A walk or drive along the tree-lined streets in São Paulo, the largest city in Brazil, you will see a number of tipuana trees (Tipuana tipu). Also called rosewood or tipu, it is a tall tree with a huge spreading canopy and is everywhere in the city.
Native to various South American countries, including Bolivia, the tipuana was first planted in São Paulo in the 1950s. Apart from providing shade and other environmental advantages, it is also a witness to the evolution of pollution in the city.
Scientists from the University of São Paulo’s Bioscience Institute (IB-USP) and Medical School (FM-USP), in partnership with collaborators at the University of Campinas (UNICAMP), used the tipuana as a marker of heavy metals and other pollutants in the air inhaled by São Paulo’s inhabitants.
Their investigation of the chemical composition of annual growth rings — concentric circles that can be seen in a cross-section of the trunk — and samples of bark from fallen tipuana trees in the west of the city revealed a decrease in the levels of pollution by nickel, cadmium, lead, and copper in the past three decades.
The outcomes of the study were reported in the Environmental Pollution journal. The study was supported by FAPESP through a Thematic Project, a regular research grant, and a postdoctoral research scholarship.
In the recent past, scientists at IB-USP and FM-USP have started the evaluation of the probability of investigating the chemical composition of tree bark and growth rings with the goal of evaluating environmental pollution levels in São Paulo over the long term. For this, they have chosen three of the most common tree species in the city: privet (Ligustrum sp.), sibipiruna or partridgewood (Caesalpinia pluviosa), and tipuana.
The tipuana was mainly introduced by real estate developer Companhia City, which, very soon after World War II, developed a number of green neighborhoods in western São Paulo, such as Pacaembu, Jardim Europa, and Alto de Pinheiros.
The scientists are of the view that the species the most appropriate for the chemical analysis of tree rings, also called dendrochemistry.
The tipuana has proved the best species both for this chemical analysis of annual growth rings and for measuring the evolution of environmental pollution.
Giuliano Maselli Locosselli, Postdoctoral Researcher, IB-USP
During tropical storms, tipuanas are often blown down, and São Paulo has started to replace them with native species. Heavy metals and other chemicals present in the atmosphere are absorbed by their roots and they fall to the ground in rainwater. The tree’s xylem cells transport these chemicals in sap, where the chemicals are stored in the wood of its growth rings.
Each growth ring represents a year of the tree’s life. The more recent rings are farther from the center and wider, and those closest to the center are narrower. The heavy metal levels in the soil year by year are reflected by the chemical composition of the tree’s growth rings, and the outcomes can be compared to ascertain the way the type of pollution has differed on a scale of decades.
“If a tree is 50 years old, for example, it will tell the story of pollution in the city during that period,” stated Locosselli.
Analysis of tipuana bark reveals the levels of atmospheric chemicals passively deposited in the external part of the trunk. The researchers will be in a position to map spatial variations in the levels heavy metals on a scale of years by evaluating the levels of heavy metals in samples of bark from trees that still stand in various neighborhoods of the city.
“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” stated Locosselli. “The result is a map of pollution by heavy metals and other chemical elements throughout the city.”
Falling Levels of Pollution by Heavy Metals
The scientists performed an initial study which involved the measurement of the levels of nickel, cadmium, sodium, copper, lead, mercury, and zinc in rings from two tipuana specimens standing in the gardens of the University of São Paulo’s Medical School in the west of the city’s central region. Their aim was to investigate the temporal changes in the levels of heavy metal pollution in this part of São Paulo.
The age of the two trees used in this study was 35 years. An instrument known as Pressler increment borer was used for taking samples from their growth rings. This device includes a hollow auger bit and is designed to extract a cylindrical section of wood tissue from a living tree through its entire radius with comparatively lesser harm to the plant itself.
The procedure can be compared to a tree biopsy.
Giuliano Maselli Locosselli
The annual growth ring samples measuring 15 mm were sent to Marco Aurelio Zezzi Arruda, a professor at the University of Campinas’s Chemistry Institute (IQ-UNICAMP). The samples were scanned by laser ablation coupled with mass spectrometry, producing software-processed images for chemical analysis.
The researchers chose the cells of interest and carried out a continuous analysis of all the annual growth rings to evaluate the levels of heavy metals absorbed by the trees in each year of their lives.
The data analysis revealed a considerable decrease in pollution by nickel, cadmium, lead, and copper in the past 30 years in the part of the city inhabited by the sampled tipuana trees, and also a more moderate reduction in the levels of zinc and sodium.
“The falling levels of lead reflected the gradual elimination of this chemical element from the composition of Brazilian gasoline,” stated Marcos Buckeridge, a professor at the University of São Paulo’s Bioscience Institute (IB-USP) and a coauthor of the study.
“The downtrend in cadmium, copper and nickel pollution probably reflects enhanced vehicle efficiency and deindustrialization in São Paulo,” Buckeridge told Agência FAPESP.
For a considerable part of the 20th century, tetraethyl lead was used across the globe as an antiknock agent in automotive gasoline to optimize engine performance and to reduce wear. The ensuing release of lead into the atmosphere through vehicle exhaust was a serious health hazard.
In 1988, the addition of tetraethyl lead to automotive gasoline was banned by Brazil.
The electronic appliance industry, lithography, plastic, solar cells, tire rubber, pigments used in dye and paint, fuels, batteries, semiconductors, photography, urban garbage, fireworks, and electroplating are the principal sources of cadmium in ambient air pollution. Metal alloy casting, burning of industrial and urban waste, and pesticides are the principal sources of copper emissions.
“The levels of these chemical elements in São Paulo’s ambient air have fallen in recent decades owing to deindustrialization,” stated Locosselli.