Using plant functional traits and life strategies of annual species to inform green roof plant selection

Abstract

Functional traits of plant species growing on green roofs can determine their delivery of ecosystem services and help guide plant selection. Flowering plants are desirable on green roofs for aesthetic reasons and plants which quickly achieve high coverage through rapid growth and high biomass help to meet green roof construction guidelines. While most extensive green roof plants are perennial succulents due to their drought tolerance, annual species which escape drought by completing their life-cycle before drought, could be advantageous for green roofs as they also typically have acquisitive or ‘fast’ traits associated with rapid growth and flowering. To quantify plant strategies of annuals and develop a trait-based approach for selecting annuals for green roofs, we undertook a glasshouse experiment with 18 understudied Australian annual plant species to investigate relationships among traits related to drought resistance and resource acquisition (specific leaf area, leaf dry matter content and leaf thickness), competitiveness (plant height) and reproductive ability (seed mass and germination attributes). We also explored how these traits relate to plant performance metrics including growth rate, shoot biomass and flowering time. Species which were more acquisitive (taller species with bigger and thinner leaves and greater seed mass) were more competitive, faster growing and produced more biomass. The annual species had two different strategies in terms of flowering time and relative growth rate; in one group flowering time decreased with an increase in growth rate (earlier flowering with faster growth), while in the other group, flowering time increased with growth rate (later flowering with faster growth). There was also a trade-off between flowering time and specific leaf area (SLA) and faster flowering species with higher SLA were considered less drought resistant (drought escapers). Faster growing and resource acquisitive plants could be more desirable on green roofs as they will achieve cover rapidly during the wetter months and have higher stormwater mitigation. However, in dry periods it is preferable to select species with a range of time to flowering and SLA, to achieve a longer flowering community with different levels of drought resistance.