Climatic isotope signals in tree rings masked by air pollution : A case study conducted along the Mont Blanc Tunnel access road (Western Alps, Italy)

Three sites at about 1400 m a.s.l., were chosen for this study along the Mt. Blanc Motorway in Italy. Chronologies of stable isotope ratios (δ 13C, δ 18O, δ 15N), total N concentration and ring width of Larix decidua Mill. were analyzed to observe changes in growth and climatic signals in tree rings after significant changes in air pollution emissions occurred locally over time. The tunnel opened in 1968 and was closed for three years from March 1999 to March 2002. The obtained series from the three sites (Close = C, High = H and Far = F from the highway) for the analyzed periods, 1950-1970 (only sites H and F) and 1985-2008 (all sites), did not show any particular long-term change except site H that showed significant changes in δ 18O (enrichment), δ 15N (depletion) and total N (increase) δ 13C values at site C were enriched in the first year of the tunnel closure (1999), showing an opposite trend in δ 13C at site C, in comparison to the two control sites H and F, which cannot be explained by climatic factors. Since no great differences in δ 18O were recorded in 1999, this enrichment in δ 13C could be related to an enhancement in photosynthetic rate during periods of low air pollution loads. Opposite to δ 15N, total N concentration shows a generally good correlation between sites, and site F was measured as the most N enriched. In a correlation analysis performed on the two study periods between the climatic parameters and ring width, we did not find any clear relationships, whereas for the series of stable isotopes and total N, we found the strongest and most significant relationships only between δ 13C and summer (June to August) temperature (positive correlations) and precipitation (negative correlations) at sites H and F. These same relationships at site C were, instead, mostly insignificant, indicating an alteration of the climatic signal recorded in the δ 13C chronology, caused by direct exposure to the high level of air pollution at this site. Given that site C is more affected by pollution with respect to the other two sites (whereas the same climatic conditions influence tree growth at all sites), the lack of a climatic signal in the δ 13C chronology at this site can be ascribed to air pollution. Few other long-term changes were recorded by tree rings (e.g. at site H), indicating that trees probably record better the pollution events or the worsening of the environmental conditions rather than a lack of pollution for a relatively short time period in a polluted environment.