Sedum —Annual plant interactions on green roofs: Facilitation, competition and exclusion

Green roofs are becoming a standard component in environmentally aware modern urban planning. In an attempt to maximize the benefits from these roofs, the choice and management of the flora on the roofs remain understudied, especially in non-temperate climates. While the perennial succulent, Sedum, is commonly planted on green roofs, the use and integration of additional plant forms can potentially create more biodiverse roofs. Here, we conducted an experiment to evaluate the interactions between the local Sedum sediforme and local annuals under a Mediterranean climate. We followed the development of S. sediforme and annuals and monitored the invasibility of the green roof over three years. We additionally monitored the substrate temperature, moisture, conductivity, pH, nitrate and percent organic matter. In an attempt to increase intraspecific competition, which is predicted to reduce interspecific competition, S. sediforme were initially arranged in two different dispersion patterns − uniform and clumped. Three levels of S. sediforme (absent, uniform and clumped) were crossed with the presence or absence of twenty species of annuals, introduced as seeds. Sedum sediforme dispersion did not differentially affect annuals, but both dispersion patterns of S. sediforme reduced biomass and diversity of annuals during the third year. Annual presence reduced clumped dispersion of S. sediforme cover and inflorescences throughout all three years, but increased total chlorophyll levels in S. sediforme leaf tissue during the third year. Sedum sediforme also reduced abundance and biomass of colonizing plants. Sedum sediforme and annuals each showed valuable enhancements to some of the most important green roof functions. Annuals increased nitrate fixation in the substrate which may have explained the increased levels of chlorophyll and carotenoids in S. sediforme in these treatments. Finally, S. sediforme interacted with annuals to reduce substrate temperature and presence of annuals reduced substrate moisture thereby improving storm-water abatement capacity. We conclude that the combination of annuals and S. sediforme in Mediterranean green roofs will improve green roof function as an important component in future sustainable urban planning.