School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemEffect of green roof design configuration and climate on rainfall retention, evapotranspiration, and plant drought stress( 2023-04)Urbanisation significantly alters the hydrological cycle through creation of impervious surfaces and removal of vegetation. Besides creating large volumes of stormwater runoff which degrades urban stream ecosystems and water sources, impervious surfaces reduce infiltration of rainfall and therefore water availability for remaining vegetation. Sustainable and resilient stormwater management techniques are required to mitigate the impacts of stormwater runoff and compensate for the loss of vegetation in cities. Green roofs are a promising green infrastructure technology with the potential to deliver these ecosystem services, however, gaps in our understanding of how they should be designed is preventing widespread uptake. Green roof substrates are typically shallow due to building weight-loading restrictions and therefore have limited water storage for reducing runoff and sustaining vegetation. Species typically used on green roofs are limited to those with the ability to survive harsh environmental conditions, such as succulent species. However, selecting plants with high water use, combined with drought resistance, such as non-succulent species, and planting at higher density in deeper substrates may improve green roof rainfall retention without substantially increasing plant drought stress. Where substrate depth cannot be increased, mimicking processes we observe in natural systems, such as redirecting rainfall towards vegetation in semi-arid banded systems, has significant potential to both increase rainfall retention and reduce plant drought stress. Climate is the primary driver of green roof performance, specifically the supply of and demand for water from the atmosphere, which in turn determines evapotranspiration, rainfall retention and plant drought stress. Ideally, by understanding the interaction of substrate depth, plant density (water use) and redistribution of water resources on green roofs, it should be possible to determine the most suitable green roof configuration for different climates, and therefore remove the key barrier to widespread green roof installation. In this thesis, I aimed to achieve this understanding through a combination of controlled experiments and water balance modelling. Firstly, a glasshouse experiment was used to understand how increasing substrate depth and plant density, as well as their interaction, impacted plant water use, drought stress and rainfall retention. Due to COVID-19 disruptions, only the first well-watered phase could be completed, after which the experiment had to be terminated without measuring water use and drought stress under water-deficit. Therefore, I used an established green roof water balance model to simulate performance under water deficit conditions. Pre-existing functions describing the plant species’ drought response were combined with well-watered plant crop factors (Kc) calculated from ET measured during the experiment, to estimate rainfall retention and the incidence of plant drought stress. Contrary to my initial hypotheses, increasing plant density did not result in a proportional increase in plant water use, even when substrate depth was doubled. Importantly, this indicated little gain in retention performance by increasing plant density. Using a water balance model to extend these findings to include performance under water deficit, I showed that rainfall retention was very high, regardless of substrate depth and plant density, as plants in the glasshouse had a very high crop factor and therefore rates of water use. With a high crop factor, all treatments from the glasshouse simulated in the model depleted the substrate water quickly, resulting in greater retention, but also significant plant drought stress. In this model, the indicator of drought stress in plants on each day of the rainfall simulation was when the depth of water (millimeter) in the substrate at the end of any given day reached zero. Importantly, increasing substrate depth showed no significant benefit to either rainfall retention or plant drought stress. Overall, planting in shallower substrates at a lower density optimised green roof performance when measured and simulated in a temperate climate. Secondly, a rainfall simulation experiment using green roof modules was conducted to understand the effect of plant density and redistribution of rainfall (runoff zones) on rainfall retention, plant water use and drought stress. In this experiment, drought stress in plants was indicated by midday leaf water potential (MegaPascal). Again, planting at a lower density (10 plants per module, approximately half the module area planted) achieved high rainfall retention and most importantly, plants experienced lower drought stress than fully-planted modules (18 plants per module). Furthermore, using