Species diversity in plant communities on ultramafic soils in relation to pine afforestation

The influence of pine afforestation on the species diversity of plant communities on ultramafic substrate was investigated in an area of Tuscany, central Italy, by means of species-area relationships, plant unit area, the Gini coefficient and the pattern of the index of Jaccard in relation to plot size. The species-area relationship was found to best fit the semilogarithmic model. Contrary to the available data for temperate ecosystems, tree canopy cover was found to increase the alpha-diversity of the understorey vegetation and its cover. The nutrient input due to the pine canopy caused an increase in the abundance of the grass Festuca inops - which was already present in the community - and subsequently the spread of several grassland species leading to a reduction in dominance concentration and to the formation of a species-rich grassland. The pine cover caused a decrease in floristic resemblance between plots, especially when the canopy cover is scattered. The increase in species richness found under the pine canopy, where the metal content in the soil is higher, suggests that potentially toxic metals are not the most limiting factor in Tuscan ultramafic soils. The typical poorness of vegetation on ultramafic soils should be first of all related to hydrological and nutritional stresses.