Substrate depth and roof age strongly affect plant abundances on sedum-moss and meadow green roofs in Helsinki, Finland

Green (i.e. vegetated) roofs are an increasingly popular measure to improve climate regulation in residential areas. However, there is limited knowledge on the effect of green roof characteristics (e.g. substrate depth and roof age, size and height), especially in northern climates, on plant species composition of the roofs, even though roof performance is mediated through the plants. We hypothesized that substrate depth and roof age, size and height shape the vegetation on green roofs. We inventoried vegetation on 51 green roofs in the metropolitan area of Helsinki in 2011 and found 230 plant species, of which seven are red-listed. We showed that substrate depth and roof age were predominant in structuring plant communities on green roofs. Thin substrates and young roofs maintained sedum and moss species, while thick substrates and older green roofs supported meadow plants. Roof size and height had variable effects on the vegetation. Our results provide knowledge on the dependence of plant species on roof parameters, which is useful in the planning for particular vegetation on roofs taking into account, e.g. load capacity and substrate depth, the wanted functional properties of the roof, and maintenance.