runoff zones to direct rainfall towards plants also reduced plant drought stress. However, the runoff zones also created preferential flow pathways and shaded the substrate surface, both of which were the likely cause of lower rainfall retention, despite the observed reduction in plant drought stress. Although, reducing plant density showed the most effective way of achieving high rainfall retention and lower drought stress, there are other ways of increasing substrates' water retention and holding capacity, such as using water retention additives to increase water available for more densely planted green roofs which would improve the ecological, environmental, and social benefits of a green roofs. While redirecting rainfall showed promising approach, further work is required to improve their design to find the optimal method to redirect more water to plants and improve the coverage of plants on green roofs. Finally, using results from both experiments, I developed and validated a new green roof water balance model to simulate long-term green roof rainfall retention and plant performance in two contrasting climates (temperate vs semi-arid climates). Green roofs showed high rainfall retention in both temperate and semi-arid climates, regardless of substrate depth, plant density and presence/ absence of runoff zones. Even unplanted roofs showed high retention in both climates, showing that evaporation is the major component of evapotranspiration and therefore a primary driver of rainfall retention. In this experiment, midday leaf water potential was used to indicate the maximum water stress experienced by plants during the day and therefore plant drought stress. Therefore, I modified the water balance model used in the first experiment, by using the relationship between midday leaf water potential (MegaPascal) and substrate water content (S) to estimate plant drought stress. As expected, green roofs in semi-arid climates had significantly greater plant drought stress (more negative water potential) as compared with those in temperate climates, with no observed benefit in rainfall retention, despite increased substrate depth. Hence, a substrate depth of 150 millimeter could achieve optimal retention in both temperate and semi-arid climates. Increasing substrate depth, plant density and the use of runoff zones was less important for improving rainfall retention than climate. The modeled results also highlight that an unplanted roof is equally good for stormwater management alone, as it can achieve similar rates of rainfall retention as compared to a planted roof. However, keeping in mind the ecological, environmental, and social benefits of vegetated green roofs with good plant coverage, it is not recommended that practitioners install non-vegetated green roofs. Overall, the results showed that green roofs perform very well for rainfall retention, in both temperate climates with a large proportion of small rainfall events, and semi-arid climates with an annually low rainfall depth. However, only one plant species was evaluated in my thesis, which would have impacted on these results as the plants were high-water using species that could effectively dry out substrates after rainfall and also tolerate drought stress in dry substrates. The design of green roofs for good plant coverage and survival in semi-arid climates is likely to be more challenging than constructing in temperate climates in real conditions due to the risk of plant death where drought periods are more severe and prolonged. In this case, it is likely to be preferrable to plant low water using succulent species. In both temperate and semi-arid climates, green roof substrate depth did not necessarily need to be deeper than 150 mm, as the increase in rainfall retention was minimal and plant drought stress could not alleviate beyond this depth. Installing runoff zones is a promising approach to changing how water is distributed on green roofs and have the capacity to reduce plant drought stress. However, they can also promote preferential flow pathways minimising the water storage capacity of substrates and decreasing the evaporation from surface of substrates underneath their structure and therefore, reducing rainfall retention. Hence, reconsideration the design for such runoff zones could be a potential avenue of future research. In the end, the practical output of this research suggests that green roofs with lower plant density such as 1 plant per 0.1 square meter and substrate depth such as 150 millimeter can effectively retain rainfall in temperate and semi-arid climates. While in temperate climates, higher water using plant species can be used, it is recommended that in semi-arid climates, green roofs are planted with low water using species such as succulents to support vegetation cover on green roofs. This means that even when green roof plant cover reduces over time, green roofs will still have high performance for rainfall retention.
