School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemEffect of elevated carbon dioxide and high temperature on major micronutrients in strawberry( 2019)In this study, four different folate derivatives (tetrahydrofolic acid – THFA, 10-formylfolic acid – 10FFA, 5-formyltetrahydrofolic acid – 5FTHFA, 5-methytetrahydrofolic acid (5MTHFA)) were identified in fresh and freeze-dried strawberry samples. The individual and interaction effects of increased [CO2] and temperature on total folates content were significant (P≤0.05), and the responses were cultivar dependant. Total folate content in strawberries varied from 52.6 ± 5.1 µg to 364.8 ± 16.0 µg/100 g FW in cultivar ‘Albion’ and from 48.6 ± 7.0 µg to 237.4 ± 23.8 µg/100 g FW in cultivar ‘San Andreas’. Although, increased temperature positively affected the total folates content under lower [CO2] levels, the effects turned negative at the highest [CO2] concentration (950 pm). Higher temperature reduced the content of total folates in strawberries by 26% and 13% in cultivar ‘Albion’ and ‘San Andreas’, respectively. Impacts of elevated [CO2], higher temperature and their interactions on total vitamin C content in strawberries were statistically significant (P≤0.05) and the responses were cultivar dependent. Vitamin C contents in cultivar ‘Albion’ and ‘SA’ fresh strawberries were in a range of 59 ± 7 mg to 133 ± 15 mg/100 g FW and 56 ± 9 mg to 132 ± 9 mg/100 g FW, respectively. Increased growth temperature to 30 °C at 650 ppm [CO2] enhanced the amounts of vitamin C significantly (P≤0.05) to a maximum by 123% and 132% in cultivars ‘Albion’ and ‘San Andreas’, respectively. However, that effect wasn’t detected when the CO2 concentration was increased further to 950 ppm, and vitamin C concentrations drastically decreased by 36% and 31% in Albion’ and ‘San Andreas’, respectively. In general, folates and vitamin C contents were significantly (P≤0.05) higher in FD strawberry than fresh fruits. The next step of the study was to study the accessibility of increased polyphenols, vitamin C and folates in the fruits of fresh and frozen strawberries using simulated in vitro gastrointestinal digestion and colonic fermentation. Elevated [CO2] (ambient to 950 ppm) and higher temperature (ambient to 30 °C) enhanced the accessibility of polyphenols, folate and vitamin C in strawberries. Bioaccessibility of Pel-3-Glu increased from 67% to 88% in fresh strawberries when exposed to elevated growth. The exact amounts of individual polyphenols in accessible fraction were significantly (P≤0.05) higher in fresh fruits of strawberries grown under elevated growth conditions. For example, the highest amounts of Pel-3-Glu (19.89±0.4 mg/100 g FW), Pel-3-Rut (2.55±0.5 mg/100 g FW), p-coumaric (0.23±0.02 mg/100 g FW), ferulic (1.33±0.05 mg / 100 g FW), quercetin (1.97±0.2 mg/100 g FW) and p- coumaroyl (0.65±0.05 mg/100 g FW) were detected in fed state simulated gastrointestinal digesta of fresh strawberry grown under elevated growth conditions. Fresh strawberries grown under ambient growth contained 93.09±6.2 µg/100g folates and 18.55±0.5 mg/100g vitamin C as bioaccessible fractions under fed state while, elevated growth enhanced soluble folates and vitamin C up to 188.63±7.5 µg/100g and 30.48±0.3 mg/100g, respectively. Fresh strawberries contained higher amounts of accessible micronutrients than frozen strawberries, while increased bile contents in intestinal fluid (fed state) facilitated the release of bioactive compounds to gastrointestinal fluid. The insoluble fraction of strawberry digests after gastrointestinal digestion was then subjected to in vitro colonic fermentation using human faecal cultures and basal media. The soluble fraction of fermented strawberry digests was extracted to analyse polyphenols, folates and vitamin C. Higher contents of folate (7.90±0.05 µg/100 g FW), vitamin C (33.6±1.0 ng/100 g FW), Pel-3-Glu (2.00±0.14 mg/100 g FW), and p-coumaric (39±5 µg/100 g FW) were observed in soluble fraction of fermented precipitate after simulated gastrointestinal digestion at fasted state in frozen strawberries. These bioactive compounds and their metabolites would play an important role in the human colon by maintaining a healthy environment via scavenging the free radicals. According to the current study, the amount of bioaccessible bioactive compounds in strawberry could vary quantitatively and qualitatively based on growth and storage conditions as well as the status of digestion (fed or fasted state). Increased carbon dioxide and temperature in the growth environment enhanced the bioaccessibility of polyphenols, folates and vitamin C in strawberries. It can be concluded that strawberry fruits grown under elevated [CO2] and temperature may not be visually attractive comparing to normal strawberries. However, considering their nutritional value, those fruits can be promoted as freeze-dried strawberry in value added foods such as dairy products. Additionally, these research outcomes would help the commercial growers to focus on the nutritional aspects of fruits and vegetables grown under such elevated and extreme environmental conditions in the future. However, as a very little information is available concerning the interactive effects of elevated [CO2] and high temperature on fruits and vegetables in the field, more researches are needed to confirm the results from glasshouse studies.
