Community assembly of mosses, lichens, and succulents across a green roof chronosequence in Malmö Sweden

Research on the flora of green roofs has mainly focused on vascular plants. However, as green roofs age, they are spontaneously colonized by mosses and lichens, with mosses often becoming the dominant lifeform. There is thus a need to document and understand how moss and lichen communities assemble over time, and how these changes influence the provisioning of ecosystem services. To fill this knowledge gap, we analysed a chronosequence of 20 extensive green roofs, ranging from 0.4 - 28 years of age. For each roof, we measured environmental variables and collected percent cover data for all observed vascular plants, mosses, and lichens. Overall, all rooftops experienced spontaneous moss and lichen colonization over time. Increased shade, organic layer depth, and age appeared to favour perennial, pleurocarpous moss species over annual acrocarpous moss species. Based on the results of our study, the pleurocarpous mosses Brachythecium albicans and Hypnum cupressiforme are well adapted to rooftop conditions. Although species-specific experiments are needed to confirm this, the dense mats that these mosses produce likely enhance green roof function by reducing the temperature of the substrate. For the lichen species observed, the cyanolichens Peltigera canina and Peltigera didactyla have the capacity for nitrogen fixation and thus can enhance green roof function by increasing the availability of nitrogen to the surrounding plants. Our findings suggest that careful selection of moss and lichen species, tailored to specific roof conditions at installation, could enhance colonization speed and success. Future research should explore how these communities interact with other components of green roof ecosystems.