• The flux of ultraviolet (UV)-B radiation to the Earth's surface is increasing, particularly in high latitudes. We studied the sensitivity of some dominant plant species of boreal and subarctic peatlands to this increase.
• Intact peat monoliths with the mosses Sphagnum balticum and Sphagnum papillosum, and cotton grass (Eriophorum vaginatum) were exposed to ambient solar UV-B or ambient solar UV-B supplemented by 30% in a field experiment in central Finland.
• Although the UV-B dose was low during the growing season, owing to frequent cloudiness, both Sphagnum species showed significantly higher membrane permeability under enhanced UV-B. In S. balticum, UV-B tended to decrease the capitulum dry mass and induced a 30–40% increase in the concentration of chlorophyll and carotenoid pigments. Enhanced UV-B had no effects on leaf morphology, chlorophyll fluorescence or stomatal functioning in E. vaginatum.
• The various UV-B responses in the Sphagnum species under investigation indicate that they may be sensitive even to small increases in solar UV-B radiation. By contrast, E. vaginatum appeared to tolerate the UV-B fluxes of the experiment.