School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemClimate and climate change effects on carbon uptake and storage in Australia’s wooded ecosystems( 2022)Forest ecosystems are central to the land carbon sector due to their capacity to store and sequester carbon. Many studies have demonstrated that forest carbon uptake and storage is strongly dependent upon climatic conditions. However, the effects of climate on forest carbon uptake and storage in different biomes are still uncertain. Climate change may alter carbon dynamics within forest ecosystems through the direct effects of increased temperature, increased CO2 concentration and changing precipitation regimes. Yet forests may also adjust to changing climate through mechanisms such as thermal acclimation. In this thesis I used three modelling approaches (machine learning, boundary-line analysis, and a land-surface model) to examine how climate of the recent past, present, and future affect carbon uptake (as Gross Primary Productivity, GPP) and storage (as above-ground biomass, AGB) in Australian forests. Furthermore, I explored how current GPP adjusted to thermal regimes and how acclimation affected carbon uptake and storage in the future. In my first quantitative chapter (Chapter 2), I explored relationships between carbon storage (as AGB) with climate and soil in Australian forests across the continent. I developed RandomForest models with climate-only, soil-only, or climate plus soil variables to examine whether climate or soils are better predictors of forest biomass at the continental scale and to identify the most important predictor variables. In this chapter I demonstrated that climate (particularly temperature and the timing of precipitation) was more important than soil for explaining variation in AGB across Australia’s forests. In Chapter 3, I used boundary-line analysis to examine the ecosystem temperature response of carbon uptake (as GPP) in 17 wooded ecosystems representing five distinct ecoregions. These responses were represented as a convex parabolic curve that was similar in shape among ecoregions – narrow in tropical forests and broader in woodlands. I then derived the thermal optima of GPP (Topt) from these curves for each ecosystem and examined the relationship between Topt and mean air temperatures across sites. My analysis revealed a strong positive linear relationship between Topt and mean air temperature that indicated GPP was optimised to the present climate. Finally, in Chapter 4, I predicted how carbon uptake and storage will be affected by climate change in these 17 ecosystems and examined the effects of thermal acclimation of photosynthesis on these predictions. I used the CABLE-POP land surface model adapted with thermal acclimation of photosynthetic functions and forced with climate projections from the extreme climate scenario RCP8.5. My simulations indicated that increased temperature, CO2 concentration and changed precipitation patterns will have a positive effect on future carbon uptake and storage in the majority of the 17 ecosystems. Furthermore, thermal acclimation of photosynthesis is likely to enhance this effect in tropical ecosystems. My results confirm that carbon uptake and storage in Australian forests are fundamentally linked to temperature and precipitation regimes, and that these forests may be capable of adjusting to climatic conditions. My research indicates that the direct effects of climate change are likely to enhance the storage and sink capacity of Australia’s forests in the future. While I did not assess the indirect effects of climate change on carbon cycles through changes to disturbance regimes, overall, my thesis suggests that carbon uptake and above-ground biomass carbon stores in Australia’s forests are likely to be resilient to climate change.
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ItemMaking the connection between history, agricultural diversity and place: the story of Victorian apples( 2016)Apple growing practices are embedded in a productivist mentality aiming for ever higher efficiency and productivity. And while the climate change impacts are to a large extent known, there is little attention given to the coupling of the social and the ecological effects. I use apple growing as a case study to explore the relationship between place, biodiversity and rural change in Victoria. My research is based on historical research; including an analysis of the Museum Victoria’s collection of wax apple models, and in-depth interviews with orchardists. By drawing on environmental history, social-ecological systems thinking and Bourdieu's theory of practice, I highlight the importance of a systems perspective and inform it by emphasis on the critical role of underlying power structures and individual dispositions, or the habitus, of the growers. These dispositions have been shaped and internalised by the growers’ histories and their physical surroundings. Orchardists have been able to respond to intensifying production requirements by utilizing technologies and scientific nous to keep up with the continuous aim for efficiency. Growers are caught up in a self-reinforcing cycle of satisfying the demand for perfect apples by adopting expensive techno-scientific approaches to enable ever more intensive production. The symbolic violence and amplified biophysical pressure orchardists experience has driven many to despair; resulting in a significant decline in small scale apple growing businesses over the last decade. I offer some suggestions for government policy and support measures and argue that any services or support programs need to be tailored to the appropriate level and need of each orchard business and the individuals who are involved. My analysis shows that those growers, who engage more closely with their biophysical place as well as their history and identity as apple growers in that place are (re-)creating another version of what it means to be an apple grower. In some cases this is resulting in resistance to the vortex of agricultural productivism that has been the basis of their existence for many generations.
