Impacts of enhanced ultraviolet-B radiation on mosses in a subarctic heath ecosystem

A subarctic heath where Hylocomium splendens and Polytrichum commune dominate the bryophyte layer was irradiated with enhanced UV-B radiation, simulating 15% ozone depletion. The extra UV-B radiation was supplied by fluorescent tubes during three growing seasons. H. splendens reacted quicker and to a greater extent to enhanced UV-B radiation than did P. commune. Annual length increment in H. splendens was unaffected in the first year but was suppressed in the subsequent years. In P. commune, annual length increment did not decrease until the third growing season. Dry mass production of H. splendens decreased in segments from all three years. Annual dry mass production in P. commune did not change due to enhanced UV-B. Shoot morphology was measured after the third growing season. In H. splendens, the segment area decreased. Leaf density along P. commune shoots increased, and the shoots became stunted. The UV-B enhancement induced a higher variance in the concentration of UV-B absorbing compounds in H. splendens after the first and the third growing season. In P. commune, the variance remained unaffected, but the concentration was significantly decreased after the third year. Indirect UV-B impacts on intra- and interspecific relations, which may be mediated by changes in shoot growth and morphology, are discussed.