Bradley Rowe and Kristin Getter. Michigan State University
Background/Question/Methods Green roofs can provide many ecological, economic, and social benefits including stormwater management, energy conservation, mitigation of the urban heat island, increased longevity of roofing membranes, reduced noise and air pollution, improved aesthetics, and improved human health. They also have the potential to sequester carbon and help mitigate climate change. By sequestering carbon in plants and soils and by lowering demand for heating and air conditioning use, less carbon dioxide will be released from power plants and furnaces. A study was conducted on a roof in East Lansing, MI, where 20 plots were established in April 2007 with a substrate depth of 6.0 cm. In addition to a substrate only control, plots were sown with a single species of Sedum (S. acre, S. album, S. kamtshaticum, or S. spurium). Plant material and substrate were harvested seven times across two growing seasons.
Results/Conclusions Results showed that above- and below-ground carbon storage varied by species with an average of 168 g C•m-2 and 107 g C•m-2, respectively. Substrate carbon content averaged 913 g C•m-2, with no species effect, which represents a sequestration rate of 100 g C•m-2 over what was already present in the soil. The entire extensive green roof system sequestered 375 g C•m-2 in biomass and substrate organic matter. Based on these numbers, if all commercial and industrial roofs in metropolitan Detroit (14,734 hectares) were covered with vegetation similar to this study, then 55,252 metric tons of carbon could be sequestered in the plants and substrates alone (not including avoided emissions). This is similar to removing more than 10,000 mid-sized SUV or trucks off the road for a year. While the embodied energy in the initial green roof system is greater than what is stored in the substrate and plant biomass at any given time, the emissions avoided due to energy savings should pay for those costs in time. When considering the greenhouse gas potential for generating electricity and burning natural gas, these figures translate to 702 g C per square meter of green roof in electricity and natural gas savings combined per year. While these figures depend on climate and green roof design, they nonetheless represent a small but significant potential for sequestering carbon in urban environments. One could expect greater carbon sequestration on roofs with deeper substrates and plants with greater biomass.