Influence of colonizing shrubs on species‐area relationships in alvar plant communities

In herbaceous plant communities of Stora Alvaret (the Great Alvar), Öland, Sweden, we studied the dependence of species‐area relationships on the canopy cover of the colonizing shrubs Juniperus communis and Potentilla fruticosa. A series of plots 0.25 ‐ 256 m2 in area were sampled in communities with varying canopy cover of Juniperus or Potentilla. Species number ‐ log (area) regressions always fitted data better than log (species number) ‐ log (area) regressions. The number of species declines more dramatically with increasing Potentilla cover than with increasing Juniperus cover. The total species number in large Juniperus plots follows a unimodal curve with a peak at about 75% shrub cover. The alvar (limestone grassland) species number in plots of all sizes declines monotonically with Juniperus cover following the regression equation S = 31.09 + 10.75 log A ‐ 0.0025 P2, where A is area and P is percentage Juniperus cover. There is a monotonic decline of species number with increasing shrub cover in Potentilla plots of all sizes. The species richness in plots with Potentilla was fitted by the regression equation S = 16.48+ 14.91 log A‐0.131 P‐0.00104 P2 log A, where P is% Potentilla cover. A faster initial decline of species number with Potentilla colonization is apparently caused by its preferential establisment in species‐rich, elevated microsites.