School of Ecosystem and Forest Sciences - Theses
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ItemPhysiological basis of genotypic variation in plant response to elevated CO2( 2013)The CO2 concentration [CO2] in the atmosphere is increasing and is predicted to double by the end of the 21st century which is likely to have a profound effect on plant growth and yield, especially for C3 plants. Rising [CO2] has direct effect on photosynthesis (A), thus its potential stimulation drives the plant growth and development. However, these responses varied among species but, little is known about the underlying mechanism of genotypic variation in plant response to elevated [CO2]. In this study, wheat (Triticum aestivum L.) was used to test the hypotheses genotypic variation in response to elevated [CO2] exists within the wheat germplasm and if genotypic variability exists, whether it is related to leaf level A and related other traits. Based on above hypotheses, one experiment under glass house and two experiments under Australian Grains Free Air CO2 Enrichment (AGFACE) condition were conducted in the year 2010 and 2011. In experiment 1, seven wheat genotypes were grown at either ambient [CO2] (~384 µmol mol–1) or elevated [CO2] (700 µmol mol–1) in temperature controlled glass houses. Grain yield increased under elevated [CO2] by an average of 38% across all seven genotypes, and this was correlated with increases in both spike number (r = 0.868) and above ground biomass (AGB)(r = 0.942). Across all the genotypes, flag leaf A increased by an average of 57% at elevated [CO2]. The response of A to elevated [CO2] ranged from 31% (in H45) to 75% (in Silverstar). Only H45 showed A acclimation to elevated [CO2] which was characterised by lower leaf maximum RuBisCO carboxylation efficiency (Vc.max), maximum electron transport rate (Jmax) and leaf nitrogen (N) concentration. Leaf level traits responsible for plant growth, such as leaf mass per unit area (LMA), carbon (C), N concentration on dry mass basis (NFLAG LEAF), N content on an area basis (NLA) and the C:N increased at elevated [CO2]. LMA stimulation ranged from 0% to 85% and was clearly associated with increased NLA. Both of these traits were positively correlated with grain yield, suggesting that differences in LMA play an important role in determining the grain yield response to elevated [CO2]. Thus increased LMA can be used as new trait to select genotypes for a future [CO2]-rich atmosphere. In the second experiment, leaf blade elongation rate (LER) and leaf C and N dynamics in expanding leaf blade (ELB, sink) and A, C and N status in the last fully expanded leaf blade (LFELB, source) were determined using two wheat genotypes, Yitpi and H45. Plants were grown at either ambient [CO2] (~384 µmol mol–1) or elevated [CO2] (~550 µmol mol–1) within the AGFACE facility. Elevated [CO2] increased the leaf area and plant total above ground dry mass by 42% and 53% respectively for Yitpi compared to 2% and 13% for H45. Both genotypes showed increased LER at elevated [CO2] which was 36% for Yitpi compared to 5% for H45. In contrast, A increased by 99% for Yitpi at elevated [CO2] whereas H45 showed no change. For both genotypes, a strong correlation (r=0.807) was observed between LER of ELB and soluble carbohydrate concentration in LFELB over diurnal cycle. In ELB, the highest spatial N concentration was observed in the cell division zone, where N concentration was 67.3 and 60.6 mg g-1 for Yitpi compared to 51.1 and 39.2 mg g-1 for H45 at ambient and elevated [CO2] respectively. In contrast, C concentration in the cell division and cell expansion zone of the ELB increased only in Yitpi suggesting that C supply from the LFELB is genotypes dependent and well associated with LER, leaf area expansion and whole plant growth response to elevated [CO2]. In the third experiment, variation in A acclimation to elevated [CO2] at different leaf ontogeny was investigated. Seven wheat genotypes with contrasting growth and yield response to elevated [CO2], Janz, Yitpi, Silverstar, H45, Drysdale, Hartog, and Zebu were grown under rain fed condition at either ambient [CO2] (~384 µmol mol–1) or elevated [CO2] (~550 µmol mol–1) in the AGFACE facility. Leaf gas exchange measurements were conducted at two stages of flag leaf blade development: pre and post-anthesis. Immediately after the gas exchange measurements, flag leaf blade was sampled for biochemical analysis. Vc.max was calculated from gas exchange data to estimate the A acclimation to elevated [CO2]. Genetic variation in A acclimation to elevated [CO2] was observed. H45 showed strong acclimation to elevated [CO2] where Silverstar showed no acclimation. A acclimation to elevated [CO2] was observed only at post-anthesis, which was characterized by both reductions of Vc.max and Jmax. In both stages of the flag leaf, soluble protein concentration was significantly decreased at elevated [CO2]. Nevertheless, association of A acclimation and soluble protein concentration was not significant. In particular, Silverstar showed a large reduction in soluble protein concentration at elevated [CO2] at post-anthesis, but still showed no significant A acclimation (no reduction in Vc.max) to elevated [CO2]. The reductions in total soluble protein content at elevated [CO2] were not similar either between growth stages or genotypes. These findings suggest that A acclimation to elevated [CO2] varied between wheat genotypes but it is not fully associated with changes in soluble protein concentration. In summary, elevated [CO2] stimulated A, growth and grain yield and found large genetic variability of CO2 response. Variation in growth and yield response to elevated [CO2] was closely associated with leaf level traits such as LMA, leaf N and A capacity suggesting that plasticity of leaf level A related traits play a key role in response to elevated [CO2]. Particularly, genotypes showed higher A response to elevated [CO2], also showed increase LER which later translate to biomass and greater grain yield under elevated [CO2].
