Evaluation of Acer rubrum Tree Bark as a Bioindicator of Atmospheric Heavy Metal Pollution in Toronto, Canada

The concentrations of heavy metal (copper (Cu), mercury (Hg), manganese (Mn), nickel (Ni), lead (Pb), and zinc (Zn)) were measured in Acer(A.) pseudoplatanus tree bark to evaluate its suitability as a bioindicator of air pollution in downtown Toronto, Canada. The analysis of digested tree bark samples was carried out by inductively coupled plasma optical emission spectrometry (ICP-OES) for Cu, Mn, Ni, Pb, and Zn, whereas a mercury analyzer was used to quantify Hg without sample pre-treatment. The concentrations of the analyzed heavy metals were found to be 26.4 μg g−1 for Cu, 51.7 μg kg−1 for Hg, 55.3 μg g−1 for Mn, 6.55 μg g−1 for Ni, 26.5 μg g−1 for Pb, and 95.2 μg g−1 for Zn. Analysis of background control tree barks (collected in a remote area) showed that maple tree barks in Toronto were strongly enriched with heavy metals, with their mean accumulation factors ranging between 1.88 (Mn) and 12.54 (Pb). The tree bark was also found to distinguish between impacted areas as the locations close to the roads with elevated vehicular traffic showed higher metal contents. Therefore, it could be concluded that A. pseudoplatanus tree bark is a good bioindicator of atmospheric heavy metal pollution.