Air pollution in the past recorded in width and stable carbon isotope composition of annual growth rings of Douglas‐fir

Tree‐ring indices (TRIs) of annual growth rings in stems of Douglas‐fir ( Pseudotsuga menziesii ) growing near a copper smelter showed reduced growth during two multi‐year time periods in the past. These periods coincided with World Wars I and II, which are known to represent periods of particularly high SO 2 emissions from the smelter. Reduced growth was correlated with less negative stable carbon isotope composition (δ 13 C) in cellulose purified from wood formed in such years. Based on current models for 13 C/ 12 C in plants, these results indicate that exposure to air pollution resulted in reduced concentration of CO 2 in the intercellular air spaces of the needles. This is consistent with the hypothesis that stomatal closure resulted in impaired photosynthesis and reduced growth during past episodes of high air pollution. The pollution‐related change in δ 13 C was superimposed on a change with time in δ 13 C, independent of growth, by ‐ 1.4 per mil from 1902 to 1984.