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.