Associate Professor The University of Melbourne, Australia
Abstract: The plant functional traits of species growing on green roofs are major factors determining the ecosystem services green roofs provide. In addition, flowering plants which can quickly achieve high cover through rapid growth and high shoot biomass are desirable for aesthetic reasons and to meet green roof construction guidelines. Annual species generally escape drought by completing their life-cycle before the onset of dry conditions and typically have an acquisitive strategy with ‘fast’ traits, rapid growth and flowering. However, annuals grow successfully in different habitats and climates and therefore, 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. 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; one group of annuals showed faster flowering with faster growth and later flowering with slower growth (species at the higher end of relative growth rate spectrum), while the other group had faster flowering with slower growth and later flowering with faster growth. 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 resistant (drought escapers), as earlier flowering and higher SLA are both evolutionary responses of annual species to escape stress. Fast growing and resource acquisitive plants could be more desirable on green roofs as they have higher water use and therefore higher stormwater mitigation ability. In addition, it is preferable to select species with a range of time to flowering onset and SLA, in order to achieve a long-term flowering community with different levels of drought resistance.
