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 Tus cany, central Italy, by means of species-area relationships, plant uni t area, the Gini coefficient and the pattern of the index of Jaccard i n relation to plot size. The species-area relationship was found to be st fit the semilogarithmic model. Contrary to the available data for t emperate ecosystems, tree canopy cover was found to increase the alpha -diversity of the understorey vegetation and its cover. The nutrient i nput due to the pine canopy caused an increase in the abundance of the grass Festuca inops - which was already present in the community - an d subsequently the spread of several grassland species leading to a re duction in dominance concentration and to the formation of a species-r ich grassland. The pine cover caused a decrease in floristic resemblan ce between plots, especially when the canopy cover is scattered. The i ncrease in species richness found under the pine canopy, where the met al content in the soil is higher, suggests that potentially toxic meta ls are not the most limiting factor in Tuscan ultramafic soils. The ty pical poorness of vegetation on ultramafic soils should be first of al l related to hydrological and nutritional stresses.