Facilitation of vascular plants by cushion mosses in high-Andean communities

Mosses are a dominant ground cover in a wide array of ecosystems, especially in those developing under cold-stressed environments such as arctic and alpine ice-melting glacial forelands. Consequently, mosses may influence the performance and distribution of other plants. Here, we assessed the nature of interactions between vascular plants and cushion-forming mosses in three alpine communities in the northern Patagonian Andes. We recorded species richness, plant abundance and cover of vascular plants within and outside moss cushions, measuring also patch area and moss layer depth. To determine the effect of mosses on vascular plant assemblages, we calculated the relative interaction index (RII) in terms of richness, abundance and cover of all vascular plants, and of individual species. Moss-cushion patches showed higher species richness, plant abundance and cover in comparison with bare ground areas. Overall, the diversity of vascular plants increased with both moss-cushion area and layer depth. Species-specific RII values revealed that the effects of moss cushions on neighboring vascular plants were predominantly positive for all three plant communities surveyed. These results highlight the role of mosses as nurse plants in alpine ice-melting glacial forelands and, thus, as ecosystem engineers that can be key in maintaining vascular plant diversity.