Green roofs are becoming increasingly prolific in the urban landscape, with the goal of minimizing impermeable surfaces to alleviate stormwater runoff burden. Aside from water runoff, the construction of green roofs can also provide additional benefits in the form of ecosystem services, such as water and air purification, biological habitat, food provisioning, and building insulation. The number and quality of services provided depends heavily on the growing media and the biological interactions that occur within it in the form of plants, microbes, and invertebrates. This study is an exploration of the relationship between abiotic and biotic components of green roofs in the Midwest. In the summer of 2017, soil and vegetation samples, as well as roof characteristics, were taken from 44 different roofs across Chicago, Minneapolis, and Cleveland. The soil was analyzed for pH, soil texture, C/N ratios, and available P, Fe, Mg, Ni, Zn, Cd, and Pb content. Plant species were identified and plant species richness, coverage, and proportion of weed species were determined. Statistical analyses were performed to examine the relationship between soil properties and plant species.
Results/Conclusions
This study represents one of the largest green roof exploratory surveys to date. Roofs of a wide range of ages (1-44 years), types (extensive, semi-intensive, and intensive), and sizes (150-500,000 ft2) were sampled. Surprisingly, plant species richness had no statistically significant relationship to roof size, age, type, or management regime (fertilizer application or irrigation). This pattern is contrary to what we would expect in natural ecological conditions, where larger or deeper areas of soil support higher amounts of plant diversity. Compared to the other two cities, green roofs located in Cleveland had the largest concentrations of available metals in the soil and the lowest pH, which was also associated with lower plant species richness on average. Chicago and Minneapolis, two cities with more established green roof markets, had greater plant diversity and more similar soil conditions to each other, although geographic location of the roof did have a significant impact on the roof plant community, indicating possible effects of climate or differences in the surrounding urban landscape. Additional research on the soil microbial communities and the soil conditions of the surrounding ground-level urban landscape is currently underway to determine their impact on green roof plant communities.