In urban areas, trees and other greenery have been proven to provide many benefits. For instance, their leaves and needles play a fundamental role in filtering air pollutants and reducing exposure to dangerous airborne substances that can increase the risk of cardiovascular and respiratory diseases. However, it is not yet clear which species of trees purify the air more effectively.
To clarify this issue, a research team from the University of Gothenburg in Sweden has collected leaves and needles from eleven different tree species from the Gothenburg Botanical Garden’s arboretum. They analyzed the samples to determine which pollutants they captured. The investigation has focused on polycyclic aromatic hydrocarbons (PAHs), a type of pollutants emerging mostly from traffic.
“Our analyses show that different tree species have different abilities to absorb air pollutants. Conifers generally absorbed more gaseous PAHs than broadleaved trees. Another advantage of conifers is that they also act as air purifiers in winter, when air pollution is usually at its highest,” reported study co-author Jenny Klingberg, a postdoctoral fellow in Biology and Environmental Sciences at the university.
While needles continue to absorb air pollutants for several years – which leaves, for obvious reasons, cannot – broadleaved trees were found to be more efficient at cleaning the air of particles, due to their larger surface area to which these particles can attach.
“The various species differed more than we expected. Larch, which is a conifer that sheds its needles each autumn, was best in test. Larch trees absorbed the most particle-bound pollutants, but were also good at capturing gaseous PAHs,” Klingberg added.
However, since needles and leaves cannot significantly break down pollutants, the soil beneath the trees can become contaminated when they shed and decompose. This can place soil ecosystems at risk. Fortunately, the pollutants do not seem 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,” explained lead author Håkan Pleijel, a professor of Applied Environmental Science at The University of Gothenburg.
These findings suggest that authorities should not simply start filling city streets with trees but rather combine different tree species to optimize air purification. They should also take into account the other functions and benefits particular trees may have. Finally, while trees and greenery can contribute to better air quality in urban settings, the most important environmental measures remain the reduction of emissions.
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