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ItemBiochemical and physiological mechanisms of legume nitrogen fixation under higher atmospheric CO2 concentrations( 2019)Atmospheric CO2 concentration ([CO2]) is expected to rise from a current level of ~400 to 550 µmol mol-1 by 2050. It is well established that elevated [CO2] enhances plant growth and yield. However, the stimulation of plant growth at elevated [CO2] requires additional nitrogen (N) and prolonged exposure to elevated [CO2] potentially risks N limitation. Legumes can overcome such limitations by fixing aerial N. Previous studies under Free Air CO2 Enrichment (FACE) have shown that elevated [CO2] can stimulate N2 fixation, but it is unknown to what extent this applies to dryland Mediterranean environments or what impact environmental interactions have. Legumes grown in dryland environments frequently experience terminal drought accompanied by high temperature during reproductive phases. It has been suggested that elevated [CO2] delays the effect of drought by conserving soil water, maintaining N2 fixation mechanisms for longer under drought. This thesis addresses this gap by investigating the growth and N economy of three legumes (lentil, field pea and faba bean) in a FACE facility in a semi-arid environment where seasonal and experimentally controlled drought was imposed. In addition to N2 fixation itself, the supply and translocation of N compounds to the maturing grain is another point of interest, because it is crucial in maintaining grain N concentration. This thesis investigated N2 fixation, remobilization and grain quality of dryland legumes under predicted future e[CO2] atmosphere conditions, including interactions with drought, heat waves, and genotypes. Free Air CO2 Enrichment technology was used to simulate future growing conditions in the field with target [CO2] as expected by the middle of this century. Elevated [CO2] stimulated N2 fixation through increased nodule number, nodule biomass, and nodule activity to a greater extent under unstressed conditions. Soil water savings under elevated [CO2] were only temporary, so that drought reduced nodule activity due to lower C/sucrose supply and therefore decreased N2 fixation. Consequently, elevated [CO2] was found to stimulate N2 fixation of all three species of legumes, but this effect was smaller under drought or heat stress. The decrease of N2 fixation under drought caused depletion of grain N concentration under elevated [CO2]. In contrast, when soil water was sufficient, N2 fixation continued throughout the grain filling period, and grain N concentration was maintained under elevated [CO2]. Traits that allow N2 fixation for longer throughout the growing season, e. g. by exploiting potential water savings mechanisms under elevated [CO2], may confer benefits under future climatic conditions. Findings of this study are now available to underpin new strategies for improvement of the N2 fixation potential of legumes as atmospheric [CO2] continues to increase in the future.
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ItemFunctional aspects of root and leaf development in dryland crop water use under elevated CO2( 2018)Atmospheric CO2 concentration ([CO2]) is rising due to anthropogenic activities and is expected to reach ~550 μmol mol-1 by 2050 and exceed ~700 μmol mol-1 by the end of this century. As the main substrate of photosynthesis, this rising [CO2] has direct implications for plant metabolism, such as stimulating net photosynthetic CO2 assimilation rate (Anet) in C3 crops and leading to greater biomass production and yield through the so-called ‘CO2 fertilisation effect’. In addition, elevated [CO2] (e[CO2]) lowers stomatal conductance (gs), and thus may reduce transpiration rate. Increased assimilation and lower transpiration result in higher leaf-level water use efficiency, which lead to the assumption that crop water use will be lower under e[CO2]. On the other hand, e[CO2] increases leaf area, which tends to increase transpiration and therefore canopy water use. Therefore, the net response of crop water use to e[CO2] is dependent on the balance between e[CO2]-induced reduction of gs and e[CO2]-induced stimulation of transpiring leaf area. These responses under e[CO2] are further complicated by other environmental variables and growing conditions. The response of crop water use to e[CO2] will be of particular interest for dryland agriculture, where water is nearly always the most limiting factor for crop production. This project investigated the functional aspects of root and leaf development on water use of dryland wheat (Triticum aestivum L.) and canola (Brassica napus L.) under a future e[CO2] using experiments with different water and nitrogen regimes, soil types and