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

Second Advisor

Professor William Stock

Abstract

In Western Australia, groundwater accounts for about 57% of Perth's water supply. The majority of this is from the Gnangara Mound, the largest superficial aquifer on the Swan Coastal Plain. Prior to the mid 1970's groundwater of the superficial aquifer reached a semi-steady state, however since this period levels have been steadily falling. This decline coincides with a dramatic change in Perth's climate, groundwater abstraction and maturation of pine plantations. The influence of pine plantations upon groundwater recharge is well understood, however there is paucity of information about groundwater use, in particular whether pines directly access shallow groundwater resources. This study used traditional dendrochronology techniques to evaluate the influence of historical climate and groundwater trends upon growth trends of Pinus pinaster Ait stands on the Gnangara Mound, Western Australia. Twenty eight (28) trees were destructively sampled, 16 from the Gnangara plantation and 12 from the Pinjar plantation. Three age classes, being young (<15 years), intermediate (15-25 years) and old (>25 years) and two depth to groundwater regimes were selected to meet the study's objectives. The Gnangara plantation sites were selected to represent Pinus pinaster Ait. Plantations which potentially have access to groundwater resources ( <5 mbgl), and the Pin jar plantation sites represented those without (>20 mbgl). From each tree a whole-tree cross-section was removed and tree growth-rings were delineated to assess annual growth trends. Standard dendrochronology techniques were used to developed tree growth-ring chronologies. These chronologies were then used to evaluate the influence of historical climate and depth to groundwater trends, and historical silvicultural treatments upon growth trends. The key findings of the tree growth-ring analysis are that growth trends of Pinus pinaster Ait occurring on the Gnangara Mound, are significantly influenced by silvicultural treatments and cumulative growing season rainfall, irrespective of the underlying groundwater trends. Thinning in particular significantly influenced ring growth. The growth increment of trees sampled from the Pinjar and Gnangara plantation sites were all responsive to rainfall trends however the trees located in the Pinjar plantations were slightly more responsive. Therefore growth trends did not indicate whether those trees underlain by a shallow groundwater resource (<5 mbgl) directly access this resource. The differential responses to rainfall were most likely attributed to site specific soil properties. There was no significant correlation identified between tree growth-ring trends and the other tested variables of minimum, maximum ar1d cumulative temperature, evaporation and depth to groundwater trends. Water use efficiency was assessed by analysing 813C signatures of whole-wood from annual tree growth-rings and from foliar material collected from the trees destructively sampled for chronology development. Analysis of foliage material revealed that needle 813C values are significantly influenced by canopy position and brunch length characteristics and arc therefore considered unsuitable for the evaluation of the influence of historical climate and groundwater trends. Nitrogen concentrations analysed from foliar material was used as a proxy for soil nutrients. Results indicated the soil nutrient conditions at the Gnangara and Pinjar plantation study sites were similar, therefore unlikely to account for differences between 813C values. It is however recommended that future assessment of 813C assess soil nutrients including nitrogen and phosphorus. Results from analysis of 813C in tree growth-rings revealed that there were no consistent correlations between 813C and historical climate and groundwater trends. Therefore 013C is not a suitable indicator of intrinsic water use efficiency. The 813C trends however did enhance the interpretation of growth responses to historical climate and groundwater trends and silvicultural treatments. From the results obtained from this study, it was concluded that Pinus pinaster Ait. Is a suitable species for chronology development and data derived from the application of dendrochronology techniques may enhance the development of future forest management programs.

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