Change in the dendrochemistry of sacred fir close to Mexico City over the past 100 years

The chemistry of the tree-rings of sacred fir (Abies religiosa) growing in the high elevation forest park, Desierto de los Leones, close to Mexico City was measured in 5 year increments dating back to 1897. Surface soil beneath sampled trees had elevated concentrations of trace metals, especially Pb, Cd and Zn, compared with a reference site at San Pablo 45 km upwind of Mexico City. High metal concentrations were associated with low soil pH. Lead and Cd increased in tree-rings formed since the 1960s, reflecting the increase in urbanization and vehicle numbers in Mexico City. Small increases in Cu and Ni during this period appear to be a natural occurrence, as similar changes were found at San Pablo. Peaks in Cd and most obviously Pb, were found in rings formed in the period 1920–1940 corresponding to the heartwood–sapwood boundary, but were most apparent in trees growing in contaminated soils with low pH. It is suggested that Pb and Cd taken up by roots of sacred fir accumulate in the heartwood, whereas Pb and Cd entering through bark are transported radially to a much lesser extent and more accurately record changes in trace metal deposition.