeneration, growth, and canopy recruitment for centuries. The age distributions of Pinus dalatensis, Keteleeria evelyniana and Fokienia hodginsii show a distinct pulse of recruitment during the 1750s and during the period 1800-1850 that consistently coincided with extreme regional droughts. Periods of sudden and sustained growth release amongst these endemic conifers were associated with regional mega-droughts in the late 18th and 19th centuries. Results from superposed epoch analysis between Palmer Drought Severity Index and discrete tree release events indicated that extreme droughts and abrupt changes in moisture availability may have contributed to increased canopy disturbance rates and tree-level mortality. The effects of drought on tree growth were typically recorded in the tree rings 1-5 years after the drought event. This indicated that extreme drought conditions may drive large-scale canopy disturbances in BNNP. My results also confirmed that growth releases occurred within ~5 years of recorded fire events. This relationship between growth releases and fire events is consistent over 250 years of tree-ring records (1770-2020). Finally, my thesis demonstrated that a number of the angiosperm tree species occur in BNNP have significant potential for tree-ring studies. This is the first study to systematically examine the potential of a large number of angiosperm species for dendrochronological study in Vietnam. Demographic information derived from tree-rings of angiosperms could provide well-dated, long-term data that complements the tree-ring record from the conifers at BNNP. This would further advance our understanding of forest dynamics and the impacts of climate change in these species-rich tropical landscapes.
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    Dynamics of a flammable species in a forest landscape: A case study on forest wiregrass Tetrarrhena juncea R.Br.
    Cadiz, Geofe ( 2022)
    Species abundance often determines the extent of influence of a species to ecosystem function and processes. Typically, the abundance of a species is constrained by environmental factors within its habitat. However, there are instances where native species becomes prolific and the shift in abundance greatly impacts the ecosystem. Such is the case when a flammable species becomes prolific within its range and alters the flammability of the ecosystem. This is a concern with climate change, as conditions might be tipped in favour of such species. Hence, it is crucial to understand the drivers of abundance to understand how native species can be released from environmental constraints of abundance to become prolific within their own range, and to predict the potential effect of changing environmental conditions on their abundance. Thus, the overarching aim of this thesis was to understand how a flammable native species can become prolific within its own range. This is achieved using a case study species – forest wiregrass Tetrarrhena juncea R.Br. (hereafter wiregrass) – an understorey native species that is of high importance to flammability in the eucalypts forests of south-eastern Australia and grows prolifically under certain conditions. The overarching aim of the thesis was addressed using a mix of research methods to identify the key drivers of wiregrass distribution and abundance. Firstly, a database of the current distribution for wiregrass were analysed using species distribution modelling to identify highly suitable habitat for wiregrass (Chapter 2). Temperature seasonality, precipitation of the driest month, rainfall seasonality, annual mean temperature, the minimum temperature of the coldest month and soil pH were strongly associated with the suitable habitat of wiregrass. The high importance of climatic factors indicates the distribution of wiregrass may be sensitive to climate change. Highly suitable habitats do not necessarily harbor abundant wiregrass because site-specific factors can also control abundance. Hence, Chapter 3 sought to identify the factors most important to wiregrass abundance in the highly suitable habitat of Mountain Ash-dominated forest. Wiregrass cover was assessed in a field survey across a chrono-sequence of 126 sites with contrasting disturbance histories. Canopy cover and net solar radiation were the most important predictors of wiregrass abundance, with wiregrass cover highest in recently disturbed areas with sparse canopy cover, high light levels, and low precipitation. The final component of the thesis used a glasshouse experiment to quantify causal links between resource availability and wiregrass abundance. Wiregrass growth was more sensitive to water availability than light, whereas biomass allocation and leaf morphology were more sensitive to light availability. Collectively, the results showed that, where wiregrass is present (distribution), three key conditions will greatly favour its prolific growth (abundance): (i) non-limiting water resource; (ii) reduced canopy cover and increased light; and (iii) recent disturbance. These key results strongly suggest wiregrass can become prolific when resources are increased, and the vegetation community is substantially disturbed. Under such conditions, increased wiregrass abundance could create a window of increased flammability for the forest ecosystem. Since climate change can alter resource availability and disturbance regime, shifts in wiregrass abundance are likely to occur under future climate scenarios.
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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.