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We need trees to fight pollution in cities—but which trees we use matters a lot

Placing trees between roads and pedestrians can make life way healthier, but some trees perform their task better than others.

We need trees to fight pollution in cities—but which trees we use matters a lot
[Source Image: ilyakalinin/iStock]

Though having a lot of greenery indoors may not significantly remove pollutants from the air of your home (though the plants do look nice), green infrastructure does have a large impact. Some outdoor vegetation does directly remove pollutants from the air, but even on the scale of an entire city, this effect is pretty negligible. Instead, what greenery can do in a specific area or on a specific street, though, is form a physical barrier between traffic emissions and pedestrians walking around, which does protect from the health effects of air pollution. It’s not that just having trees somewhere in a city helps to make the air less polluted; it’s more about having the right kinds of trees in the right places.

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Trees that are part of these green barriers do directly capture some pollutants. They also divert and dilute plumes of polluted air, even affecting wind flow depending on how porous or dense the green infrastructure is. In a paper recently published in Climate and Atmospheric Science, two experts from the Global Center for Air Research (GCARE) analyzed scientific literature on what aspects of green infrastructure influence ambient air quality, and put together information about 12 influential traits for 61 tree species to help urban planners and landscape architects pick which trees to plant to be the best barrier against pollutants.

Properties including small leaf size, high foliage density, and trees with long “in-leaf” periods like evergreens make a tree a more effective barrier against roadside pollution. Also, a rough leaf texture, such as a “hairy” or waxy surface—rather than a smooth leaf surface—is more effective in terms of capturing pollutants that are directly deposited onto the leaves.

Of the 61 species the researchers identified, Prashant Kumar, founding director of GCARE and one of the paper authors, says in an email, “all are capable of forming a moderately dense barrier, and are therefore effective as a physical barrier between road traffic emissions and pedestrians.”

One of the key messages of this paper, Kumar notes, is that the context of where these trees will be planted determines whether or not a specific species is going to be beneficial or detrimental to the air quality. He and his coauthor, Yendle Barwise, did identify which species were potentially the most effective overall; of the 61 species included in the paper, there are 12 standouts, including seven pines (black, maritime, monterey, eastern white, scot, stone, and bhutan).

There are also some of those potentially harmful trees on the list, too, which is why the researchers emphasize that the most effective plant really depends on the environmental context—for example, whether, it will be used in a deep (like a city commercial center) or shallow (a residential road) street canyon or in an open road environment. That list of 12 standouts also included Turner’s oak and Spanish oak, which can emit a lot of volatile compounds and so the researchers caveat that these should not be planted in great quantities, as well as common yew, which is poisonous and so shouldn’t be planted near playgrounds or livestock.  “Those species (even if they are native) should not be planted in vast swathes,” he says, “and some of the species are high pollen emitters (e.g. western red cedar), and so those species even if they are native should not be planted near vulnerable populations.

By providing more information on what makes a tree a potential great barrier against air pollution, and what may make a tree harmful to the ambient air quality, the researchers hope urban designers and landscape architects can make more informed and strategic decisions about what kind of green infrastructure they plant, to benefit a city’s population as well as the climate. “Rapid urbanization and the ever-growing need for housing have turned large areas of many cities into concrete landscapes that lack greenery. Green infrastructure can support human health, provide socioeconomic and environmental benefits, and bring color to an otherwise grey urban landscape,” Kumar says. “There is no standard, but an informed decision on their deployment is key for exploiting the full potential of them.”

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