Tree rings reveal the reduction of Cd, Cu, Ni and Pb pollution in the central region of São Paulo, Brazil.

The concern about environmental pollution has risen in the last decades because of its effects on human's health. However, evaluation of the exposure to certain pollutants is currently hampered by the availability of past environmental data. Tree rings are an alternative to reconstruct environmental variability of pre-instrumental periods. Nevertheless, this approach has some reported limitations including migration of chemical elements in the tree rings. The aim of this study was to evaluate the distribution of Cd, Cu, Hg, Na, Ni, Pb, Zn in the tree rings of Tipuana tipu (Fabaceae) to aid the reconstruction of past environmental pollution. We sampled trees in the central region of the city of São Paulo, Brazil, and scanned their tree rings using LA-ICP-MS. We used these data to evaluate the temporal trends of chemical elements under investigation. Results show a non-random distribution of these chemical elements within the tree rings, with higher content in the cell-walls of vessels and lower content in the fibers. Sodium was the only element intimately related to the axial parenchyma cells. Due to differences in elemental composition of xylem cells, temporal trends where evaluated using distinct quartiles of data distribution in each tree ring. The first quartile represents the lower content found in fibers and parenchyma, while the third quartile corresponds to the higher content found in vessels. Data from vessels better represent the decreasing trend of Cd, Cu, Pb, and Ni in the last three decades. This reduction is less significant for Na and Zn. Our results highlight the potential to improve the records of environmental pollution using data from different cells. Pronounced reduction in Pb may be attributed to the lead phase-out in gasoline, while the decreasing trend of Cd, Cu, Ni pollution is probably related to increasing efficiency of vehicles and the deindustrialization of São Paulo. Chemical elements are non-randomly distributed in tree rings. Chemical content of vessels cell-walls is a reliable record of metal pollution, which is decreasing in São Paulo.