The ecology of Cycas armstrongii and management of fire in Auatralia's tropical savannas

  • David Thomas Liddle

    Student thesis: Doctor of Philosophy (PhD) - CDU


    Cycads are an ancient and interesting group of plants that are in decline in many parts of the globe due to human activities. In the tropical savanna of north em Australia cycad populations are large and the status of most species less tenuous. However, the possible initiation of a grass-fire cycle with the invasion of alien grasses and loss of habitat due to land clearing threaten many elements of the biota, including cycads.

    I investigate the response of Cycas armstrongii to fire. Treatments included "ambient" fire with fuel loads made up of native species and "intense" fire representative of communities invaded by alien grasses. Data from 1126 marked plants of various size classes that were reassessed and measured over 3 years, were used to construct stage transition population models in RAMASGIS.

    Seed viability declined markedly following scorching by fire and stem mortalities were increased by intense fire. Cycas armstrongii populations were remarkably resilient to a broad range of fire frequencies under ambient fire but populations are unlikely to persist with intense fire more frequent than around 1 in 10 years. Under plausible scenarios of land (and fire) management, projected population decline exceeded thresholds for classification as vulnerable to extinction. Given C. armstrongii is considered a fire tolerant plant, impacts of changing fire regimes on biodiversity and density of woody plants are likely to be severe. Adding harvest of various (relatively conservative) levels to simulations did not materially affect outcomes, with the impacts of harvest being swamped by effects of habitat loss and modification.

    The projected decline of C. armstrongii raises complex resource management issues involving interactions among international agreements, national and local legislation, land clearing, fire regimes and weed control. Extraordinarily rigorous approaches to regulation of trade will have no affect on the real threats, and may in fact inhibit improved management of cycad habitat by reducing incentives to maintain the resource.
    Date of Award2004
    Original languageEnglish
    SupervisorPeter Whitehead (Supervisor), Sharon Chirgwin (Supervisor) & Greg Leach (Supervisor)

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