Date of Award

2002

Degree Type

Thesis

Degree Name

Bachelor of Science Honours

Faculty

Faculty of Communications, Health and Science

First Advisor

Dr Eddie van Etten

Abstract

The current paradigm of biodiversity conservation requires the assessment of alien plant invaders, and their potential negative impacts on indigenous species and communities. Leptospermum laevigatum (Gaertn.) F. Muell. (Victorian tea tree/ Coast tea tree) is indigenous to eastern Australia. It has invaded ecosystems within its natural biogeographic range, within new ranges in Australia and overseas. The species is listed as a high priority weed in the Environmental Weed Strategy for Western Australia. However, the basis for its listing has been casual observation rather than focussed research. This study of L. laevigatum is unique in being the first to create a comprehensive synthesis of the ecology and management of L laevigatum in Australia, combining scientific investigation with information from land managers in eastern and Western Australia. The study combined experiments and vegetation sampling with information from researchers, council staff and bush regenerators to provide baseline data about the species' ecology and phytogeography in Western Australia. Sites were chosen where L laevigatum appeared to be invading remnant vegetation; at these sites morphological and life history characteristics of L. laevigatum were assessed. Soil and plant litter variables and floristics were examined, comparing invaded areas with the indigenous vegetation. Predation rates and seed viability were also investigated. The results of the study show that L. laevigatum is able to occur on a variety of soil types in the southern part of Western Australia; its distribution is apparently restricted to areas with approximately 400mm average annual rainfall. The species is spreading regionally and locally in Western Australia, with long distance dispersal probably effected by road vehicles. L laevigatum is able to invade bushland in good condition with minimal disturbance and of high conservation value. Fire is identified as r: major factor in enhancing invasion by L. laevigatum. Where fire occurs and a seed source is adjacent, resultant recruitment appears to be immense, resulting in high density thickets of L laevigatum. Such thickets are subject to intense intraspecific competition and density dependent mortality. Within thickets, survivorship of indigenous species appears to be low in general, yet some indigenous species are able to co-exist: usually those with a life form substantially different from L. laevigatum. Through chronosequence analysis, three hypotheses about L. laevigatum's impacts on vegetation have been generated (1) L. Iaevigatum enhances levels of leaf litter underneath its canopy, leading to differential recruitment by other species and alterations in community composition; (2) increases in litter result in increased soil moisture, thereby favouring mesophyte establishment; and (3) where dense canopies of L. laevigatum form, L. laevigatum homogenises photosynthetically active radiation over a large area, thereby reducing the variability in microsites with respect to light conditions. Photophilic species are repressed. Investigations of the biology of the species have allowed recommendations to be made about current and potential control strategies. Tree injection with herbicide leaving the dead plant in place is the least invasive method in sensitive vegetation communities. Despite which method is used, site revisits are required within approximately four years of the first control program, to remove regenerating seedlings.

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