Plant species turnover in an alvar grassland under different environmental conditions and at different scales

Abstract

Nutrient and water supply, light availability and removal of biomass were manipulated in a limestone grassland community on the Swedish island of Oland from 1990 to 1993 in order to investigate how these factors influence species richness and year-to-year turnover at fine scales. Turnover, broken down into disappearance (‘extinction’) and appearance (‘immigration’) of vascular plant species, was measured at different spatial scales in plots ranging from 4 cm2 to 0.25 m2. For most treatments, turnover was highest at the finest scale, both measured as absolute and relative numbers. Turnover rate increased at all scales when the treatment implied such a change in the environment, where application of fertilizer was combined with shading. In these cases, disappearance of species largely exceeded appearance in a predictable manner, and resulted in species-poor assemblages at all scales. In plots treated with either fertilizer or shade, there were large differences in the direction of the turnover (i.e. whether immigration or extinction dominated) between years, apparently depending on the weather conditions in the respective year. After drought spells total turnover was also high for such treatments, whereas plots regularly receiving additional water supplies had rather similar turnover rates during the whole experiment.