cultivars. Free Air CO2 Enrichment (FACE) technology was used to simulate future growing conditions in the field with a target atmospheric [CO2] expected by the middle of this century. This was supplemented by glasshouse studies to investigate crop physiological response to e[CO2] under more controlled conditions. Increased leaf-level water use efficiency under e[CO2] stimulated biomass and yield per unit water used, but this commonly resulted in little change in seasonal water use in this dryland, terminal drought environment. However, the dynamics of crop water use during the growing season varied depending on [CO2], whereby early in the season greater stimulation of leaf growth counteracted the increased leaf-level water use efficiency and resulted in greater water use under e[CO2] relative to a[CO2]. Under field conditions, the accumulated water use at the end of the season was then similar both under a[CO2] and e[CO2], pointing to the overriding effect of the seasonal conditions. Under water-limited conditions, e[CO2]-induced stimulation of root growth especially in the deeper soil layers maintained plant physiological processes by improving access to deeper soil water. This greater assimilation rate later in the season ensured better assimilate supply to the developing grains, which resulted in better yield benefits from the ‘CO2 fertilisation effect’. In addition, this thesis shows that interactions between growing conditions (experimental water and N regimes) and expression of genotypic traits (cultivars contrasting in vigour, transpiration efficiency and N use efficiency) play a decisive role in determining potential biomass and yield benefits from rising [CO2]. Observed genotypic variability in response to e[CO2] suggests a potential breeding opportunity to maximise the benefit from ‘CO2 fertilisation effect’.
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ItemThe economic impact of climate change on perennial crops: the coconut industry in Sri Lanka and the value of adaptation strategies( 2016)Coconut is an important food crop in the Sri Lankan economy which utilises nearly 20 percent of its arable lands. This industry is shifting from an export orientation to a domestic industry; mainly due to the population-driven domestic demand and stagnating coconut production. Nearly 65 per cent of annual production is consumed domestically as fresh coconut, while the remaining 35 per cent is utilised by the processing sector for products including coconut oil, desiccated coconut and copra. Coconut production fluctuates with climate. This yield fluctuation has considerable impact on industry stakeholders due to inelastic supply of and demand for fresh coconuts. Government interventions are ad hoc and numerous. Further, the effectiveness of these strategies is questioned in the absence of a consistent analytical framework. This study developed an economic framework; an equilibrium displacement model for the Sri Lankan coconut industry which analyses the impact of different policy interventions. The model was tested for seven hypothetical scenarios of external shocks. The model was subsequently used to analyse the likely impact of climate change in this study. An analytic hierarchy process was used to estimate the biophysical impact of coconut yield under future climatic scenarios. Climate, soil and topography were the main considerations of the model. The outcomes of this model as yield changes were used as a supply shift in the economic model. The total change in economic benefits and distribution of these benefits were estimated to find out the magnitude of the economic shock and impact on different stakeholders. The findings show that the coconut industry in Sri Lanka will face a loss equivalent to 4,795 Rs.Million which is nearly 5 percent of the total value of the industry at equilibrium. The mostly affected stakeholders are wholesalers and domestic coconut consumers. Then the impact of different adaptation options and cost effectiveness were considered to address the impact of climate change. Among these yield increasing adaptation practices, irrigation during dry periods was promising the highest productivity levels. However, the investments were not cost effective for large scale irrigation systems and availability of a water source was a major concern. Fertilizer application and moisture conservation were also identified as cost effective practices that would offset the yield loss and provide extra gain. Development of a heat tolerant cultivar would be a long term sustainable solution with the observed and expected increase in maximum temperature. However, this may take several years and still worthy to invest on. The findings are useful in assessing potential future impacts and directing the industry policies.
