Concerns about increased stratospheric ozone depletion increasing ambient levels of ultraviolet-B radiation (UV-B), and the fact that some ecosystems are naturally exposed to high levels, has resulted in an increased interest in the effects of UV-B on plant communities. Despite this, there has been a paucity of studies into its effects on plant competition. Artificial plant communities consisting of Lolium perenne and Lotus corniculatus and a sub-montane community consisting of Agrostis tenuis, Festuca ovina and Galium saxatile (also including different nitrogen levels) were created using the response surface approach. The long-term effects of UV-B were also studied on a natural sub-Arctic community in Abisko, Sweden. In addition, all plant samples were analysed by Fourier-Transform Infrared Spectroscopy (FT-IR) to obtain a ‘metabolic fingerprint’ which was used to detect chemical differences to the whole biochemical complement of the sample. The results showed that enhanced UV-B altered the competitive interaction of Lolium perenne and Lotus corniculatus in favour of Lolium perenne although ambient levels of UV-B did not elicit an effect in the sub-montane community. Only one dwarf shrub species in the sub-Arctic experiment, Vaccinium myrtillus, was negatively affected by UV-B. In most cases, elevated UV-B elicited a change in the metabolic fingerprint in the samples and in some cases an alteration in competitive stress altered the metabolome. This suggests that FT-IR can be used as a screening tool to detect for both abiotic stress and competitive biochemical alterations. In addition, this thesis proposes that the facilitative effect between the grass-legume mixture of Lolium perenne and Lotus corniculatus is not related to nitrogen fixation in the early stages of competition which has traditionally been believed.