Determinants of plant species richness in an alpine meadow

1. Nutrient availability, defoliation and soil disturbance are important factors that influence the richness of plant communities. However, few studies have examined the interactions between these factors, especially in harsh environments. We therefore examined the effects of fertilization, defoliation and soil disturbance on plant species richness in an alpine meadow in northern Sweden. 2. From 2002 to 2005, plots were fertilized, mowed and disturbed annually in a factorially designed experiment. Plant species richness, colonization and extinction were recorded in permanent subplots. Seedling densities and recruitment from seed of Potentilla crantzii were also estimated in order to examine the mechanisms whereby these treatments influence colonization by new species. 3. Species richness peaked in the absence of disturbance in unfertilized plots and with light disturbance in fertilized plots. Colonization by new vascular plant species, seed recruitment and seedling density were all increased by soil disturbance, suggesting that colonization was increased because conditions were more favourable for seed recruitment following the disturbance treatment. Extinction of species was highest in the mowed, intensively disturbed and unfertilized plots and lowest in the fertilized, lightly disturbed and unmowed plots. 4. Local extinction was negatively correlated with both moss and vascular plant biomass. Local colonization, seedling density and recruitment of P. crantzii were all strongly negatively correlated with the biomass of mosses. In contrast, the biomass of vascular plants was only weakly correlated with local colonization, seedling density and recruitment of P. crantzii. 5. This study suggest that the level of disturbance at which species richness peaks should move towards higher disturbance levels when productivity increases. However, lower extinction rates rather then higher colonization rates seem to be causing the increase in species richness following disturbance.