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ItemStudies of Ascochyta rabiei in Australia( 2005)Ascochyta rabiei (teleomorph: Didymella rabiei) which causes ascochyta blight is the most serious disease of chickpea (Cicer arietinum) in Australia as it causes significant losses in crop yield and quality. Although A. rabiei is heterothallic and genetically diverse elsewhere in the world, a study carried out on Australian isolates collected between 1995 and 2000 identified only one mating type and a low level of genetic diversity within the Australian A. rabiei population. In 2002, ascospores of Didymella rabiei, the sexual state of A. rabiei, were trapped in a discharge chamber, from chickpea stubble naturally infected with ascochyta blight in Western Australia. Examination of the stubble revealed pseudothecia typical of Didymella rabiei. The reported presence of the teleomorph in Western Australia indicated that the second mating type had been introduced into Australia or that the pathogen was capable of a low level of homothallic compatibility, previously undetected. The aims of this research were, to undertake a new survey of Australian A. rabiei isolates, to test for the presence of the second mating type, to determine if there has been a change in the diversity of the Australian population and to investigate if pathogenic variability was displayed among isolates. Sixty-seven isolates collected from chickpea fields in South Australia, New South Wales, Queensland, Victoria and Western Australia during the 2003 cropping season were single spored and confirmed as A. rabiei using a PCR test. The isolates were typed for mating type using MAT gene specific PCR primers and compared with tester isolates from USA. This test revealed that all the 67 isolates belonged to mating type 2 (MAT 1-2), thus, the presence of mating type 1 (MAT 1-1) in Australia could not be confirmed. Sequence Tagged Micro Satellite (STMS) markers were used to examine the genetic diversity of the A. rabiei isolates. The isolates were assessed for allelic variation at 19 microsatellite loci, each of which amplified a single locus. Seven of the loci were polymorphic across all the 67 isolates, while the remaining twelve were monomorphic. Based on the allele profiles at the seven polymorphic loci, 19 distinct A. rabiei haplotypes were identified with a total of 33 alleles. One haplotype constituted 35.8 % of the population and was found in the collections from South Australia, New South Wales, Queensland and Victoria. Cluster analysis did not show a clear distinction between isolates based on the state from which they were collected. Polymorphism across the 19 microsatellite loci revealed a slight elevation in diversity in the 2003-2004 population (Ht = 0.07; compared to 0.02 in the 1995 to 2000 collection) and an increase in the number of haplotypes compared with that detected in the previous study of Australian isolates. To examine the pathogenic variability of the Australian population of A. rabiei, nine isolates were inoculated on five chickpea differentials, ranging from highly susceptible to resistant, under controlled conditions optimal for A. rabiei growth and infection. Eight of the isolates were virulent on the susceptible and intermediate chickpea cultivars but not the resistant cultivar and one isolate was only virulent on the susceptible cultivar. Based on these results the isolates were classified into two pathotype groups. The results obtained from the study of the population structure and the pathogenic variability of A. rabiei in Australia will enable the Australian chickpea breeders to understand the A. rabiei population better for formulating management and breeding strategies.