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ItemAssessing site quality of South Australian radiata pine plantations using airborne LiDAR data( 2011)Site quality information underpins many aspects of radiata pine plantation management in South Australia. Site quality assessment is essentially a problem of assessing the spatial variation of standing volume in unthinned stands at reference age nine or ten. Data collected during three experimental trials and two operational surveys were used to research a new site quality assessment system relying on airborne LiDAR data. A two-stage method was adopted characterised by the calibration of predictive models relating forest and LiDAR variables, and the subsequent application of these models to predict the forest variable across an area of interest. The four parts of the study examined the properties and behaviour of LiDAR prediction models, the sampling design and timing of field data collection, the spatial resolution of model application and the special case of partially thinned plantations. Single-variable, linear models, fitted to high altitude/low density LiDAR data captured across 20 sites scattered over an area of 10,000 km2 , had RMSE of 10-11% and 3-4% for stand volume and predominant height respectively. Evidence of site effects in the models was inconclusive. Models fitted to four LiDAR and field datasets acquired in 2002, 2006, 2007 and 2009 had consistent structure but model parameters were sensitive to the differences in operational LiDAR campaign parameters, indicating that prediction models should be re-calibrated each survey. Fifty field plots were found to be adequate to fit a regionally applicable volume prediction model. Sample selection methods only influenced model precision when sample sizes were small (less than twenty plots). The correlation between forest and LiDAR variables remained strong when field and LiDAR data were collected several years apart, but model parameter values changed rapidly as a result of tree growth. A time lag between LiDAR and field data collection can be tolerated but field measurements, once commenced, should be concentrated in time. The LiDAR predictor variable for volume was found to be insensitive to changes in the reference plot area indicating that volume prediction models may be applied in partitions (spatial units) with areas different than those of the calibration field plots. Comparison of alternative configurations of size, shape and arrangement of partitions, coupled with one of four spatial interpolation techniques, demonstrated significant differences in precision of predicted volume surfaces, with key factors being the dimensions of the interpolation neighbourhood and LiDAR data density. Several configurations closely approximated or, in the case of low density data, exceeded the precision of the prediction models. A method for assessment of partially thinned plantations appeared effective but requires further validation. The results of this study will be used to guide the second operational LiDAR-based site quality survey in South Australia, scheduled for early 2012.
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ItemFactors affecting microwave modified wood permeability and strength( 2013)Microwave treatment can be used to increase wood permeability, speeding subsequent drying or impregnation processes with potential savings in time, energy and cost. However microwave treatment can reduce the strength of the treated timber. Treatment pressure can be used to alter the boiling point of water, which could have a significant effect on the resultant strength and permeability. Three methodologies were used to investigate the relationship between microwave treatment conditions, in particular pneumatic pressure, and the resultant strength and permeability of the timber. A device was designed and constructed with the aim of enabling the simultaneous measurement of the permeability of small cylindrical samples during microwave treatment. Testing of the individual components proved successful, however moisture vapour generated during microwave heating of the timber samples presented problems during combined testing. Permeability measurements were performed separately in the second and third methodologies and utilised larger samples. Controlled temperature and controlled power density treatments were trialled with the controlled power density treatment providing more consistent results. Bending testing and an impregnation cycle were found to be the most reliable methods of assessing strength and permeability respectively. High power microwave treatment increased the permeability and reduced the strength of the treated timber. Treating the samples under elevated pressure, followed by an immediate pressure delta minimised this strength loss. A ranking system was proposed to assess the combined strength and permeability resulting from treatment. Using this ranking system, an optimum balance between high permeability and maximum strength was obtained using a high power, Elevated Pressure Microwave treatment followed by an Immediate Pressure Delta (EPMIPD). This treatment performed better than atmospheric pressure microwave treatments conducted as part of this research.
