Changes in plant community composition and functional plant traits over a four-year period on an extensive green roof

Constructed ecosystems like green roofs are increasingly deployed in cities to mitigate issues associated with urbanization. To minimize the cost of green roof infrastructure, shallow growing media (substrate) for plants is often employed. Spatial heterogeneity in substrate depth has also been hypothesized to allow greater plant species diversity without adding to the weight. Stress and competition can change green roof plant communities after initial planting, but little is known about the long-term effects of spatial heterogeneity on vegetation composition and functional characteristics. Our goal was to determine how green roof plant communities and, in turn, functional plant traits, change over time in response to environmental stress and substrate heterogeneity. This four-year experiment used four substrate depth treatments: three with homogenous substrate depths of 5 cm, 10 cm, and 15 cm, and one treatment with a heterogenous substrate depth that varied between 5 cm and 15 cm (5/15 cm). The volume of the substrate in the 10 cm treatment and 5/15 cm treatment was equal. By the end of this four-year experiment, variation occurred between treatments for community composition and functional diversity, with the greatest species richness observed in the least stressful treatment (15 cm) and the greatest functional diversity and evenness observed in the most stressful treatment (5 cm). Additionally, each treatment had lower functional diversity after four years compared to the initially planted community. When the heterogenous 5/15 cm treatment was compared to the homogenous 10 cm treatment, there were no differences in the number of plant species, but the treatments contained two distinct plant communities. Furthermore, the 5/15 cm treatment supported taller species, a trait value associated with reduced stormwater runoff and substrate temperature. This finding indicates that creating green roofs with a heterogenous substrate depth could improve overall green roof function without increasing roof weight. Substrate depth can be manipulated by green roof designers to alter vegetation characteristics, but species and functional diversity showed opposite trends along the depth gradient.