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ItemAntioxidant defence systems and symptom expression of wheat infected with Barley yellow dwarf virus and grown under elevated CO2( 2016)Barley yellow dwarf virus (BYDV) is regarded as the most significant viral pathogen of wheat worldwide. Symptoms produced during viral infection may have an interactive effect with environmental conditions expected under future anthropogenic climate change, including the rising atmospheric CO2 concentration. In particular, antioxidant defence systems – including the key non-enzymatic antioxidants ascorbate and glutathione – play an important role in regulating potentially harmful reactive oxygen species (ROS) produced during plant-virus interactions. However, the role of ascorbate and glutathione during systemic virus infection and growth under elevated CO2 (eCO2) is not well understood. This thesis investigated BYDV infection of three Australian wheat cultivars: the BYDV-susceptible spring wheat ‘Yitpi’, the susceptible winter wheat ‘Revenue’ and the resistant winter wheat ‘Manning’. In addition, the system was investigated under eCO2 to determine any interactions with infection on symptom expression and antioxidant defence capacity. Studies were performed within controlled environment chambers and the field at the Australian Grains Free Air CO2 Enrichment (AGFACE) facility located in the semi-arid grain-growing region of Horsham, Victoria, Australia. The response of plants to virus infection and eCO2 was assessed by measurement of the total concentration and redox state of ascorbate and glutathione. In addition, symptom expression was measured including growth, photosynthesis, stomatal conductance, leaf chlorophyll and nitrogen, and disease incidence and severity. BYDV infection was associated with an imbalance in antioxidative metabolism, which is an indicator of oxidative stress. Greater ROS turnover is the likely cause of the observed decrease in total ascorbate and glutathione and increase in the oxidised fraction of ascorbate after infection. In particular, a decrease in total ascorbate was the most consistent response to infection by all cultivars grown in both chambers and the field. The present research demonstrates that the observed imbalance in non-enzymatic antioxidant metabolism can be used as a marker for oxidative stress during systemic BYDV infection of wheat. The antioxidant response of both the BYDV-susceptible and resistant winter wheat cultivars was similar. Oxidative stress was not influenced by the putatively different virus concentration between these cultivars, but simply by virus infection alone. Infection was also associated with decreased biomass and height in both these cultivars and in both chamber and field studies, which indicates a sensitivity of the resistant cultivar to infection regardless of a putatively lower virus concentration. Despite few interactive effects between virus and eCO2 treatments on symptom expression, eCO2 altered the expression of yellowing disease symptoms in virus-infected plants, although not consistently between cultivars and environmental growing conditions. In addition, although there were significant changes to antioxidants in plants grown under eCO2, results were not consistent between studies. Research into this topic increases our understanding of how plants respond to virus infection and oxidative stress, and how plant-virus interactions may change under future eCO2. With the findings presented in this thesis, I have furthered the knowledge of this area by elucidating the response of ascorbate and glutathione during systemic wheat-BYDV interactions, and reinforced the potential use of these metabolites as markers of oxidative stress.
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ItemThe influence of climate factors and plant physiological responses on the accumulation of rotundone in Vitis vinifera cv. Shiraz grapevines( 2015)Rotundone is a sesquiterpene compound that gives grapes and wine a ‘black pepper’ aroma and flavour, which is stylistic important to many Australian Shiraz wines. This PhD project determined the factors important for the concentration of rotundone in Shiraz grapes and wine, including vineyard environmental parameters and grapevine physiological factors. Wine produced from the same Shiraz vineyard over 15 vintages (The Old Block, Mount Langi Ghiran, Victoria, Australia) were analysed for rotundone concentration and compared to historical weather data. Results showed that the highest concentrations of rotundone were consistently found in wines from cool and wet seasons, while the concentration of rotundone in wine was negatively correlated with daily solar exposure and grape bunch zone temperature (veraison to harvest), and positively correlated with vineyard water balance. The influence of vineyard temperature on the concentration of rotundone was further investigated by quantifying rotundone variability within-vineyard, within-vine, and within-bunch. Occurrence of the highest concentration of rotundone was observed in shaded bunch sectors and vines, and from higher vigor vines in the southern-facing areas of the vineyard, which correlated to the lower grape surface and bunch zone air temperature. Modelling further showed that berry temperature exceeding 25 °C negatively affected the rotundone concentration in grape berries. The influence of solar exposure on the concentration of rotundone was investigated by excluding sunlight from grape bunches at different stages of berry development. Significantly lower concentration of rotundone and its precursor -guaiene was observed in grape berries with sunlight exclusion at all stages. This indicated that sunlight illumination was important to determine the concentration of rotundone in grapes at harvest, and the biosynthesis pathway from -guaiene to rotundone might be light sensitive. Grapevine phenological stage is the most important physiological factor affecting the concentration of rotundone in grape berries. The evolution of rotundone and other terpenoids in grape berries from 4 weeks post-flowering to maturity was investigated. Results showed that different terpenoids had different production patterns during berry development depending on their biosynthesis pathway, and terpenoids derived from the same biosynthesis pathway shared similar production pattern. The concentration of rotundone and α-guaiene in grape berries gradually decreased at pre-veraison stages until veraison and increased afterward until maturity. The influence of grapevine non-grape organs on the concentration of rotundone in wine was also investigated. Non-grape organs contained significantly higher amount of rotundone than berries. However, rotundone could not mobilize from these organs into grape berries via the phloem. Despite this, non-grape organs could still be used as a source of rotundone, especially for wineries conducing whole-bunch fermentation and harvest by machine. This project also showed that herbivore activity did not significantly modify the concentration of rotundone in grapevine leaves, and therefore rotundone may not be a member of the herbivore-induced terpenoids. This project investigates numerous factors affecting the production of rotundone in Vitis vinifera CV. Shiraz grape and wine, which is very important for wineries and wine regions to establish the ‘terroir’. The knowledge developed from this project could help Australian grape growers and wine industry produce grapes and wine of better quality.
