As surprising as it may seem, the trees that make our cities greener can also contribute to poorer air quality by promoting ozone formation. And some tree species contribute more than others, according to a study published last month in the scientific journal Science Advances.
Ozone, a gas that is toxic to our lungs, is formed through complex chemical reactions that occur in sunlight between volatile organic compounds (VOCs) released by vehicles, chemical industries and vegetation with the nitrogen oxides produced by high-temperature combustion, including in vehicle engines.
The researchers set out to estimate the contribution of various VOC sources to ozone production in Beijing, China. To their surprise, they found that urban vegetation — trees lining streets, as well as those in parks, urban forests and other green infrastructure — generated more than 50 per cent of the chemical compounds, or VOCs, involved in the reactions that lead to ozone formation. They also found that isoprene, a highly reactive compound, was by far the VOC most commonly released by this vegetation.
The high concentrations of isoprene in the Chinese megacity are attributable to “vegetation cover that has steadily increased over the past two decades, while vehicle VOC emissions have declined significantly,” the authors wrote.
According to their analysis, Beijing’s high isoprene emissions can be explained by an urban forest made up largely of weeping willows and poplars, both isoprene-emitting species that have been favoured because of “their rapid growth and resilience to climate change.”
Climate warming
In a second stage, the researchers estimated isoprene emissions in other major cities around the world using tree inventories and datasets on urban vegetation planning. They found that some of these cities had higher isoprene emissions than Beijing, including Melbourne and Sydney, Australia. They also noted that cities with particularly high isoprene emissions — such as Los Angeles, Atlanta, New York, Berlin and Singapore — tend to have urban vegetation made up largely of tree species that emit high levels of isoprene: eucalyptus in Australia, oak in the United States and willow and poplar in northern China.
The researchers predict that climate warming will only intensify ozone production in cities, since both the concentrations and the reactivity of VOCs emitted by urban vegetation increase as temperatures rise. Vegetation’s contribution to the chemical reactions that lead to ozone formation rises from 21 per cent at 20 C to 74 per cent at 35 C.
The study’s findings show that replacing even a small proportion of isoprene-emitting trees in urban areas can significantly reduce overall isoprene emissions. The researchers recommend that planners overseeing the greening of major urban developments choose tree species based on their VOC emissions to prevent air quality from deteriorating in the future.
Putting ‘the right tree in the right place’
According to biologist Anthony Daniel, an urban forestry planning adviser with the City of Montreal, ozone production in Beijing is largely driven by air pollution — particularly the presence of nitrogen oxides generated by road transportation and industry — as well as by the city’s higher temperatures.
“Beijing is much more polluted than Montreal, and temperatures there are higher. So ozone production is not as much of a problem in Montreal as it is in Beijing,” he said.
Between 2015 and 2024, no fewer than 260 different tree species were introduced into Montreal’s urban landscape. Among the species planted most frequently are several varieties of oak, Dutch elm disease-resistant elms, honey locusts, Kentucky coffeetrees, several species of maple, ginkgo trees and serviceberries, says Laurence Larose, an urban forestry planning adviser with the City of Montreal.
“We don’t particularly favour fast-growing species [as China has done]. We do plant some poplars and willows, but less often,” Anthony Daniel added.
The goal is above all to put “the right tree in the right place,” Larose said. For example, if a site is even somewhat shaded, a Carolina hornbeam may be planted. Along heavily travelled roads, as well as downtown where soils are compacted, the city selects species that can tolerate urban stress, pollution, soil compaction and road salt, such as honey locust, Kentucky coffeetree, ginkgo and certain elms. Heat- and drought-tolerant species, including bur oak, honey locust, Kentucky coffeetree, common hackberry and ginkgo, are chosen to green highly paved areas and urban heat islands.
At La Fontaine Park, very old poplars that were losing branches had to be cut down because they “posed a safety hazard to people using the park. We replaced them with red oaks,” Daniel said.
“Oaks are large trees that grow well in parks. They provide good canopy cover, which is in line with the city’s goal of expanding the urban canopy to reduce heat islands,” Larose added.
“Our planting strategies today emphasize diversity to avoid problems such as the emerald ash borer and Dutch elm disease, which devastated some city streets that had been planted as monocultures,” Daniel said.
“And even though some of the species we plant produce isoprene, I would say the benefits provided by trees, particularly their ability to reduce ambient temperatures, will naturally help offset the problem of ozone production,” he explained.
