School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemStormwater runoff retention and tree growth in passively irrigated street tree systems( 2023-11)Urbanisation creates extensive impervious surface cover which alters natural hydrological cycles. Impervious surfaces generate large volumes of stormwater runoff, which degrades receiving waterway ecosystems when conventionally drained. To protect urban streams, runoff volumes need to be significantly reduced to restore pre-development flow regimes. Street trees have considerable potential to increase the demand for stormwater, by transpiring large volumes of water if irrigated with runoff via ‘passive irrigation’. Passive irrigation systems can also benefit trees by reducing drought stress and increasing growth and therefore providing shading and cooling benefits. However, lack of knowledge on system design and tree species selection is preventing widespread uptake of passive irrigation systems for street trees. This thesis aimed to identify the drivers of runoff retention, tree growth, water use and drought response, to inform the design of and tree species selection for passive irrigation systems. Runoff retention and tree growth were quantified for alternative system designs with different inlet types and storage volumes in the field. Using this data, runoff retention was also modelled for a range of soil exfiltration rates, climates and storage sizes. Tree water use, drought response and growth were assessed in a glasshouse study to inform species selection in relation to system storage. In the field, systems with inlets designed to exclude sediment captured less runoff. However, the exfiltration rate of the surrounding heavy clay soil ultimately limited runoff retention. Therefore, inlet selection will likely be driven by maintenance requirements and cost, rather than runoff retention. However, where storage volume was large relative to the contributing catchment area, or where exfiltration rates were faster, runoff retention was substantially greater. Passively irrigated trees in the field grew faster than control trees; however, this was likely due to trees having both adequate drainage, to avoid waterlogging, as well as an internal water storage. In the glasshouse study, trees with an internal water storage and low stomatal sensitivity showed lower drought induced leaf loss compared to those without, whereas trees with high stomatal sensitivity and an internal water storage showed lower drought stress and recovered rapidly after drought. This thesis demonstrated that passive irrigation systems have significant potential to reduce runoff volume entering waterways. However, large storage volumes will be required to facilitate exfiltration in heavy-textured soils. Passive irrigation systems can improve tree growth when waterlogging is avoided and internal water storage provided, which will likely increase evapotranspiration, runoff retention and canopy growth in the future. As per other stormwater control measures, maintenance requirements must be considered during design and greater oversight during construction is required to ensure systems perform as intended. Designs need to flexible and adapt to local soil and climate conditions. Matching tree species, or more accurately, tree water use strategies, to system design is required to ensure systems maintain function in future climates.
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ItemLinking fire, climate, connectivity and animal conservation( 2023-05)Australia’s forests and woodlands have been severely fragmented by agriculture, urbanisation and forestry, resulting in loss of habitat for native species. Habitat loss and fragmentation affect landscape structure, defined as the composition and configuration of land cover types. Species persistence in modified landscapes depends on the landscape structure and the availability of resources, which may be altered through management practices, such as prescribed fire. Fire can be used in the ecological management of many terrestrial ecosystems, where its application or suppression alter habitat structure and the availability and configuration of key resources. Current approaches to ecological fire management do not consider the influence of the surrounding landscape or the effect of fire on animal movement (connectivity) within or among habitat patches. The implications of current fire regimes for long-term population persistence are also overlooked. Connectivity is linked to persistence because it maintains dispersal, gene flow and genetic diversity, helping species combat environmental change and avoid extinction. Incorporating connectivity into fire management will help conserve biodiversity in fragmented landscapes. In my research program I investigated how 1) species richness and mammal community composition and 2) individual species respond to habitat, fire and landscape structure using data from remote-sensing cameras collected in south-eastern Australia. Additionally, I combine fire simulation modelling and connectivity analyses using genetic data to 3) compare future connectivity for two small mammals, yellow-footed antechinus (Antechinus flavipes) and heath mouse (Pseudomys shortridgei) under alternative fire regimes and predicted climate scenarios. These findings will inform an understanding of how fire, habitat and climate influence mammal communities, and species’ distributions, connectivity, and persistence. In Chapter 2, my first data chapter, I evaluate the relative influence of fire (time since fire and fire frequency), vegetation type, land use diversity and annual rainfall on ground-dwelling mammal community composition and species richness. Findings suggest that vegetation type and rainfall have the greatest influence on the mammal community and the vegetation type treeless heath is of great importance to critical weight range mammals. In Chapter 3, I focus on 1) the influence of vegetation structure, fire, and annual rainfall on the