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ItemClimate-smart agriculture: a review assessing the merits and future applications of a holistic agricultural paradigm that addresses climate change and development challenges.( 2015)Agriculture is facing significant challenges into the future, especially related to food security when faced with climate change. Climate-Smart Agriculture seeks to address these challenges by providing a holistic framework for agricultural adaptation to, and mitigation of climate change, while also alleviating food security problems in a triple-win context. Climate-Smart Agriculture has origins in other agricultural paradigms, such as the Green Revolution, Conservation Agriculture and Sustainable Intensification. However it has the advantage of amalgamating the best responses from all three. Theoretically, Climate-Smart Agriculture is a universal paradigm able to solve all agricultural issues. In practice, Climate-Smart Agriculture has significant flaws. It has broad principles that cause conflicts and trade-offs even when a triple-win outcome is promised. Climate-Smart Agriculture also suffers from significant funding, insurance and technical issues, while there is little detail as to how the three arms of Climate-Smart Agriculture (adaptation, mitigation and food security) works together in practice. With some perseverance, Climate-Smart Agriculture can be the paradigm to address all these concerns, especially in a developing country context. Climate-Smart Agriculture has the platform for success, through policy formulation, pushing technologies and intervening in local agricultural systems. Even with potential trade-offs, Climate-Smart Agriculture utilises a best-practice approach to increase agricultural resilience through improving ecosystem service by utilising agroforestry and other methods. Climate-Smart Agriculture can still achieve its goals, through a mixture of novel approaches and proven techniques, it can also succeed further with biodiversity and poverty alleviation gains. However for this to occur Climate-Smart Agriculture must improve evidence building, local effectiveness, climate and agricultural policy cohesion and funding. Scientific endeavour must be prioritised with Climate-Smart Agriculture becoming more water, energy and nutrient-smart. Further research needs to occur into potential synergies and trade-offs, with more Climate-Smart Agricultural involvement in food systems, with a push for more integrated food-energy systems. Furthermore barriers to adoption need to be better understood. Funding has been highlighted as the most important issue facing Climate-Smart Agriculture. Overall funding is not well targeted, nor coalesced between adaptation and mitigation strategies. There is a lack of accountability regarding adaptation funding and an overall disjoin between Climate-Smart Agriculture, climate finance and carbon markets that must be rectified. Overall agriculture requires significant transformation. Climate-Smart Agriculture provides the framework to do this. Yet in its current state, Climate-Smart Agriculture provides nothing new, it faces significant problems that must be rectified if it is to become more than just another theoretical agricultural concept.
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ItemWheat grain quality dynamics under elevated atmospheric CO2 concentration in Mediterranean climate conditions( 2013)Since 1959, carbon dioxide concentration [CO2] in the atmosphere increased from 315 µmol mol-1 to approximately 389 µmol mol-1 by 2009 in a rate of 1.5 µmol mol-1 per year. Within the next 50 years, atmospheric [CO2] will likely to rise to 550 µmol mol-1. Carbon dioxide is a greenhouse gas and a major factor that contributes to global warming. In parallel, global temperature is predicted to increase by an average of 1.5-4.5 ºC with more frequent occurrences of extreme climatic events such as heat waves and/or drought by the mid of this century. There is a limited understanding on the impact of elevated atmospheric [CO2] (e[CO2]) on wheat grain quality in semi-arid and Mediterranean cropping systems. The research reported in this thesis investigated the effects of e[CO2] on wheat grain physical, chemical, flour rheological properties under two main climate conditions: semi-arid and Mediterranean which represent the water-limited “mega-environment 4”, larger wheat grown area in the world as defined for wheat (Braun et al., 1996). The experiments were carried out using state art technology of free- air CO2 enrichment (FACE) facilities located in Walpeup and Horsham, Victoria, Australia. (See thesis for full abstract)