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ItemThe role of the antioxidants ascorbate and glutathione, in the plant-pathogen interaction between Phytophthora cinnamomi Rands and susceptible and resistant Eucalyptus species( 2013)This study investigates the role that the low molecular weight antioxidants, ascorbate and glutathione, play in the plant-pathogen interaction between Eucalyptus spp. and Phytophthora cinnamomi - Rands. The study has shown that plant responses (i.e. antioxidant concentration changes, rates of photosynthesis, stomatal conductance, quantum efficiency of photo-system II and carbohydrate levels) to P. cinnamomi root infection in roots and leaves differ significantly between susceptible and resistant Eucalyptus species. Antioxidant concentration decreases and an increased sensitivity to leaf photoinhibition were associated with infection in susceptible E. sieberi. In contrast, early antioxidant increases were associated with resistant E. sideroxylon.
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ItemEffects of wildfire on forest structure and plant functioning in resprouting forests: implications for catchment water balance( 2013)Globally, forests release large amounts of water that are critically important for urban and industrial water supply. Forests also account for the majority of terrestrial carbon sequestration. In southern Australia, much of the water supply comes from protected catchments vegetated by eucalypt forests. The primary natural disturbance in these forests is wildfire, with close to 3 million hectares burnt over the last decade in the state of Victoria (DSE 2013). Given that forest water-use (the volume of water released to the atmosphere) can change following disturbance, thereby changing the volume of streamflow discharge, understanding the effects of wildfire on forest water-use is vital for water resource planning. Concern around the effects of wildfire on water supply were heightened in 2009 when, following a prolonged drought that necessitated water restrictions, the Black Saturday wildfires burnt through 430 000 ha, including 30% of the water supply catchments for Melbourne, Australia’s second largest city. There are two different ecological responses of eucalypt forests to wildfire. The mixed eucalypt species forests are facultative resprouters, meaning they primarily regenerate vegetatively, with some seedling recruitment. In contrast, ash-type forests are obligate seeders, meaning they are largely killed by fire and regenerate from seed. Despite mixed eucalypt species forests dominating southern Australia’s water catchments, the effect of wildfire on evapotranspiration and streamflow from this forest type is unknown. Research to date has instead been focused on ash-type forests where post-fire evapotranspiration can be up to double that of long unburnt forest, leading to a corresponding reduction in streamflow. The central aim of this thesis is thus to elucidate how post-fire changes in forest structure and plant functioning in resprouting forests affects catchment water balance. Focusing research on the resprouting mixed eucalypt species forests not only addresses this knowledge gap in ecohydrology in Australian catchments, but also provides an opportunity to investigate the effects of altering foliage distribution, while largely maintaining other attributes of forest structure. The overall approach of the thesis is to evaluate the influence of post-fire changes in forest structure and functioning on evapotranspiration and streamflow. This is a process based approach which identifies the mechanisms underlying any observed changes in catchment water balance. This approach is critical for modelling the impacts of topographic driven variability in forest type and variability in fire severity on catchment water balance; and for predicting changes in evapotranspiration under circumstances other than those measured, such as at other locations or under a different climate. Such an approach also provides insights into the functioning of other resprouting vegetation types, which are found across the globe. The research presented in this thesis found that observed post-fire evapotranspiration was a function of both fire severity and landscape position. In forest subject to high intensity wildfire (100% canopy scorch), evapotranspiration was substantially less than in unburnt forest, over 1-3 years post-fire. The magnitude of change in evapotranspiration in forest burnt at moderate severity (<30% canopy scorch), was much less than for high intensity fire. Evapotranspiration was consistently lower in damp forest (located on southern slopes and gullies) than dry forest, although the rate of recovery was similar. These reductions in evapotranspiration in burnt forest were driven by lowered stand-scale transpiration. This was a function of partial tree mortality, 100% shrub mortality, and reduced transpiration within surviving trees. Reductions in stand-scale transpiration were partially offset by increased interception and evapotranspiration close to the forest floor, this in turn was driven by regenerating seedlings which increased the total leaf area