Marie-Curie IAPP ‘Green Roof Systems’ Project
The Green Roof Research Conference 18-19 March 2013, Sheffield
Dynamics of green roof vegetation over time Christine E. Thuring1, Dr. Nigel Dunnett2 1
University of Sheffield, Landscape Department, c.thuring@sheffield.ac.uk University of Sheffield, Landscape Department, n.dunnett@sheffield.ac.uk
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Introduction Since their development in the 1980s, extensive green roofs (EGR) have become increasingly popular as mitigation tools for urban environmental issues around the world. The early EGR pioneers experimented with analogue plant communities of Central Europe, with a focus on species-rich dry meadows (Minke and Witter, 1983) and on species ensuring consistent cover. One community, the Sedo-Scleranthetea, was deemed optimally suited and Sedum species/ cultivars have indeed become synonymous with EGRs. This research explores vegetation on EGRs that were installed over 20 years ago in southwest Germany. At the time of installation, many of the roofs surveyed were prototypes, some of which have become commonplace to the green roof industry, while others are discontinued relics. Although of interest, the development of vegetation on these systems over time has been conjecture at best due to issues like accessibility and deficient scientific method. In the context of the Marie Curie IAPP, this work relates long-term species persistence and performance to the establishment and screening trials of Work Package 1.
Overview of Methodology The traditional tool of quantitative plant ecology, a 1 m2 square quadrat, was used for floristic description, specifically cover abundance. Nine roofs were sampled from early June to midJuly over two field seasons (2010, 2011). Further to vegetation, plots were sampled for substrate characteristics like depth and physical and chemical properties.
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Figure 1 – Data collection with a 1 m quadrat
Marie-Curie IAPP ‘Green Roof Systems’ Project
The Green Roof Research Conference 18-19 March 2013, Sheffield
Key Findings Older roofs had significantly less cover and diversity by grasses (rho = -.716, n = 136, p < .001; rho = -.595, n = 136, p < .001, resp.). Of the 12 grass species sampled, 5 were colonisers with minimal cover and stature; the small annual grasses (Vulpia myuros, Setaria viridis) are easily overlooked, while the larger species (Agrostis stolonifera, Arrhenatherum elatius, Poa angustifolia) only occur in special conditions (e.g. deeper soils). Older roofs had significantly shallower depths (rho = -.481, p < .01, n = 134) and lower pH (rho = -.336, p < .01, n = 118) but more organic content (rho = .338, p < .01, n = 118). In spite of the small sample size and the variability amongst roofs, the oldest roofs had mean depths of between 53 and 61 mm, while the youngest roofs had between 70-85 mm.
Figure 2 – Mean substrate depth decreases on EGRs over time
Along with the combined and complex effects of substrate development, older roofs supported fewer species and mainly succulents. Sedo-Scleranthetea appears to function as a climax community on EGRs, and Sedum is clearly the champion. Greater species diversity can be achieved by providing more substrate depth, at least in mounds, and other additions of physical heterogeneity as recommended by Thommen (1998), Riedmüller (1994), Mann (1996), Buttschardt (2001), Brenneisen (2004), and others.
Further Reading BRENNEISEN, S. Green roofs - How nature returns to the city. International Conference on Urban Horticulture, SEPT 2-6, 2002 2004 Wadenswil SWITZERLAND. PROCEEDINGS OF THE INTERNATIONAL CONFERENCE ON URBAN HORTICULTURE, 289-293. BUTTSCHARDT, T. K. 2001. Extensive Dachbegrünungen und Naturschutz. PhD Dissertation, Universitaet Karlsruhe (TH). MANN, G. 1996. Die Rolle begrünter Dächer in der Stadtökologie. Biologie in unserer Zeit, 5, 292-299. MINKE, G. & WITTER, G. 1983. Häuser mit grünem Pelz: ein Handbuch zur Hausbegrünung, Frankfurt/ Main, Fricke Verlag. RIEDMÜLLER, J. 1994. Untersuchungen zur Anlage, Besiedlung und Vernetzung von anthropogenen Sekundärbiotopen auf Dachflächen. Ruprecht-Karls-Universität. THOMMEN, M. S. 1998. Pflanzengemeinschaften natürlich besiedelter Kiesdächer und extensiver Dachbegrünungen. Diplom (Master's), Universität Basel.