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ItemThe impact of fire disturbance and simulated climate change conditions on soil methane exchange in eucalypt forests of south-eastern Australia( 2013)Soils in temperate forest ecosystems globally act as sources of the greenhouse gas carbon dioxide, and both sinks and sources of the greenhouse gases nitrous oxide and methane (CH4), with well-drained aerated soils being one of the most important sinks for atmospheric CH4. Soil CH4 uptake is driven by aerobic CH4 oxidation through methanotrophic bacteria that oxidize CH4 at atmospheric to sub atmospheric concentrations with soil gas diffusivity being one of the key regulators of soil CH4 uptake in these systems. Climate change predictions for south-eastern Australia indicate a high probability of increasing temperatures, lower average rainfall and an increase in the frequency and severity of droughts and extreme weather events. As a further consequence of climate change in south-eastern Australia, there is a predicted increase in days with high fire risk weather and an increased probability of severe wildfires. In response to these predictions, the use of planned burning as a management strategy within Australian temperate forests and woodlands has increased significantly in an attempt to mitigate this risk of uncontrolled wildfire. Changes in soil moisture regimes, temperature regimes and soil disturbance have the potential to alter soil CH4 uptake, however this has generally been studied in the deciduous and coniferous forests of the northern hemisphere. Currently there is a lack of knowledge regarding temporal and spatial regulators of soil CH4 uptake in temperate Australian forest systems and results from northern hemisphere studies cannot be confidently applied to the eucalyptus dominated Australian forests. Consequently, it is difficult to assess how climate change might affect this important soil based CH4 sink, resulting in significant uncertainty around the magnitude and future trends of the CH4 sink strength of forest soils in south-eastern Australia. To help address this uncertainty, this study investigated both the seasonal drivers of soil CH4 uptake and the sensitivity of soil CH4 uptake to altered soil conditions caused by wildfire, planned burning or simulated climate change scenarios in south-eastern Australian temperate eucalypt forests. This thesis encompasses four field studies: (i) To investigate the possible impacts of the predicted decrease in average rainfall and increase in temperature on soil CH4 uptake we measured soil CH4 flux for 18 months (October 2010 – April 2012) after installing a passive rainfall reduction system to intercept approximately 40% of canopy throughfall (as compared to control plots) in a temperate dry-sclerophyll eucalypt forest in south-eastern Australia. Throughfall reduction caused an average reduction of 15.1 ± 6.4 (SE) % in soil volumetric water content, a reduction of 19.8 ± 6.9 (SE) % in water soil filled pore space (WFPS) and a 20.1 ± 6.8 (SE) % increase in soil air filled porosity (φair ). In response to these changes, soil CH4 uptake increased by 54.7 ± 19.8 (SE) %. Increased temperatures using open top chambers had a negligible effect on CH4 uptake. Relative changes in CH4 uptake related more to relative changes in φair than to relative changes in WFPS indicating a close relationship between φair and soil gas diffusivity. Our data indicated that soil moisture was the dominant regulating factor of seasonality in soil CH4 uptake explaining up to 80% of the seasonal variability and accounting for the observed throughfall reduction treatment effect. This was confirmed by additional soil diffusivity measurements and passive soil warming treatments. We further investigated non-linear functions to describe the relationship between soil moisture and soil CH4 uptake and a log-normal function provided best curve fit. Accordingly, soil CH4 uptake was predicted to be highest at a WFPS of 15%. This is lower than in many other ecosystems, which might reflect a drought tolerant local methanotrophic community. However, the applicability of the log-normal function to model CH4 uptake should be evaluated on global datasets. Soil moisture during our study period rarely fell below 15% WFPS and the observed mean was approximately 40% WFPS. It is therefore likely that soil CH4 uptake will increase if rainfall reduces in the dry-sclerophyll forest zone of Australia as a consequence of climate change. (ii) Planned burning is a management strategy applied in south-eastern Australia that aims to reduce fuel loads and therefore mitigate the risk of large, uncontrolled wildfires. Recent government policy changes have led to a significant increase in the total area of public land subject to planned burning activities within the region. To investigate the impact of fire frequency (as a result of planned burning) on soil CH4 uptake, soil methanotrophic activity and soil CO2 fluxes we measured these three variables in six campaigns across all seasons (March 2009 – February 2011) in a dry sclerophyll eucalypt forest in the Wombat State Forest, Victoria. Three different fire frequency treatments had been applied since 1985: planned burning in autumn i) every 3 years, ii) every 10 years, and iii) not burned since before 1985. Mean soil CO2 emissions were significantly higher in the planned burn treatments compared to the unburnt treatments. In contrast, soil CH4 oxidation did not show the same response to planned burning. Our data indicate that differences in soil CO2 fluxes in response to planned burning might be driven by increased autotrophic root respiration most likely related to decreased nutrient and water availability to overstorey plants. This theory contrasts with alternative explanations that focus on post fire changes in soil nitrogen dynamics, increased heterotrophic respiration and increase soils surface temperatures. Given the long-term nature of the applied burning treatments (implemented for over 25 years) it is therefore unlikely that increases in planned burning will have an