occurrence of 18 mammal species in two vegetation types (heathy woodland and treeless heath) at the local and landscape scale and 2) if species’ life history traits influence their responses to local and landscape scale predictors. At the local scale understory complexity influenced the occurrence of nine species in both vegetation types. In the woodland species occurrences were also influenced by annual rainfall and basal area while in treeless heath time since fire and fire frequency were important drivers. At the landscape scale, the extent of mature vegetation was most influential in the treeless heath while fire age-class diversity and the extent of native vegetation was most influential in the woodland. All tested species life history traits (size, diet, nest requirement, mean annual offspring and native status) were found to influence species responses at the site scale while only one trait (mean annual offspring) was found to predict species responses at the landscape scale. In Chapter 4, I combine fire simulations and connectivity analyses to compare future connectivity under four alternative fire regimes (0%, 1%, 2% and 4% of total available area burnt per year) and predicted future climate, for two small mammals, heath mouse (Pseudomys shortridgei) and yellow-footed antechinus (Antechinus flavipes). Genetic modelling found that heath mouse and yellow-footed antechinus considered the early post-fire age class low resistance (high connectivity) and pastural farmland to be high resistance (low connectivity). The results of the simulations suggest that the composition of the future landscape will shift towards more recently burnt vegetation, increasing connectivity for the heath mouse and yellow-footed antechinus. Overall, my research has provided new insights into how vegetation type and structure, fire and landscape structure influence mammal species distributions and contributed new information towards an important knowledge gap; how fire regimes influence animal movement and connectivity. Collectively, the findings will inform how fire regimes may influence species distributions and connectivity for long-term persistence in fire-prone landscapes.
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ItemClimate and climate change effects on carbon uptake and storage in Australia’s wooded ecosystems( 2022)Forest ecosystems are central to the land carbon sector due to their capacity to store and sequester carbon. Many studies have demonstrated that forest carbon uptake and storage is strongly dependent upon climatic conditions. However, the effects of climate on forest carbon uptake and storage in different biomes are still uncertain. Climate change may alter carbon dynamics within forest ecosystems through the direct effects of increased temperature, increased CO2 concentration and changing precipitation regimes. Yet forests may also adjust to changing climate through mechanisms such as thermal acclimation. In this thesis I used three modelling approaches (machine learning, boundary-line analysis, and a land-surface model) to examine how climate of the recent past, present, and future affect carbon uptake (as Gross Primary Productivity, GPP) and storage (as above-ground biomass, AGB) in Australian forests. Furthermore, I explored how current GPP adjusted to thermal regimes and how acclimation affected carbon uptake and storage in the future. In my first quantitative chapter (Chapter 2), I explored relationships between carbon storage (as AGB) with climate and soil in Australian forests across the continent. I developed RandomForest models with climate-only, soil-only, or climate plus soil variables to examine whether climate or soils are better predictors of forest biomass at the continental scale and to identify the most important predictor variables. In this chapter I demonstrated that climate (particularly temperature and the timing of precipitation) was more important than soil for explaining variation in AGB across Australia’s forests. In Chapter 3, I used boundary-line analysis to examine the ecosystem temperature response of carbon uptake (as GPP) in 17 wooded ecosystems representing five distinct ecoregions. These responses were represented as a convex parabolic curve that was similar in shape among ecoregions – narrow in tropical forests and broader in woodlands. I then derived the thermal optima of GPP (Topt) from these curves for each ecosystem and examined the relationship between Topt and mean air temperatures across sites. My analysis revealed a strong positive linear relationship between Topt and mean air temperature that indicated GPP was optimised to the present climate. Finally, in Chapter 4, I predicted how carbon uptake and storage will be affected by climate change in these 17 ecosystems and examined the effects of thermal acclimation of photosynthesis on these predictions. I used the CABLE-POP land surface model adapted with thermal acclimation of photosynthetic functions and forced with climate projections from the extreme climate scenario RCP8.5. My simulations indicated that increased temperature, CO2 concentration and changed precipitation patterns will have a positive effect on future carbon uptake and storage in the majority of the 17 ecosystems. Furthermore, thermal acclimation of photosynthesis is likely to enhance this effect in tropical ecosystems. My results confirm that carbon uptake and storage in Australian forests are fundamentally linked to temperature and precipitation regimes, and that these forests may be capable of adjusting to climatic conditions. My research indicates that the direct effects of climate change are likely to enhance the storage and sink capacity of Australia’s forests in the future. While I did not assess the indirect effects of climate change on carbon cycles through changes to disturbance regimes, overall, my thesis suggests that carbon uptake and above-ground biomass carbon stores in Australia’s forests are likely to be resilient to climate change.