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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 …