VEGETATION OF TUSCAN ULTRAMAFIC SOILS TN RELATION TO EDAPHIC AND PHYSICAL FACTORS

Vegetation and soil sampling were carried our in 80 plots located in f ive different ultramafic (serpentine) sites of Tuscany, central Italy. The physical and chemical features of each plot were determined and t he species composition and cover recorded. The exchangeable fraction o f soil;metals was analysed because it gives a measure of their concent rations available to plants. The plots were classified by cluster anal ysis and ANOVA was used to compare the environmental variables of the groups of plots. Canonical correspondence analysis was used to detect the principal factors for gradients of species composition within the plant communities. A higher content of exchangeable metals was found u nder the more evolved and structured plant communities, suggesting tha t serpentine vegetation of Tuscany is not strongly limited by soil met als, such as chromium, cobalt, nickel and magnesium, typically associa ted with ultramafic soils. The low nutrient content of the soils and d rought stress mainly due to topographical features, appear to have a m ore significant role in determining the typical scattered vegetation o f the Tuscan ultramafics.