School of Ecosystem and Forest Sciences - Theses

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    Using field experiments to test associations between plant traits and fire regimes
    Kreger, Isaac Daniel ( 2021)
    Conservation of biodiversity in fire-prone landscapes requires a thorough understanding of how plants are impacted by fire regimes. Altered fire regimes, due to changes in climate and land use, will likely impact many plant species in arid and semi-arid ecosystems. The study of functional traits – including fire-related traits that help plants to persist in areas subject to recurrent fire - provides a way to understand how plants respond to contemporary and future fire regimes. In my first chapter I studied the influence of a large-scale fire and climate gradient on variation of functional traits of a serotinous tree species, Callitris verrucosa. I measure intraspecific variation in fire-related traits including level of serotiny, bark thickness and tree size on C. verrucosa populations along a 340 km north-south gradient in semi-arid southeastern Australia. Level of serotiny was strongly associated with fire frequency: tree-level of serotiny increased at stands estimated to experience higher fire frequency, while the variation in level of serotiny at standsincreased with lower fire frequency. After accounting for tree size, relative bark thickness was also strongly associated with fire frequency and average annual rainfall. Thicker bark is allocated to smaller stems in areas that experience more frequent fire. In my second chapter, I used a field experiment to explore topkill of resprouting eucalypts after a planned burn in a semi-arid mallee woodland. I measured fire-related traits of eucalypt trees and stems including bark thickness, basal diameter, and canopy height, as well as elements of fuels including litter depth, ground cover, and vertical strata. The probability of topkill declined with taller stems (>3 m): taller mallee eucalypts appear to ‘escape’ low intensity fire. Fuels were also important, with topkill higher in areas with increased cover of hummock grass. Overall, my research demonstrates clear relationships between plant functional traits -including serotiny, 3 height and bark thickness - and fire regimes. This knowledge can be used to inform manipulations of fuels, habitat structure and plants through planned burning, to achieve a wide range of social and environmental objectives. Moreover, the demonstrated variation in C. verrucosa fire-related traits could be used to guide restoration efforts. For example, seeds sourced from southern locations, where populations have higher levels of serotiny, may promote resilience under a scenario of more frequent fire. While the extent to which fire regimes will change is uncertain, field experiments help us learn about contemporary processes and forecast future changes.
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    The role of tubulins in secondary cell wall deposition in woody tree species
    Machado Tobias, Larissa ( 2020)
    Woody trees are an essential source of timber, pulp, paper and biofuel, and advances in biotechnology provide opportunities for the improvement of traits of interest for specific end uses. Cellulose microfibrils, the basic structural component of plant cell walls, are responsible to a large degree for wood mechanical and physiological properties. The angle between the direction of the helical windings of cellulose microfibrils in plant secondary cell walls, or microfibril angle (MFA), plays critical roles in a tree’s development and has become a subject of major interest in forest biotechnology, particularly in detailed studies of the secondary cell wall of xylary (wood) cells. While our knowledge of how exactly the cellulose synthase complex (CSC) acts in response to environmental and genetic cues remains sketchy, guidance of cellulose deposition has been repeatedly accredited to microtubules, a cytoskeleton component formed of protein dimers of alfa- and beta-tubulin. Nevertheless, few studies explore the cytoskeleton roles in secondary cell wall deposition in woody tree species. Reaction wood (RW) develops in response to gravitational stimulus through a series of changes at the cellular and molecular levels. Tubulin genes have been previously reported to be upregulated during RW formation and differences in their expression might lead to differences in microtubule assembly. This differential microtubule organisation might be related to changes on cell wall morphology, including MFA. In this study, cortical microtubule array organisation was therefore assessed in samples from trees forming RW and stems growing upright (normal wood). To further investigate if perturbation of microtubule organisation would impact wood formation, microtubule-interacting drugs were applied to wood tissue depositing SCW in vivo and in vitro. Together, results indicate that tubulins play an essential role in cellulose deposition in the secondary cell wall of woody tree species to ensure appropriate microfibril orientation.