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News Article

Trees reveal changes in environmental pollution

Chemical analysis of tipuana growth rings shows falling heavy metal pollution in São Paulo

Concern surrounding environmental pollution and its impact on human health has increased significantly in recent years, although the lack of historical environmental data has meant that it is often difficult to assess the exposure of humans to certain pollutants, which is where tree-rings come in.  They can help reconstruct the variation in the environment during periods where data is unavailable.  A recent study, published in the journal Environmental Pollution, has found that the Tipuana tipu tree, native to Bolivia, is the most suitable tree for measuring long-term environmental pollution levels in São Paulo. 

Researchers analysed the chemical composition of tree bark and growth rings of three different tree species in the central region of São Paulo, which included the distribution of cadmium, copper, mercury, sodium, nickel, lead and zinc, in order to reconstruct temporal trends of these chemical elements.  The tests were carried out to compare the performance of the most common tree species in the city; privet (Ligustrum sp.), Sibipiruna or partridgewood (Caesalpinia pluviosa) and Tipuana tipu.  The Tipuana tree was considered to be the most suitable tree for measuring environmental pollution levels in the city, by analysing the chemical composition of the growth rings and tree bark.

Heavy metals and other chemicals that are present in the atmosphere and fall to the ground as rainwater, are absorbed by the roots of the Tipuana tree.  The chemical compounds are transported in sap via the tree’s xylem cells and then stored in the wood of its growth rings.  Samples from the trees were taken using a pressler increment borer, which extracts a cylindrical section of the wood tissue, with very little injury to the tree itself.  The process is similar to a tree biopsy.

The tree rings were then scanned using LA-ICP-MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry) which generated images used for chemical analysis.  Cells of interest were then scrutinised further and a continuous analysis of all the growth rings was performed to determine the level of heavy metals that were absorbed by the rings each year. 

The results pointed to a reduction in the levels of pollution by cadmium, copper, nickel and lead in the west of São Paulo over the last 30 years, as well as a more moderate decrease in the levels of sodium and zinc.  It is thought that the reduction in lead may be attributed to the phase-out in gasoline, while the decreasing trend of cadmium, copper and nickel is likely to be related to the increasing efficiency of vehicles and the deindustrialisation of the city.

The chemical composition of the tree’s growth rings is an indication of the levels of heavy metals in the soil during that year and the results can then be compared to determine how this type of pollution has varied over decades.

In addition, analysing Tipuana bark shows the levels of atmospheric pollutants that have been passively deposited in the external part of the tree trunk.  By monitoring the amount of heavy metals present within the bark of trees located in different areas of the city, enables the researchers to map spatial variations over the years.

"It's easier to obtain samples of bark than annual growth rings, and the chemical analysis of bark is less costly, so we can analyze samples from many trees and cover a large area," said study first author Giuliano Maselli.  "The result is a map of pollution by heavy metals and other chemical elements throughout the city."

Subscribers to Environmental Impact can access over 350 records on ("air pollution" OR "heavy metals") AND "growth rings".  A selection of these records can be found in the further reading section below.

Journal reference

Giuliano Maselli Locosselli, Katherine Chacón-Madrid, Marco Aurelio Zezzi Arruda, Evelyn Pereira de Camargo, Tiana Carla Lopes Moreira, Carmen Diva Saldiva de André, Paulo Afonso de André, Julio M. Singer, Paulo Hilário Nascimento Saldiva, Marcos Silveira Buckeridge. Tree rings reveal the reduction of Cd, Cu, Ni and Pb pollution in the central region of São Paulo, BrazilEnvironmental Pollution, 2018; 242: 320 DOI: 10.1016/j.envpol.2018.06.098

Article details

  • Author(s)
  • Stephanie Cole
  • Date
  • 26 September 2018
  • Source
  • Fundação de Amparo à Pesquisa do Estado de São Pau
  • Subject(s)
  • Dendrochronology