Carbon Sequestration Potential of Extensive Green Roofs KRISTIN L. GETTER,* ,† D. BRADLEY ROWE, † G. PHILIP ROBERTSON, ‡ BERT M. CREGG, † AND JEFFREY A. ANDRESEN § Departments of Horticulture and Geography, Michigan State University, East Lansing, Michigan 48824, and Department of Crop and Soil Sciences, W.K. Kellogg Biological Station, Michigan State University, Hickory Corners, Michigan 49060 Received May 26, 2009. Revised manuscript received August 10, 2009. Accepted August 17, 2009. Two studies were conducted with the objective of quantifying the carbon storage potential of extensive green roofs. The first was performed on eight roofs in Michigan and four roofs in Maryland, ranging from 1 to 6 years in age. All 12 green roofs were composed primarily of Sedum species, and substrate depths ranged from 2.5 to 12.7 cm. Aboveground plant material was harvested in the fall of 2006. On average, these roofs stored 162 g C · m -2 in aboveground biomass. The second study was conducted on a roof in East Lansing, MI. Twenty plots were established on 21 April 2007 with a substrate depth of 6.0 cm. In addition to a substrate only control, the other plots were sown with a single species of Sedum ( S. acre, S. album, S. kamtshaticum, or S. spurium). Species and substrate depth represent typical extensive green roofs in the United States. Plant material and substrate were harvested seven times across two growing seasons. Results at the end of the second year showed that aboveground plant material storage varied by species, ranging from 64 g C · m -2 ( S. acre) to 239 g C · m -2 ( S. album), with an average of 168 g C · m -2 . Belowground biomass ranged from 37 g C · m -2 ( S. acre) to 185 g C · m -2 ( S. kamtschaticum) and averaged 107 g C · m -2 . 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 the 2 years of this study. The entire extensive green roof system sequestered 375 g C · m -2 in above- and belowground biomass and substrate organic matter. Introduction Establishing green roofs, or vegetated roofs, can improve stormwater management (1-4), conserve energy (5, 6), mitigate urban heat island effects (7), increase longevity of roofing membranes (8), improve return on investment compared to traditional roofs (9), reduce noise and air pollution (10, 11), increase urban biodiversity (12, 13), and provide a more aesthetically pleasing environment (14, 15). Green roofs are either “intensive” or “extensive”. Intensive green roofs may include shrubs and trees and appear similar to landscaping found at natural ground level. As such, they require substrate depths greater than 15 cm and have “intense” maintenance needs. In contrast, extensive green roofs consist of herbaceous perennials or annuals, use shallower media depths (less than 15 cm), and require minimal maintenance. Due to building weight restrictions and costs, shallow substrate extensive green roofs are more common than deeper intensive roofs and will be the focus of this study. Although green roofs are often adopted for energy savings and heat island mitigation, rarely has this technology been promoted for its ability to mitigate climate change. By lowering demand for heating and air conditioning use, less carbon dioxide is released from power plants and furnaces. Sailor (6) integrated green roof energy balance into Energy Plus, a building energy simulation model supported by the U.S. Department of Energy. This simulation found a 2% reduction in electricity consumption and a 9-11% reduction in natural gas consumption. Based on a model of a generic building with a 2000 m 2 of green roof, these annual savings ranged from 27.2 to 30.7 GJ of electricity saved and 9.5 to 38.6 GJ of natural gas saved, depending on climate and green roof design. When considering the national averages of CO 2 produced for generating electricity and burning natural gas (16, 17), these figures translate to 637-719 g C per m 2 of green roof in electricity and 65-266 g C per m 2 of green roof in natural gas each year. Another 25% reduction in electricity use may additionally occur due to indirect heat island reduction achieved from large-scale green roof implementa- tion throughout an urban area (18). Green roofs may also sequester carbon in plants and soils. Photosynthesis removes carbon dioxide from the atmosphere and stores carbon in plant biomass, a process commonly referred to as terrestrial carbon sequestration. Carbon is transferred to the substrate via plant litter and exudates. The length of time that this carbon remains in the soil before decomposition has yet to be quantified for green roofs, but if net primary production exceeds decomposition, this man- made ecosystem will be a net carbon sink, at least in the short term. However, this ecosystem will not likely sequester large amounts of carbon due to the types of species used and shallow substrate. Many species used on extensive green roofs exhibit some form of Crassulacean acid metabolism (CAM; 14). CAM photosynthesis operates by opening stomata during the night to uptake CO 2 and storing it in the form of an organic acid in the cells’ vacuoles. During the following daylight period, stomata remain closed while stored organic acid is decarboxylated back into CO 2 as the source for the normal photosynthetic carbon reduction cycle (19). When operating in CAM mode, rates for daily carbon assimiliation are 1/2 to 1/3 that of non-CAM species (20). The goal of this research was to evaluate the intrinsic carbon storage potential of extensive green roofs and the effect of species selection on carbon accumulation. Two studies were conducted in the United States to meet these objectives. Materials and Methods Study 1. Aboveground biomass was determined on eight Sedum based extensive green roofs in Michigan and four roofs in Maryland (Table 1). Roofs ranged from 1 to 6 years in age and from 2.5 to 12.7 cm in substrate depth. Above- ground biomass was sampled in quadruplicate on each roof with a 13.0 cm ring during the fall of 2006 (see Table 1 for specific dates). Any aboveground biomass that was within the ring was clipped at substrate level, placed in paper bags, and dried in an oven at 70 °C for 1 week. Samples were then * Corresponding author e-mail: smithkri@msu.edu. † Department of Horticulture, Michigan State University. ‡ Department of Crop and Soil Sciences, Michigan State University. § Department of Geography, Michigan State University. Environ. Sci. Technol. 2009, 43, 7564–7570 7564 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 19, 2009 10.1021/es901539x CCC: $40.75  2009 American Chemical Society Published on Web 08/25/2009 Downloaded by MICHIGAN STATE UNIV on September 30, 2009 | http://pubs.acs.org Publication Date (Web): August 25, 2009 | doi: 10.1021/es901539x