of burnt forest. Despite lower transpiration per unit sapwood area in surviving trees, transpiration per unit leaf area was higher compared to unburnt trees. This was related to the post-fire canopy architecture of surviving trees, with more foliage located at lower heights than in unburnt trees. This lower foliage had higher rates of gas exchange, consistent with the hydraulic limitation hypothesis which predicts higher gas exchange in lower foliage due to a shorter hydraulic path length and lesser gravitational force. Recovery of evapotranspiration is predicted to occur within 10-15 years post-fire, over which time net evapotranspiration, when compared with unburnt forest, is predicted to be lower in forest burnt at high severity, but higher in forest burnt at moderate severity. These changes in forest water-use are expected to result in net streamflow increases following severe wildfire, but net decreases following moderate severity fire, subject to soil water storage dynamics. Streamflow observations from a mixed eucalypt species forested catchment burnt at light-moderate severity supports these predictions, with streamflow lower than expected due to fire induced changes in evapotranspiration dynamics over the initial five years post-fire. The research findings presented in this thesis demonstrate the importance of both forest structure and plant functioning in governing catchment water balance. These findings also demonstrate that the ecological response of species to disturbance is a critical factor in determining rates of recovery of carbon and water fluxes.
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ItemThe potential use of LiDAR and digital imagery in selection of suitable forest harvesting systems( 2013)Major factors affecting the productivity and efficiency of mechanized forest harvesting systems include stand conditions (e.g. tree form, tree size, crown size and the type and density of trees), terrain conditions (e.g. slope, ground roughness, ground strength, road and drainage features, etc.), yield, operator performance (e.g. experience, skill and work technique), and machinery limitations or design. The purpose of the study was to examine whether ‘tree size’ and ‘slope’ information derived from LiDAR (Light Detection And Ranging) data and multispectral imagery, could be used to predict the productivity and efficiency of forest harvesting equipment. Tree size is known to be the biggest influence on harvester productivity. The study aimed at developing a productivity model for a harvester operating in a 35-year-old radiata pine (Pinus radiata) plantation in South Australia using data obtained from low density LiDAR (Light Detection And Ranging) (2.6 points / m2) and high resolution Quickbird imagery (60 cm). Tree size extracted from a harvester onboard computer system (OBC) was used to estimate tree size impact on harvester productivity by conducting a time study. LiDAR-derived tree heights were not found to be significantly different (p < 0.05) from field measured tree heights and the absolute mean underestimation of LiDAR-height was 1.3 m. LiDAR-derived tree height estimates were found to be poorly related to tree volume and hence to harvester productivity. This was believed to be the result of the stand’s thinning history reducing the range of tree sizes i.e. removal of trees in the thinning operations. An attempt, therefore to estimate crown diameter from Quickbird imagery and or low density LiDAR was made which, in combination with LiDAR height, might be used to estimate tree volume and hence harvester productivity. Slope is a major terrain factor affecting harvester productivity. A study in Tasmania examined the ability of LiDAR to derive terrain slope over large areas and to use the derived slope data to model the effect of slope on the productivity of a self-levelling feller-buncher in order to predict its productivity for a wide range of slopes. Low intensity LiDAR (>3 points / m2) flown in 2011 over the study site was used to derive slope classes. A time and motion study carried out for the harvesting operation was used to evaluate the impact of tree volume (estimated from manual tree measurements) and slope on the feller-buncher productivity. The study found that productivity of the feller-buncher was significantly greater on moderate slope (11-18°) than on steep slope (18-27°). This difference in productivity resulted from operator technique differences related to felling. The productivity models were tested using LiDAR-derived slope and trees not used in the model development and were found to be able to accurately predict the effect of slope on the productivity of the feller-buncher. To better understand the drivers of harvesting productivity, a detailed comparative study of two single-grip harvesters was carried out in Australian Pinus radiata clearfell harvesting operations. Significant differences in productivity between the harvesters were found to be largely due to operator working technique differences. This factor cannot be determined through remote sensing. However, its influence can be reduced by using a general productivity model obtained from multiple operators.