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ItemThe ecological costs and benefits of urban stormwater wetlands to frogs( 2018)The speed and scale at which humans are altering natural systems creates novel challenges for many species. Some species can cope with human-induced rapid environmental change by exhibiting adaptive behavioural or phenotypic plasticity. Many others, however, respond maladaptively in ways that can impact individual fitness. When rapid environmental change triggers mismatches between perceived and actual habitat quality, animals can prefer inferior habitats, that are known as ecological traps. Using a meta-analysis, I show that ecological traps are an unexplored but potentially important conservation risk to animals within wetland habitats (Chapter 2). Focusing on urbanisation and stormwater wetlands as a case study, I assess how anthropogenic environmental change affects frogs, in terms of the environmental variables influencing species occurrence (Chapter 3), the capacity of individuals to make adaptive habitat selection decisions (Chapter 4), and the fitness and behavioural consequences of these decisions (Chapter 4 and 5). I show that frogs occupied wetlands across a broad spectrum of pollution levels, including even the most contaminated, and that pollution exposure reduced survival and impaired predator avoidance behaviours. Breeding frogs did not avoid wetlands where these fitness reductions occurred, demonstrating that stormwater wetlands can function as ecological traps. Collectively, my results highlight the need for a greater focus on individual-level metrics (e.g. fitness and habitat preferences) in addition to the more commonly measured population- and community-level metrics (e.g. richness and abundance). Based on my research, I propose three key recommendations to maximise biodiversity at wetlands within urban landscapes. Firstly, appreciate that poor water quality at stormwater wetlands may impact resident wildlife, and attempt to reduce the causal factors. Second, despite this, do not ignore the potential value of stormwater wetlands in providing habitat and enhancing connectivity amongst aquatic habitats, particularly when they are appropriately designed and managed. Finally, it is important to design and construct wetlands for wildlife that are not connected to stormwater networks, with their placement within the landscape carefully considered.
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ItemFire effects on pollinators and pollination( 2016)One of the most important knowledge gaps currently inhibiting biodiversity conservation in fire-prone landscapes concerns the interactions between fire and other ecosystem processes. Animal-mediated pollination, whereby animals transport pollen between the male and female parts of flowers, is an important ecosystem processes as it is required for approximately 80-90% of the Earth’s flowering plants to reproduce sexually. The aim of my thesis is to better understand fire effects on pollinators and pollination and their role in fire management. First I synthesise the literature to develop a conceptual model, and then describe my empirical work exploring the ideas underlying this model. The central idea is that fire can influence plant-pollinator interactions through multiple processes operating over different spatial scales, and this was supported by my empirical work (and a study (Ponisio et al. 2016) from North America published as I was finalising my thesis). I collected data from the Mediterranean climate zone of south-eastern Australia, employing a space-for-time substitution design with spatially independent sites along a 75 year post-fire successional gradient. I monitored pollinator visitation to the sexually-deceptive orchid Caladenia tentaculata, capsule set of the food-deceptive orchid Diuris maculata sensu lato, and sampled aerial invertebrate assemblages (focusing on known fly and wasp pollinator groups) across these sites. I found that visitation to C. tentaculata was greatest when the site was recently burnt but the surrounding landscape was long-unburnt. Diuris maculata s.l. capsule set was influenced mostly by rainfall in the growing and flowering season (winter and spring). Flies and wasps exhibited only moderate or weak responses to post-fire age. Interestingly, though, species richness was positively related to fire age diversity within 800 m of sample locations but negatively related to fire age diversity within 200 m. My model could form the basis for simulations of plant and pollinator population dynamics in fire-prone landscapes, parameterised with spatially-explicit empirical data as presented in my thesis.
