Short communicationThe effects of substrate depth heterogeneity on plant species coexistence on an extensive green roof

- We created green roof plots with homogeneous and heterogeneous substrate depths.
- We used two plant species associated with different optimal soil depths.
- Heterogeneous plots showed greater total plant coverage.
- Substrate depth heterogeneity decreased reciprocal invasion of species' patches.
- Substrate depth heterogeneity may encourage green roof plant species coexistence.

Green roofs are often planted with mixtures of plant species. Studies of green roof plant community composition over time show that species diversity often declines, in part due to competition between plant species. The incorporation of substrate depth heterogeneity into green roof designs is expected to increase habitat heterogeneity, and could reduce interspecific competition among plants, but this has not been tested empirically. Two species with contrasting responses to substrate depth and water availability, Festuca rubra and Sedum acre, were planted in a rooftop substrate depth heterogeneity experiment. There were a total of four treatments: three homogeneous treatments (substrate depth 5 cm, 10 cm, and 15 cm) and one heterogeneous treatment (patches with substrate depths of 5 cm and 15 cm), with the same substrate volume and average depth as the 10 cm homogeneous treatment. By the end of the study period there was little difference in the ratio of species' cover between the 10 cm and 5/15 cm treatment. However, there was significantly less spread of each species into areas planted with the other species in the 5/15 cm treatment compared to the 10 cm treatment, and the 5/15 cm (heterogeneous treatment) had significantly greater overall plant cover. The results suggest that, while the effects are subtle, soil depth heterogeneity could allow coexistence of species associated with different conditions for longer than homogeneous conditions, and could result in greater plant species diversity in green roof ecosystems.