As urbanisation proceeds and species diversity declines, the potential of extensive green roofs to support biodiversity gains increasing interest. However, little is known about the long-term function of these systems. How do green roof vegetation and substrates develop, decades after installation? How do the physical and chemical properties of green roof substrates change?
This paper addresses these questions through field surveys of some of the oldest extensive green roofs (EGR) in the world. The study was conducted in south-west Germany, where the EGR industry began in the 1980s and where the first commercial installations occurred. The roofs surveyed were between 21-33 years old, and were selected according to criteria of accessibility, age, and location (Stuttgart region). While they featured a range of construction types (two were sloped), all were based by typical EGR substrates (as defined by the FLL specifications); all employed multiple-layered systems; all were predominantly full sun; and all featured dominant Sedum coverage with varying cover by flowering herbs and grasses.
The FLL is a not-for-profit body within the Federation of German Industries and its “Guidelines for roof greening” were designed to create a standard for quality in central Europe. The first of 7 editions was published in 1982. In addition to best practices with reference to standards that interface with green roofs (e.g., DIN, ISO), each edition reflects the state-of-the-technology. The FLL developed precise guidelines and requirements for green roof substrates on the basis of plant health and drainage and retention of stormwater runoff.
Although the sample size is small, the results were substantiated by other German studies. The nutrient concentrations on the roofs sampled were in suitable concentrations for plant growth, and in line with the FLL specifications to which the roofs were designed. However, the pH of the substrates on most roofs was quite acidic by contrast with the FLL recommendations; similarly all roofs had excessive soil organic content as per FLL criteria. Low pH is known to inhibit nutrient availability for many plant taxa, with the exception of stress tolerant Sedum spp. that comprise the majority of plant cover on these mature systems. Since original species lists and documentation from those roofs indicate that plant species diversity had declined for all eight roofs since the time of installation, we hypothesise that Sedum dominance on extensive green roofs might actually result from a plant-soil feedback mechanism.
