Rooftop ecology: Using Sedum as a nurse plant to increase plant diversity on a green roof

Background/Question/Methods

In this work, we examine the ability of Sedum to facilitate the growth of plants that are attractive to insect pollinators, thereby enhancing the value of green roofs by providing additional habitat for these species. On traditional green roofs, potential plant diversity and habitat value is constrained by high levels of abiotic stress, including water deficit and high temperatures. Among the plants able to survive these challenging conditions is Sedum (Crassulaceae). While the direct value of Sedum to pollinators is limited due to their short flowering time, we hypothesize that these species may provide an indirect benefit to pollinators by facilitating the growth of long-flowering species that would otherwise be unable to grow in such a harsh environment.  To examine this hypothesis, we compared the growth of Agastache rupestris (Lamiaceae) and Asclepias verticillata (Asclepiadaceae) grown alone and with Sedum album on the Tisch Library Green Roof at Tufts University in Medford, Massachusetts.  
Results/Conclusions

In areas of the green roof with soil only, soil reached temperatures of up to 40 C.  Soil in areas planted with Sedum species was often 6-8 C cooler.  S. album did not increase survival of either species because both species had nearly 100% survival when grown with and without S. album.  However, after 20 days with minimal rain, A. rupestris grown with S. album maintained more aboveground plant cover than conspecifics grown alone.  After three months (June-August 2008), S. album had a net negative impact on biomass accumulation of A. rupestris.  Results from A. verticillata are less conclusive, possibly due to a heavy aphid load which stunted growth and prevented flowering.  These results suggest that S. album may facilitate survival of neighboring plants during dry periods, but may act as a competitor when water is abundant.  By understanding the ecology of these plants, we will be better able to maximize the habitat value of green roofs.