impact on the CH4 uptake capacity of these fire resistant eucalypt forests. (iii) Wildfire is the most important disturbance event that alters composition and stand age distribution in forest ecosystems in south-eastern Australia. Wildfire impacts often alter environmental conditions that influence CH4 uptake of forest soils. The impact of wildfire on the CH4 uptake capacity of forest soils is currently unknown. In 2010/2011 we measured soil atmosphere CH4 exchange along a chronosequence in a Tasmanian wet sclerophyll eucalypt forest where the time since the last stand-replacing disturbance ranged between 11 years and approximately 200 years and was due to either wildfire or wildfire emulating harvest operations. Our results indicate an initial increase in soil atmosphere CH4 uptake from the most recently disturbed sites (11 years post-disturbance) to ‘mature’ sites (46 and 78 years post-disturbance). This initial increase was followed by a time-since-last-disturbance (TSLD) related decrease in soil atmosphere CH4 uptake. Our data indicate the initial increase in CH4 uptake is related to a decrease in soil bulk density and an associated increase in soil gas diffusivity. However, the subsequent decline in CH4 uptake with increasing TSLD (from 78 to 200 years) was more likely driven by an increase in soil moisture status and a decrease in soil gas diffusivity. We hypothesize that the observed increase in soil moisture status for the stands aged 78 years and older was driven by forest succession related changes in soil organic matter quality/quantity, an increase in throughfall and an overall decrease in stand water use as demonstrated for tall mixed wet sclerophyll eucalyptus forests elsewhere. (iv) In order to gain a better understanding of seasonal and inter-annual variation in soil CH4 exchange for temperate eucalypt forests in south-eastern Australia, we measured soil CH4 exchange in high temporal resolution (every 4 hours or less) over two consecutive years (March 2010 – March 2012) in the Wombat State Forest, Victoria and over one year (October 2010 – February 2012) at the Warra, Tasmania. These two sites are both temperate Eucalyptus obliqua (L. Her) dominated forest systems however they have contrasting annual precipitations (Victoria Site= 870 mm yr-1, Tasmania Site = 1700 mm yr-1). Both systems were continuous CH4 sinks with the Victorian site having a sink strength of -1.79 kg CH4 ha-1 yr-1 and the Tasmanian site having a sink strength of -3.83 kg CH4 ha-1 yr-1 in 2011. Our results show that CH4 uptake was strongly regulated by soil moisture with uptake rates increasing when soil moisture decreased, which explained up to 90% of the temporal variability in CH4 uptake at both sites. Furthermore, when soil moisture was expressed as soil air filled porosity (φair) we were able to predict the CH4 uptake of one site by the linear regression between φair and CH4 uptake from the other site, indicating a generic relationship. Soil temperature only had an apparent control over seasonal variation in CH4 uptake during periods when soil moisture and soil temperature were closely correlated. The natural fluctuation in generally low soil nitrogen levels did not influence soil CH4 uptake at either site. Comparing our measured site data to modelled data utilising a process based methane uptake model (Curry 2007), our two sites showed reasonable agreement providing scaling factors used to account for soil temperature (rT) response and moisture response (rSM) of methane oxidation rate (k) were forced to unity. Under these conditions CH4 uptake was primarily regulated by diffusivity in the model, indicating that observed seasonal variability in soil CH4 uptake at both sites was primarily regulated by soil moisture related changes in soil gas diffusivity. This study filled some important knowledge gaps with regards to information about magnitude and controls of temporal variability but also with regards to climate changes sensitivity of soil CH4 uptake in temperate eucalypt forests in south-eastern Australia and provides important datasets that will enable better predictive modelling of changes in soil CH4 uptake across the temperate forest landscape in south-eastern Australia. The results indicate it is likely that soil CH4 uptake will increase if rainfall reduces in the dry-sclerophyll forests of Australia as a consequence of climate change. Our findings on the impact of wildfire on soil CH4 exchange highlight the potentially large spatial variability in CH4 uptake across the landscape within the same forest and soil type, a factor that would need to be accounted for in global CH4 uptake models. This issue could be partially addressed for tall wet temperate eucalypt forests in case the here theorized relationship between forest succession and CH4 uptake can be verified in further studies.The finding that low intensity planned burning does not have an effect on soil CH4 uptake suggests that fire may need to be of a particular severity before changes in soil properties and the associated changes in soil CH4 uptake can be observed. Our long term monitoring results further highlight the importance of long-term field measurements in establishing relationships between soil environmental drivers and soil CH4 uptake and are therefore useful for the calibration of models that calculate the global CH4 sink distribution and magnitude.
