School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemThe mechanisms through which fire shapes plant life cycles in heathlands( 2023-11)Fire is a key driver of plant diversity, and many plants have adaptations that help them thrive in fire–prone ecosystems. However, changes to fire activity threaten thousands of plants worldwide. To understand the future of plant populations under fire regime changes, empirical research on fire’s influence on demographic processes is required. This thesis explored how patterns of fire influence plant species across their life cycle, from seeds to mature life stages, and how this relates to plant functional traits. I examined a Mediterranean-type heathland ecosystem as a case study, to examine the mechanisms through which fires impact plants at different life-stages, including those that take place above and below ground. I established 57 study sites in Gariwerd, southeastern Australia, which has experienced substantial variation in fire history. First, I investigated whether knowledge of plant traits can be used to make robust predictions for how fire influences plant relative abundance. I deductively assigned species to plant functional types, based on their persistence traits, establishment capacity, and the timing of key life stages, and made a priori predictions on how relative abundance changes as a product of time since fire. Using empirical data I collected on species relative abundance, I then built nonlinear models to test species’ model conformity to a priori predictions for plant functional types. Predictions of the direction of changes in relative abundance (increase or decrease from 0-81 years since fire) were correct for 18 of 24 species modelled. Predictions of the shape of changes in relative abundance were not as accurate, but still useful: 13 out of 24 species showed ‘excellent’ conformity with shape predictions, 7 ‘good’ conformity, and 4 ‘poor’. This suggests plant functional types can be used to generalise fire responses across species that share similar traits, and thus inform fire management and biodiversity conservation. Second, I examined how fire severity and time since fire interact to influence plant maturity. I collected data on the proportion of plants that had reached reproductive maturity at a site. I used this field data, alongside satellite-based fire severity mapping, to build non-linear models of plant-fire relationships. The results indicated that the proportion of mature plants was influenced by time since fire, regardless of fire severity. For example, for Banksia marginata, the proportion of mature plants increased from 13% (1-year post-fire) to 58% (15 years post-fire), and maturity of this species showed minimal variation between low and high severity fire. Interestingly, no relationships were observed between time since fire and the relative abundance of plants. That is, only when plant life stages were considered, did I detect an effect of fire on plants. Ecological studies that distinguish between plant life stages will help to predict the impacts of fire on populations and enhance decision-making. Third, I investigated how time since fire and mean fire interval influence canopy seedbank production, based on a suite of plant traits. I surveyed all individual plants with canopy cones present at each of the 57 study sites. On each mature individual, I measured plant height and width, and counted the number of cones. I sampled a subset of these cones across individual plants, and then germinated them in a laboratory trial. I used regression models to explore the relationship between fire frequency and variables relating to different aspects of canopy seedbank production. The interval between fires influenced canopy seedbank production and viability. For example, no canopy cones were observed on plants at short mean fire intervals: such as fire intervals more frequently than every 18 years for the obligate seeder tree Callitris rhomboideia. Quantifying the fire intervals which supports canopy seedbanks provides a new understanding of an important above ground process and helps to determine how frequently to burn ecosystems containing serotinous species. Last, I examined how time since fire and fire frequency influence the occurrence of different species in the soil seedbank and, again, examined ecological relationships through the lens of plant traits. I sampled the soil seedbank at 57 sites, treated soil samples with heat and smoke product to promote germination, and grew seedlings in a germination trial lasting 14 months. I used non-linear modelling to explore relationships between fire and species occurrence. Fire frequency influences the occurrence of species in the soil seedbank, and the nature of these relationships depends on plant traits such as plant and seed longevity. For example, frequent fires (every <15 years) will reduce the occurrence of herbaceous species with long-lived seed. However, for other types of plants, such as perennials with short-lived seed, I observed no relationship between fire and soil seedbank occurrence, demonstrating many species have soil seedbanks resilient to frequent fires. Overall, my research advances understanding of how fire impacts different species and groups of plants across their life cycle. Notably, a mix of field research, laboratory studies and empirical models provide evidence that the traits of plants can be used to identify how fire affects species in the soil and canopy seedbanks, and as juvenile and mature plants. By examining plant life stages above and below ground, this work also helps to define the fire regimes that support plants in the heathy woodlands of Gariwerd. Because it is based on mechanisms, I anticipate that the trait-based approaches I have developed and tested could be used to understand and predict fire-related changes in plant populations in a wide range of ecosystems.
