CHANGES IN PLANT-SPECIES RICHNESS IN A CALCAREOUS GRASSLAND FOLLOWING CHANGES IN ENVIRONMENTAL-CONDITIONS

This study presents results of a three and a half year experiment in a calcareous grassland on the Baltic island of Oland, south-eastern Sweden. Environmental conditions, viz. nutrient regime, water availability, light conditions and grazing pressure were changed in a randomised experimental design with ten different combinations of these factors, including a control. Weather conditions during the study period fluctuated considerably. Only the combination of reduced light and increased soil nutrient availability decreased species richness significantly, presumably by inducing asymmetric competition for light by competitively strong species. Physical space also became less available for weaker competitors. In cases where only nutrients or only water were supplied in sufficient amounts, competitive exclusion was inhibited by either low nutrient or low water availability. Under conditions which combined high levels of nutrient and water supply with regular disturbance (i.e. simulated grazing) high species richness was maintained. Although there was a significant negative correlation between species richness and amount of above-ground biomass, the number of species was not significantly reduced in the treatment which combined fertilisation and irrigation. One reason for this may be that the heterogeneous light regime in this treatment still allowed species persistence and coexistence.