Recent increase in plant species richness in mountainous wetland habitats

Calcareous fen meadows in the pre-alpine region of Switzerland are rare wetland habitats of high conservation concern because they can support a high diversity of rare plant and animal species. An important determinant of habitat quality in fen meadows is nutrient availability. We surveyed 36 fen meadows differing in management regimes (mown and grazed) in Switzerland in 1995/96 and again in 2005/2006 along an altitudinal gradient from 800–1400m and determined species density in 2m by 1m plots of vascular plants and bryophytes together with abiotic environmental conditions. Vascular plant species density was consistently higher in mown fens, which had the same nutrient conditions as grazed fens. However, vascular plant species density strongly decreased with increasing soil nitrogen among replicate plots within fens (P<0.0001). Total soil carbon influenced vascular plant species richness positively (P<0.04). Bryophyte species richness was not significantly affected by these factors. Although nutrient conditions of the 36 fens did not change over the 10-year observation period, vascular plant species density significantly increased by one species from 32.3 to 33.2/2m² (and bryophyte species richness remained relatively constant at 11.8 m^–2). We interpret this increase in vascular plant species richness as a potential effect of global warming which allowed several lowland species to expand their ranges towards higher altitudes.