Air pollution affecting growth of trees

Washington: Air pollution is not just responsible for affecting human health, it can also affect the growth of trees, a recent study suggests. The study was published in the journal, 'Science of the Total Environment'

The research, conducted by researchers of a university in Sao Paulo in Brazil, has shown that atmospheric pollutants restrict tree growth and the ecosystem services provided by trees, such as filtering pollution by absorbing airborne metals in their bark, assimilating CO2, reducing the heat island effect by diminishing solar radiation, mitigating stormwater runoff, and controlling humidity.

"We found that in years when levels of particulate matter in the atmosphere were higher, for example, the trees grew less. As a result, they started later in their lives to provide ecosystem services that play an important role in reducing urban pollution and mitigating or adapting the city to climate change," said Giuliano Maselli Locosselli, one of the researchers and first author on the study.

Using as a model the Tipuana tree (Tipuana tipu), also known as Rosewood or Tipu, a tall tree with a large spreading canopy, the researchers measured the impact of air pollution and weather on the tree's growth in Sao Paulo. They analysed samples of 41 Tipuanas aged 36 on average located in the vicinity of an industrial area.

Samples were taken from the trees' growth rings using an instrument called a Pressler increment borer, which has a hollow auger bit and is designed to extract a cylindrical section of wood tissue from a living tree throughout its radius with a relatively minor injury to the plant. All samples were taken at chest height, approximately 1.3 m from the topsoil.

By analysing the chemical composition of the bark and the size of the growth rings, the researchers were able to measure variations in air pollution levels based on the various chemical elements to which the trees were exposed during their development and to estimate how this factor influenced tree growth.

"The tipuana is an excellent marker that clearly represents levels of air pollution by heavy metals and other chemical elements in the city," Locosselli said. Metals and other chemicals suspended in the air are absorbed by bark. Particulate matter is deposited on leaves, increasing their temperature and reducing the supply of light for photosynthesis.

Growth rings indicate how pollution has affected the life of the plant year by year. Thicker rings indicate years of vigorous growth and lower levels of pollution, while thinner rings indicate the reverse.

Analysis of growth rings showed that these tipuanas grew faster in the warmer parts of Capuava with higher levels of phosphorus in the air. Phosphorus is a known macronutrient for plants and acts as the basis of their energy metabolism via photosynthesis and respiration.

On the other hand, trees close to traffic and exposed to high levels of aluminium, barium and zinc associated with the wearing of automotive parts (such as tires, brake linings and clutch plates) displayed less growth over time.

"Trees exposed directly to high levels of pollution from the factories in the area grew less in terms of trunk diameter development throughout their lives than plants exposed to medium and low levels," Locosselli said. "Under normal growth conditions, a tipuana's chest-height diameter can reach 1 meter."