Using sugar maple tree rings to trace historic lead pollution in eastern Canada temperate forest

Abstract

The use of tree rings to record changes in lead (Pb) deposition levels and pollution sources has produced contrasted results depending on tree species and sites. In this study, Pb concentrations and isotopic ratios (206Pb/207Pb, 208Pb/206Pb and 206Pb/204Pb) were analyzed in the exchangeable (0.05M HCl extraction) and the residual (concentrated HNO3 extraction) fractions of sugar maple’s tree rings at four eastern Canada forest sites. Above 90% of tree ring Pb was associated with the residual fraction, reflecting strong binding to wood, thus suggesting low radial mobility in sugar maple. After an increase in the first part of the 20th century, wood Pb concentration declined from the 1970s–80s to nowadays for most sampled trees to reach levels similar or even lower than in the early 20th century. This decline was paralleled by a slight decrease in 206Pb/207Pb ratio, reflecting a decrease in Pb pollution in the last decades. Our isotopic data suggest the contribution of USA sources declined by 20–40% at three out of the four study sites since the early 1900’s. These observed trends align with findings reported in previous studies and reveal that sugar maple may be a good tree species model to record Pb atmospheric pollution. Further study must be conducted to validate these findings, especially to estimate the contribution of root vs. aboveground biomass to Pb uptake.