Water vapour and CO2 exchange were measured in moss dominated vegetation using a chamber at 17 sites near Abisko, Northern Sweden and 21 sites near Longyearbyen, Svalbard, Northern Norway to quantify the contribution of mosses to ecosystem level fluxes. With the help of a simple light-response model we showed that the moss contribution to ecosystem carbon uptake varied between 14 and 96%. This moss contribution could be related to vegetation canopy properties like normalized difference vegetation index (NDVI) and leaf area index (LAI). NDVI was a good predictor of gross primary production (GPP) of mosses and of the whole ecosystem, across different moss species, vegetation types and two different latitudes. NDVI was also correlated with thickness of the active green moss layer. Finally, mosses played an important role in water exchange; they are expected to be most important to gas exchange during spring when leaves are not fully developed.