Vegetation composition of old extensive green roofs (from 1980s Germany)

Christine E Thuring,Nigel Dunnett

doi:10.1186/2192-1709-3-4

Abstract

Since their development in the late 1970s in Germany, extensive green roofs (EGR) have become increasingly popular as mitigation tools for urban environmental issues around the world. EGRs are planted with select species, which ensure consistent cover and performance over time. This research presented herein is part of a systematic re-evaluation of EGR technology since the German industry began. Given the opportunity to access a small sample of old EGRs installed over 20 years ago in south-west Germany, this research surveyed the vegetation and substrate with an interest in describing these parameters with time-through-space substitution. Similar to previous studies, this preliminary work found correlations between roof age with vegetation (cover abundance and species diversity) and substrate properties (e.g., depth, organic content, pH, and nutrients). Roof age had positive relationship with soil organic content (Corg), and negative relationships with substrate depth and soil pH. These soil variables are inter-related, as shallow acidic substrates create unfavourable conditions for decomposition and thereby the accumulation of duff. Substrate variables correlated with EGR vegetation, suggesting a trend of simplified species composition over time. Indeed, Corg had a negative relationship with cover and species diversity of most life forms; only Sedum species had positive associations with Corg. Considering the dynamics associated with shallow mineral substrates, and the greater floristic diversity of younger roofs, simple Sedum-based vegetation may represent a steady state for conventional EGRs.

Highlights

Since their development in the late 1970s in Germany, extensive green roofs (EGR) have become increasingly popular as mitigation tools for urban environmental issues around the world
In spite of the differences in location, age, area and slope, the roofs are all based by typical EGR substrates, and field observations confirmed the use of multiplelayered systems
Given the small sample size and the lack or incomplete quality of original documentation, we can only speculate about how the vegetation and substrate properties develop on EGRs over time

Summary

Introduction

Since their development in the late 1970s in Germany, extensive green roofs (EGR) have become increasingly popular as mitigation tools for urban environmental issues around the world. Growing plants on roofs is an ancient concept common to many cultures and climates. Grass (or sod) roofs have a long and global lineage, too, often for areas lacking building materials yet requiring insulation from exposure and extreme climates (e.g., Scandinavia, sub-Saharan Africa, American Midwest) (Adler, 2005; Arhendt, 2007; Grant, 2006). In the late 18th to mid-19th centuries, tar-paper-gravel (TPG) roofs became popular in many German cities (e.g., Berlin, Göttingen, Osnabrück, Karlsruhe) to stop the spread of fire and provide insulation (Köhler and Poll, 2010). The vegetation that colonised these early systems and/or which developed following sowings in the 1980s, range from xeric Sedo-Scleranthetea communities on shallow depths to grassy Festuco-Brometea communities on deeper substrates (Bornkamm, 1961; Bossler and Suszka, 1988; Buttschardt, 2001; Poll, 2008)

Methods

Results

Conclusion