Trees and pollution: Investigating the impact of sulfur dioxide using ring widths and stable isotopes

Excessive sulfur dioxide (SO2) pollution released into the atmosphere from mining and smelting operations in northeastern Ontario, Canada has devastated surrounding forest ecosystems. Over the course of the 20 th century, smelting operations in Sudbury, Ontario released more than 100 million tonnes of SO2 into the atmosphere. At peak production in 1960, Sudbury smelters generated 2.56 million tonnes of SO2 that represented approximately 4% of global sulfur emissions. Past studies have shown that in high enough doses, SO2 acts as a toxic environmental pollutant that can be associated with decreased growth in the annual growth rings of trees. However, the application and use of stable carbon-13 (delta 13C) and sulfur-34 (delta34S) isotopes in dendrochemical studies involving sulfur pollution is still relatively new. This study aims to better understand the influence of atmospheric SO2 pollution loading on nearby forest ecosystems and tree physiology -- from both a spatial and temporal perspective by combining tree-ring width and isotope data. Three sample sites along a 110-kilometer northeasterly transect from Sudbury to Temagami were chosen in order to quantify the interplay of pollution and climate. Ring-width and isotope data provided useful information that allowed this study to comment on the magnitude of influence and impact that SO2 pollution had on eastern white pine (Pinus strobus) trees throughout Sudbury's entire smelting history, as well as the success of Vale's Emission Reduction Program (ERP) that has reduced overall SO2 emissions by 90% over the last 45 years. Results indicated that the closest study site to Sudbury displayed growth ring trends that were influenced by SO2 during the peak emissions period and isotope data revealed a weakened relationship with climate variables before drastic emission reductions in the 1970s. Ultimately, this study provided results in favour of white pine trees being good biorecorders of anthropogenic SO 2 pollution. Because of the influence of SO2 emissions on tree growth and chemistry, failure to identify the associated signals of pollution in tree-ring widths and stable isotope records may lead to biased and inaccurate paleoclimatic reconstructions from regions affected by an increased pollution load.