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ItemEcosystem goods and services in production landscapes in south-eastern Australia( 2013)Ecosystem goods and services (EGS), the benefits that humans obtain from ecosystems, are vital for human well-being. As human populations increase so do demands for almost all EGS. Managing changing landscapes for multiple EGS is therefore a key challenge for resource planners and decision makers. However, in many cases the supply of different types of goods and services can conflict. For example, the enhancement of provisioning services can lead to declines in regulating and cultural services, but there are few tools available for analysing these trade-offs in a spatially-explicit way. This thesis developed approaches and tools for spatially explicit measurement and management of multiple EGS provided by production landscapes. These were used to assess the impacts of land-use change and to provide a basis for managing these trade-offs using case studies in two contrasting production landscapes in south-eastern Australia. Both landscapes have been subject to extensive clearing of native vegetation, which is now present in remnant patches. One study landscape had a concentration of commercially-valuable hardwood and softwood plantations, and the other was dominated by land traditionally focused on agricultural production that is currently being re-configured to provide for more sustainable farming practices and to increase provision of multiple ecosystem services. The study involved five components: (i) development of a novel, qualitative approach for rapid assessment of EGS in changing landscapes that was used to assess observed and potential changes in land use and land cover and their impact on the production of different EGS (Chapter 2); (ii) development and testing of an approach for assessing multiple EGS across space and time using a case study of six key EGS in a sub-catchment in Lower Glenelg Basin, south-western Victoria that demonstrated landscape-scale trade-offs between provisioning and many regulating services (Chapter 3); (iii) an economic valuation of EGS using market and non-market techniques to produce spatial economic value maps (Chapter 4); (iv) spatial assessment of the biodiversity values that underpin provision of many ecosystem services utilising a variety of readily available data and tools (Chapter 5); and (v) assessment of trade-offs and synergies among multiple EGS under current land use and realistic future land-use scenarios (Chapter 6). Results indicate that EGS can be assessed and mapped in a variety of ways depending on the availability of data, time, and funding as well as level of detail and accuracy required. A qualitative assessment can be useful for an initial investigation (Chapter 2) while quantitative and monetary assessments may be required for detailed landscape-scale planning (Chapters 3, 4). In addition, the provision of EGS by production landscapes can vary considerably depending on land use and land cover, and management choices. The study demonstrates that landscapes dedicated mostly to agricultural production have limited capacity to produce the range of ecosystem services required for human health and well-being, while landscapes with a mosaic of land uses can produce a wide range of services, although these are often subject to trade-offs between multiple EGS (Chapters 2, 3). Furthermore, the study demonstrated that spatial assessment and mapping of biodiversity value plays a vital role in identifying key areas for conservation and establishing conservation priorities to allocate limited resources (Chapter 5). There is potential for an improved balance of the multiple EGS required for human health and well-being at the landscape scale, although the economic incentive to adopt more sustainable land use practices that produce a wide range of services are compromised due to the lack of economic valuation of public ecosystem services (Chapter 6). High hopes have been placed by researchers on spatial assessment, mapping and economic valuations of ecosystem goods and services to influence policy makers for coping with the accelerating degradation of natural capital. The approaches and tools used in this thesis can potentially enhance our collective choices regarding the management of landscapes for multiple values and can help policy makers and land managers to enhance the total benefits that landscapes provide to societies through the provision of an optimal mix of goods and services.
