Effect of substrate depth on plant community development on a Michigan green roof

Green roofs are gradually being accepted and implemented across North America as they can mitigate many of the negative effects that urban areas have on humans and the environment. As rooftops are filled with living plants, individual species need to be tested for longevity and suitability. Twenty-two plant species including herbaceous perennials, grasses, and sedum were evaluated on a green roof at substrate depths of 4.5, 10, and 20 cm over an eight-year period in East Lansing, Michigan. Plants were irrigated when needed during the first two years, but had to rely on natural rainfall thereafter. All five species of sedum were still present at the end of the eight-year period, but only seven of the 17 herbaceous perennials and grasses survived. Of the herbaceous perennials and grasses, Allium cernuum was by far the dominant species in both the 10 and 20 cm deep zones. Sporobolus heterolepis and Koeleria macrantha also established themselves in the 20 cm zone, but all other species were either only present in trace amounts or had completely disappeared. Substrate moisture was probably the major factor in terms of species survival and a drought during 2016 was likely the cause for the large loss of species. Deeper substrates supported a larger variety of species, but plant species diversity decreased over time at all substrate depths. This study emphasizes the fact that if herbaceous perennials and grasses are to be successful on green roofs, then irrigation may be critical to mitigate drought and maintain species diversity. It also points out the importance of long-term studies to accurately evaluate green roof plant communities and provides the framework for a continuation of the same study in other areas of the world.