Species interactions in green roof vegetation suggest complementary planting mixtures

Facilitation between species could be used to increase the number of plant species that can survive on an extensive green roof. Green roofs are associated with high substrate temperatures and water loss which can lead to plant death. Therefore, species that are known to cool substrate temperatures and reduce water loss may be able to act as facilitators. Three drought tolerant, mat-forming species native to Nova Scotia were tested. Species included Cladonia spp. (lichen), Polytrichum commune (acrocarpous moss), and Danthonia spicata (bunchgrass). The target species was the forb Solidago bicolor. Additional neighbour treatments included an artificial plant, conspecific neighbours (S. bicolor surrounded by 8 S. bicolor), and a no-neighbour control. This study was conducted over a two year period in green roof modules. Overall, none of the neighbour treatments appeared to facilitate the growth of S. bicolor species. However, the S. bicolor in both the moss and lichen treatments had similar growth measurements to the control, likely due to a mulching effect, an indication that these species can successfully co-exist. Additionally, the substrate temperature in the lichen and moss treatments was significantly cooler than the control. The bunchgrass treatment had a net competitive interaction with S. bicolor. However since no S. bicolor died in this treatment the smaller stature of the S. bicolor in these treatments may improve drought tolerance. Overall, the incorporation of functional diversity, especially varied growth forms, increases the diversity of green roofs potentially improving the resilience and performance of green roof systems over the long term.