Mosses inhibit germination of vascular plants on an extensive green roof

Mosses are capable of tolerating harsh conditions, and are commonly found on both conventional bare roofs and engineered green roofs. Intentional planting of mosses on green roofs has shown that mosses can improve ecosystem services such as stormwater retention and microclimate cooling. Past studies of moss interactions with vascular plants on green roofs and in other habitats demonstrate both positive and negative effects on juvenile and adult tracheophytes. Moss can aid vascular plants by buffering environmental extremes, facilitating seed establishment, and improving nutrient and water availability. Conversely, moss can also hinder young seedling germination by blocking access to the soil column and/or light. In the green roof context, few studies have examined the relationship between moss and vascular plant germination. Since mosses commonly colonize green roofs it is important to understand the effects of established moss colonies on incoming plant propagules. The purpose of this study was to examine how four moss species, Atrichum undulatum (Hedw.) P. Beauv., Polytrichum commune Hedw., Polytrichum piliferum Hedw., and Racomitrium lanuginosum (Hedw.) Brid. affected the germination of weedy and native vascular plant seeds in a green roof system. Seeds were added to six pre-established moss treatments: monocultures of each moss species, a mixture which included all four moss species, and a substrate only control in pots on a rooftop. Overall, the control (moss-free) treatment resulted the highest germination rates for the majority of seeded species examined, an indication that for these vascular plants, mosses tend to inhibit germination of incoming seeds.