School of Geography - Theses
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ItemUnderstanding the response of Tasmanian rainforest to climate change in the absence of human influence( 2019)The predicted increase of climate-driven wildfires poses a threat to the endemic rainforest species of Tasmania. In order to sustainably conserve and manage these threatened ecosystems in the future, it is crucial to understand the natural response of western Tasmanian vegetation to rapid climate change. While previous research at the Lake Selina site in the region has produced a paleoenvironmental reconstruction of the environmental response to climate shifts during a period in which historical indigenous land management practices were in effect, there is a knowledge gap regarding the response of vegetation to rapid climatic warming in the absence of these practices, which describes the situation in western Tasmania today. As such, this thesis seeks to understand the post-glacial response of vegetation to warming Holocene climates in the absence of anthropogenic fire regimes. To do so, a multi-proxy analysis of lake sediments from Darwin Crater in western Tasmania is conducted in order to facilitate comprehensive palaeoenvironmental reconstruction of a post-glacial environment. After establishing the suitability of conducting a comparison between the selected sites, this research goes on to determine the differences in the response of Tasmanian vegetation in the presence or absence of fire-based land management. The findings from this research identified a clear relationship between anthropogenic fire regimes and the response of western Tasmanian vegetation and can thus be used to project the future responses of vegetation in the region in the absence of indigenous land management practices.
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ItemA northward shift of the Southern Westerlies during the Antarctic cold reversal: evidence from Tasmania, Australia( 2018)The Southern Hemisphere Westerlies are one of the most important components of the Earth’s climate system: they are the primary driver of Southern Hemisphere climate, they modulate global ocean circulation patterns, and they are a critical natural driver of atmospheric CO2 variation. Despite their clear importance, their dynamics in response to rapid changes in climate boundary conditions are poorly understood. Critical to this lack of understanding is (1) an absence of robust proxy-data from the Australian sector of the Southern Hemisphere, which hampers attempts at predictive modelling, and (2) a lack of consensus within the palaeoclimate literature as to how the Southern Westerlies have responded to past periods of rapid climate change. A case in point is the behaviour of the Southern Westerlies during the Antarctic Cold Reversal (ACR; 14,000 – 13,700 years ago), a millennial-scale climate event that punctuated the termination of the Last Ice Age in the Southern Hemisphere. A thorough understanding of how this critical climate component changed during the ACR is hampered by the only available proxy-dataset from the Australian sector of the Southern Hemisphere, which disagrees with records from other regions, and with the leading conceptual understanding of Southern Westerly dynamics. To address this discord, this thesis sought to reconstruct the dynamics of the Southern Westerlies in the Australian sector by developing two robust terrestrial proxy-datasets from Tasmania, Australia, covering the ACR. The results from this thesis demonstrate that the Southern Westerlies responded to the climatic changes of the ACR as predicted by the leading conceptual understanding of their dynamics, and also revealed that they responded symmetrically across the Southern Hemisphere, coincident with substantial changes in atmospheric CO2 variation. This thesis supports the hypotheses that the Southern Westerlies are the primary determinant of long-term Tasmanian climate variation and are a critical regulator of long-term global atmospheric CO2 variation.
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ItemDirect and indirect effects of long-term climatic change on terrestrial-aquatic ecosystem interaction in Tasmania( 2018)Climate influences aquatic ecosystems through two important pathways: (1) directly through temperature or changes in the precipitation/evaporation balance and/or (2) indirectly mediated by changes in the terrestrial environment. However, the indirect impacts of climate on aquatic ecosystems are poorly understood. The aim of this thesis is to better understand how aquatic ecosystems respond to past climate change, using two lakes in western Tasmania as case studies. Palaeoecological research on two multiproxy lake sediment records (Paddy’s Lake and Lake Vera) were used to reconstruct chronology (radiometric dating, i.e. 14C); fire regimes (charcoal); vegetation dynamics (pollen); nutrient dynamics (C%, N%, C/N, δ13C, and δ15N); catchment geochemistry (µXRF scanning); and aquatic response (diatoms and cladocerans) to determine the impact of climate change on these aquatic ecosystems. Results from Paddy’s Lake reveal long-term changes in the cladoceran community are indirectly driven by climate through changing vegetation productivity and available 14N altering the trophic status of the lake. Following the invasion of sclerophyll vegetation caused by increased fire frequency, the indirect climate influences on the aquatic system break down and the cladocerans appear complacent to changing vegetation productivity. At Lake Vera, diatoms respond indirectly to climate through changes in the acidity and dystrophic conditions of the lake with catchment peat formation. An increase in climate variability at ca. 5 ka caused declines in lake level resulting in a shift to a direct response in the diatoms to climate. During a period of increased drying at ca. 2.4 to 0.7 ka, increased fire activity adversely impacts the aquatic system causing a non-linear transition in the diatom community. The findings from this thesis show aquatic ecosystems of Tasmania are predominantly indirectly driven by climate through the formation of thick organic peats. Shifts in vegetation composition alter the surrounding soils and catchment dynamics impacting aquatic ecosystems trophic status and pH. Fire is another important driver of aquatic ecosystem response that causes changes in vegetation composition, altering the nutrient profile of soils and increasing erosion and sediment delivery. Aquatic ecosystems respond with increased pH, disturbance taxa and a shallowing of lake mixing depth in the diatom community. These terrestrial-aquatic ecosystem interactions have the potential to be more widespread across Southern Hemisphere biomes and temperate peatlands worldwide that share similar vegetation-soil dynamics.