Using recycled aggregates in green roof substrates for plant diversity Chloe J. Molineux a,b, *, Alan C. Gange a , Stuart P. Connop b , Darryl J. Newport b a School of Biological Sciences, Royal Holloway, University of London, Egham, Surrey TW20 0EX, United Kingdom b Sustainability Research Institute, University of East London, Docklands Campus, 4-6 University Way, London E16 2RD, United Kingdom A R T I C L E I N F O Article history: Received 28 January 2015 Received in revised form 5 May 2015 Accepted 27 May 2015 Available online xxx Keywords: Recycled aggregates Growing substrate Extensive green roof Biodiversity Plant assemblages A B S T R A C T Extensive green roofs are becoming a popular tool for restoring green infrastructure in urban areas, particularly biodiverse habitats such as post-industrial/brownfield sites. This study investigated the use of six recycled lightweight aggregates and combinations of them in green roof growing substrate, to determine their effectiveness for enhancing plant abundance and species diversity. In two separate experiments, we examined the roles of substrate type and depth on the establishment of a perennial wildflower mix over a 15-month period. We found that some of the alternative substrates are comparable to the widely used crushed red brick aggregate (predominantly found in commercial green roof growing substrate) for supporting plant establishment. For some materials such as clay pellets, there was increased plant coverage and a higher number of plant species than in any other substrate. Substrates that were produced from a blend of two or three aggregate types also supported higher plant abundance and diversity. Generally, increasing substrate depth improved plant establishment, however this effect was not consistent across substrates. We conclude that recycled materials may be viable constituents of growing substrate for green roofs and they may improve green roof resilience, through increased plant cover and diversity. The results could provide evidence to support the construction of mosaic habitat types on single roofs using various substrate blends. ã2015 Elsevier B.V. All rights reserved. 1. Introduction Green roofs—rooftops that have been purposefully vegetated (Oberndorfer et al., 2007) either with low growing Sedum plants, wildflowers, grasses or shrubs and trees, are an emerging green technology that is becoming increasingly popular in urban environments due to the many benefits they provide. One such benefit is their potential to restore biodiversity in urban landscapes (Gedge, 2001; Grant et al., 2003; Sadler et al., 2011; Ishimatsu and Ito, 2013; Madre et al., 2014). There is an increasing body of evidence demonstrating that green roofs are able to support high biodiversity if designed appropriately (Brenneisen, 2006; Kadas, 2007; Baumann and Kasten, 2010; Tonietto et al., 2011) and increasing recognition that rich biodiversity in cities can have enormous potential to mitigate the effects of climate change through the enhancement of urban resilience and sustainability (Niemelä, 2014). Extensive green roofs are generally designed with a substrate layer (up to 150 mm deep) that contains a high (up to 90%) percentage of aggregate and a small amount of organic material. This not only provides a low nutrient growing substrate ideal for green roof vegetation (Molineux et al., 2009; Molineux, 2010; Nagase and Dunnett, 2011) but also reduces extra roof weight. Problems can occur with either the addition of ‘soil’ and its attending clay fraction causing reduced water transmissivity or excessive compost/organic matter risking substrate shrinkage (Snodgrass and Snodgrass, 2006). Extensive green roofs are often vegetated using blankets, comprised of up to 12 different Sedum species and are rolled out over the substrate layer to provide an instant ‘green’ effect (Emilsson and Rolf, 2004). Other types of planting that are popular include wildflower and grass blankets, plug-planted systems (with either Sedum or wildflower species) and seeded systems. Biodiversity roofs tend to use both plug- plants and seeds and often support local species that naturally invade the roof (Bates et al., 2013) such as Buddleia,Chenopodium spp., Trifolium spp., tree species seedlings (Salix spp.) and various grass species. These types of roofs are generally designed to mimic natural wasteland areas where bare ground can be colonized by wildflowers and grasses, with succession proceeding to scrub and finally woodland, allowing a wide range of wildlife to become established (Gibson, 1998; Angold et al., 2006). These roof level habitats often naturally retard succession due to limitations of substrate depth, water holding capacity and nutrient availability * Corresponding author at: School of Biological Sciences, University of East London, Egham, Surrey TW20 0EX, United Kingdom. Tel.: +44 208 223 7931; fax: +44 208 223 3327. E-mail address: c.molineux@uel.ac.uk (C.J. Molineux). http://dx.doi.org/10.1016/j.ecoleng.2015.05.036 0925-8574/ ã 2015 Elsevier B.V. All rights reserved. Ecological Engineering 82 (2015) 596–604 Contents lists available at ScienceDirect Ecological Engineering journal homepage: www.elsevier.com/locate/ecoleng