How substrate depth shapes spontaneous plant colonisation on Mediterranean green roofs: A case study

Green Roofs (GRs) have been recognized by the EU as key Nature-based Solutions providing environmental and human benefits in cities. Mediterranean GRs (MGRs) are still rare and little is known about the successional dynamics and environmental factors shaping plant community composition. In the current study, we monitored spontaneous plant communities colonising two MGRs different in substrate depths and composition, and year of installation in the city of Genoa (NW Italy). Taxonomic composition variation over time was described using Non-Metric Multidimensional Scaling (NMDS) and species turnover rates. A nested ANOVA was used to test differences in plant biomass. Finally, functional composition variations were investigated by calculating CSR ecological strategies of species using the StrateFy tool, which identifies competitive (C), stress-tolerant (S) and ruderal (R) plant strategies. Both the taxonomic and functional structure of spontaneous plant communities were strongly dependent on MGR characteristics, with S species usually dominating on MGRs with shallower substrate, while deeper MGRs were also able to support more C species. However, the temporal variation in biomass and CSR strategy distributions indicates that substrate effects might be modulated by external interactive factors such as climate conditions and human disturbance. Our results show that, although described as extreme habitats, MGRs can support resilient, heterogeneous and dynamic spontaneous plant communities and that substrate plays a key role in driving their temporal evolution. From a planning perspective, our study demonstrates the importance of implementing a functional trait-based approach for species selection in MGRs, considering substrate-related factors and management practices.