Recent increases in species richness and shifts in altitudinal distributions of Norwegian mountain plants

Opportunities for observing long-term changes in natural biota are rare. Observations on the distribution and frequency of vascular plants were performed on 23 mountains situated along a west-east gradient in Jotunheimen, central Norway, where detailed site descriptions and species fists exist from AD 1930-31. The sites were resurveyed during the summer of 1998, to examine possible changes in species richness and species distributions along the altitudinal gradient during a 68-year period. Increased species richness was found on 19 of the mountains and was most pronounced at lower altitudes and in the eastern areas. Lowland species, dwarf shrubs and species with wide altitudinal and ecological ranges showed the greatest increases in abundance and altitudinal advances since the 1930-31 study. Species with more restricted habitat demands, such as some hygrophilous snow-bed species, have declined. High-altitude species have disappeared from their lower-elevation sites and increased their abundance at the highest altitudes. Climatic warming occurring in the last 100 years might have allowed the invasion of lowland and lee-slope species. Increased competition at sites where such species have invaded may have led to a decreased abundance of the less competitive species and a concentration of high-altitude species on the highest ridges. Natural succession since the 'Little Ice Age', increased deposition of nitrogen during recent years and changes in grazing and tourism might have influenced some of the species turnovers, but recent climatic changes are considered to be the most likely major driving factor for the changes observed.