Green Roofs as Urban Ecosystems: Ecological Structures, Functions, and Services

摘要

Green roofs (roofs with a vegetated surface and substrate) provide ecosystem services in urban areas, including improved storm-water management, better regulation of building temperatures, reduced urban heat-island effects, and increased urban wildlife habitat. This article reviews the evidence for these benefits and examines the biotic and abiotic components that contribute to overall ecosystem services. We emphasize the potential for improving green-roof function by understanding the interactions between its ecosystem elements, especially the relationships among growing media, soil biota, and vegetation, and the interactions between community structure and ecosystem functioning. Further research into green-roof technology should assess the efficacy of green roofs compared to other technologies with similar ends, and ultimately focus on estimates of aggregate benefits at landscape scales and on more holistic cost-benefit analyses.nnBuildings change the flow of energy and matter through urban ecosystems, often causing environmental problems. These problems can be partially mitigated by altering the buildings' surficial properties. Roofs can represent up to 32% of the horizontal surface of built-up areas (Frazer 2005) and are important determinants of energy flux and of buildings' water relations. The addition of vegetation and soil to roof surfaces can lessen several negative effects of buildings on local ecosystems and can reduce buildings' energy consumption. Living, or green, roofs have been shown to increase sound insulation (Dunnett and Kingsbury 2004), fire resistance (Köhler 2003), and the longevity of the roof membrane (Porsche and Köhler 2003). They can reduce the energy required for the maintenance of interior climates (Del Barrio 1998), because vegetation and growing plant media intercept and dissipate solar radiation. Green roofs can also mitigate storm-water runoff from building surfaces by collecting and retaining precipitation, thereby reducing the volume of flow into storm-water infrastructure and urban waterways. Other potential benefits include green-space amenity, habitat for wildlife, air-quality improvement, and reduction of the urban heat-island effect (Getter and Rowe 2006). Architects have applied green-roof technology worldwide, and policymakers and the public are becoming more aware of green-roof benefits. Although green roofs are initially more expensive to construct than conventional roofs, they can be more economical over the life span of the roof because of the energy saved and the longevity of roof membranes (Porsche and Köhler 2003).nnAlthough green roofs represent a distinct type of urban habitat, they have been treated largely as an engineering or horticultural challenge, rather than as ecological systems. The environmental benefits provided by green roofs derive from their functioning as ecosystems. The first goal of this article is to describe the history and components of living-roof ecosystems; the second is to review the ways in which the structure of a green roof—including vegetation, growing medium, and roof membrane—determines its functions.