Factors affecting the species richness of old permanent semi-natural grasslands in North-East Scotland
The successful management for conservation and restoration of semi-natural ecosystems may be achieved only after the factors which regulate plant biodiversity and species composition have been identified. This study investigates the floristic composition and site characteristics of old permanent semi-natural grasslands in North-east Scotland and tests using pot and field experiments the role of soil fertility in determining vascular plant species richness, relative abundance and dynamics of communities. Results from field surveys show that, of the site characteristics quantified, vascular plant species richness is most strongly correlated (negative relationship) with extractable soil phosphorus. Where species richness is high (>40 in a study stand and >17 in a 1 m2 quadrat) extractable soil phosphorus levels (determined by ammonium acetate-acetic acid/polyacrylamide solution extraction and using inductively coupled radio frequency plasma spectrometry by the atomic emission method) are consistently very low (<1.9 mg 1-1 soil) by local agricultural standards. In pot experiments using soil from a site where high species richness is maintained, the loss of forb and graminoid species from synthesised communities may be shown to increase along a nutrient gradient created by incorporating increments of phosphorus. Despite a general increase in the above ground biomass of species which survive, those with the ability to form nitrogen-fixing nodules grow to dominate the community. By comparison, a nutrient gradient created by the incorporation of potassium has little effect on the species richness of synthesised communities. At field sites where the level of extractable soil phosphorus approaches that above which species-rich semi-natural grassland communities are not found to exist, the addition of nitrogen leads to species loss through the dominance of graminoid species. This effect may be intensified by applying phosphorus.