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ItemEffects of adding nutrients on soil chemistry and tree growth in native Eucalyptus forests of south-eastern Australia( 2007)The decreasing area available for timber extraction in south-eastern Australia, due largely to social pressure to reserve greater areas of forest, has led to the consideration of fertiliser-application to increase wood output from the remaining available forest. Potentially deleterious effects of fertilising on water quality must be assessed before implementation on a wide scale. This is in accordance with relevant forest management policies. This study examined the effects of applying fertilisers containing nitrogen and phosphorus, on soil and soil-water chemistry in two pole-sized stands of mixed Eucalyptus spp in the Wombat Forest, in the Midlands Forest Management Area, Victoria, Australia. The findings are synthesised and discussed in relation to management of regenerating mixed-eucalypt forests in south-eastern Australia. Fertiliser treatments were none (R); 400 kg N ha-1 as ammonium-sulphate (N); or 400 kg ha-1 plus 202 kg P ha-1 as triple superphosphate coated with 10% sulphur (NP). It was calculated that incidental additions of S were 1371 kg ha -1 (N treatments), and 1696 kg ha-1 (NP treatments). It was expected that P would be principally adsorbed on soil surfaces; N immobilised in the soil organic pool and that metallic cations would enter the soil solution to varying degrees. Fertiliser-addition increased both plot-basal-area (BA) growth and the rate of stand self-thinning. In 3.8 years, BA in reference (R) plots at two sites increased by 7.3% and 23.4%. Where N alone was added, BA increased by 14.2% and 27.1%, while in NP plots BA increased by 17.1% and 42.7% respectively. Mortality was 9% in untreated plots compared to 14% in NP plots. Estimated increases in biomass growth equated to additional above-ground nutrient accumulation of 0.4 to 1.5 kg ha-1 of P, and 5.5 to 20.8 kg ha-1 of N. This represented only 0.2 to 0.7% of added P, and 1.4 to 5.2% of added N. Soil solution was extracted from 10 and 50 cm with porous-ceramic-cup tension-lysimeters (-0.6 kPa). Concentrations of P and N were low both before and after adding fertiliser. Across all treatments the maximum median PO43- concentration in soil-water at 50 cm was 0.12 ppm (mean 0.28 ppm). Typically PO43- concentrations were not higher than 0.03 ppm. The 400 kg ha-1 of added N was rapidly immobilised in the soil organic pool. The greatest mean NH4' concentration from a single sampling occasion was 1.1 ppm. The mean NO3 concentration at 50 cm was never higher than 0.26 ppm. After adding N in fertiliser the proportion of NO3- relative to NH4* in soil-water increased and was correlated with decreasing soil-water pH. Less than 1% of added P and N was recovered from soil solution at 50 cm. The largest pool of added P recovered was PO43- adsorbed to soil between 0 and 20 cm, due to the soil adsorption capacity being well in excess of the applied 202 kg P ha-1. Phosphate desorption using sequential extractions with a mild acid extractant (0.3M NH4F, 0.1M HCI) recovered between 25% and 116% of added P. Differences were attributed to both the amount of P added and the effect of time since treatment at different sites. Soil disturbance during sampler installation was found to be more likely to raise soil-water P concentrations at 50 cm than would adding up to 202 kg P ha-1. Among the ions in solution. SO42- and CI' were the dominant anions while Cat+ dominated the cation chemistry. In untreated forest 5042- in soil-water ranged from 7.7 to 16.0 ppm at 10 cm and 7.9 to 12.2 ppm at 50 cm. In fertilised plots up to 100.5 ppm SO42 was measured in soil-water at 50 cm depth. In the N treatment at 50 cm, SO42- in soil-water accounted for 9.4 % of applied S. compared to 14.0 % in NP. In untreated forest, soil-water Cl- and SO42- accounted for over 98% of the total soil-water anions, in roughly equal proportions at 10 cm, and CI- slightly higher at 50 cm. Following fertiliser-application soil-water pH at 10 cm fell from 6.3 in R to as low as 4.81 (N) and 4.45 (NP). At 50 cm pH never dropped below 6 and there were no visible departures from reference concentrations. Relative activities of K+ and Mg2+ in solution increased with decreasing pH, indicating increased leaching potential. Sulphate in soil-water increased total anion charge further in NP than in N. Total charge (cmolc L-1) for cations followed anions. A slight deficit in anion charge was likely due to the unquantified contribution of organic anions. These results confirm that despite the quantity of fertilisers added in this trial being double likely operational quantities, the forest and associated soils had the capacity to retain these nutrients through a variety of processes. The study validates the environmental sustainability of proposed intensive management practices including fertiliser-application in this forest type. It also emphasises the importance of understanding fundamental forest nutrient cycling processes when aiming to carry out intensive forest management practices in an environmentally sensitive manner.
