Date of Award

1-1-2004

Degree Type

Thesis

Degree Name

Master of Science

School

School of Natural Sciences

Faculty

Faculty of Computing, Health and Science

First Advisor

Dr Eddie van Eiten

Second Advisor

Dr Ray Froend

Abstract

Tuart (Eucalyptus gomphocephala DC.) is an ecologically and culturally important tree species that grows across a narrow 400 km long coastal belt, from the Sabrina River south of Perth, to Jurien Bay in the north. Unfortunately, Tuart is also a species under considerable threat due to clearing, lack of recruitment and canopy decline. Canopy decline is of particular concern, being increasingly reported throughout the Tuart distribution. Despite this, previous studies of Tuart have generally been limited to localised canopy decline events. This two-phased study firstly involved an assessment of Tuart canopy condition at 46 sites across the species distribution. At each study site the canopy condition of no Jess than 20 Tuart trees was assessed using estimations of canopy completeness, measures of canopy size reduction, and through the scoring of canopy condition indices. The second phase of the study involved collecting data on a wide selection of environmental factors considered to be important to canopy condition. These included factors of stand structure, understorey composition,landform (soil type, geology, topography, and geography), climate (gradients and change), hydrology (depth, depth change, and chemistry), fire regimes, pollution and anthropogenic disturbance (fragmentation and site disturbance). Assessment of canopy condition indicates that most the Tuart distribution has slight to moderate canopy decline, and is characterised by a mean canopy completeness of 70 percent. Most of this canopy decline is considered to be background (stand level) decline and likely to be a characteristic of the species. However, the lack of comparative studies using similar methodology makes the determination of the current state of Tuart canopy condition difficult. Long-term monitoring of Tuart canopy condition is therefore recommended to determine temporal trends in canopy condition. The main area of concern for Tuart conservation was found to be the Yalgorup region, where trees had typically less than 30 percent canopy completeness. Severe canopy decline in this region is similar to dieback reported in other eucalypt species from across Australia, that is, gradually receding canopy foliage leaving stag-headed trees and abundant epicormic foliage. Localised cases of canopy decline, similar in structure to that of the Yalgorup region, were found at Yellagonga, Neerabup and Ludlow National Park. Investigations into environmental factors found severe canopy decline in the Yalgorup region is associated with higher rainfall, finer and shallower soil, higher groundwater alkalinity and salinity, and greater rates of groundwater salinity increase. Elsewhere Tuart canopy decline was found to be associated with nutrient enrichment (high topsoil ammonium nitrogen) and high levels of fragmentation. Environmental factors other than those mentioned do not appear to be causing Tuart canopy decline across the distribution, for example understorey competition, altered fire regimes and climate change. Future management of Tuart canopy decline needs to focus on the severe canopy decline in the Yalgorup region, and build upon the hypotheses proposed in this study to explain canopy decline. Low recruitment should also not be forgotten as a major cause of the species decline.

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