Green roofs are associated with a number of different ecosystem services, leading to an increasing demand for their construction. The majority of extensive green roofs, which have a substrate depth ≥ 15cm, are typically constructed with succulent species from the genus Sedum, and rarely include spatial heterogeneity into their design. However, previous research has demonstrated the benefits of species diversity and green roof spatial heterogeneity. Here we describe two separate green roof studies investigating the impact of spatial heterogeneity on survival and growth of green roof vegetation. In the first study we compare species growth and diversity in green roof plots with a heterogeneous soil depth and a homogenous soil depth. In the second study we characterize the role of soil heterogeneity and surface features (logs and pebbles piles) in altering substrate conditions and discuss the resulting impacts on seedling survival and species richness.
In the first study we found that soil depth heterogeneity led to increased growth, biomass and species diversity. However, more time is necessary to see if this trend continues. In the second study we found that areas on the green roof with deep soil and surface features supported greater seedling density and species richness, while pebble piles delayed seedling mortality and species loss during drought. However, we did not observe differentiation of seedling community composition. Overall, these two studies indicate that minor alterations to green roofs, such as variation in soil depth and/or inclusion of surface features, have the potential to better support green roof vegetation. Further research is needed to understand the role of these design modifications in altering long-term plant diversity.