Passive monitoring of trace elements by means of tree leaves, epiphytic lichens and bark substrate

Total concentrations of Al, Cu, Fe, Hg, Mn, Mo and Zn in the epiphytic lichen Parmelia caperata, substrate bark and leaves of oaks (Quercus cerris and Q. pubescens) from a background area in Tuscany (central Italy) were compared. The elemental composition of unwashed samples, especially bark, was clearly affected by soil dust contamination, thereby leading to incorrect interpretations of baseline concentrations and relationships between elements. The normalization of total element concentrations to the surface soil composition (digested and analyzed by the same procedures), using Al as reference element, gave more reliable baseline values and allowed comparison between different biomonitors. The suitability of lichens, leaves and barks for studying the environmental distribution of trace elements in terrestrial ecosystems is discussed.