Coarse- and fine-scale plant species mixtures to optimize green roof ecosystem functioning

Green roofs provide ecosystem services in cities. The role of plant species choice to maximize green roof benefits is now a major research area for many horticultural scientists and urban ecologists. Past work indicates that increasing species diversity on green roofs can improve ecosystem functioning. These studies were carried out in modular, replicated extensive green roof systems (substrate depth <15 cm) and use unplanted modules as controls. It was shown that the positive effects of plant diversity are related to plant-plant interactions; combining sets of monocultures did not have the same effect on ecosystem functions. On green roofs, where many plant species are often deployed in monospecific patches, the implication is that while roof-level plant diversity may be high the improvement of ecosystem services that related to growth, biomass production and canopy characteristics may only occur when the species are planted in mixtures at fine spatial scales. In contrast, spatial heterogeneity in soil and canopy characteristics was higher among monocultures of 9-15 species compared with fine-scale mixtures of the same species. This is likely due to the species-specific effects on the local environment; in fine-scale mixtures where multiple species share space in close proximity, the effects of any one species are likely diluted. A testable hypothesis is proposed that different ecosystem services should be maximized by mixed plantings at different spatial scales. The "mixture of monoculture patches", which is already adopted by many horticulturists, could maximize habitat heterogeneity so that a green roof designed in this way might increase the number of associated animal species using the roof. In contrast, fine-scale mixtures seem to maximize functions related to biological production rather than resource heterogeneity. Overall, this work emphasizes the key role of ecologists in urban horticulture and the benefits of cross-disciplinary collaboration.