School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemInvestigating the seed ecology, functional traits and ecological strategies of Australian annual plants to create diverse, flowering green roofs( 2022)A diverse vegetation layer on green roofs is important to achieving and maximising green roof ecosystem services. Previous green roof research has mostly examined succulents and perennial plants with less attention to annuals and few green roofs incorporate annuals in the planting designs. This is despite annual species having an important role in some natural habitats with conditions that are analogous to green roofs. Their ability to escape seasonally harsh conditions as seeds, fast and cheap establishment via sowing, rapid growth, and the biodiversity and aesthetic value of their colourful flowers are also potential advantages of using annual plants on green roofs. Australia is a continent with a diverse annual flora with colourful and attractive flowering species and high potential to be used in horticultural applications such as green roofs. Therefore, the objective of this project is to develop an attractive flowering, resilient plant palette of Australian annual species suitable for green roofs in Mediterranean-like climates. However, to achieve this, we need more knowledge regarding annual species seed germination, functional traits and ecological strategies. Seed germination capacity is one of main constraints in seed-based projects. A high proportion of wild plant species in Australia produce seeds that are dormant upon maturity and will not germinate without a suitable pre-treatment. Increasing knowledge about factors such as seed mass and climate variables that may influence or predict germination percentage is helpful if species are to be used in horticultural applications, like green roofs. To do so, we commercially sourced seeds of 58 Australian annual species from a wide range of habitats and performed germination tests using different germination stimulants and temperatures in growth chambers. Only 20 out of 58 species germinated regardless of the applied treatments and no optimal treatment was found for all species. No relationship was found between seed mass, and germination percentage and speed. Lower germination was observed in species from hotter and drier environments as well as environments with more variable rainfall. Moreover, germination speed was greater in species from drier environments and environments with lower climate variability. Finding significant relationships between climate variables and germination attributes, and no relationship between seed mass and germination, shows that climate is a better indicator of germination and could be a more useful way to select annual species for horticultural applications than seed mass. The plant functional traits of plant species growing on green roofs are major factors determining the ecosystem services green roofs provide. Moreover, flowering plants which can quickly achieve high cover, growth and biomass are desirable for aesthetic reasons and to meet green roof construction guidelines. Annual species are generally drought escapers with acquisitive strategy and ‘fast’ traits, rapid growth and flowering. However, since annuals grow successfully in different habitats and climates, it is expected that they have different strategies to live and complete their life-cycle. To quantify plant strategies of annuals and evaluate the ecosystem services they may offer on green roofs and to develop a trait-based plant palette of annuals for green roofs, I undertook a common garden experiment with 18 annual plant species (successfully germinated in the previous experiment). It explored the relationships among traits related to drought resistance and resource acquisition, competitive and reproductive ability and also aimed to determine whether differences in plant strategies would influence their growth rate, shoot biomass and flowering time. Species with higher acquisitive strategy were more competitive, fast growing and produced higher final biomass. Two opposing strategies were observed in the studied annual species in regard to their flowering time and relative growth rate. Plant species also showed a trade-off between their flowering time and specific leaf area (SLA). Fast flowering species with higher SLA were considered less drought resistance (drought escapers), as earlier flowering and higher SLA are both evolutionary responses of annual species to escape stress. Fast growing species with resource acquisitive strategy could be more desirable on green roofs as they have higher water use and therefore, stormwater mitigation ability. It is also more preferable to select species with a range of time to flowering onset and SLA to achieve a long-term flowering community with different levels of drought resistance. Finally, I created a seed mixture using 16 species which we found had higher than 50% germination in the earlier study. I then sowed them in extensive green roof modules, to assess the effects of water availability on annual plant growth and diversity. Species abundance and richness were not significantly different between irrigation levels but showed a significant reduction through the experiment. Plant cover was significantly higher when the mixes were irrigated at a higher frequency; however, > 80% plant cover was achieved in all irrigation treatments. At the community level, functional diversity was unaffected by irrigation frequency. This study showed that annual species seed mixes can have good germination and establishment on green roof substrate and attain high species and functional diversity and cover with little irrigation.
