Question: How did the vascular plant species composition of a nunatak in the alpine vegetation belt change over a time span of one century?
Location: A 0.056-km2 nunatak, Isla Persa in the Swiss Alps, that remained ice free during the last maximum glacier advance in the 1850s and is today partly covered with climactic alpine grassland and dwarf heath shrubs.
Methods: Floristic inventories in 1906, 1927, 1972, 1995, 2003 and 2004 and a comparative analysis of the species composition over the period 1906-2004.
Results: Thirty-one species that were not recorded in the first inventory were found in the following surveys. However, among them only six were common by 2004. On average, the new species prefer warmer conditions than those previously present and most newcomers are associated with montane or subalpine grasslands and woodlands. In particular, the observed increase of Vaccinium myrtillus and the arrival of shrub and tree species further substantiate a trend towards vegetation composition of the lower altitudinal belt. Ferns represented 24% of the newcomers, probably due to the high dispersal ability of their lightweight spores. The observed species enrichment was globally small compared to previously inventoried summits.
Conclusion: Floristic change strongly suggests warmer climatic conditions as the main factor contributing to species compositional change. The relative stability of species richness may be explained by several factors: the isolation of the nunatak and the difficulties for plants to reach the site, the colder local climate, a limited available species pool and interactions of established alpine plants with newly immigrating taxa. Supplementary data collected at about the same altitude would be necessary to better understand the influence of climate change on alpine grasslands.