Soil formation on green roofs and its contribution to urban biodiversity with emphasis on Collembolans

SummaryIn urban areas, green roofs are part of the so-called environmental “green lung” providing important environmental, economic and technical advantages compared to conventional flat roofs. We hypothesised that soil formation occurs in the growing medium of extensive roof greening, and that the successive development in the growing medium promotes urban biodiversity and counters habitat loss. To demonstrate this, we selected 10 representative green roofs of two different age classes within the inner urban area of Hanover (Germany). “Old roofs” were constructed between 1990 and 1994 and “young roofs” between 1998 and 1999. During summer 2002, the roofs were sampled to determine abundance and species diversity of collembolans and to measure selected soil properties. Discriminant and cluster analyses, considering abiotic properties only, separated the growing media of the roofs into the groups “young roofs” and “old roofs”. The results indicated a more stable environment in the substrate of old roofs due to advanced soil formation and improved niche occupancy of collembolans. In the mature growing media of old green roofs, pH was lower, while Corg and Nt contents, as well as dehydrogenase activity, were higher compared to young green roofs. The cluster analysis on the substrate properties revealed young roofs to be more similar compared to old roofs, which are characterised by increasing dissimilarities. The collembolan densities were only slightly higher on old roofs (57,000 ind. m−2) compared to young roofs (55,000 ind. m−2). Also, differences in species richness were small with 26 and 24 species on young and old roofs, respectively. However, differences became obvious at the species level. Species diversity was highly dynamic over time, undergoing successive development comparable to that occurring in extreme soil environments like newly reclaimed mining areas. We conclude that extensive roof greening promotes urban biodiversity but does not replace nature.