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ItemEcological benefits of termite soil interaction and microbial symbiosis in the soil ecosystem in two climatic regions of Australia( 2015)Termite soil interaction is a multidimensional process, the interphase between the surface and subsurface being the most prominent location termitaria and other termite structures usually occupy. Genetic and environmental conditions, including soil type and moisture content, in different climatic regions affect this interaction. There is scant information on termite preferences, foraging behavior within these conditions and impact on soil profile and associated symbiont microorganisms. Foraging activity of termites (Coptotermes frenchi), depth and changes in soil profile with layers of top soil, fine sand, coarse sand and gravel, was studied using a test tank in a laboratory. Termite activities were intensive in only the longest foraging galleries via which they reached and foraged up to the edge of the tank. Wood stakes inserted vertically at three different depth level intervals (0-100, 100-200, and 200-300 mm), visual observations of soil profile samples taken using auger and excavated cross sections of the soil profile all confirmed presence of termite activity, transport and mixing of soil up to the lowest horizon in the otherwise uniform sandy or gravely lower horizons. However, termite activity did not result in complete mixing of soil horizons within the study period. Termites (Coptotermes acinaciformis) were tested for their preference topsoil, fine sand, potting mix and peat, in a laboratory condition at soil moisture contents of 0, 5, 10, 15 and 20% for 30 days. The experimental apparatus involved termite colonies foraging from nesting jars connected to four sets of standing perspex tubes filled with each soil type and moisture content combination attached to the jar lid on top. Soil type had a significant effect on termite preference whereas soil moisture content did not. At lower moisture levels of 0 and 5%, termites preferred fine sand while topsoil was preferred at 10, 15 and 20%. Soil heterogeneity and textural variability with respect to particle size distribution due to termite activity was investigated in two climatic regions of Australia. Mound and surrounding soils of Coptotermes lacteus in Boola Boola State Forest, Victoria, and Amitermes laurensis and Nasutitermes eucalypti in Gove, Northern Territory were studied. The residual effects on bacteria and fungi counts were also investigated in the former. For C. lacteus and A. laurensis mounds the very fine particles sizes (< 0.045 mm) were significantly higher than that of the surrounding soil while the reverse was true for the 2 - 1 mm particle size ranges. For the Nasutitermes mound, however, they recorded significantly higher 2 - 1 mm particle sizes and significantly lower < 0.045 mm particle size ranges than the surrounding soils. For the other particle size ranges in both sites no significant difference was observed between the mound and surrounding soils except for the 0.5 – 0.2 and 0.20.063 mm ranges in the A. laurensis mound which were significantly higher than surrounding soil. Average moisture content of the surrounding soils was significantly higher than that of the mound surfaces which could have resulted in the higher bacteria and fungi counts (cfu/ml) in the surrounding soils.
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ItemFire, resources and behavioural responses of ground-dwelling mammals( 2015)Planned fire is commonly used to reduce adverse effects of bushfire to human life and property, but may also be used to conserve biodiversity. However, there is a dearth of information regarding the effect of these fires on fauna. I investigated the response of ground-dwelling mammals to a planned fire event in the Otway Ranges, south-eastern Australia. Bush rats (Rattus fuscipes) and swamp wallabies (Wallabia bicolor) were chosen as study species given that they are expected to be affected by a change in vegetation resources due to fire. The differences in body size of the two species provided the scenario to study fire effects at two different spatial scales. At a small scale, I studied changes in abundance of bush rats as a consequence of fire, plus the role of unburnt areas as refuges. I used microsatellite markers to study movement of individuals between slopes and gullies. At a larger scale, I used GPS technology to track swamp wallabies before, during and after fire, to study changes in home range and habitat selection, as well as behaviour during fire. The studies included different temporal levels as well, the study on abundance and movement of bush rats, and on habitat selection of swamp wallabies, compared data from two months before with two months after fire. Home range data of swamp wallabies compared data two months before fire with data from up to eight months after the fire. Finally, the study on movement of wallabies during the fire, included data from 36 hours when the fire was burning compared to pre-fire data. The study on bush rats corresponded to a Before-After Control-Impact (BACI) design using a paired catchment approach, while the study on wallabies corresponded to an Impact Analysis (IA) design, comparing responses not only before and after, but also during fire. There was no strong relationship between the different responses that were quantified and the amount of area that was burnt within transects and home ranges, possibly because there were enough unburnt areas available in the post-fire landscape, emphasizing the importance of keeping areas of unburnt vegetation when applying planned fires. Yet, the impact of fire was presumably larger on bush rats; this was reflected in the reduction of abundance of individuals, while all swamp wallabies survived. The impact of fire varied between the two study species, reflecting the importance of investigating the effect of planned burns at different spatial and temporal scales. The strategy used in the fire event that I studied, i.e. of low intensity, progressively and in patches across the targeted area did not have major effects on the study species. The information provided in this thesis intends to improve the capacity for land managers to consider the ecological effects of planned fire by adding to current knowledge linking fire, resources and behavioural responses. Further assessment involving more intense fire would be necessary to assess the response of bush rats and swamp wallabies, and to predict the possible consequences that a wildfire could have on these species.
