Green roofs for hot and dry climates: Interacting effects of plant water use, succulence and substrate

Abstract

Green roofs are increasingly being constructed in urban environments to provide a range of environmental benefits. However, little is known about how they will perform in hot and dry climates where water is often limiting and drought tolerance determines plant survival. We evaluated the effects of severe drought (113 days without water) on growth, water use and survival of five succulent species (Sedum pachyphyllum, S. clavatum, S. spurium, Disphyma crassifolium and Carpobrotus modestus) planted in three different green roof substrates (growing media) differing in water holding capacity. Plants survived 12 days longer in substrates with higher water holding capacity. Water use determined survival under severe drought with species with higher water use (D. crassifolium and C. modestus) dying at least 15 days earlier than Sedum species which were conservative water users. Increased survival was not related to increased leaf succulence but was related to reduced biomass under drought. Under well-watered conditions, water use was greatest in species with lower leaf succulence in substrates with increased water holding capacity. To maximise survival, green roofs in year round or seasonally hot and dry climates should be planted with species that have high leaf succulence and low water use in substrates with high water holding capacity.

Introduction

Green roofs are increasingly being built to provide a diverse range of environmental benefits. These include energy conservation through improved building insulation and energy efficiency (Sailor, 2008), mitigation of the urban heat island effect (Bass and Baskaran, 2003), noise attenuation (Van Renterghem and Botteldooren, 2009), biodiversity habitat provision (Brenneisen, 2006) and urban stormwater management (Berndtsson, 2010, VanWoert et al., 2005). Green roofs are constructed profiles made up of layers including water-proofing, drainage (gravel or proprietary system) and substrate (growing media) layers in which plants are grown. Weight loading restrictions on buildings limit the depth of substrate (often <20 cm) on retrofitted green roofs. This makes green roofs difficult environments for plant growth and survival as water availability fluctuates dramatically between rain events (Nagase and Dunnett, 2010, Oberndorfer et al., 2007). Consequently, survival during drought periods determines plant species suitability for green roofs (Bousselot et al., 2011), especially in hot and dry climates.

Survival on green roofs is determined by substrate depth and physical properties, particularly water holding capacity. Drought tolerance of Sedum species in response to substrate depth has been widely investigated, with increased survival in greater depths (Durhman et al., 2007, Getter and Rowe, 2009, VanWoert et al., 2005). However, there has been little comparison of species performance under drought conditions in different substrates with different physical properties. For long term success, green roof substrates need to balance a number of competing and sometimes contrasting properties. Good aeration and low bulk density are needed to ensure the substrate is free draining, lightweight and facilitates plant respiration, yet this must be balanced against sufficient water retention for plant growth and survival (Nektarios et al., 2004, Rowe et al., 2006, Thuring et al., 2010). These properties can be achieved with light weight components; however, many components, particularly organic materials, shrink and/or decompose over time, therefore green roof substrates are largely mineral based. Mineral based substrate composition differs according to local availability and cost, and many include recycled or waste products to maximise the environmental benefits of green roofs (Molineux et al., 2009). Most green roof substrates are developed according to specified performance guidelines and standards, notably the widely used German FLL guidelines (FLL 2008) or the more recent American Standard Testing Methods (ASTM 2009a,b,c, ASTM 2010). Both specify value ranges and limits for different substrate properties and the required testing methodologies.

The plants most commonly used on European and North American green roofs in temperate climates are succulents from the genus Sedum (Oberndorfer et al., 2007, Snodgrass and Snodgrass, 2006). Sedum species are considered ideal for green roofs due to their low spreading habit, providing good lateral cover, and drought tolerance (Nagase and Dunnett, 2010, VanWoert et al., 2005). Their drought tolerance is largely due to high leaf succulence and physiological adaptations such as CAM (Crassulacean Acid Metabolism) photosynthesis (Butler, 2012, Durhman et al., 2006). CAM plants have greater water use efficiency than C3 plants as transpiration per unit CO₂ fixed is reduced due to stomata opening at night for CO₂ uptake (Sayed, 2001). Some Sedum species are also considered facultative CAM (Kluge, 1977), shifting from C3 to CAM photosynthesis under stressful conditions such as drought, salinity or elevated temperature (Sayed, 2001). Leaf succulence enables plants to survive periodic drought by providing usable water when soil water conditions prevent uptake by roots (von Willert, 1992). Many Sedum species used on green roofs have been selected from alpine areas for their frost tolerance and enhanced survival during winter (Durhman et al., 2007), including Sedum album, S. acre, S. reflexum and S. spurium. As a consequence of higher frost tolerance, these species exhibit reduced leaf succulence (Osmond et al., 1975, Teeri et al., 1981). As the degree of leaf succulence directly influences drought tolerance (von Willert, 1992), these Sedum species may be less suitable in hot and dry climates than species with greater succulence (Williams et al., 2010b).

Despite widespread implementation in cooler northern hemisphere climates, there are very few extensive green roofs in hot and dry climates (Williams et al., 2010b). Successful implementation of green roofs in hot and dry climates is important as the environmental benefits are likely to be far greater in than in temperate climates (Alexandri and Jones, 2008). However, it is problematic to rely on temperate northern hemisphere green roof practices without scientific testing (Williams et al., 2010b), due to climatic differences, access to suitable substrate and plants and limited information on plant performance under drought conditions (Bousselot et al., 2011). To date little research has been done to determine drought tolerance of green roof succulents and the suitability of different substrates in year round or seasonally hot and dry climates.

This paper describes an experiment that determined the effects of drought on plant growth, survival and water use of five succulent species, with varying degrees of leaf succulence, grown in three different substrates that were developed according to the FLL guidelines. This drought experiment had two objectives: (i) to determine how substrate water holding capacity affects plant water use and survival; and (ii) to determine whether leaf succulence affects plant survival.