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ItemThe role of climate and tree nutrition on the occurrence of the southern greater glider (Petauroides volans) and its implications for conservation planning( 2021)The southern greater glider (Petauroides volans) is southeastern Australia’s largest gliding marsupial and widely distributed, but the species has recently experienced drastic declines in population numbers. Its association with hollow-bearing trees, used for nesting, has made it an important flagship species for the conservation of old-growth forest ecosystems. Stand replacing or altering wild- or planned fires and timber harvesting have been identified as major threats to the greater glider, but individuals and populations have over time disappeared from areas that have experienced neither, raising questions about the role of other factors in their decline. Habitat suitability is determined by three distinct factors: A narrow thermotolerance makes greater gliders vulnerable to high ambient temperatures. A specialised diet consisting entirely of nutrient-rich Eucalyptus leaves confines them to certain forest types and the need for multiple large tree hollows for denning confines populations to mature forests. Therefore, different spatial scales need to be considered when determining likelihood of occupancy and habitat suitability for greater gliders. Using high spatial and temporal resolution climate data, I identified that climate is driving greater glider occupancy across the landscape. At a stand scale, I developed new methods of remotely sensing feeding and nesting resources using field sampling and multispectral imagery collected by an unoccupied aerial vehicle (UAV). My findings have important implications for greater glider management and conservation of their habitat and associated forest types. At the landscape scale, I identified climatic refugia that may allow greater gliders to persist into a future under climate change if properly protected. At a stand scale, remotely sensed estimates of feeding and nesting resources may enable forest managers to better retain highly suitable habitat or assess remaining habitat suitability and likelihood of persistence after disturbance. The gathered knowledge enables us to spatially extrapolate field observations and model findings to test how different landscape, resource and disturbance configurations affect population density and persistence, which should further enhance our ability to protect southern greater gliders in southeastern Australia.
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ItemQuantifying fire-severity patterns using optical remote sensing data in temperate eucalypt forests of south-eastern Australia( 2020)Wildfires have significant biophysical and ecological impacts on ecosystems worldwide from local to regional and national scales. The magnitude of such impacts is related to wildfire severity. Recent increases in wildfire occurrence have been associated with climate change, however whether there has also been a change in fire severity remains underexamined in many biomes. Better understanding of fire-severity patterns is required for effective wildfire management, particularly in the fire-prone landscapes of temperate south-eastern Australia, which support a diversity of forests varying in species composition, structure, and post-fire regeneration strategies. Thus, the overarching aims of my Thesis were to accurately quantify wildfire severity at landscape scales and to examine spatial and temporal variation in wildfire severity across a range of forest types in Victoria, south-eastern Australia. To meet the overarching aims, my Thesis involves: (1) identification of optimal optical spectral indices for mapping fire severity across the dominant and most fire-prone forest types in Victoria; (2) a comparison of the accuracy of two different fire-severity mapping approaches, namely single spectral indexing thresholding and machine learning; (3) using the acquired knowledge, the development of fire-severity maps for large (>1000 ha) wildfires occurring in Victoria between 1987 and 2017, and a retrospective analysis of changes in spatial patterns of high-severity fires over that period; and (4) an analysis of the relative importance of four groups of environmental variables (namely fire weather, fuel, topography and climate) as predictors of high-severity fire extent and landscape configuration. My evaluation of remote sensing based spectral indices indicated that the best-performing indices of fire severity varied with forest type and forest functional group, but that there is scope to group forests by structure and fire-regeneration strategy to simplify fire-severity classification in heterogeneous forest landscapes. Results from my comparative analysis confirmed that machine learning outperformed the spectral index thresholding approach for mapping fire severity in most cases, increasing overall accuracy by 11% on a forest-group basis, and 16% on an individual wildfire basis. My results also confirmed that the accuracy achieved with a reduced set of predictor variables that included the previously identified optimal indices of fire severity was not improved by adding more variables. Greater overall accuracies (by 12% on average) were achieved when in-situ data (rather than data from other fires) were used to train the machine-learning algorithm. As such, my study demonstrates the utility of machine-learning algorithms for streamlining a robust fire-severity mapping approach across heterogeneous forested landscapes. Analysis of spatial patterns highlighted that high-severity wildfires in temperate Australian forests have increased in extent and aggregation in recent decades. The total and proportional high-severity burned area increased through time from 1987 to 2017. While the number of high-severity patches per year remained unchanged in that period, the variability in high-severity patch size increased, and high-severity patches became more aggregated and more irregular in shape. Finally, key findings from my models on the relative importance of environmental drivers (climate, fire weather, fuel, and topography) were that fuel type and fire weather were the most important predictors of the extent and configuration of high-severity fires in Australian temperate forests. My Thesis presents one of the most comprehensive analyses of fire-severity patterns from remote sensing data in Australia. My research results support the reliable estimation of wildfire severity from optical images using machine-learning algorithms once optimal spectral indices are identified and when in-situ training data are available for individual fires. Importantly, the quantified shifts in fire regimes across Victoria’s forested landscapes may have critical consequences for ecosystem dynamics, as fire-adapted temperate forests are more likely to be burned at high severities relative to historical ranges, a trend that seems set to continue under projections of a hotter, drier climate in south-eastern Australia. It is therefore critical that forest scientists and land managers continue to acknowledge and quantify changing wildfire-severity patterns so that they are better informed to address the ecological consequences.