School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemImpacts of short-interval wildfires on tree demography and forest structure in temperate Australia( 2019)Fire is a powerful agent of disturbance in terrestrial ecosystems, and it shapes vegetation composition and patterns globally. This is particularly true in south-eastern Australia where forests are dominated by species of the genus Eucalyptus, many of which have the capacity to recover from high-severity fire by resprouting from epicormic or basal buds. Climate change is predicted to yield more severe fire weather and lengthen fire seasons in temperate Australia, leading to increased wildfire frequency in these forests. While increased fire frequency – resulting in wildfire intervals of under a decade – are known to negatively affect fire-sensitive eucalypts (obligate seeders which have a juvenile period of 10 – 15 years) less is known about how such changes impact fire-tolerant, resprouting eucalypts. This Thesis examines the impacts of recent wildfires in south-eastern Australia, where a series of large wildfires burned over four million hectares of land, leading to the burning of different types of fire-tolerant eucalypt forests (basal resprouters, epicormic resprouters) by high-severity wildfires once, twice, and sometimes three times between 2003 and 2013. In the context of this massive natural experiment in the landscape, my overarching aim was to quantify the impacts of short-interval wildfire on eucalypt tree demography and regeneration, to improve understanding of potential fire-related changes to the structure and resilience of fire-tolerant forests. Short-interval high-severity wildfires significantly increased whole-tree mortality and decreased the abundance of both resprouts and seedlings in basally resprouting eucalypt forests. In these sub-alpine forests, dominated by snow gum (E. pauciflora), more frequent wildfire (two and particularly three short-interval fires) also increased the cover of grasses at the expense of shrubs. In mixed-species eucalypt forests, which occupy extensive tracts of low elevation landscapes in south-eastern Australia, resprouting occurs from both basal and epicormic buds. In these forests, the dynamics of both topkill (i.e. stem, but not whole-tree, mortality) and whole-tree mortality have important ramifications for forest structure. After a single high-severity wildfire, small-diameter stems were typically topkilled; after two short-interval wildfires, the diameter of stems topkilled increased. Additionally, the overall likelihood of either basal or epicormic resprouting decreased after two short-interval wildfires. This decline in resprouting capacity indicated that the size class most vulnerable to ‘resprout failure’ after multiple wildfires was intermediate sized stems (in the vicinity of 20 - 30 cm DBHUB), rather than smaller or larger stems. Seedling regeneration also decreased in these forest types after short-interval wildfires, suggesting that, as for fire-sensitive forests, immaturity risk may be a relevant factor for fire-tolerant forests. Short-interval wildfires reduced the total and aboveground carbon stocks of mixed-species forests, while also increasing the proportion of carbon mass in the dead pool, indicating that resprouter forests might not be perpetually secure carbon stocks under emerging fire regimes. My Thesis highlights that fire-tolerant forests may not be as invulnerable to changes in fire frequency as widely assumed, and that management interventions will likely be required to counteract increasing tree mortality and decreasing tree regeneration if predictions of more frequent and severe wildfires in temperate Australia are realised.
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ItemLong-term effects of frequent burning on fungal communities and the role of fungi in fire-prone forests( 2007)Bushfire is an integral part of the Australian environment. Animals and plants show adaptations to and dependence on fire and prescribed burning is an important management tool in eucalypt forest ecosystems. Responses of flora and fauna to fire regimes have been extensively examined in Australian forests, however one aspect of the biota abundant within all forest types that has received little consideration is fungi. Despite their undoubted ecological significance, little is known regarding the taxonomy, biology and ecology of fungi, let alone the impact of fire upon fungal communities. Knowledge of the responses of fungi to fire is of intrinsic interest and is essential for effective forest management. Fungi have significant roles in transporting, storing, releasing and recycling nutrients. Consequently, disturbances such as fire that impact upon fungi and their ability to perform these ecosystem processes may be of importance to forest structure, health, productivity and sustainability. The aim of this study was to investigate the effects of repeated low-intensity prescribed burning on various aspects of the fungal community in two Australian eucalypt forests. Such research was deemed necessary to fill a significant gap in current knowledge regarding fungal ecology and to provide forest managers with recommendations for use of prescribed burning to enhance fungal biodiversity. Current knowledge of fungal community structure, function and contributions of fungi to forest ecosystem processes was explored, with a range of traditional and new techniques used to assess quantitative, qualitative and functional aspects of above- and belowground fungal communities. The diverse methods used enabled comprehensive assessment of numerous community dynamics and their application throughout the study was evaluated. Cost analysis showed that assessment of diversity and functional diversity of aboveground sporocarps was far more expensive than analysis of belowground fungal diversity. It was therefore suggested that future research should consider the relevance of aboveground sporocarps in the overall fungal community and that more attention should perhaps be given to diverse, abundant and functionally significant soil fungi. Assessment of fungal communities was undertaken in relation to experimental burning treatments within eucalypt forests and the influence of fire on vegetation, fuel and soil characteristics. Little overall difference was observed in richness and diversity of sporocarp morphotypes and functional groups among treatments in the Wombat Forest. No significant differences were observed among soil fungal biomass as indicated by ergosterol concentration in either Bulls Ground or Wombat Forest soils. In addition, molecular data showed that richness and diversity of soil fungi among treatments were similar and that no specific fungal community was associated with soils of any particular treatment in the Wombat Forest. Such findings suggest that low-intensity prescribed burning has little long-term effect on these aspects of the fungal communities investigated in Wombat Forest and Bulls Ground study areas. However, given the critical roles of fungi within ecosystem processes it could be assumed that even minor changes in community dynamics may be of functional significance within forests. It was therefore considered too presumptuous for the current study to offer management recommendations based on these findings and was suggested that further understanding of relationships between fungal diversity, functional groups and ecosystem function is necessary for appropriate management decisions and development of sustainable forests.
