As researchers continue to investigate the effects of various green roof components and system performance (Berndtsson et al., 2006; Getter et al., 2007; Getter and Rowe, 2008; Mentens et al., 2006; Molineaux et al., 2009; Rowe et al., 2006; Teemusk and Mander, 2007; VanWoert et al., 2005a), the total green roof area in North America increases (Erlichman and Peck, 2013). As the layer that supports the biological function of any green roof system, GRSs retain water for plant growth, allow air movement for root gas exchange, offer stability and structure for root anchoring, and provide nutrients for plant uptake. Although substrates retain a proportion of any rainfall (buffering immediate storm water runoff), plants provide the additional ecosystem service of storm water removal via transpirational water loss. In this way, water held in the GRS is taken up through the roots and cycled directly back into the atmosphere as water vapor, decreasing the water content of the GRS and allowing water retention from the next rain event. Although water does leave the substrate through evaporative losses, Starry et al. (2014) demonstrated that with the exception of large (>62.5 mm) rain events, green roof platforms planted in P. kamtschaticus in the mid-Atlantic region were 30% more efficient at removing storm water through ET compared with evaporation alone from unplanted platforms. This contradicted VanWoert et al.’s (2005a) conclusion that brown or unplanted experimental roof platforms were as effective at evaporating storm water as planted experimental platforms. Given plants’ substantial influence on ET water loss from a green roof system, the effects of GRS composition on plant growth and ET should be investigated to enhance storm water retention predictions and inform green roof system design.
In general, any soilless substrate should be consistent in composition, free of pathogens and weed seed, and provide adequate water, air, and nutrients for plant survival and growth (Handreck and Black, 2007). In addition to these properties, GRS must also have an adequate bulk density to resist wind uplift without surpassing roof structural live load limits for the roof; they must also be engineered to rapidly drain to avoid ponding. In the early 19th century, green roofs in Berlin did not use engineered media; rather, construction rubble was spread over tar paper roofs and the living systems developed overtime (Kohler and Poll, 2010). Modern GRS composition is largely based on recommendations in the Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau (FLL), the German landscape industry’s guidelines for the design, planting, and maintenance of green roof systems. The FLL makes recommendations for particle size distribution and organic content as well as specific physical properties such as water holding capacity, bulk density, and total porosity (FLL, 2008).
Beyond the basic FLL recommendations, GRS composition varies internationally and regionally, usually due to raw material availability; however, the FLL recommendations have been adopted by municipalities and public entities around the world and applied to green roof components not considered in or by the FLL. North American GRS are largely composed of manufactured lightweight aggregates—usually slate, shale, or clay that has been kiln fired to create expanded mineral particles (Ampim et al., 2010). Particles of varying diameter are mixed together to achieve appropriate particle size distribution and physical properties such as water holding capacity, total porosity, and bulk density (Handreck and Black, 2007). Although the North American green roof industry largely uses manufactured aggregate for GRS, research from other countries indicates efforts to use lower carbon recycled or natural materials for the inorganic component of GRS. For example, New Zealand GRSs are largely composed of naturally occurring zeolite and volcanic rock (Fassman-Beck et al., 2013). A study based in northern Italy used a blend of locally available naturally occurring mineral materials as the extensive GRS (Nardini et al., 2012). Molineaux et al. (2009) reported in the United Kingdom, broken brick is a commonly used mineral portion of extensive GRSs. In Sweden, extensive GRSs were traditionally natural soil amended with naturally occurring lava or scoria, and Emilsson (2008) reported the results of a study using broken roof tiles as a component of extensive GRSs as an alternative to those mined minerals.
The organic content of GRS typically varies depending on the design intent of the green roof system; however, most ready-to-plant blends roughly follow the FLL guidelines of ≤ 65 g/L (FLL, 2008). This gravimetric recommendation is based on verification via loss on ignition. However, in practice, horticultural substrates are generally mixed volumetrically. The FLL guideline is a weight per volume metric—a value that could therefore vary widely depending on the bulk density of the blend if it is mixed volumetrically, as a typical horticultural substrate. Griffin (2014) demonstrated that given the differences in densities of the mineral and organic portions of GRS, a substrate could have up to 40% OM (volumetrically) and still fall within the FLL guidelines. Since OM provides cation exchange and water holding capacity, varying from the organic content of a GRS could have significant impacts on substrate water holding capacity, plant growth, and ET.
Green roofs present a unique engineered environment for plants—a thin substrate layer requires a fibrous, nonaggressive root system to avoid compromising the integrity of the waterproof membrane of the roof; the reduced rooting zone also limits the volume of water that can be stored after rain events. Green roof plants must tolerate extreme diurnal temperature ranges, direct sun exposure, and high wind exposure. All these factors combine to provide a drought prone system even in climatic areas with relatively consistent rainfall. Although green roofs are most often found in urban areas, the environmental challenges they present to plants are in many ways comparable to deserts or rocky outcroppings, and the plants that are most often used in extensive green roof systems are succulent species that have evolved physiological responses to extreme heat and drought conditions.
One such mechanism is a variation on the traditional C3 photosynthetic pathway termed the CAM. CAM allows for a water use efficiency, or the weight of plant material per volume of water used, 6-fold greater than C3 plants (Nobel, 1996) because carbon uptake occurs nocturnally. CAM plants are adapted to keep their stomata closed during the day to prevent water loss—carbon dioxide (CO2) is sequestered at night when stomata open, and is stored as malic acid until sunrise. Even though stomata are closed during the day (primarily for water conservation), photosynthesis can continue during the day (albeit at a reduced rate) by converting the malic acid back into CO2 for use in photosynthesis (Taiz and Zeiger, 2010). Various degrees of CAM expression exist—“CAM cycling” refers to the internal refixation of carbon stored as malic acid, whereas “CAM” indicates nocturnal carbon fixation via the enzyme phosphoenolpyruvate carboxylase with the potential for periods of stomatal opening at the beginning and end of the day. “CAM idling” refers to stomatal closure for the entire 24-h day, where no new carbon is metabolized but malic acid is still created nocturnally via the recapture of respiratory CO2 (Borland et al., 2011).
Starry et al. (2014) evaluated P. kamtschaticus for CAM metabolism and found it to be less drought resistant with less evidence of CAM metabolism than Sedum album, but did report some CAM activity for P. kamtschaticus. This supported Butler’s (2011) findings that different succulents commonly found on green roofs can express variation in the extent to which they use CAM. Regardless of the photosynthetic pathway, the effects of GRS water availability on plant growth and ET of green roof plants has not been studied in depth. In this study, the effects of substrate organic content on green roof plant growth and ET were evaluated by growing P. kamptschaticus in four different substrates in a growth chamber for 16 weeks, culminating with a series of three stress (dry-down) periods where water was withheld for 12 d (after the first dry period) or 10 d (after the second dry period), to gain a better understanding of how substrate composition may affect the growth of green roof species and the consequent effects on storm water mitigation.
Our hypotheses were as follows:
HO: P. kamptschaticum root and shoot growth is unaffected by substrate OM content.
HA: P. kamptschaticum root and shoot growth is affected by substrate composition, with 40% OM substrate producing greater root and shoot biomass than 20% and 10% OM content, due to the additional cation exchange and water holding capacity provided by the OM.
HO: GRS organic content will not affect evapotranspirational water loss from pots planted with P. kamptschaticum.
HA: GRS organic content will affect evapotranspirational water loss from pots planted with P. kamptschaticum, since shoot growth is expected to increase with increasing proportions of OM, which should lead to greater leaf area and canopy volume and thus greater daily ET.
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