Date of Award

1998

Degree Type

Thesis

Degree Name

Bachelor of Science Honours

Faculty

Faculty of Communications, Health and Science

First Advisor

Dr Ray Froend

Abstract

The Gnangara Mound is a large shallow groundwater aquifer which occurs in the superficial formations bounded by the Swan River, Ellenbrook, Gingin Brook, Moore River and the Indian Ocean on the northern Swan Coastal Plain. To protect terrestrial vegetation on the Gnangara Mound. groundwater levels must be maintained to allow plants access to water which is required for their survival. In areas with a shallow depth to groundwater, studies have shown that groundwater drawdown (reduction of the water table by any processes or factors) has a high potential to impact on the vegetation (Havel. 1968: Aplin. 1976: Heddle. 1980a: Dodd et al. 1984: E.M. Mattiske and Associates. 1993. 1995). Water level fluctuations and a decrease in availability can result from land-use changes, climatic factors and groundwater abstraction (Water and Rivers Commission. 1997). B. ilicifolia (Proteaceae) has been identified as a tree species being responsive to fluctuations in groundwater levels (Heddle. 1980a: Havel Land Consultants. 1988: Dodd and Heddle. 1989: Arrowsmith. 1992: Thomas. 1993: E.M. Mattiske and Associates. 1995) but there is only anecdotal evidence to support a relationship between spatial distribution and depth to groundwater. The principle aim of this study is to investigate the spatial distribution and temporal change in B. ilicifolia on the Gnangara Mound as related to groundwater regime. Specifically the project aims to: 1. Identify and classify floristic communities that contain B. ilicifolia on the Gnangara Mound and investigate the current relationship between the distribution of B. ilicifolia communities and depth to groundwater. 2. Examine the long-term changes in the abundance, vigour and distribution of B. ilicifolia and co-occurring B. attenuata adults and seedlings due to altered groundwater regime. climate, fire and disease. 3. Identify the changes that may have occurred in B. ilicifolia populations over time due to altered groundwater regime and compare the characteristics of populations in areas subject to groundwater drawdown to areas of the Gnangara Mound less affected by drawdown. B. ilicifolia was found between 1.5-9.5 metres depth to groundwater on the Water and Rivers Commission transects on the Gnangara Mound. There was a general decrease in abundance and vigour of those individuals located downslope, with B. attenuata being distributed in areas where B. ilicifolia had shown substantial vigour loss. Comparisons between populations of B. ilicifolia of drawdown and less drawdown sites showed a significant difference in the mean Crown Assessment Index, size class and height of B. ilicifolia individuals. In comparison, there was only a significant difference in the mean Crown Assessment Index of B. attenuata between sites of drawdown and sites of less drawdown. Overall, the results of the study supported trends noted elsewhere. It can be concluded from these results that B. ilicifolia is a suitable indicator of vegetation response under drawdown conditions and the long-term monitoring program on the Gnangara Mound is only superficially representing this change.

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