Understanding Invasion Patterns and Processes Associated with Garden Escapees in an Urban Environment

  • Rakhi Palit

    Student thesis: Doctoral Thesis

    Abstract

    Horticulture is a primary source of global weed introductions, especially in urban areas.
    Approximately, 75% of the world’s naturalized plants are garden escapees, while 66% of
    invasive plants in Australia are garden plants. Invasive plant species cause significant
    ecological and economic damage by affecting both managed and natural ecosystems. The most
    efficient way to alleviate such damage is to prevent the introduction of potentially invasive
    non-native species and to identify and eradicate any naturalized species before they can spread
    widely or become invasive. While pre-border weed risk assessment can screen for species with
    weedy potential, this does not address the issue of a large number of non-native plant species
    already present in gardens that could become invasive in the future. The primary objective of
    this thesis was to determine the key factors that influence the naturalization and invasion of
    woody, non-native, horticultural plants in an urban landscape. Factors that potentially influence
    naturalization and invasion success include propagule pressure, or planting effort, residence
    time, certain species traits, and climatic tolerances. I examined the importance of these
    variables on the naturalization of 1439 horticultural, woody, non-native species introduced to
    Canberra, Australia over 150 years. Canberra is a unique city for this purpose because it was a
    planned city with extensive horticultural plantings originating primarily from government
    nurseries, meaning there are records documenting planting effort from the time the city was
    established.
    I showed that the introduction and naturalization of introduced species increased with
    the rising urbanization and population growth, and species with greater planting effort, longer
    residence time, smaller seeds, and with obvious dispersal mechanisms were more likely to
    naturalize. Cold hardy and tall plants were also more likely to naturalize, but cold hardiness
    and height mostly affected naturalization success indirectly via planting effort. Introduced
    species tended to have a long lag-phase between introduction and naturalization (20-170 years). Species planted in consistently higher numbers per year had a shorter lag-phase, while taller
    species had a more extended lag phase. In addition, there was a marked increase in
    naturalizations associated with increased plantings due to population growth and urban
    development in Canberra peaking in the 1970s. Hence, the long lag-phase of many species may
    reflect early introduction and low planting rates early on, with naturalization rates increasing
    as the planting effort increased as the population expanded. I used a model for naturalization
    developed using the Canberra data, along with other weed risk assessment approaches to try
    and predict naturalization outcomes at a specific location (Bendora Arboretum in the Australian
    Capital Territory), which was planted in the 1940s. The Canberra model failed to predict
    outcomes at Bendora Arboretum, perhaps because the model was applied to a very restricted
    set of species, which required a longer lag-phase to naturalize. Overall, the findings of this
    study will be useful in developing a predictive model to quantify the potential invasion risk of
    exotic woody horticultural species
    Date of Award2020
    Original languageEnglish
    SupervisorRichard Duncan (Supervisor) & Paul Downey (Supervisor)

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