Impacts of increased nitrogen supply on Norwegian lichen-rich alpine communities: a 10-year experiment

Abstract

Summary 1 Species cover was tested during a 10-year fertilization experiment in the low-alpine Cetrarietum nivalis community and the middle-alpine Phyllodoco-Juncetum trifidi community in the Dovre mountains of south-central Norway. Nitrogen was added at 7, 35 and 70 kg N ha−1 year−1, with the highest dose corresponding to approximately 3.5 times the annual deposition in south-west Norway. 2 Both communities are dominated by lichens (Cladonia spp. and Cetraria spp., respectively), have a patchy structure and are ‘conservative’ as regards species content. 3 Lichens, which showed a decrease in cover and size, and after some years developed discoloured thalli, are the best organisms for monitoring changes in alpine vegetation that is exposed to increased nitrogen deposition. The most sensitive species in Cetrarietum nivalis appeared to be Alectoria nigricans and Cetraria ericetorum, but more abundant species (Cladonia mitis, C. stellaris and Cetraria nivalis) are likely to be more reliable indicators. Cetraria delisei seems to be a reliable indicator species for monitoring in Phyllodoco-Juncetum trifidi. 4 Fertilization had no significant effect on the vascular plants (dwarf shrubs and a few graminoids) in either community, except for Festuca ovina, the cover of which increased slightly. 5 Nitrogen pollution may affect oligotrophic, alpine communities differently, depending on their species composition and horizontal structure (patchiness). 6 It is suggested that other factors, such as climate, soil properties and community structure, may be more important than long-range nitrogen pollution for determining species composition and species cover in many of the oligotrophic, alpine communities in southern Norway. However, in lichen-rich communities, critical loads have already been exceeded in the most polluted areas of south-west Norway.