Date of Award

2004

Degree Type

Thesis

Degree Name

Bachelor of Science (Hons.)

Faculty

Faculty of Computing, Health and Science.

First Advisor

Dr Ray Froend

Abstract

Water is considered to be the major limiting resource to plant growth and survival in regions with a Mediterranean-type climate, particularly during dry summer periods when low water potentials develop (Poole et al., 1981; Miller et al., 1983-84; Mooney and Miller, 1985; Stock et al., 1992). The Swan Coastal Plain is situated within this climatic region and interactions between the climate, soil and geology has an important bearing on the water requirements of the associated Banksia woodlands (Dodd and Heddle, 1989). The assessment of the condition of any vegetative community and in this case, the resilience of Banksia woodlands to groundwater drawdown, is dependent upon a complete understanding of the factors influencing that particular plant community (Sharma, 1989). The main determinates influencing the location of different plant communities on the Gnangara Mound arc the underlying site conditions and depth to groundwater, both of which fluctuate on a seasonal basis. For example, Banksia ilicifolia can be found in the middle to lower slopes and depressions where depth to groundwater is relatively low, and other species such as Banksia antemata and Banksia menziesii are found across a topographical gradient as they can tolerate a greater range in conditions (Allen, 1981 ). The aim of this thesis is to assess the resilience of Banksia woodlands to sudden groundwater decline events, by incorporating the main results obtained from the three specific aims in the project. These were: I. Identify and describe the hydrological and climatic regimes associated with sudden decline events and recovery of a Banksia woodland community. 2. Examine the floristic changes and recovery in a Banksia woodland community impacted by a sudden groundwater decline event. 3. Assess the resilience of Banksia woodland communities to sudden groundwater decline episodes. The Bassendean dune system forms a part of the northern Swan Coastal Plain under which lies a large shallow unconfined aquifer, the Gnangara Groundwater Mound. Groundwater and soil moisture levels have been gradually decreasing in most areas of the Gnangara Mound since the 1970s, as a combined result of a number of years of below average rainfall and increased groundwater abstraction (Davison. 1995). The Gnangara Mound is the largest and most important shallow underground water resource in the Perth region and it supplies substantial amounts of water to meet Perth's current water demands, and is therefore, a vital resource to Perth and the surrounding regions (Heddle. 1986). In conjunction with this use, the Gnangara Mound also represents a significant water resource to native phreatophytic (groundwater dependent) vegetation. To safeguard terrestrial vegetation, groundwater levels must be maintained to allow plants access to water which is required for their growth and continued existence. In many areas throughout the Gnangara Mound, studies by Havel and Mattiske have indicated that watertable drawdown has a high potential to impact on phreatophytic vegetation. A lowering of the watertable level and the climatic changes over the last 30 years has resulted in measured changes in community composition to more drought tolerant species. This was observed at the long-term monitored sites that were examined as part of this study (Neaves and Yea! Swc.1mp). The survival of groundwater dependent vegetation to drawdown depends on a species' capacity to adjust to reduced water availability. In conclusion, the research documented in this thesis has used both experimental data and data derived from the existing Waters and Rivers Commission's long-term vegetation database. From the results, it can be concluded that on the whole, Banksia woodland communities are resilient to large-scale drawdown events and will recover to an equivalent state if time permits.

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