School of Agriculture, Food and Ecosystem Sciences - Theses
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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.