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ItemWinter and spring phenology of Australian pome fruit in historical and future climates( 2013)Successful agricultural production relies on favourable environmental conditions in combination with sound management strategies and practices. The advent of anthropogenically induced climate change is likely to modify climate variables, potentially influencing the productivity of these systems. Pome fruit trees are vulnerable to future climate changes due to their perennial nature, extended juvenile phase and long productive lifespan (decades). Australian growers are further susceptible as Australia has been identified as particularly exposed to future climate change. To explore this potential exposure, interpretation of expected changes to the climate as applicable to Australian pome fruit trees was considered in this thesis. Specifically, two key phases in the growth cycle were evaluated, winter chilling and spring flowering phenology. These phases are both temperature dependent and therefore implications of changes to temperature conditions were the focus of the research. Prior to consideration of climate future impacts, assessment of plant responses under current conditions is required. For the two phenophases considered, substantial knowledge gaps persist in relation to physiological mechanisms as driven by environmental conditions. As a result, many methods have been developed to estimate these processes based on observed responses to temperature. In this study, several methods were applied in all the analyses to consider the implication of methodological selection on the results obtained. Winter chill was analysed at 13 locations across southern Australia representing the major pome fruit growing regions. Four different models were used to calculate chill historically and into the future (0-7.2°C, Positive Utah, Modified Utah and Dynamic models). Historically, many locations recorded notable declines in accumulated chill regardless of chill model selection (Orange, Lenswood, Tatura, Yarra Valley and Bacchus Marsh). Other locations have remained stable with no location exhibiting a consensus increase in chill across the tested models. Large season-to-season variability was observed at all locations and for all chill models. Comparison of output between the chill models indicated that the 0-7.2°C model frequently behaved differently to the other models. Projected changes in winter chill were assessed according to +1, 2 and 3°C increases to mean global temperatures. Reduced chill accumulation in the future was found for all locations with notable regional differences. The sites in Western Australia were found to bear the greatest impact with a substantial reduction in chill expected. The chill models tended to interpret changes differently with the 0-7.2°C model generally being most sensitive to changes in temperature followed by the Utah models and then the Dynamic model. The impact of climate on spring flowering was also considered. Collation of historical flowering phenological records of pome fruit in Australia was an important contribution of this research. No such information has been analysed for Australia and only one other study exists for the Southern Hemisphere. Observed relationships indicated flowering advancement of between 4.1 to 7.7 days per degree Celsius increase in mean spring temperature. Compared with similar phenological research in the Northern Hemisphere, the changes in timing in relation to temperature were often shallower in Australia, although the significance of observed hemispheric differences was not clear. To determine relationships to temperature, a sequential chill-growth model as well as correlation to mean springtime temperatures were used. The sequential chill-growth approach proved superior, with coefficients of determination between 0.49 and 0.85, indicating the inclusion of chill conditions are important for spring phenology modelling. Projections of future flowering timing were created using the relationships determined in the historical analysis combined with +1, 2 and 3°C increases to mean global temperatures. Both of the previous sequential chill-growth and the springtime temperature modelling approaches were used. The springtime temperature model predicted advancement in flowering timing for all datasets. With 1°C warming the sequential chill-growth model recorded similar flowering timing to current conditions for many of the species. Greater warming, however often tended towards later emergence. The timing of projected flowering was related the relative dominance of the protraction of the chill period versus the contraction of the growth phase. Finally, the implication of changes to flowering timing was considered in relation to frost risk. Results from projections of both phenology modelling approaches were used in combination with two assumptions about future frost conditions; current conditions or frost is modified with climate change. Depending on the combination of phenology method, frost assumption and level of warming, frost risk was predicted to notably increase, remain similar or decrease. Overall, chill accumulation in Australia is likely to decline. The changes reported are sensitive to chill model selection for Australian conditions, consistent with previous assessment made for other regions. Further research using the 0-7.2°C model is not recommended whilst greater uptake of the Dynamic model is encouraged. Modelling of flowering phenology, especially under climate perturbed conditions, illustrated flowering phenology model selection is important. Although the sequential chill-model model performed better than the springtime temperature approach, reservations still remain in employing this method for climate impact statements. Future research directed at greater understanding of physiological processes is required to better characterise current physiological responses and produce robust climate impact assessments.