Urban Forestry & Urban Greening 12 (2013) 98–108 Contents lists available at SciVerse ScienceDirect Urban Forestry & Urban Greening j ourna l ho mepage: www.elsevier.de/ufug Vegetation development over four years on two green roofs in the UK Adam J. Bates a,* , Jon P. Sadler a , Rae Mackay b a School of Geography, Earth & Environmental Sciences, The University of Birmingham, Birmingham, West Midlands B15 2TT, UK b Geotechnical & Engineering Research Group, Monash University, Gippsland Campus, Victoria 3800, Australia a r t i c l e i n f o Keywords: Brown roof Eco-roof Greenroof Living roof Rooftop garden Vegetation composition a b s t r a c t Carefully designed green roofs have the potential to be used as mitigation for habitats lost at ground level. The development of plant assemblages on two green roofs designed to emulate diverse brownfield habitats (brown roofs), by using recycled demolition aggregate as part of a low-fertility growth substrate, were studied over the first four years of their development. The cover-abundance of flowering plants and habitat structural components (e.g. bare ground, moss) were measured on the Domin-Krajina scale within all identified microhabitats. Drought disturbance was one of the main controlling factors on assemblage development. Annual plants were abundant and successful in the first growth season, and thereafter only re-appeared in any numbers following drought disturbances in subsequent years. Moss and Sedum acre L. increased through the study period until these plants dominated coverage. The cover-abundance of perennial wildflower species was strongly influenced by drought disturbance. The influence of drought disturbance varied between different brown roof microhabitats, with plant assemblages in coarser and less fertile microhabitats more resistant to these disturbances. Observed responses to drought were consistent with the following two hypotheses: (i) Areas of coarse substrate can act as disturbance refugia for plants during drought by helping preserve pockets of water under large clasts and within absorbent materials such as brick. (ii) The plant assemblages living in areas of more fertile substrates, which grow more luxuriantly when water availability is high, are more vulnerable to drought disturbance. Green roofs should be designed to include a range of substrate types to create several microhabitats that will collectively support more species than any single microhabitat. © 2013 Elsevier GmbH. All rights reserved. Introduction Green roofs are roofs of buildings covered with a growth substrate and plants, which are also known as roof gardens, living- roofs, and eco-roofs. There are two main types: (1) intensive green roofs, which are essentially heavily landscaped ‘gardens’ that have a growth substrate depth greater than 20 cm and typically require structural reinforcement of the roof and regular maintenance; and (2) extensive green roofs, which have a growth substrate depth less than 20 cm and usually require little roof structural reinforcement and maintenance (Oberndorfer et al., 2007). Extensive green roofs have received the most research focus because they offer the most replicable design solutions and have more potential to be retro- fitted on a large scale (Dunnett and Kingsbury, 2004; Getter and Rowe, 2006; Rowe, 2011). Extensive green roofs are the focus of this research. Public and research interest in green roofs has increased in recent years, most probably as a result of the variety of environ- mental advantages that are frequently attributed to them. These * Corresponding author. Tel.: +44 0 121 4146167; fax: +44 0 121 4144942. E-mail address: a.j.bates@bham.ac.uk (A.J. Bates). include: removal of air pollution, increased roof water-proofing longevity, urban cooling and reduction of roof storm-water runoff (Wong et al., 2003; Bengtsson, 2005; Mentens et al., 2006; Saiz et al., 2006; Oberndorfer et al., 2007; Yang et al., 2008; Castleton et al., 2010; Rowe, 2011). However, it is becoming increasingly well- appreciated that the strength of these environmental benefits are dependent on the design of the green roof used (Bates et al., 2009; Rowe, 2011), and that more research is needed before these poten- tial advantages are fully understood and quantified (Dunnett and Kingsbury, 2004; Oberndorfer et al., 2007; Rowe, 2011). A key environmental benefit of installing green roofs is the cre- ation of habitat for wildlife, which could potentially mitigate for habitat lost at ground level (Donovan et al., 2005; Baumann, 2006; Brenneisen, 2006; Kadas, 2006; Francis and Lorimer, 2011; MacIvor and Lundholm, 2011; Sadler et al., 2011). However, research on this benefit remains in its infancy, and the best way to design green roofs for habitat creation it is not yet clear. Desirable ecological charac- teristics of green roof habitats include: (i) high species diversity, (ii) the provision of habitat for rare and endangered species, and (iii) the provision of habitat for species with a high fidelity for the habitat lost at ground level. Some of the most diverse and valu- able wildlife habitats in urban areas are found on brownfield sites (demolition and post-industrial sites) (Gilbert, 1989; Spalding and 1618-8667/$ – see front matter © 2013 Elsevier GmbH. All rights reserved. http://dx.doi.org/10.1016/j.ufug.2012.12.003