Growing plants on rooftops is an old concept that has evolved from sod roofing to roof gardens and today's lightweight extensive green roofs. Green roofs serve as partial solutions for many problems created by the built environment, and have become an important factor for urban planning in modern Europe (Brenneisen, 2004). A relatively new concept in North America, the known benefits provided by green roofs have initiated research to promote the integration of green roofs into current and future building developments.
Despite their shallow substrates, extensive green roofs have numerous benefits such as reducing the volume of runoff entering stormwater infrastructure (DeNardo et al., 2005; Liesecke, 1998; VanWoert et al., 2005a). Indeed, stormwater management is the main reason green roofs are so widely implemented in Germany (Keeley, 2004). Green roofs also improve air quality by trapping dust and airborne particles, and can mitigate the urban heat island effect by moderating ambient temperatures (Dimoudi and Nikolopoulou, 2003). Through direct shade, evapotranspiration, and insulation by plants and substrate, green roofs can reduce the demand for air conditioning and hence also greenhouse gas emissions (Liu and Baskaran, 2003). Finally, green roofs can offer ecological compensation for the building footprint by providing habitat and refuge for birds and invertebrate life (Brenneisen, 2004).
Extensive green roofs are low-maintenance, low-input systems containing between 50 and 200 mm of substrate, and are typically planted with drought-tolerant perennials (Kolb and Schwarz, 1999). Suitable plants for extensive green roofs are shallow-rooted, low-growing perennials that are tolerant of heat, sun, wind, drought, salt, insects, and disease. The factors that influence green roof plant selection, such as substrate depth and local climate, can likewise determine the lifespan of perennials (White and Snodgrass, 2003).
Green roofs with less than 100 mm of substrate and lacking irrigation systems support mainly stonecrops and few other perennials (Liesecke, 1998), especially in regions with summer droughts. Many stonecrops avoid desiccation in drought by switching to crassulacean acid metabolism (CAM) and/or by C3-CAM shifting (Ting, 1985). CAM plants take up carbon dioxide (CO2) at night and then, with closed stomata, fix carbon via the CAM photosynthetic pathway by day. Some CAM plants, such as white stonecrop, are facultative and can switch from the C3 to the CAM pathway in response to drought, salt stress, or changes in the duration of photosynthesis (Sayed et al., 1994).
With increasing substrate depths and/or supplemental irrigation, green roof plant selection broadens. In temperate climates with hot summers and cold winters, stonecrops are perfect for substrate depths of less than 100 mm. However, in deeper substrates, 200 mm for example, stonecrops do not benefit from additional water as do other species (Dunnett and Nolan, 2004). By contrast, many herbaceous perennials perform well only when irrigation is provided, even at greater substrate depths (Dunnett and Nolan, 2004; Durhman et al., 2007; Monterusso et al., 2005; Rowe et al., 2006). While 80 to 100 mm of substrate will support stonecrops and grasses, 100 to 120 mm can support a stonecrop-grass-herbaceous community, and 120 to 150 mm can support a grass-herbaceous community with stonecrops interspersed (Liesecke, 1998).
Plant lists from European green roofs are often used in eastern North America, but evaluation of these species in the Americas is limited. In some cases, these selections are based purely on their success on European green roofs (E. Snodgrass, personal communication). In a German study of substrate type, depth, and drought, white stonecrop was found to remain photosynthetically active after 100 d without water (38 d of which had temperatures higher than 18 °C) (Lassalle, 1998). Studies in Michigan and Maryland have observed similar responses for white stonecrop, while also testing native perennials in various green roof scenarios (Durhman et al., 2007; Monterusso et al., 2005; Murphy and Snodgrass, 2006). Using native plants can add bioregional value to green roof projects, with potential benefits for local ecosystems and food webs.
This study was designed to evaluate the effects of substrate type and three depths on the establishment and early growth of five plants that are frequently found on green roof plant lists in North America. Two main hypotheses anticipated that 1) early drought is more harmful for plants grown in shallow (30 mm) rather than deeper (120 mm) substrate depths, and 2) plants that survived early drought conditions would produce less shoot biomass than those subjected to late drought. By observing survival and quantifying shoot biomass, the objective of this study was to determine the conditions and combination of factors in which these popular green roof plants perform the best.
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