Biodiversity could be the secret to creating a successful “green roof” according to new research published in the April issue of Annals of Botany. The research, led by Scott MacIvor and Dr. Jeremy Lundholm at Saint Mary’s University, compared various dryland and wetland plant species used to make ‘green roofs’, plant-covered roofs that can insulate buildings. The team found that the challenging conditions on roofs are best tackled by a mix of dryland species but adding wetland species to the mix could improve water capture on roofs.
Scott MacIvor said: “Retrofitting existing buildings with green roofs can be difficult. They often need to be lightweight and that means minimizing the soil depth on the roof. This is a very different environment than ground level, where the soil is often much deeper. This means you can’t always just plant local species and expect them to thrive.”
To test which plant species performed best, the team created modular green roofs with a mix of plant types adapted to rocky, exposed environments. In some modules they planted just a single species, either a grass, shrub or berry suited to dryland or wetland habitats. In other modules they planted a mix of dryland plants, or wetland plants. Finally they mixed various combinations of dryland and wetland plants in modules to see if a combination of plants worked better. Then they observed the results over two years.
Scott MacIvor said: “What we found is that survival and plant cover were highest when a mix of dryland plants were used. Dryland plants can cope with periods of drought better than wetland plants, so they’re more at home on an exposed rooftop. Dryland plants provided better insulation, reducing the need for air conditioning in summer and heat in winter.
“However what surprised us was that combinations of dryland and wetland species were among the best surviving treatments, and highest for water capture. We don’t know why that is yet. One possibility is that dryland and wetland species together have complementary water uptake strategies, thereby improving conditions for one another. Designing green roofs with biodiversity in mind may lead to other synergisms between plant groups that increase green roof longevity and habitat value”.
Performance of dryland and wetland plant species on extensive green roofs by J. Scott MacIvor, Melissa A. Ranalli and Jeremy T. Lundholm. doi: 10.1093/aob/mcr007