Trait-environment relationships over short taxonomic and abiotic gradients on Sedum/moss roofs

One commonly constructed green roof type, the Sedum /moss roof, usually has a substrate depth less than 5 cm and thus can only support a limited plant community, typically dominated by drought-tolerant succulents. Despite this restricted plant community, variation in succulent community composition exists, likely influenced by roof characteristics such as age, slope, and shade. Since different plant species and traits are associated with different ecosystem services, there is a need to understand how even minor variation in green roof design or environmental setting can influence community composition and trait values. In this study, we examine a chronosequence of 31 Sedum /moss extensive green roofs, built in a similar manner, in Malmö, Sweden and Helsinki, Finland. The purpose of this research was to understand how slight changes in green roof environmental/abiotic features affect (1) plant community-level traits (means and diversity), as well as (2) intraspecific trait variation, that is, how traits of individual species (here Sedum album , Phedimus spurius , and Phedimus hybridus ) vary along an environmental gradient. Based on our results, taller plant species are more likely to be observed on older roofs, with deeper substrate, less solar exposure, and on shorter buildings. Deeper substrates also promoted plants with higher specific leaf area (SLA). Furthermore, small changes in roof attributes led to intraspecific trait variation, with taller individuals of P. hybridus observed on roofs with a deeper substrate; taller individuals of S. album observed on younger roofs; and higher values of SLA for P. hybridus and P. spurius observed on roofs with higher solar exposure. Since both SLA and plant height have been associated with stormwater retention and thermal cooling, key green roof ecosystem services, our findings demonstrate the importance minor variation in environmental conditions can have on the benefits provided by vegetated rooftops.