Bryophytes play an important role in green roof ecosystems. Their presence can facilitate neighboring plant species and improve green roof function. Both lichen and moss decrease the presence of bare substrate, which can reduce substrate temperature and increase stormwater retention. Some Bryophytes also increase the availability of soil nutrients. This can occur through habitat provision, such as that observed between moss and N-fixing microbes, or in the case of Peltigera lichens, N-fixation by the bryophyte itself. However, little is known about the factors influencing the colonization of green roof bryophytes. The purpose of this study was to determine what environmental variables influence the cover, richness, and identity of green roof bryophytes. This study examines a green roof chronosequence spanning 19 green roofs in Malmo Sweden. These roofs were built between 1994 and 2022 by the same company, and are classified as Moss/Sedum extensive green roofs. They have a substrate depth between 1-5 cm and are dominated by bryophytes and succulents. For each roof, percent cover data was collected for individual vascular species, bryophytes, and bare substrate in July 2022. This was done by randomly placing 0.5 x 0.5 m quadrats on each roof. For each roof, five roof variables were collected: age, slope, area, height, and shade; and seven substrate variables were collected: depth, OM content, total nitrogen, 15N, total carbon, 13C, and C/N ratio. We observed eight different lichen species, with the most common being the N-fixing Peltigera canina. Lichen richness increased as roof age and substrate 13C increased and substrate 15N decreased. Lichen cover was highest on older roofs and those with a shallow slope. Trends in the % cover of specific lichen groups were observed for Peltigera species. and Cladonia species. The % cover of Peltigera species increased with depth and total nitrogen; and the % cover of Cladonia species increased with exposed substrate and 13C. For the mosses, 28 different species were observed, with the most common being Ceratodon purpureus. In our study moss cover increased as 15N decreased. Additionally, the % cover of feathermosses increased with roof age, and on roofs with a lower substrate depth and slope. Overall, roof characteristics substantially influenced the % cover of colonizing moss and lichen species. However, more research is needed to understand the underlying mechanisms behind the patterns observed.