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ItemInteractions between fire, environmental heterogeneity and ground-dwelling mammals( 2014)Environmental heterogeneity is known to influence a range of ecological processes at various spatial scales, from individual habitat selection and interspecific interactions, to species’ distributions and diversity. Fire regimes can influence environmental heterogeneity by altering the spatial and temporal distribution of resources. In this thesis I used the ground-dwelling mammals of south-eastern Australia as a focal group to explore the role of fire-mediated heterogeneity in driving individual species distributions, abundance and species diversity. I focussed on two aspects of fire regimes as drivers of environmental heterogeneity at different spatial scales; time since fire at a landscape scale and spatial extent of fire within an individual planned burn. As a secondary objective I also evaluated techniques used to detect ground-dwelling mammals. I investigated relationships between fire-mediated heterogeneity and species diversity at the landscape scale. I compared heterogeneity defined by spatial pattern metrics based on fire age and vegetation type, versus heterogeneity derived from site-based habitat structural measurements. I used two complementary diversity metrics, species richness and beta diversity as response variables. Heterogeneity defined by habitat structural contrasts was positively correlated with beta diversity, however heterogeneity defined by mapped post-fire age classes and vegetation types did not influence beta diversity, and neither measure of heterogeneity was related to species richness. The mammal communities in our study area were influenced by environmental heterogeneity but only if it was present in specific structural attributes of the environment. This suggests that relationships between heterogeneity and diversity depend on how variables representing these properties are quantified. The spatial pattern metrics based on fire age and vegetation type did not reflect physical contrasts that are important for maintaining ground-dwelling mammal diversity. Building on the knowledge at the mammal community level, I investigated individual species responses to time since fire at the landscape scale. Specifically, I used a space for time substitution to investigate interrelationships between the occurrence of eight ground-dwelling mammals, time since fire, and structural resources. Individual species distributions were not well predicted by time since fire. Time since fire was moderately correlated with habitat structure yet was a poor surrogate of mammal occurrence. Variables representing habitat structure were better predictors of mammal occurrence than time since fire for all species considered. These results suggest that time since fire is unlikely to be a useful surrogate for ground-dwelling mammals in heterogeneous landscapes. At a smaller spatial scale, I used a before-after-control-impact experiment, focussed on a planned fire, to investigate the role of unburnt patches in providing post-fire refugia for Agile Antechinus Antechinus agilis and Bush Rats Rattus fuscipes. The two species responded differently to the presence of unburnt patches associated with wet gullies in the burnt landscape. Relative to controls, fire had little effect on Bush Rat abundance in unburnt gullies. In contrast, the fire caused Agile Antechinus abundance to increase in gullies, indicating a shift of individuals from burnt parts of the landscape. Bush Rats that previously occupied burnt parts of the landscape most likely perished in the aftermath of the fire. These differences are likely driven by differences in habitat use and intraspecific competition between these species. I evaluated the three techniques used to detect mammals, live trapping, camera trapping and hair detection. The camera traps detected more species than the other two techniques but live trapping consistently complemented the cameras by detecting unique species. Furthermore the effectiveness of the different techniques varied across the landscape, with live trapping detecting more unique species in wetter, more productive vegetation types, whereas in dry vegetation types the camera trapping alone detected all species present in the sample. I also evaluated two different camera trap models. I found that Reconyx cameras consistently detected more species than Scoutguard camera, mostly because they detected small and medium species more frequently. The results showed that the use of Scoutguard cameras in isolation would have led to erroneous conclusions about the main drivers of species distributions across the landscape.
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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.