INFLUENCE OF COLONIZING SHRUBS ON SPECIES-AREA RELATIONSHIPS IN ALVAR PLANT-COMMUNITIES

In herbaceous plant communities of Stora Alvaret (the Great Alvar), Oland, 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 logA - 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.