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 Award | 2020 |
|---|---|
| Original language | English |
| Supervisor | Richard DUNCAN (Supervisor) & Paul Downey (Supervisor) |