Infrastructure Engineering - Theses
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ItemStable isotope dendrochronology of eucalypts: implications for riparian zone hydroecology( 2002)The aim of this study was to apply stable isotope dendrochronology to Australian ecosystems where there have been enormous changes in hydrology. It was anticipated that these massive changes in hydrological regimes would have resulted in significantly large isotopic differences compared to natural conditions. Thus, given the extent of hydrological change it was expected that this would be reflected in the tree ring isotope composition of Eucalyptus spp. originating from these ecosystems. This is the first study in Australia to consider the use of stable isotopes in the context of tracking hydrological change at the catchment scale, as opposed to the detailed internal physiological processes. The advantage of the catchment approach, if successful, is that it will enable water managers access to biophysical information of catchment responses to hydrological change that is not available by any other means. Currently, there is an absence of data and adequate studies to demonstrate the outcomes of environmental flow allocations. It is envisioned that the results from this study will contribute to improving this knowledge base. In order to produce a chronology, the tree ring sequences first had to be dated. The year of tree regeneration was unknown for many of the trees and therefore sophisticated dating techniques were required. These involved using radiocarbon and radium. These methods worked well when growth was annual, or close to. However, when growth was less than annual the techniques were inconclusive, although it was not clear why. The dating techniques indicated that most of the trees in this study produced annual rings. There were a small number of trees where the growth rate was still uncertain. Stable isotope analysis of a Eucalyptus sieberi stand in south-eastern Australia revealed that population variability is much smaller than the differences between water sources measured in other Australian ecosystems. Much of the variability observed in δ18O was related to sap flow, however, the cause of variability in δ18C was not clear. The results from this study imply that sampling strategies be based on mature trees with well developed root systems, trees with high sap flow (compared to other trees in the stand), and that population variability is unlikely to be a major source of error in data interpretation. Detailed investigations in riparian zones, where large changes in hydrology have occurred due to river regulation, were conducted on E.largiflorens from Chowilla Anabranch and E.camaldulensis from Barmah Forest. The stable isotope technique did detect differences in historical water use strategies between trees, however, the isotopic composition of tree ring cellulose was not recording the large shifts in hydrological and environmental conditions. Causes of the observed isotopic variability were investigated in detail. This detailed analysis did not reveal any generic catchment scale relationships that could be used to reconstruct specific hydrological or physiological information. This was because the individual trees responded to environmental factors differently. These different responses were primarily related to site specific characteristics of water availability. These findings were consistent between field sites, and for both δ18O and δ18C. The findings of this research highlight the need for a better understanding of the processes involved in the stable isotope composition of tree rings before stable isotope dendrochronology can be used to reconstruct catchment information in Australian ecosystems. In particular, more detailed physiological type experimentation, both field based and under controlled conditions, is necessary under a range of growth conditions. While the results of this study were often not clear, especially at the catchment scale, this research has provided a solid base from which further development and application of this technique can be targeted in order for future analyses to be successful.