Effects of enhanced ultraviolet-B radiation on subarctic ecosystems

Sammanfattning: Biologically harmful ultraviolet-B radiation (UV-B, 280-320 nm) is increasing at the Earth´s surface owing to stratospheric ozone depletion. This is of global concern due to potential impacts of enhanced UV-B radiation on the biosphere. The aim of this thesis was to investigate effects of enhanced UV-B radiation on components (bryophytes) and processes (decomposition, productivity, species interactions) in ecosystems. In the Subarctic, a bog and a birch-heath ecosystem were exposed to enhanced UV-B radiation for two and three growing seasons, respectively. A 15% ozone depletion was simulated. Bryophytes (Sphagnum fuscum, Hylocomium splendens, Polytrichum commune), representing different morphological ?types? and growth strategies, responded to UV-B enhancement with altered shoot morphology and reduced stem increment. Changed growth was not clearly linked to alterations in pigmentation, maximum net photosynthesis or dark respiration. Feedbacks of growth alterations on intra- and interspecific interactions are discussed. Productivity per unit area was not affected in S. fuscum. The main variables determining productivity (length increment, shoot mass relations, spatial shoot density) were affected although in opposite directions. UV-B-induced changes in shoot biometry and reduced spatial shoot density imply long-term effects on peat structure with possible feedback on productivity and decomposition. In H. splendens, enhanced UV-B radiation reduced current year´s shoot biomass production by 25% and tended to decrease spatial shoot density. The effect on these variables caused an amplified effect at the community level as productivity per unit area was reduced by 34%. Decomposition studies were done with dwarf shrub litter in the laboratory and in the heath. Altered leaf litter quality, which was induced during leaf growth under enhanced UV-B radiation, caused a decrease in microbial decomposer activity and delayed litter turnover. UV-B radiation applied during decomposition reduced microbial decomposer activity and decreased the abundance of fungi in the decomposer community but did not delay litter turnover. The potential delay was possibly offset by photodegradation of litter. Changes in species composition in the heath (including phanerogams and cryptogams) were not detectable after four growing seasons. Because of differences in UV-B responsiveness between species with different growth strategies, a shift in species composition could occur over the long-term perspective.