Persistence, loss and gain : characterising mature green roof vegetation by functional composition

Like any constructed ecosystem, the vegetation of extensive green roofs (EGRs) will change over time. Although this may influence the desired function and performance, little work has examined the floristic dynamism of EGRs over the long-term. Variations in species composition may be associated with original species (persistent or lost), colonisers (gained), or the effects of spatial heterogeneity. This paper reports on floristic variation of two unmanaged German EGRs twenty years after installation. To evaluate floristic change, the analyses focused on functional composition and plant strategies sensu Grime's CSR theory, referring to the basic adaptive strategies of competition, stress-tolerance and ruderality, and their derivatives. With reference to original documentation, less than half the original species persisted. In spite of the losses, both roofs had entire cover, or nearly so, thanks to colonising species. The generalist strategy (CSR strategists) was the most important functional trait in the observed vegetation, followed by stress-tolerance and then by variations in stress-tolerant ruderality. The functional composition of colonising species was chiefly ruderals, followed by stress tolerators and generalists. The drivers behind these changes relate to the pressures of stress, disturbance and competition, as well as spatial heterogeneity and strategies for dispersal and regeneration, seedbank and propagule sources. This study suggests that long-term floristic diversity may be facilitated by ensuring a diversity of traits and species from the start, by providing spatial heterogeneity, and by considering the mechanisms that support persistence and those which determine colonisation.