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ItemPredicting redistribution of species and communities under environmental change: Improving the reliability of predictions across time( 2023-04)Ecological models used to forecast range change (range change models; RCM) have recently diversified to account for a greater number of ecological and observational processes in pursuit of more accurate and realistic predictions. Theory suggests that process-explicit RCMs should generate more robust forecasts, particularly under novel environmental conditions. RCMs accounting for processes are generally more complex and data-hungry, and so, require extra effort to build. Thus, it is necessary to understand when the effort of building a more realistic model is likely to generate more reliable forecasts. During my thesis, I investigated how explicitly accounting for processes improves the temporal predictive performance and transferability of RCMs. I first identified key knowledge gaps, and the challenges of evaluating temporal predictive performance and transferability. One of the main challenges is the lack of robust metrics to assess predictive performance and transferability. To address this I implemented and tested the use of new emerging tools to enable fair comparisons of predictive performance across samples with varying degrees of imbalance (e.g. species with low and high observed prevalence). I then tested a couple of hypotheses related to whether modelling observational processes explicitly results in better forecasts. In particular, I evaluated under what circumstances the benefits of explicitly accounting for imperfect detection and allowing information sharing across multiple species are retained when the models are extrapolated to generate predictions beyond the training temporal window. The findings should shed light on how to address remaining knowledge gaps, and how to generate more reliable forecasts on species’ responses to global change scenarios.
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ItemTrees Need Closure Too: Unveiling The Molecular Control Of Wound-Induced Secondary Vascular Tissue Regeneration In Trees( 2023-09)Trees play a pivotal role in terrestrial ecosystems and are an important natural resource. These attributes are primarily associated with the capacity of trees to continuously produce woody tissue from the vascular cambium, a ring of meristem cells located just beneath the bark between phloem and xylem tissue layers. Long-lived trees are exposed to a myriad of biological and environmental stresses that may result in wounding, leading to a loss of bark and the underlying vascular cambium. This affects both wood formation and the quality of timber arising from the tree. In addition, the exposed wound site is a potential entry point for pathogens that cause disease and may even lead to the death of the whole plant. In response to wounding, trees have the capacity to regenerate lost or damaged tissues at a wound site. Investigating gene expression changes associated with different stages of wound healing reveals complex and dynamic changes in the activity of transcription factors, signalling pathways and hormone responses. This thesis investigated molecular regulators of wound-induced secondary vascular tissue (SVT) regeneration. It summarises current literature on primary and secondary vascular tissues and bark wounds and related revascularisation processes, specifically on genes and hormones. Using this information, eight genes from Eucalyptus, including WUSCHEL RELATED HOMEOBOX 4 (EgrWOX4), Arabidopsis thaliana HOMEOBOX GENE 8 (EgrATHB8), CORONA (EgrCNA), PHABULOSA/PHAVOLUTA (EgrPHX), REVOLUTA (EgrREV), AUXIN RESPONSE FACTOR 5 (EgrARF5), PIN-FORMED 1 and 3 (EgrPIN1 and EgrPIN3) were chosen for subsequent experiments on wound-induced SVT regeneration. During these in-planta experiments, Induced Somatic Sector Analysis (ISSA) was used as a molecular tool to assess promoter activity and gene function of these candidate genes in wild-type stems and those where auxin transport was chemically inhibited. Endogenous auxin (IAA) concentrations were quantified using LC-MS to understand how varying auxin concentrations might be required for proper vascular tissue patterning during various stages of regeneration. Results show that the remaining xylem tissues on the wound surface regenerate all lost tissues in a four-step process. EgrPIN1/3 are expressed in all tissue types, EgrWOX4, EgrARF5 and EgrREV predominantly in cambium tissues and EgrATHB8, EgrCNA and EgrPHX in cambium and xylem tissues. Overexpressing micro-RNA-resistant REV leads to faster regeneration rates, while over-expressing miR166 and chemical inhibition of polar auxin transport leads to slower regeneration rates. Samples from overexpression experiments and auxin inhibition also lead to defects in cell anatomies, arrangement, and organisation. Quantification of IAA levels suggests alternating high and low auxin signalling during different stages of regeneration. Together, this thesis provides novel insights into spatial-temporal expression patterns of the selected molecular regulators and discusses how they relate to our current understanding of vascular cambium formation and xylem differentiation during secondary growth. Based on the findings, I propose a model for wound healing that provides the conceptual foundations for future studies aiming at understanding this intriguing process.
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ItemSalinity control, water reform and structural adjustment : the Tragowel Plains Irrigation District(University of Melbourne, 1999)
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ItemMycorrhizal associations of Prasophyllum R.Br. (Orchidaceae) and the conservation of its threatened species(University of Melbourne, 2010)
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ItemSustainable use of recycled water for irrigating lettuce(University of Melbourne, 2009)
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ItemInvestigations into rhizoctonia root rot resistance in aegilops tauschii and other wild wheat relatives(University of Melbourne, 2005)
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ItemNo Preview AvailableEpidemiology of mint rust and variation in the Pathogen, Puccinia menthae Pers(University of Melbourne, 1998)
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ItemThe effects of exogenous abscisic acid application on the quality of crimson seedless grapes(University of Melbourne, 2009)
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ItemNo Preview AvailableGenetics of resistance to Heterodera avenae in Triticum tauschii and its transfer to bread wheat (Triticum aestivum)(University of Melbourne, 1995)