Every year, air pollution is responsible for seven million deaths around the world. Recent studies have emphasised how trees and green spaces are essential to maintaining good air quality in urban areas.
Now new research from Sweden explains that mixing species of trees could be optimal for helping improve the air we breathe.
The research from the University of Gothenburg in Sweden, published in the journal “Ecological Indicators”, shows that mixing tree types can help purify air in urban spaces. Looking at the different benefits and functions of various trees, they evaluated their abilities to absorb air pollutants in urban areas.
According to the study, the leaves and needles of trees filter air pollutants and reduce exposure to hazardous substances in the air.
The researchers firstly collected leaves and needles from 11 different types of trees growing in the same location in the arboretum of the Gothenburg Botanical Garden. They then analysed these samples to determine the pollutants they had captured.
Among 32 air pollutants taken into account, the researchers focused on polycyclic aromatic hydrocarbons (PAHs). In urban settings, PAHs are largely produced by incomplete combustion from car traffic.
The study found that the accumulation of pollutants in leaves varied greatly among species. Conifers, which include firs, pines and larches, found to absorb more gaseous PAHs – and continued to do so for several years – compared to deciduous trees such as oaks and poplars.
But the latter, which have well-developed leaves, have the advantage of being efficient in cleaning up polluting particles from the air. According to experts, this is due to the fact that the leaves have a larger surface area, which the particles can attach to.
“The various species differed more than we expected. Larch, which is a conifer that sheds its needles each autumn, was best in testing. These trees absorbed the most particle-bound pollutants and were also good at capturing gaseous PAHs,” said Jenny Klingberg, a co-author of the study.
But weaknesses were noticed: trees absorb pollutants but don’t destroy them. This means there’s a potential risk of the soil around the trees becoming contaminated when leaves and needles fall and decompose, although this was not studied in this investigation.
“The pollutants do not appear to impact the trees’ photosynthesis; leaf chlorophyll content is just as high in the most polluted areas of Gothenburg compared with trees that grow in less polluted environments.
“But this likely looks different in cities with even worse air quality,” noted project leader Håkan Pleijel, professor of applied sciences at the University of Gothenburg.
While the research focused on trees more commonly found in the West, the findings could apply to urban planning all over, including in Malaysia.
The study outlines that careful planning in cities is in order and, by combining different tree species, better air purification can be facilitated. For example, on a narrow street it may be preferable to have hedges rather than a line of trees, which could actually result in reducing airflow and promoting greater pollution on neighbouring streets.
To remedy the problem, researchers advocate low vegetation on narrow streets that are sheltered from the wind.