An ecological approach to green roof ecosystem functioning

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The long-term goal of my research program is to determine how the structure of ecological communities affects the provisioning of ecosystem services. Cities are characterized by hard surfaces, leading to problems with excessive stormwater runoff and urban heat islands. Green roofs consist of a vegetation layer planted in a shallow growing medium and can address multiple urban environmental problems. Past work shows that increasing plant species diversity on green roofs can reduce stormwater runoff, but many other functions might also be optimized by carefully selected species mixtures. Short-term objectives 1. Predict which two-species mixtures will outperform single species plantings for a variety of ecosystem functions in green roofs. 2. Determine the role of spatial environmental heterogeneity on species coexistence and ecosystem functioning in green roofs. 3. Determine whether a history of growing together in the same place allows populations of two species a higher likelihood of long-term coexistence and increased ecosystem functioning. Approach My students and I will conduct green roof and greenhouse experiments comparing species mixtures with monocultures. For each single species planting (monoculture) and species mixture we will quantify a suite of benefits, including thermal functions that reduce building energy consumption, hydrological functions that mitigate stormwater runoff, habitat provisioning for invertebrates, carbon storage, and resilience against drought. We will compare mixtures of species with contrasting functional traits to those exhibiting similar traits. To test whether a spatially heterogeneous environment can support more species and higher levels of ecosystem functioning, we will compare green roof systems with variation in soil depth to homogeneous systems with the same total amount of soil. We will also examine the role of the spatial scale of planted species diversity in creating environmental variability, which in turn may increase the value of green roofs as habitat for invertebrates. To determine whether a history of coexistence influences how populations of two species interact, we will select two widespread species that live together in the same habitat (rock barrens) and collect seed from each species pair at three sites in different regions. These will be grown in a common green roof environment where we will examine coexistence and ecosystem services in experiments pairing each combination of populations. This research will lead to a tangible improvement of green roof performance and identify fundamental principles for engineering optimal species mixtures. This work will provide a stimulating training platform for two PhD students, four MSc students, and at least six undergraduate honours students.