Chemical elements and heavy metals in European larch tree rings from remote and polluted sites in the European Alps

Abstract

Air pollution dispersal in the European Alps has been studied both for glacial and forest environments. In this study, chemical elements and heavy metals in the tree rings were analyzed for seven sites of European larch (Larix decidua Mill.) in the Italian European Alps. At three sites in the proximities of the Mont Blanc Tunnel (MBT) entrance the analyses were performed at the yearly scale on the periods 1950-1970 (comprising the MBT opening in 1965) and 1985-2008 (comprising the 3-yr MBT closure after the 1999 car accident) with the aim to check if trees recorded at the yearly scale variations in chemical elements and changes in heavy metals concentrations over time. At the regional scale, the analyses on heavy metals were conducted without annual resolution for most sites on the common periods 1950-1970 / 1985-1998 with the aim to detect possible ongoing trends and differences between some Alpine sites. Chemical elements concentrations at Entrèves (EN) sites varied significantly between heartwood and sapwood with generally higher concentrations in sapwood. At EN sites no clear patterns were found for heavy metals before and after the MBT opening and during its 3-yr closure. We found that the “high” site (ENH) was generally less polluted than the “close” (ENC) and the “far” (ENF) sites. At site ENC we found higher values of Cr, Ni and Cu, whereas at site ENF we found higher values for all the other elements analyzed. The analysis of heavy metals at the regional scale revealed generally no significant temporal changes in concentrations except for Cr and Cu, showing higher values in the recent period. On comparing the heavy metals concentration between the seven sites, the Palud site showed almost always the highest concentrations, except for Ni and Cu that were higher in two remote sites close to glacial environments in the Gressoney and Valtellina valleys. Dendrochemical analysis revealed that the temporal information in the tree rings is covered by too many signals and no environmental changes are recorded at the yearly scale by European larch. However tree rings may provide useful information on ongoing long-term trends and on the spatial definition of pollutant dispersal in the Alpine environment.