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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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ItemMethods of improving the preservative treatment of radiata pine and eucalyptus with emphasis on the use of novel boron compounds( 1998)This thesis identifies the effect of raw materials and processing parameters on the application of TMB on wood and wood products. It also describes the effect of TMB on the properties of wood and wood products. This work proposes the application of TMB by momentary immersion following by storage in a close container. The effect of wood moisture content, temperature and time were studied. Low wood moisture content (3%) is required to produce high penetration of the TMB in radiata pine (Pinus radiata D. Don) sapwood. High temperatures (40°C) during storing stage can result in adequate penetration. The storage stage is described in two phases, the emission of chemicals from the wood and the diffusion of the chemical from the head space of the container into the wood. Parameters affecting the process include temperature, wood moisture content and time. Wood moisture contents as low as 3% were found necessary to achieve high penetration of chemicals into wood. During the storage of treated timber, high temperatures reduced the emission and vapour diffusion phases. The levels of TMB and methanol in the headspace of the container were lower at high temperatures. The concentration of TMB and methanol in the head space of the desiccator achieved an equilibrium concentration at 100 hrs for samples stored at 40°C and 200 hrs for samples stored at 20°C. Methanol emission produced by the interaction between water in the wood and TMB was found in low levels. Samples of radiata pine were preconditioned to 3% wood moisture content, treated by momentary immersion in TMB and then stored at 20°C. After vapour diffusion the samples were submitted to an airflow of 0.25 L/min for 24 hrs. Emissions were collected in a water trap. For samples treated at 3% moisture content and stored at 20°C, the analysis showed that only 9.6% of the total production of methanol was found in the emissions. For radiata pine this equals to 28 g/m3/hr. Methanol in the wood is removed easily by water. However a strong bond between wood and methanol is formed. Samples of different species were saturated with methanol and then dried at 105°C for 24 hrs. Twenty nine grams of methanol was found in radiata pine after this process. Pre-conditioning of wood prior to momentary immersion improves the penetration and retention of TMB. Several pre-conditioning processes were studied. Steaming reported better absorption and penetration than microwaving, high temperature and air drying. TMB penetration and a retention of 2.48L/m3 was achieved when steamed radiata pine samples were immersed in TMB for 30 secs. These values were statistically different from values obtained with other processes. Momentary immersion of particleboards in TMB following by a storing stage in a close container was studied. Wood moisture content required to achieve total penetration was 8% for particleboards of thickness 12 and 18 mm. Thicker panels required lower moisture contents. Stiffness (MOE) and strength (MOR) was reduced as maximum as 13 and 17% respectively. Momentary immersion in TMB following by vapour diffusion in a close container was successfully trialed in sapwood of radiata pine, messmate (Eucalyptus obliqua L'Herit), jarrah (Eucalyptus marginata Donn. Ex Sm) and blue gum (Eucalyptus globulus Labill). The effect of non-polar solvents as carriers upon vacuum-pressure impregnation of timber with TMB was studied. Total penetration in sapwood of radiata pine was achieved at wood moisture contents below 14%. The rate of hydrolysis of TMB in non- polar systems was measured and compared with other systems. Non-polar systems have a rate of hydrolysis 26 times slower that polar systems. This allows TMB to penetrate deeper into the timber before hydrolysis begins. Different polymers of trymethylborates were also tested in these systems. Methylpolyborates, a polymer of TMB, were used in the manufacture of pastes and rods for remedial treatments of wood. Methylpolyborates in combination with copper and fluoride compounds were used as active in the manufacture of pastes and rods. Diffusion in stubs of red gum (Eucalyptus camaldulensis Dehnh.) after 60 days demonstrate the feasibility of these TMB polymers as diffusible wood preservatives.
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ItemA study of the species of Mycosphaerella on eucalypts in Australia and the impact of Mycosphaerella leaf diseases on Eucalyptus globulus Labill( 2000)In this study, the taxonomy of the species of Mycosphaerella that cause leaf diseases of eucalypts was reviewed, the effect of these diseases on E. globulus in plantations was quantified, and the use of silvicultural methods, tree selection and resistance breeding to reduce the impact of Mycosphaerella leaf diseases in commercial plantations was investigated. Over 30 species of Mycosphaerella cause leaf spots and defoliation on a wide range of eucalypt species (Eucalyptus and Corymbia), but the taxonomy of some of these is incomplete, and there is confusion in species identity between morphologically similar species. Therefore, herbarium specimens, fresh collections and previous records of Mycosphaerella species from eucalypts in Australia and elsewhere were examined. Widespread surveys of Mycosphaerella species of eucalypts in plantations and native forests were conducted in Western Australia, South Australia, Victoria, Tasmania and New South Wales. A new species of Mycosphaerella (M. vespa) was described from plantations, a previously described species (M. gregaria) was revised following further observations and investigations, further host and location records were reported, and the first record of M. suberosa outside Brazil (its place of description) was made. A detailed list of the species of Mycosphaerella in Australia was compiled, with descriptions, photographs, and illustrations of each species. A table of important taxonomic characteristics of Mycosphaerella species on eucalypts was compiled, and a taxonomic key developed to aid in identification of the species in Australia. (Part of the summary only)