Using tree rings to assess the air quality in Klosters, Switzerland

Abstract

Tree rings are widely recognised as passive environmental archives able to record the atmospheric composition and reflect the signal of the major pollutants with yearly resolution. This characteristic of trees and its efficacy has been tested in the current investigation. The case study of the village of Klosters was chosen to check the hypothesis that two groups of Norway spruce trees growing in relatively similar environmental conditions but exposed to two different quantities of atmospheric pollution from traffic exhausts, will store a dissimilar amount of pollutants within their tree rings. In order to verify this hypothesis, a combination of multiple analytical techniques was implemented to identify the signal of specific elements and isotopes usually associated with road traffic pollution. The Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) was performed to verify the presence and signal of several heavy metals, often found in vehicles exhausts or released by the wear of the engines. The nitrogen isotope analysis gave insights into the concentration of the δ15N isotope, which depending on the emitting sources can be more or less depleted. Finally, the 14C analysis depicted the sites’ dilution effects generated by the emissions of radioactively dead CO2. However, the radiocarbon analysis revealed that in the site exposed to lower quantities of traffic pollution the growth of Norway spruces is limited by the availability of CO2 rather than nitrogen, which is very often the case for most of the forested ecosystems. The results of the three analyses did not reflect the expectations of the initial hypothesis, since changes and differences in traffic volumes do not seem to produce major discrepancies in the recorded data of tree rings. Such lack of response in tree rings is possibly ascribable to the various limitation encountered in the current research for each of the applied method