Approaches to Understand Historical Changes of Mercury in Tree Rings of Japanese Cypress in Industrial Areas

Abstract

Historical changes of mercury (Hg) concentrations in tree rings of Japanese cypress (Chamaecyparis obtusa) and the role of dendrochemistry over the last 50 years in Yeosu and Gwangyang National Industrial Complexes of South Korea were evaluated. Mercury uptake in trees were found to be bidirectional and variable depending on atmospheric Hg conditions. With low atmospheric Hg concentrations, Hg concentrations in tree rings were significantly influenced by soil Hg concentrations via roots. With high atmospheric Hg concentrations, Hg concentrations in tree rings were dominated by atmospheric Hg uptake via foliage. Patterns of Hg concentration in sampling sites were divided into: (1) a linear increase in low concentration of Hg originated from soils via roots during 1967–1977 and (2) an elevated and constant concentration with spatial variation of Hg concentration due to foliar uptake from atmosphere during 1978–2014. Between 1967 and 1977, when shrubs and vegetation senesced each year, there was an annual source of Hg in soils due to continued deposition of Hg to soil via litterfall and debris. Thus, Hg concentration was increased over time. During these periods, Hg concentrations in tree rings reflected uptakes of Hg through roots under young forest and low atmosphere Hg conditions. Whether tree rings can serve as reliable proxies for atmosphere Hg concentrations remain unclear due to Hg uptakes from soils and limits from atmosphere under low atmospheric Hg conditions. Intensified chemical plants and steel mills have continued throughout Yeosu and Gwangyang industrial areas since late 1970s, resulting in high Hg emissions. Hg concentrations in tree rings during 1978–2014 showed elevated and constant levels. In addition, tree ring Hg concentrations at study sites were increased gradually with decreasing distance from industrial areas, with a high concentration of 11.15 ng/g at the Yeosu site located the nearest to industrial areas and a low concentration of 4.34 ng/g at the Suncheon site which was the farthest away from industrial areas.