An assessment of pollen limitation on Chicago green roofs

Abstract

With increasing urbanization and creation of novel habitat types, green roofs can provide habitable space for many species. To date, most research on green roofs has focused on minimizing environmental impacts of buildings and little is known about the ecological services they may provide. Previous research has found a deficiency of pollinating bees on green roofs, which could result in pollen limitation, poor seed production and reproductive failure of many plant species requiring bee pollination. This study aims to determine whether pollination services on modern green roofs are sufficient for these novel habitats to function sustainably. Nine native Illinois prairie plant species and their pollinator communities were studied on green roofs and ground-level locations in the Chicago area. Pan traps were used to assess pollinator communities and supplemental pollination treatments were used to evaluate pollen limitation. All species showed significantly reduced seed set when pollinators were excluded but few significant differences were observed between supplemental and open pollination treatments. Seed set differed by habitat type in that green roofs had a higher overall mean percent maximum seed set compared to ground-level sites. Our results support previous studies, showing lower numbers and diversity of bees on green roofs compared to the ground level. Together, these data suggest that although green roofs contain a smaller and less diverse community of pollinators, the insects that are present provide sufficient pollinator services for many native plants. This study therefore supports the use of biotically pollinated native forbs in future green roof design.