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ItemEffects of organic applicants in a southern Victorian vineyard( 2007)Mulch is a material applied to the surface of the soil to reduce weed growth and reduce soil moisture loss through evaporation from the soil surface. The use of organic mulches will alter the soil environment. This alteration may include reducing soil temperature fluctuations, increasing soil organic matter, increasing soil microflora populations modifying soil chemical properties and increasing soil moisture retention. An experiment was conducted to compare composted green waste mulch and barley straw mulch with herbicide as alternative means of maintaining the undervine strip in a cool climate vineyard. Plant and soil responses to the different undervine treatments were monitored. Grapevine budburst was retarded, however, by the fourth week of vine growth there were no observable differences in grapevine growth stage. Both mulches stimulated grapevine growth and increased yield, with the compost mulch increasing vegetative growth with respect to fruit yield. The fruit quality parameters juice pH and titratable acidity were not significantly altered by the different undervine treatments. In the second year of the experiment the juice soluble solids were lower on the straw mulched grapevines. The compost mulch increased soil pH and carbon levels. The straw mulch improved soil water retention and the mass of soil fungal hyphae. Both mulches increased soil cation exchange capacity. The straw mulch increased soil exchangeable Mg to a greater extent than was predicted from straw nutrient content. The significant variations seen in soil cation content under the mulch treatments were not observed in the tissue analysis. Mulch can be used in a cool climate vineyard to increased yield without deleterious side effects.
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ItemRural credit and farm viability : a study of the Shelford soldier settlement scheme( 1973)The accelerated fall in wool prices towards the end of the 1960's precipitated the emergence of a financial crisis in the sheep industry with high farm debts and low farm incomes. The subject of rural credit, which had provoked only minor discussion throughout the buoyant income periods of the 1950's and 1960's, became increasingly debated by farm leaders who challenged the suitability of the existing credit market to meet the increasing capital needs of the rural sector. Since the implementation of the rural reconstruction scheme in 1971 which provides assistance for a limited number of unviable farmers, farmer demands for more general concessional credit facilities have intensified. Recently, following submissions by several farmer organizations to the Government for assistance, the subject of rural credit has rapidly developed into a political issue. This thesis is a study of farm indebtedness and the rural credit market in a woolgrowing community of soldier settlers in the Leigh Shire of the Victorian Western District. The area was chosen because it was thought that the settlers, hard hit by the 1967-68 drought, and heavily dependent on the profitability of woolgrowing, would be suffering severe financial problems. The economic analysis was restricted by the lack of suitable empirical data. The original survey data, collected by the Agricultural Extension Section of the University by personal interviews with farmers, did not include farm costs or inventory levels. Because this was a pilot study, the data was cross-sectional, referring only to the survey year. Before proceeding with the analysis, it was necessary to obtain additional financial information from the taxation records of co-operating farmers. The thesis was written before but submitted after the release of a Bureau of Agricultural Economics report (B.A.E. 1972) advising the Government on the adequacy of existing credit facilities. Consequently, no reference has been made elsewhere in the thesis to this report. The thesis is organized as follows. Chapters 1 and 2 provide a general introduction to farmers' borrowing behaviour, the existing rural credit market, and the debt position of the rural sector, especially the sheep industry. In chapter 3, the Shelford settlement is discussed with emphasis on the debt position of Shelford farmers and their use of credit facilities. In chapter 4, an attempt is made to relate the financial position of farmers to their physical resource levels and social characteristics. The variance in debt and income are analysed. Chapter 5 contains a discussion of some past influences on the current debt position, especially the 1967-68 drought. In chapter 6, the present and future prospects of farm financial viability are studied. Some commonly used indicators of viability are evaluated within the Shelford context. The dependence of settlers on short term lenders and the future role these lenders are likely to play at Shelford and elsewhere are discussed in chapter 7. Finally in chapter 8, the role of credit in agricultural adjustment is discussed. A solution for unviable Shelford farmers is proposed and some brief comments on the shortcomings of the present credit market are advanced. Conclusions are presented throughout the thesis, and chapter 7 and 8 form a summary of the rural credit situation in general and at Shelford.