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ItemThe effects of fire on bark habitats and associated beetle assemblages( 2013)Structurally complex habitats can often support more diverse animal assemblages than simpler habitats. Additionally, changes in habitat structure can alter assemblage composition. Structural changes can occur due to fire, and over time since last fire (TSLF), which may have implications for biodiversity management in fire-prone environments. The bark of Eucalyptus trees is readily modified by fire, but also provides habitat for a diverse fauna, including beetles (order Coleoptera). In a fire-prone forest type in south-east Australia, hypothesised relationships between TSLF, bark complexity and bark-associated beetle assemblages were investigated on two bark types: fibrous bark (typified by Eucalyptus obliqua) and ribbon bark (smooth bark that peels to form loose ‘ribbons’, typified by E. cypellocarpa). The research involved both a long-term (chronosequence ranging from 1 to 72 years postfire) and a short-term component (treatment-control study, comparing sites < 1 year post-fire with sites that had not been burnt for 27 years). Based on ecological theory it was expected that habitat complexity would change with TSLF, and that biodiversity would respond to complexity. The chronosequence study investigated whether bark complexity increased with TSLF; whether beetle richness and Simpson’s diversity relates to bark complexity and/or TSLF; whether TSLF affects assemblage composition; and whether assemblage responses to complexity were stronger than to TSLF. Bark-associated beetles were collected and a range of bark variables were assessed from study trees (of both bark types) at sites belonging four TSLF categories (1- 5 years; 27 – 29 years; 43 – 49 years; 72 years). Several aspects of bark complexity on fibrous-barked trees related to TSLF, but none on ribbon-barked trees. On fibrous-barked trees, Simpson’s diversity (but not richness) correlated modestly with the one element of bark complexity (surface cover of loose bark flaps), but with no others, nor with TSLF. On ribbon-barked trees, richness (but not Simpson’s diversity) was modestly related to the surface cover of loose ‘ribbons’. No other relationships with bark complexity or TSLF were found. On neither bark type was a TSLF effect on assemblage composition apparent; with many common morphospecies detectable throughout the chronosequence. Composition did not differ between the two bark types, and many morphospecies used both, suggesting that many beetles in this system can tolerate substantial differences in bark structure. The short-term comparative study was adopted in order to clarify the effects of very recent fire on bark complexity, and because some fire effects on beetle assemblages were anticipated to be short-lived (< 1 year). Burnt sites were found to have less complex bark than unburnt sites, and differences in assemblage composition (but not richness or Simpson’s diversity) were detected. Despite the detection of short-term compositional differences, the lack of longer term differences, and paucity of strong relationships with complexity, suggested that these assemblages were resilient, rather than responsive, to fire-related habitat change. This was contrary to hypothesised relationships between structural complexity and biodiversity, but consistent with suggestions that assemblages in fire-prone regions will exhibit a degree of resilience to fire impacts.
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ItemPost-fire debris flows in southeast Australia: initiation, magnitude and landscape controls( 2013)Surface runoff and sediment availability can increase after wildfire, potentially resulting in extreme erosion, flash floods and debris flows. These hydro-geomorphic events supply large amounts of sediment to streams and can represent a hazard to water supply systems, infrastructure and communities. This thesis combines observations, measurements and models to quantify and represent the post-fire processes that result in hazardous catchment responses. The processes that constitute risk to water quality and infrastructure were identified through field surveys of burnt catchments in the eastern upland of Victoria (southeast Australia) where impacts had occurred. The survey established that the majority of high-impact events after wildfire were linked to runoff-generated debris flows, a process previously undocumented in the region. The debris flows were initiated through progressive sediment bulking, and occurred in response to short duration and high intensity rainfall events, within one year after wildfire. Debris flows were confined to dry sclerophyll forests that had been subject to crown fire. Wet forest types displayed comparatively subdued responses, a pattern attributed to the relatively high infiltration capacity in these systems. Infiltration and sediment availability were isolated as the key hillslope components that were sensitive to burning and which strongly influenced catchment processes and debris flow susceptibility. The aims of subsequent work were therefore to develop models of infiltration and sediment availability as controls on hillslope response and use these to quantify changes in key parameters during recovery from wildfire. Infiltration was modelled as function of surface storage (H), matrix flow (Kmat) and macropore flow (Kmac). Macropore flow was found to be the main parameter driving the temporal trends in infiltration capacity during recovery from wildfire. Water repellency was ubiquitous in headwater recovering from wildfire, although the strength diminished during prolonged wet weather conditions, a dependency which could be modelled as a function of monthly weather patterns. Sediment availability was highly variable with soil depth, a feature which contrasts with assumptions underlying commonly used erosion models, typically developed in agricultural systems. The majority of erosion following wildfire was found to occur in a shallow layer of highly erodible material which could be represented through dnc, a parameter describing the depth of non-cohesive soil. This depth of available soil decreased exponentially during recovery. The models of sediment availability and infiltration were effective at capturing both spatial variability and recovery processes and form a basis on which to model debris flow initiation and magnitude in variable landscapes during recovery from wildfire.
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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.